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The effects of weathering on the strength and chemistry of Columbia River Basalts
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and their implications for Mars Exploration Rover Rock Abrasion Tool (RAT)
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results
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B. J. Thomson1, J. A. Hurowitz2, L. L. Baker3, N. T. Bridges4, A. Lennon4, G. Paulsen5,
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and K. Zacny5
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1
Center for Remote Sensing, Boston University, 725 Commonwealth Ave. Rm. 433,
Boston, MA 02215, USA.
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2
Department of Geosciences, Stony Brook University, Stony Brook, NY 11794, USA.
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3
Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, ID
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83844, USA.
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4
Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA.
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5
Honeybee Robotics, Pasadena, CA 91103, USA.
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Corresponding author: Bradley J. Thomson
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Email: [email protected]
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Office phone: 617.353.5148
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Number of Pages: 54
Number of Figures: 10
Number of Tables: 6
Date: 30 January 2014
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Abstract
Basalt physical properties such as compressive strength and density are directly
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linked to their chemistry and constitution; as weathering progresses, basalts gradually
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become weaker and transition from intact rock to saprolite and ultimately, to soil. Here
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we quantify the degree of weathering experienced by the Adirondack-class basalts at the
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Mars Exploration Rover Spirit site by performing comparative analyses on the strength
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and chemistry of a series of progressively weathered Columbia River Basalt (CRB) from
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western Idaho and eastern Washington. CRB samples were subjected to compressive
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strength tests, Rock Abrasion Tool grinds, neutron activation analysis, and inductively
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coupled plasma optical emission spectroscopy. Analyses of terrestrial basalts indicate
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linked strength-chemical changes, as expected. Weathering sufficient to induce the loss
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of more than 50% of some cations (including >50% of MgO and MnO as well as ~38%
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of Fe2O3 and 34% of CaO) was observed to weaken these samples by as much as 50% of
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their original strength. In comparison with the terrestrial samples, Adirondack-class
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basalts are most similar to the weakest basalt samples measured in terms of compressive
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strength, yet they do not exhibit a commensurate amount of chemical alteration. Since
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fluvial and lacustrine activity in Gusev crater appears to have been limited post-basalt
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emplacement (most of the aqueous activity in Gusev crater predates the eruption of flood
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basalt lavas), the observed weakness is likely attributable to thin-film weathering on
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exposed, displaced rocks in the Gusev plains (in addition to some likely shock effects).
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The results indicate that Adirondack-class basalts may possess a several mm-thick weak
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outer rind encasing an interior that is more pristine than otherwise indicated, and also
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suggest that long rock residence times may be the norm.
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Keywords: Mars, Earth, Mars Exploration Rovers, basalt, weathering, rock strength,
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physical properties
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1
Introduction
Understanding the material strength of rock, outcrop, and soil targets encountered
53
by landed spacecraft on Mars is an essential part of our knowledge of present and past
54
surface and subsurface environmental conditions. When considered together with a
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rock’s chemistry and mineralogy, information about the physical properties of materials
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(including parameters such as texture, strength, hardness, and particle size, shape, and
57
sorting) provides an additional means to help reconstruct its geologic history. Of
58
particular relevance is the role of rock alteration. As chemical weathering progresses,
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primary igneous minerals are altered and replaced by secondary phases to a degree
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dictated by limiting factors such as temperature, time, kinetics, and/or the availability of
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water. The complex assemblage of primary and secondary materials created by these
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processes yields a unique fingerprint that can be used to constrain the modification
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processes. The strength and physical properties of geologic materials vary in tandem with
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these mineralogical alterations, but, until now, such strength changes have received little
65
attention.
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The purpose of this study is to investigate the linked chemical and strength
67
changes undergone by basalt during weathering. Basalt is an important crustal component
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that represents a significant fraction of the surface area of terrestrial bodies [Basaltic
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Volcanism Study Project, 1981]. Martian basalts have been identified from orbital
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remote sensing measurements [e.g., Mustard et al., 1997; Christensen et al., 2000;
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Bandfield, 2002], chemical, spectral, and textural analyses from landed spacecraft [e.g.,
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Clark et al., 1982; Bell et al., 2004; Christensen et al., 2004; Gellert et al., 2004;
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Herkenhoff et al., 2004; McSween et al., 2004], and within the Martian meteorite suite
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[e.g., McSween, 1994]. Indeed, basalt is the most abundant rock type on Mars [McSween
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et al., 2009]. Given its wide abundance on Earth, Mars, and the other terrestrial planets,
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basalt makes an ideal target material for comparative physical property studies.
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Establishing an empirical linkage between strength and chemistry of basalt undergoing
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weathering will permit greater constraints on the nature and degree of weathering
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processes on Mars as inferred from remote measurements.
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1.1
Previous work
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The chemical and mineralogical effects of basalt weathering have been the
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subjects of numerous investigations. These include studies of water-limited basalt
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weathering [e.g., Elwood Madden et al., 2004; Hurowitz and McLennan, 2007], acid
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weathering [e.g., Tosca et al., 2004; Niles and Michalski, 2009; Hurowitz et al., 2010],
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high water-rock ratio weathering in surficial environments [e.g., Eggleton et al., 1987;
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Nesbitt and Wilson, 1992], hydrothermal alteration [e.g., Kristmannsdottir, 1979;
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Jakobsson and Moore, 1986; Baker et al., 2000], and impact-induced alteration [e.g.,
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Newsom, 1980]. Yet the specific effects of these processes on basalt strength that occur
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with mineralogical changes have been little addressed.
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In a prior study, we used data from Rock Abrasion Tool (RAT) grinds to infer the
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compressive strength of rocks at the Mars Exploration Rover Spirit site in Gusev crater
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[Thomson et al., 2013]. The RAT is a small rotary grinder carried at the end of the each
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rover’s robotic arm (additional details are given in §2.3.2). Designed to abrade and
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remove the outer layers of rocks in order to expose fresher interior surfaces [Gorevan et
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al., 2003], the power consumed during grinding combined with the grinding time can be
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linked to rock strength [Myrick et al., 2004; Thomson et al., 2013]. The results from our
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initial study constrain the range of strengths over which the RAT effectively operates,
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and provides a means to link the specific grind energy to compressive strength. At the
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MER Spirit site, the results confirm and expand previous initial indications that many
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intact martian materials are significantly weaker than their fresh terrestrial counterparts
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[Arvidson et al., 2004; McSween et al., 2006; Squyres et al., 2006; Wang et al., 2006],
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implicating alteration effects. Whereas these results demonstrate a robust method for
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inferring bulk strength, this earlier work did not address the specific nature of the
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weathering. Herein, we derive a relationship between the type and extent of weathering
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and the measured strength, a correlation that holds promise for inferring alteration
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processes from drilling and abrasion tools on current and future planetary rovers.
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1.2
Goals of this study
The goal of this effort is to understand and quantify the geochemical and
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geomechanical changes in typical basalt weathering profiles as a proxy for Mars. The
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sampling locale is the Columbia River Basalt Province, a flood basalt province with
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numerous interflow weathering horizons. This study extends our previous work in which
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the RAT specific grind energy was used infer the strength of rock targets [Thomson et al.,
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2013]. Here, we quantify the systematic changes in basalt physical properties with degree
5 116
of weathering. The results from this study will provide independent constrains on the
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nature and vigor of martian weathering processes.
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2
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2.1
Approach
Sample localities
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All of the samples described in this work were obtained from flows of the
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Columbia River Basalt Group (CRBG) located in eastern Washington and western Idaho
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(Figure 1). Erupted during the Miocene, the Columbia River Basalt Group is divisible
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into four main stratigraphic formations. From oldest to youngest, these are the Imnaha
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Basalt Formation (erupted ~17.5 Ma), the Grand Ronde Basalt Formation (~16.5 and
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15.6 Ma), the Wanapum Basalt Formation (~15.6 and 14.5 Ma), and the Saddle Mountain
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Basalt Formation (~14 and 6 Ma) [Swanson et al., 1979; Camp and Hooper, 1981; Tolan
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et al., 1989]. Grande Ronde Basalts constitute the overwhelming majority (87 vol%) of
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the total erupted volume (1.74×105 km3) [Tolan et al., 1989], and have been divided into
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four magnetostratigraphic units: R1, N1, R2, and N2, from oldest to youngest. Here, the
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designators R and N refer to reverse and normal magnetic polarity, respectively.
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Within this stratigraphy, our focus is in the entombed paleosol horizons in the
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Grande Ronde and Wanapum Basalts. These paleosols were developed on top of basalt
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flows during interflow intervals, and their subsequent burial in later flows provides a
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unique snapshot of weathering processes from that interflow interval [e.g., Sheldon,
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2003; Hobbs, 2010]. Compared to other paleosol surfaces, interflow paleosols confer
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several advantages. First, the protolith material can be directly determined, rather than
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inferred from the general surroundings. Second, the element of time is also tightly
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constrained: paleosol development is bracketed by the ages of the host basalt and the
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successive, capping flow. Finally, as these deposits are presently exposed only in recent
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road cuts, there has been effectively no modern re-activation of these paleoweathering
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horizons. Limited post-burial alteration effects include compaction due to the overburden
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and oxidation of organic matter in the uppermost soil layers [e.g., Retallack, 1991].
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2.1.1
Lawyer Canyon
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The Lawyer Canyon site is located south of the town of Craigmont, ID along US-
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95. Hosted by the N1 magnetostratigraphic unit of the Grande Ronde Basalt, the paleosol
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layer consists of a well-defined, 2 m thick paleosol underlain by a saprolite with a
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gradational lower boundary into progressively less weathered host rock. The Basalt of
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Icicle Flat, equivalent to the Basalt of Dodge (Eckler Mountain Member of the Wanapum
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Basalt [Kauffman, 2004; 2009]), caps the paleosol layer [Bush et al., 2004]. This broad,
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gradual transition presents an ideal locale for numerous macroscopic samples (in other
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localities, the saprolite-basalt transition is commonly compact, i.e., less than several cm
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across). Basalt corestones are observed both in the paleosol layer and the saprolite layer;
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in cross-section these are rounded, isolated masses of less weathered protolith material.
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The total exposed stratigraphic thickness is about 10 m.
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2.1.2
Kendrick Grade
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At this locality, a ~2 m thick paleosol layer (montmorillonite-kaolinite) developed
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on Grande Ronde N1 Basalt and is capped by the Priest Rapids Member of the Wanapum
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Basalt [Lewis et al., 2005]. Beneath the soil layer, a thick saprolite zone transitions down
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over about 2 m through weathered corestones into unaltered basalt parent material.
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Vesicularity increases upward through the saprolite. The paleosol is capped with a
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blackened layer that may be composed of vegetation carbonized during emplacement of
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the overlying basalt flow.
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2.1.3
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Upper Shumaker
At Upper Shumaker, a thin, 0.03 to 0.1 m paleosol layer is present within the
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Eckler Mountain Member of the Wanapum Basalt Formation. The weathered sequence is
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capped by the Shumaker Creek Member of the Wanapum (the intervening Frenchman
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Spring Member is not present at this locality; it pinches out well to the west). Beneath the
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thin paleosol, a broad saprolite zone is present that is more highly vesicular (~20%) than
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the basalt or saprolite present at Lawyer Canyon. Protolith material is accessible only as
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basalt corestones, not as an intact layer of massive, unaltered basalt. The total exposed
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stratigraphic thickness at this exposure is about 4 m.
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2.2
Sample acquisition strategy and preparation procedure
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At each road cut site, relatively intact blocks of basalt were selected at multiple
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heights. These were photodocumented in place, and the surrounding context was noted.
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Blocks were then dislocated and transported down to road level where drill cores were
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extracted using a portable gasoline-powered, wet core drill (the core drill used water to
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cool the diamond bit segments and remove cuttings). Two people were required to extract
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the cores: one to operate the drill and another to supply water using a manual pump. At
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least two drill cores were extracted per sampled block. This approach was deemed safer
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than moving the drill and water-cooling assembly up and down the road cut face due to
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the unstable nature of the surface slopes.
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To avoid complications in the grind tests induced by irregular rock textures, the
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samples were prepared at the University of Idaho. Two parallel rock saw cuts were sliced
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from each block to extract hand samples bounded by flat planes. The resulting hand
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samples were approximately 2 cm in thickness and 10 cm in minimum extent to
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accommodate the outer butterfly contact points of the RAT instrument [Gorevan et al.,
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2003]. Sample bulk densities were recorded using the glass bead displacement method
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[Macke et al., 2010; Macke et al., 2011].
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2.3
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2.3.1
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Experimental testing procedures
Compressive strength tests
Cylindrical cores were cut from the basalt samples using a diamond coring bit.
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The resulting cores had a mean diameter of 24.6 mm (±0.11 mm). Following the
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procedure established in our initial study [Thomson et al., 2013], the ends of each core
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were trimmed and ground such that they were flat, parallel, and roughly perpendicular to
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the cylindrical axis of the core. No additional grinding was performed on the cylindrical
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wall of the core. The target length-to-diameter (L/D) ratio for these specimens was 2.0,
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but fracturing of the cores resulted in a range of final L/D ratios of 0.87 to 1.86. The
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difference in L/D ratio introduces an error of approximately 4% [Thuro et al., 2001].
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Grinding was accomplished using a diamond masonry blade mounted in a wet saw with a
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linear translation table, and a custom jig for fixing the cylindrical axis of the specimen
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perpendicular to the direction of the table’s travel through the blade.
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All of the specimens were individually measured for length and diameter with a
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digital caliper. Each nominal dimension was taken as the mean of either three (for
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diameter) or four (for length) measurements at different locations and orientations. The
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nominal specimen diameter was used to calculate the nominal cross-sectional area for
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converting compressive force into compressive stress. The nominal length was used to
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check for correlation between the deviation of specimen failure strength from the mean
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and L/D ratio. No significant correlation was found, indicating that it is unlikely that end
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effects skewed the compressive strength results more for short specimens than for longer
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specimens. The nominal length was also used as the gauge length for converting
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compressive displacements into compressive strain.
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Compressive strength of the basalt samples was measured using an Instron 8502
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servohydraulic load frame equipped with a 250 kN load cell and 64 mm (2.5”) diameter
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spherical compression platens. The compression tests were conducted without radial
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confinement, so the failure stress of the specimens is reported as the unconfined
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compressive strength (UCS). Tests were run in displacement control at a constant strain
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rate of 10-4 s-1, which required modifying the displacement rate for each test based on the
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specimen length, and a data sampling rate of 10 Hz.
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2.3.2
RAT instrument overview
The Rock Abrasion Tool is an engineering instrument and vital component of the
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in situ sensing portion of the Athena science payload carried onboard the Mars
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Exploration Rovers [Squyres et al., 2003]. Attached to the end of the robotic arm (IDD or
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Instrument Deployment Device), the RAT was designed to abrade and remove the outer
10 231
weathered layers of rocks, exposing fresher material beneath [Gorevan et al., 2003]. The
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RAT fits within a cylindrical volume 8.5 cm in diameter and 12.8 cm in length, and has a
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mass of 0.687 kg. It consists of three actuators: one to rotate a diamond impregnated
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“paddle wheel” at high speed (about 3000 rpm) against the target; a second to revolve the
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paddle wheel about the RAT’s central axis at lower speeds (<2 rpm); and a third to raise
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or lower the grinding assembly against the target (termed the Z-axis motor). Typical step
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sizes of the Z-axis motor are 0.05 mm per revolution for dense targets. Prior to operation,
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the IDD must be placed firmly against a target with a preload force of about 10 to 40 N
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(typically 30 N). When the rover is on a slope, lower preload is used to reduce the risk of
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rover slip downhill. A successful grind operation produces an abraded region 4.5 cm in
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diameter and nominally 2-5 mm deep, though the actual depth attained depends on the
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rock hardness.
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Numerous engineering data are recorded during grind operations, including the
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current draw from the grinding motor. The integrated power consumption over the last
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0.25 mm of a grind combined with the grinding time has been termed Specific Grind
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Energy (SGE, units J/mm3) and is an estimation of the target’s strength and resistance to
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abrasion.
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2.3.3
Rock Abrasion Tool grind tests
To quantify the grinding operation, two terms are used: Specific Grind Energy
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(SGE) and the G-Ratio. The SGE-term is the energy used by a grinder to remove a unit
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volume of material; its units are J/mm3 [Teale, 1965]. This energy parameter is a function
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of a number of system-specific variables such as grinding parameters, the state of the
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abrasive pad, rock physical properties, cleaning efficiency, cuttings removal, and friction.
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Because SGE is a composite quantity that is a function of many variables, it does not
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necessarily capture the full spectrum of rock mechanical properties by itself, and different
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rock types may yield non-unique, overlapping SGE values.
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To partially address this issue, we also consider the G-ratio, which is a ratio of the
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volumetric wear of rock divided by a volumetric wear of abrasive material (i.e., the pads
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of the grinding bit itself; G-ratio units are mm3/mm3). For example, when grinding an
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abrasive rock (e.g., sandstone) and a non-abrasive rock (e.g., limestone), the SGE values
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may be the same. However, because the grinding bit wears faster in an abrasive rock, the
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G-ratio derived for sandstone would be lower than for limestone. Thus, the G-ratio helps
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differentiate between different types of rocks with similar SGE values.
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Measuring wear of a grinding bit and volume of rock removed on Mars is
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complicated by the lack of direct measurement of these parameters [Myrick et al., 2004];
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they must be inferred from visual images (or more preferably, from stereo image-derived
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topography [e.g., Herkenhoff et al., 2006]). In addition, the values of SGE are RAT-
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specific, and as such, any data interpretation has to be calibrated just for the RAT.
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For this project, we used the RAT brassboard model to conduct grind tests. From
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a design and performance standpoint, the RAT brassboard, Engineering Model (EM), and
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the Flight Models (FM) are all the same. The main difference is that the EM and FM have
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flight-qualified actuators and vacuum-grade lubricants. In terms of the critical factor of
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power consumption, however, the models are identical in performance.
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Grinding test protocols followed the same procedure as RAT grinds conducted on
Mars, including the same software and command sequences. Initially, the RAT executed
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a ‘seek-scan’ routine whereby the grinding wheel rotates at a low speed while the Z-axis
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moves the bit towards the surface. Once an elevated current is detected on the grinding
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axis (meaning the grinding wheel has contacted and stalled on the rock surface), the Z-
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axis stops and backs off the surface a small distance until the grinding wheel starts
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spinning again. At this point the scan part of the routine is executed. During the scan
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routine, the grinding wheel continues to spin at a slow rate. Meanwhile, the revolve axis
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starts to rotate at 1 rpm to sweep or “scan” the surface for the highest point on the rock. If
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a stall is detected on the grinding axis during this scan, the revolve axis stops and the Z-
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axis backs off the surface until the grinding axis is free to rotate. Once the grinding axis
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begins to rotate, the revolve motion begins again. The stall detection and retract process
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repeats until one full revolution is made. This Z-axis position is then set to zero depth and
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the grinding operation commences. The grinding operation consists of a bit spinning at
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3000 rpm and revolving at up to 2 rpm (the actual revolving rate is dependent on torque
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applied by the grinding wheel). After each revolution, the Z-axis advances the bit 50
291
microns. A typical grinding operation lasts 1-3 hours. Once the grinding is complete, the
292
grinding telemetry is analyzed to determine SGE. In a few cases the G-ratio was also
293
acquired by estimating the volume of the ground hole as well as measuring the wear of
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the grind pad. The latter was achieved by taking a photo of the profile of the grind bit
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before and after the test. However, the steps taken to acquire the G-ratio presented
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additional operation complexity, took extra mission time, and were not very accurate due
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to insufficient special resolution for bit imaging. For that reason this is no longer being
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done on the MER mission though bit profile images are still being captured to monitor bit
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life over several grinds.
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2.3.4
Major and trace element chemistry, mineralogy
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Sample slabs were prepared using a rock saw at the University of Idaho
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Department of Geological Sciences from the large blocks of sample that were cored in
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the field (Section 2.3). When possible, slabs were collected from immediately adjacent to
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locations on the rock where slabs were collected for the purposes of Rock Abrasion Tool
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grind tests (Section 2.2.2). When this was not possible (generally due to difficulty
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orienting the sample block for adjacent cuts), slabs were collected from a different
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location on the same block that was used for preparation of a sample for RAT grind
309
testing. In this manner, we have attempted to maximize the similarity between sample
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geochemistry and physical properties (as deduced from rock crushing and RAT grinding
311
experiments). In addition to competent slab samples, two samples of loose, disaggregated
312
paleosol material that were too friable for mechanical strength or RAT testing were
313
collected from Lawyer Canyon and Shumaker Grade for geochemical analysis.
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Whole sample slabs, weighing between 10 and 20 grams, were sent to Activation
315
Laboratories of Lancaster, Ontario, CA for preparation and analysis of major, minor, and
316
trace elements. Sample preparation entailed crushing the entire sample to minus 10 mesh
317
(1.7 mm) followed by pulverization to at least 95% minus 150 mesh (106 microns). All
318
sample crushing and grinding equipment is made of mild steel, which can add up to 0.2
319
wt% Fe; pure quartz sand was run through the sample preparation equipment between
320
each sample in order to ensure cleanliness and minimize sample-to-sample cross
321
contamination.
14 322
After powder preparation, samples were analyzed by a combination of
323
instrumental neutron activation analysis (INAA, for As, Au, Br, Ce, Co, Cr, Cs, Eu, Hf,
324
Hg, Ir, La, Lu, Nd, Rb, Sb, Sc, Se, Sm, Ta, Tb, Th, U, W, Yb, Zr) and inductively
325
coupled plasma optical emission spectroscopy (ICP-OES). Sample data are tabulated in
326
Tables 4-6; detection limits for each element are also reported.
327
For INAA, a 1 g aliquot is encapsulated in a polyethylene vial and irradiated with
328
flux wires and an internal standard at a thermal neutron flux of 7×1012 n cm-2 s-1. After a
329
7-day period to allow for the decay of Na-24 the samples are counted on a high purity Ge
330
detector with resolution of better than 1.7 KeV for the 1332 KeV Co-60 photopeak.
331
Using the flux wires, the decay-corrected activities are compared to a calibration
332
developed from multiple certified international reference materials. The standards are
333
only a check on accuracy and are not used for calibration purposes. The method is
334
described in detail in Hoffman [1992].
335
For analysis by ICP-OES, a 0.2 g sample is mixed with a mixture of lithium
336
metaborate/lithium tetraborate and fused in a graphite crucible. The molten mixture is
337
poured into a 5% nitric acid solution and shaken until dissolved (~30 minutes). The
338
samples are run for major oxides (Si, Al, Fe, Mg, Mn, Ca, Ti, Na, K, P) and selected trace
339
elements (Ba, Be, Sr, V, Y, Zr) on a combination simultaneous/sequential Thermo
340
Jarrell-Ash Enviro II ICP-OES. For the remaining trace elements (Ag, Bi, Cd, Cu, Mo,
341
Ni, Pb, S, Zn) a 0.25 g sample is digested with four acids beginning with hydrofluoric,
342
followed by a mixture of nitric and perchloric acids, heated using precise programmer
343
controlled heating in several ramping and holding cycles which takes the samples to
344
dryness. After dryness is attained, samples are brought back into solution using
15 345
hydrochloric acid and analyzed using a Varian Vista ICP-OES. With this digestion,
346
certain phases may be only partially solubilized. These phases include zircon, monazite,
347
sphene, gahnite, chromite, cassiterite, rutile and barite, and only sulphide sulfur is
348
solubilized. Loss on Ignition (LOI) was determined by measuring the mass lost from
349
samples after heating them for 2 hours at 1050°C in an open crucible, providing
350
information on the amount of H2O, CO2, S, and other volatile compounds that are present
351
in the samples.
352
Mineral phase identification and quantification was carried out on a powdered
353
sample of LC-01 that was mixed with a powdered alumina standard in known
354
proportions. The mixture was then analyzed on a Bruker HiStar X-ray diffractometer
355
using Cu k-alpha radiation (40kV, 20mA), over a 2-theta range of 10-70 degrees. Powder
356
diffraction data were then reduced using the Crystallographica search-match software
357
program to what mineral phases were present and what their abundances were.
358
359
3
360
3.1
361
Results
Individual strength test results.
A total of 24 compressive strength tests were conducted (Table 1): 7 sample sets
362
from Lawyer Canyon road cut with >2 sample cores per set (N=16 crushing tests); 2
363
samples from Kendrick Grade (N=4 crushing tests); and 2 samples from Shumaker Grade
364
(N=4 crushing tests). A third sample from Shumaker Grade (SH-03) was too friable for
365
compressive strength testing and failed prior to loading. During the course of Rock
366
Abrasion Tool testing, 42 grinding tests were performed in 3 different rock types (see
367
summary Table 2). These included 7 sample sets from Lawyer Canyon road cut (N=22
16 368
grind tests); 2 samples from Kendrick Grade (N=7 grind tests); and 3 samples from
369
Shumaker Grade (N=13 grind tests). The number of tests per rock sample ranged from 3
370
to 7. These data sets were used to determine the average and standard deviation for each
371
set of conditions.
372
Figures 2 and 3 present univariate data plots of compressive strength and grind
373
energy, respectively, ordered by decreasing mean value. In the compressive strength
374
results (Fig. 2), there is reasonably good separability between samples that have
375
experienced different degrees of weathering (assigned here based on qualitative visual
376
and textural field evidence). Mean UCS values vary between 225 to 70 MPa from the
377
freshest to most weathered samples, spanning the rock mechanical strength categories of
378
“very strong” (100-250 MPa) to “strong” (50-100 MPa) (using categories of Hoek and
379
Brown [1997]). The seven sample points from the Lawyer Canyon site define an array or
380
sequence going from stronger, less weathered samples (LC-01, LC-02) to progressively
381
more weathered ones (LC-06, LC-07). Sample from Kendrick Grade samples tend to fall
382
in the intermediate to high range, while sample from Shumaker Grade span the range of
383
observed strengths. The most weathered samples from each locality have compressive
384
strengths that range from ~30 to 50% of the most pristine samples (i.e., 46% for Lawyer
385
Canyon, 77% for Kendrick Grade, and 32% for Shumaker Grade).
386
In Figure 3, there is much less separation evident between grind values for
387
different rock types compared to Fig. 2, and also less separation between samples from a
388
given sample locality. Kendrick Grade samples are the most grind resistant (1st and 4th
389
strongest in Fig. 3), whereas in terms of compressive strength they are 3rd and 6th
390
strongest, respectively (Fig. 2). Repeat grinds of individual targets have a high degree of
17 391
scatter. Overall, the grind results have an average standard deviation of 44.5% (expressed
392
as a percentage of the mean value), compared to an average standard deviation of 15.8%
393
for the compressive strength tests.
394
The weakest SGE values of different targets have a high degree of overlap; in
395
contrast, the strongest recorded values for each sample show greater consistency. Despite
396
the relatively high uncertainties in Fig. 3, a general trend in the Lawyer Canyon samples
397
is evident, i.e., LC-02, LC-01 > LC-05, LC-07. As with the compressive strength data,
398
samples from Shumaker Grade span the range of grind values. However, the weakest
399
UCS samples, SH-02, is the second strongest in terms of SGE. (Note sample SH-03 is not
400
represented in the crushing tests as it was too weak for this type of test). Weathered
401
samples for each locality have grind values from ~40 to 80% of the most pristine sample
402
(53% for Lawyer Canyon, 78% for Kendrick Grade, and 38% for Shumaker Grade).
403
It is noteworthy that the degree of scatter exhibited by the SGE values of this suite
404
of basalts is higher than for the rock assemblage measured previously [Thomson et al.,
405
2011; Thomson et al., 2013]. A likely controlling factor of this variability is the high
406
overall strength of these samples. Our prior work indicates that the advancement of the
407
RAT grinding bit into the target rock operates with reduced effectiveness for rocks with
408
UCS values greater than ~150 MPa. For samples at or exceeding this strength threshold,
409
the RAT’s rate of penetration into the target is minimal. As a check on the grind depth
410
values recorded by the Z-axis motor, independent depth measurements were made using a
411
depth micrometer. A comparison of these two sets of depth values is given in Table 3. It
412
is apparent that the independently measured depths are significantly shallower than those
413
recorded by the Z-axis stage (they are, on average, 73% shallower). This difference is
18 414
likely attributable to compliance in the RAT system, i.e., strain accommodated in the
415
RAT motor, housing, and mounting stage assembly. In two thirds of the grind tests, the
416
micrometer measurements indicate that a grind depth of 0.25 mm was not achieved. This
417
means that the actual SGE values are higher than those reported in Table 2. Yet we
418
report the Z-axis depths and associated SGE values because these are most comparable to
419
the results from Mars. System compliance is an issue with data from both the brassboard
420
and flight models, and therefore cannot be summarily excluded from consideration.
421
We also note that the specific grind energies we derived here and in Thomson et
422
al. [2013] are significantly lower (~20-30% lower) than previously reported SGE values
423
[e.g., Arvidson et al., 2004; McSween et al., 2006; Squyres et al., 2006; Wang et al.,
424
2006; Herkenhoff et al., 2009]. A combination of factors accounts for the differences, and
425
they relate to how data reduction is performed. Previous analyses used a standard 26.5 V
426
as the operating voltage for the grind axis. Here, we compute the voltage based on the
427
average velocity of the grind axis, resulting in a value that is typically around 10% lower.
428
Also, the average no-load current for the previously computed data were calculated based
429
on a fit produced over the minimum currents recorded in the grind data product (as
430
opposed to the calibration data product) until this minimum value settled. The difference
431
for some cases is as high as 40% over the last 0.25 mm. In the data presented here, the
432
no-load current is strictly from the calibration data collected prior to the physical grinding
433
of the rock. SGE values for the lab tests presented here are thus consistent with both the
434
lab and martian SGE values reported in our initial work [Thomson et al., 2013].
435
436
3.2
Comparison of grind and compressive strength tests
19 437
When comparing the grind test results to the compressive strength test results, the
438
samples from Lawyer Canyon are reasonably consistent with the previously determined
439
correlation [Thomson et al., 2013] (Figure 4). As expected, stronger samples (LC-01,
440
LC-02) plot toward the upper right portion of the plotted field, while weaker samples
441
(LC-05, LC-07) tend to fall toward the lower left. Samples from Kendrick Grade
442
(denoted by filled red circles in Fig. 4) conform to this general trend but are offset to
443
higher SGE values. The most anomalous data point is SH-02, a slightly more weathered
444
sample from the Upper Shumaker site. This sample has a low compressive strength yet
445
unusually high grind resistivity. It may be that macroscopic weakness (such as porosity or
446
pre-existing fractures) lowered the compressive strength but did not comparably affect its
447
grindability. Opaline silica was also observed to cement the paleosol soils at the Upper
448
Shumaker site, and it is possible that opal was deposited in pore spaces of this rock
449
sample and affected its grind resistivity.
450
Also given in the right-hand panel of Figure 4 is a shaded rectangular field
451
indicating the measured SGE values of Adirondack-class basalts and interpreted UCS
452
values from Thomson et al. [2013]. The Adirondack-class basalts Adirondack,
453
Humphrey, and Mazatzal have recorded SGE values that range from ~30 to 45 J/mm3;
454
their inferred compressive strengths range from about 70-130 MPa. In terms of strength,
455
Adirondack-class basalts are similar to samples LC-05 and LC-07, which are the most
456
weathered but intact basalt samples measured in this study (excluding paleosols or
457
saprolite).
458
459
3.3
Chemical index of alteration (CIA)
20 460
To help further quantify the degree of alteration, we calculated the chemical index
461
of alteration (CIA = molar Al2O3/(Al2O3+Na2O+CaO+K2O)) from the elemental
462
abundances determined by ICP/OES (Table 4). Figures 5a-b give the CIA as a function
463
of compressive strength and grind energy, respectively. The highest recorded CIA values
464
were found in paleosol layers; these are excluded from Fig. 5 because they were not
465
sufficiently competent to conduct crushing tests or grind tests. However, based on their
466
observed field characteristics (i.e., they crumbled under blows by the point of a geologic
467
hammer), their UCS value could be generally inferred to be less than about 5 MPa, and
468
their expected SGE values would be accordingly very low, ~1-2 J/mm3, consistent with
469
very weakly consolidated material. Hence, these paleosols would plot above the upper
470
left corner of both Fig. 5a and 5b.
471
In Fig. 5a, there is a distinct separation between less weathered samples (LC-01,
472
LC-02, LC-03) and more weathered samples (LC-04 to LC-07). This general trend of
473
weathering is consistent with the position of most weathered samples (paleosols) plotting
474
above the upper left corner of the plot. In comparison, although in Fig. 5b some
475
separation is still evident between the most weathered samples (LC-05, LC-07) and least
476
weathered (LC-01, LC-02), there is much more overlap for intermediate cases (LC-03,
477
04, 06) compared to Fig. 5a. ΔCIA values, defined as the difference between the most
478
pristine and most weathered sample from a given locality, are 5.2 for Lawyer Canyon, 2.0
479
for Kendrick Grade, and 0.3 for Shumaker Grade (SH-02 minus SH-01). Samples SH-03
480
has a CIA value of 62.2, which is intermediate between the rock samples and the
481
paleosols at Lawyer Canyon and Shumaker Grade that have CIA values in the range of
482
87 to 95.
21 483
Sample SH-03 was too weak to conduct compressive tests on but sufficiently
484
competent for grind tests; its value lies in between paleosols and more competent basalts.
485
But apart from this sample, there is a large gap in terms of strength parameters and CIA
486
values between paleosols and intact but weathered basalt samples. It is likely that the
487
collection and measurement of additional samples at a finer sampling interval (e.g., every
488
2 cm below paleosol) might reveal more transitional states.
489
490
3.4
Bulk Density.
491
Systematic trends in bulk density were also noted with increasing degree of
492
weathering (Figure 6), particularly for the Lawyer Canyon (LC) samples. The difference
493
between the most pristine and most weathered samples is about 0.4 g/cm3, equivalent to a
494
loss of ~15% of the initial bulk density of 2.7 g/cm3. This density change is comparable
495
to the difference in density between fresh and slightly weathered samples of 2 Ma old
496
basalts (with densities of 2.8 and 2.1 g/cm3, respectively) measured by Moon and
497
Jayawardane [2004].
498
499
500
3.5
Depth Profile and Mobility Ratios.
In this section, we focus our attention on the samples from Lawyer Canyon (LC).
501
Samples LC-01 to LC-07 were obtained in a vertical profile of progressively weathered
502
basalt, starting at the bottom from LC-01, the least weathered sample of the protolith,
503
extending 8 meters upwards to sample LC-07, the most weathered. However, because
504
weathering into the intact basalt proceeded in a dendritic pattern along fractures and
505
defect planes (i.e., the weathering front is not strictly planar), the sequence of weathering
22 506
states may not exactly match the stratigraphic height-ordered sequence. Above these rock
507
samples, the paleosol layer was also sampled (LC-paleosol).
508
Figure 7 gives major oxides as a function of depth for the Lawyer Canyon series.
509
The profile exhibits a steady loss of MgO, CaO, and K2O as one approaches the paleosol
510
layer, consistent with downward leaching and loss of mobile cations. Total iron exhibits a
511
more complicated pattern. It generally declines, but it is slightly elevated in samples LC-
512
05 and in the paleosol relative to the materials directly beneath them (though still below
513
the initial value in the protolith). Al2O3 steadily increases in proportion toward the
514
paleosol layer, presumably as more mobile cations are preferentially removed. Silica is
515
strongly depleted in the paleosol layer but slightly enhanced in the two samples below it,
516
suggesting perhaps both mobilization and deposition of silica at different rates in
517
different portions of the column.
518
An alternative method of examining elemental changes or fractionation is to
519
compute the constitutive mass balance [e.g., Brimhall et al., 1992; Sheldon, 2003]. By
520
indexing the concentration of a given element with respect to a relatively immobile one,
521
the gains and losses of mobile elements compared to the immobile element can be
522
determined. The mass transport function for the jth element in the weathered sample (w) is
523
defined as follows:
𝜏!,! =
524
!! !!,!
!! !!,!
𝜀!,! + 1 − 1
(1)
525
where ρp and ρw are the densities of the parent and weathered material, respectively, and
526
Cj,p and Cj,w are the weight percentages of the jth element in the parent and weathered
527
material, respectively. Strain (ε)i,w is defined by the volumetric change during
528
weathering:
23 𝜀!,! =
529
530
531
∆!
!
!! !!,!
=!
! !!,!
− 1.
(2)
Substituting and rearranging equations 1-2 yields an expression for the mobility
ratio (MR) of the jth element without density or volume-dependent terms:
𝑀𝑅!,! =
532
533
!!,! /!!,!
!!,! /!!,!
− 1.
(3)
In Eq. 3, we use the highly immobile element Zr as the index element Ci. The
534
resulting mobility ratios plotted as a function of sample depth (Figure 8) indicate that all
535
major cations have been depleted relative to the least weathered parent material. Total
536
loss relative to Zr due to dissolution in the most weathered non-paleosol sample (LC-07)
537
includes 65% MnO; 54% MgO; 38% Fe2O3(T); 34% CaO; 23% Na2O; 17% SiO2; and
538
12% Al2O3. Minor oxides K2O, TiO2, and P2O5 also exhibit losses of 13%, 12%, and 6%,
539
respectively.
540
Supporting this interpretation is a ternary diagram designed to evaluate the major
541
element changes that occur with weathering. Figure 9 gives the molar proportions of
542
Al2O3, (CaO+Na2O+K2O), and (FeOT+MgO). The weathering trend observed in the
543
Lawyer Canyon series follows two distinct stages or segments. In the first segment, the
544
trend is radially away from the FeOT+MgO apex, a trend consistent with early removal of
545
ferromagnesian minerals (clinopyroxene or groundmass olivine) or glass during incipient
546
alteration. In the second segment, the trend is nearly orthogonal to the first segment,
547
consistent with bulk removal of soluble elements and production of clays.
548
549
550
551
3.6
Mineralogy from X-Ray Diffraction
Analysis of three separate aliquots of sample LC-01 (the presumed protolith)
reveals that the sample is composed predominantly of glass (58-70%), clinopyroxene (19 24 552
25%), plagioclase (10-17%), and ilmenite (0-2%). The structural data for clinopyroxene ,
553
plagioclase, and ilmenite that best fit the LC-01 XRD pattern are from Takeda [1972],
554
Stewart et al. [1966], and Wechsler and Prewitt [1984], respectively.
555
556
4
557
4.1
558
Discussion.
Styles of Alteration
Several contrasting views of martian weathering processes have been advanced.
559
Based on the fact that the secondary mineral budget at the MER landing sites is
560
dominated by Mg, Fe, and Ca sulfates plus Fe-oxides, one view is that limited chemical
561
weathering proceeded under a low-pH, sulfuric acid-rich environment [e.g., Tosca et al.,
562
2004; Ming et al., 2006; Hurowitz and McLennan, 2007; Hausrath et al., 2008].
563
Alteration is inferred to have occurred in water-limited, rock-dominated conditions, akin
564
to a closed system model. Alternatively, a second perspective is a pedogenic model
565
where aqueous alteration of the soil driven by atmospherically-sourced water, resulting in
566
top-down weathering and vertical partitioning of atmospheric solutes [Amundson et al.,
567
2008]. Such an open system of chemical alteration is inferred to have been more active
568
earlier in Gusev history, but with continued accumulation of soluble SO3 and Cl-bearing
569
salts. A third perspective stresses the importance of physical fractionation processes in
570
accounting for certain observed gains or losses of iron and magnesium in local soils
571
[McGlynn et al., 2011; 2012].
572
In terms of style of alteration, some aspects of the weathering trend at Lawyer
573
Canyon are consistent with open system alteration. All cations are not fully accounted
574
for, meaning that cation loss from the system via migrating fluids seems inevitable. Yet
25 575
the high water:rock ratios necessary to mobilize Al in circum-neutral pH fluids are not
576
necessarily required to account for the degree of weathering-induced weakening observed
577
in the vertical profile below the saprolite layers. True open system weathering is more
578
like that required to form the (heavily weathered) soils and paleosol layers, and
579
comparable, deeply weathered soils (e.g., fines dominated by clay minerals) do not
580
appear widespread on Mars. Kaolinite-bearing areas have been identified in regions like
581
Mawrth Vallis, but outside a limited number of regional exposures, they appear restricted
582
to isolated outcrops associated with impact craters. As calculated by McGlynn et al.
583
[2012], soils cannot be completely explained by open system or acid-sulfur alteration;
584
some compositional modification by hydrodynamic sorting and admixture of secondary
585
components is necessary (i.e., a non-local origin).
586
What can we infer about the nature of weathering on Mars from the strength and
587
chemistry of Adirondack basalts? In terms of mechanical strength, Adirondack basalts are
588
most similar to the weakest, most weathered non-paleosol samples measured in our study
589
(LC-05, LC-07). Strength measurements indicate that comparably weathered terrestrial
590
basalt samples from Lawyer Canyon only retain 40 to 50% of their intact strength, 15%
591
of their initial bulk density, and have also lost upwards of half of some mobile cations,
592
such as 54% of MgO, 38% of Fe2O3(T), and 34% of the initial CaO.
593
To assess the degree of chemical alteration that Adirondack basalts have
594
experienced, we recast the mobility ratios using Ti as an index element in Figure 10 to
595
facilitate direct comparison with the CRB samples. While Ti is not completely immobile,
596
it is among the least mobile minor oxides [e.g., Amundson et al., 2008]. The gray shaded
597
field in Fig. 10 gives the range of mobility ratio values of the brushed versus RAT-
26 598
abraded surfaces of Adirondack and Humphrey; most cations are within plus or minus 0.2
599
(excluding enrichments in K2O, which are interesting in their own right). Considering Mg
600
as our most sensitive tracer of alteration, Adirondack-class basalts are therefore
601
intermediate in Mg loss (mobility ratio of -0.14) between LC-03 and LC-04 (mobility
602
ratios of -0.05 and -0.3, respectively). It is clear from these data that the brushed vs.
603
abraded surfaces of Adirondack-class basalts do not encompass the same degree of
604
alteration evidenced by the transition from LC-01 to LC-07.
605
606
4.2
Mismatch between physical and chemical weathering.
607
We can envision three possible explanations for the observed weakness of the
608
Adirondack class basalts. First, the weakening may be entirely attributable to physical
609
weathering such as shock effects [e.g., French, 1998] during the emplacement process or
610
subsequent salt weathering [Malin, 1974]. Second, Adirondack basalts may be samples
611
from a soil-like weathering profile developed in a basalt layer reflective of deep (i.e.,
612
meter length scale) downward percolation of fluids [e.g., Amundson et al., 2008],
613
followed by later disaggregation into isolated blocks. A third alternative is that the
614
observed weakening is limited to a several mm to cm-thick rind on exposed basaltic
615
clasts sampled by Spirit. In this case, we are looking at a moderately weathered, weak
616
layer that encases more pristine (and unsampled) corestones.
617
Although we consider it unlikely that shock-induced weakening is solely
618
responsible due to geometric considerations (especially given the self-similarity of the
619
four Adirondack basalt samples), some degree of shock-related weakening is to be
620
expected. Some inferences on the strength of shocked samples can be gleaned from
27 621
terrestrial meteorite strength data. Although such data is sparse (due to an understandable
622
reluctance to perform destructive strength tests on rare meteorite samples), the limited
623
data available suggests average compressive strength values of ~200 MPa for stony
624
meteorites [e.g., Buddhue, 1942; Petrovic, 2001; Popova et al., 2011]. Indeed, the
625
compressive strength of the strongest stony meteorite samples overlaps with the weakest
626
iron meteorites, despite the fact that iron meteorites are typically more than twice as
627
strong as stony meteorites. Strength data for meteorites is clearly somewhat biased
628
toward higher values since only fragments that survive ejection, atmospheric entry, and
629
landing are available for study. Nonetheless, the results suggest that the low to moderate
630
levels of shock do not uniformly weaken the target material, and thus support our
631
contention that not all of the observed weakening is necessarily shock-induced.
632
The effects of salt weathering on individual rock clasts are more difficult to
633
assess. Dissolved salts can percolate into even nominally nonporous media, and
634
subsequent pressure from salt crystal growth, absorption of water by anhydrous salts, or
635
thermal cycling can exert pressure and induce cracking on infiltrated surfaces [e.g.,
636
Rodriguez-Navarro and Doehne, 1999]. Small veins filled with light-toned material were
637
observed on brushed and abraded surfaces of Adirondack basalts [e.g., Herkenhoff et al.,
638
2004], and polygonal cracks filled with secondary minerals observed in Meridiani
639
sandstones have been suggested to be the result of cyclical salt-atmosphere water
640
exchange, i.e., repeated episodes of condensations, dissolution, and reprecipitation
641
[Chavdarian and Sumner, 2006]. Salt weathering may therefore play a role in the
642
weakening of the outer surfaces of rocks on Mars [e.g., Malin, 1974], though little data
643
exists to help quantify the potential magnitude of this process.
28 644
Assuming that all of the weakening is not attributable to physical weathering, the
645
reason that a distinction between the second and third scenarios matters is one of timing.
646
If Adirondack basalts were weathered and weakened in a soil-like context, then these
647
effects are likely related to an early phase of weathering on Mars (but still in the
648
Hesperian since it had to have occurred after the lava flows that filled Gusev crater were
649
emplaced [Milam et al., 2003]). However, if the third scenario is correct, this provides
650
little to no age control for the weathering. It could be recent, or it could be significantly
651
older. Exactly how old it could be is unclear, as this has to do with how long one would
652
expect a rock to last on the surface of Mars. On the Moon, this age is only a few Ma due
653
to micrometeorites [e.g., Horz, 1977], but the atmosphere of Mars shields surface rocks to
654
some degree and therefore much older rock ages are likely permissible. Using a reaction-
655
transport model, a minimum cumulative exposure age of 2.2×105 years has been
656
estimated for the formation of the weathering rind on Humphrey (an Adirondack-class
657
basalt) [Hausrath et al., 2008]. If the material assumed to be the unaltered protolith is
658
itself weathered to the degree such that it is mechanically similar to samples LC-05 and
659
LC-07 from this work, the required weathering time would be longer, perhaps at least by
660
a factor of two (and potentially >106 years).
661
A key discriminator between the second and third scenarios is the mismatch
662
between strength and chemical data. If the physical and chemical weathering were due to
663
the development of a soil-like weathering profile in a basalt layer, we would not expect a
664
mismatch between the degree of weathering implied by the mechanical strength versus
665
that implied but the degree of chemical alteration. Though such a process may have been
29 666
active, one or both of the other two explanations (i.e., additional physical weathering or
667
thick rinds) are necessary to explain the totality of available evidence.
668
In summary, this third hypothesis (i.e., Adirondack basalts are encased in a
669
several mm-thick weak weathering rind) appears most consistent with the available
670
evidence and known history of activity in Gusev crater. Considering this latter possibility,
671
we can infer some general characteristics of this hypothetical parent composition, but
672
acknowledge that our ability to make firm inferences about its precise nature is hampered
673
by several factors. First, the parental and weathered bulk compositions of the CRB and
674
Adirondack basalt are different, and second, their mineralogies are likely dissimilar as
675
well. These two factors mean that alteration would proceed along different geochemical
676
pathways in the two rocks even if the style of weathering were identical, particularly if
677
weathering proceeded under water-limited, low pH conditions [e.g., Nesbitt and Wilson,
678
1992; Hurowitz and Fischer, 2014].
679
If we set aside these caveats and assume all of the observed weakening is due to
680
substantially similar chemical weathering, the inferred Adirondack protolith would have
681
higher Fe, Mg, Mn, Si and proportionally lower amounts of the remaining cations. Total
682
iron would exceed 20 wt%, and MgO and MnO would roughly double. In a total alkali
683
versus silica diagram, the inferred compositions would fall within the picrobasalt field
684
(40-45 wt% SiO2). McSween et al. [2006] inferred similar but not-quite-as-silica-poor
685
‘endmember’ compositions for Adirondack, Humphrey, and Mazatzal with SiO2 values
686
ranging from 45.3 to 45.9 wt%. These endmember compositions were attained by
687
extrapolating linear trends from the brushed to RAT-abraded surfaces such that residual
688
sulfur was reduced to 0.3 wt%, akin to subtracting a portion of the brushed composition
30 689
values from the abraded values. On Earth, comparable volcanic rocks with magnesium-
690
rich, low-silica compositions are largely confined to the Archean period (e.g.,
691
komatiites), and are associated with large degrees of partial melting of an undepleted
692
mantle source [e.g., Nisbet, 1982]. Without independent corroborating evidence,
693
however, it would be an overstatement to conclude that these results constituted evidence
694
for martian komatiites. Nevertheless, the results hint at the fact that the unweathered
695
cores of basalts on Mars may trend towards more ultramafic compositions than otherwise
696
indicated.
697
698
4.3
Suggestions for grinders on future Mars missions
699
Given the successful operation of the RAT instrument on both Spirit and
700
Opportunity, we recommend including a grinder on future missions (e.g., Mars2020) in
701
order to better interrogate rock coatings and weathering rinds (the Mars Science
702
Laboratory rover Curiosity posses a brush but no grinder [Grotzinger et al., 2012], and
703
the rover’s drill system does not retain sample cuttings from the first ~1.5 cm of target
704
penetration [Anderson et al., 2012].) Based on MER RAT experience, future grinders
705
should be designed to penetrate to a minimum of ~2-3 mm depth in all rock types, which
706
will facilitate removal of cuttings and allow for more accurate estimates of specific grind
707
energy (a proxy for rock hardness). In addition, rock hardness can be further quantified
708
using by computing G-ratio values based on abrasive pad loss. In general, high-resolution
709
images of both the RAT hole (to assess the volume abraded using stereo
710
photogrammetry-derived DEMs) and the grinding pads (to calculate G-ratio based on pad
711
wear) would provide more information about the target and permit more careful
31 712
monitoring of grinding pad status. For the latter calculation, images with a spatial
713
resolution <30 microns per pixel (but preferably higher) of the grinding pads are
714
recommended, especially in targets with SGE values >20 J/mm3 where more bit wear is
715
expected.
716
717
5
Conclusions
718
Investigations of the linked chemical and strength changes experienced by
719
Columbia River Basalts help inform our understanding of martian weathering processes.
720
Our results provide a basis for comparison with the MER Spirit results, and there is a
721
notable mismatch in the degree of alternation implied by rock strength inferred from
722
RAT grinds versus that indicated by APXS data recast into mobility ratios (with the
723
former indicative of greater alteration than the latter). These results suggest that
724
Adirondack-class basalts on the plains of Gusev crater have experienced a significant
725
degree of weathering-induced weakening (in addition to some likely shock effects and
726
potentially salt weathering). Underneath a thin rind of sulfur-rich dust, the basalts
727
sampled by Spirit appear encased in a weaker, reduced-density rind that is at least several
728
mm thick.
729
730
Acknowledgements. We thank Bill Phillips and the Idaho Geologic Survey for loaning
731
us a portable drill corer for sample collection, Bill Rember (U. Idaho) for assistance in
732
field site identification and sample collection, and Robert Macke (Boston
733
College/Vatican) for conducting density measurements. Constructive comments by
734
reviewer Paul Niles helped sharpen several aspects of this study. This work was
32 735
supported by a NASA Fundamental Research Program grant to BJT. Special thanks to
736
David Nava (NASA/Goddard) for having twice helped facilitate the transfer of funds
737
from one institution to another.
738
33 739
References
740
Amundson, R., S. Ewing, W. Dietrich, B. Sutter, J. Owen, O. Chadwick, K. Nishiizumi,
741
M. Walvoord, and C. McKay (2008), On the in situ aqueous alteration of soils on
742
Mars, Geochimica et Cosmochimica Acta, 72(15), 3845-3864,
743
doi:10.1016/j.gca.2008.04.038.
744
Anderson, R. C., L. Jandura, A. B. Okon, D. Sunshine, C. Roumeliotis, L. W. Beegle, J.
745
Hurowitz, B. Kennedy, D. Limonadi, S. McCloskey, M. Robinson, C. Seybold, and
746
K. Brown (2012), Collecting Samples in Gale Crater, Mars; an Overview of the Mars
747
Science Laboratory Sample Acquisition, Sample Processing and Handling System,
748
Space Science Reviews, 57-75.
749
Arvidson, R. E., R. C. Anderson, P. Bartlett, J. F. Bell, D. Blaney, P. R. Christensen, P.
750
Chu, L. Crumpler, K. Davis, B. L. Ehlmann, R. Fergason, M. P. Golombek, S.
751
Gorevan, J. A. Grant, R. Greeley, E. A. Guinness, A. F. C. Haldemann, K.
752
Herkenhoff, J. Johnson, G. Landis, R. Li, R. Lindemann, H. McSween, D. W. Ming,
753
T. Myrick, L. Richter, F. P. Seelos, S. W. Squyres, R. J. Sullivan, A. Wang, and J.
754
Wilson (2004), Localization and physical properties experiments conducted by Spirit
755
at Gusev Crater, Science, 305(5685), 821-824.
756
Baker, L. L., D. J. Agenbroad, and S. A. Wood (2000), Experimental hydrothermal
757
alteration of a Martian analog basalt: Implications for Martian meteorites, Meteoritics
758
and Planetary Science, 35(1), 31-38.
759
Bandfield, J. L. (2002), Global mineral distributions on Mars, J. Geophys. Res., 107(E6).
760
Basaltic Volcanism Study Project (1981), Basaltic Volcanism on the Terrestrial Planets,
761
1286 pp., Pergamon Press, Inc., New York.
34 762
Bell, J. F., S. W. Squyres, R. E. Arvidson, H. M. Arneson, D. Bass, D. Blaney, N. Cabrol,
763
W. Calvin, J. Farmer, W. H. Farrand, W. Goetz, M. Golombek, J. A. Grant, R.
764
Greeley, E. Guinness, A. G. Hayes, M. Y. H. Hubbard, K. E. Herkenhoff, M. J.
765
Johnson, J. R. Johnson, J. Joseph, K. M. Kinch, M. T. Lemmon, R. Li, M. B. Madsen,
766
J. N. Maki, M. Malin, E. McCartney, S. McLennan, H. Y. McSween, D. W. Ming, J.
767
E. Moersch, R. V. Morris, E. Z. Noe Dobrea, T. J. Parker, J. Proton, J. W. Rice, F.
768
Seelos, J. Soderblom, L. A. Soderblom, J. N. Sohl-Dickstein, R. J. Sullivan, M. J.
769
Wolff, and A. Wang (2004), Pancam Multispectral Imaging Results from the Spirit
770
Rover at Gusev Crater, Science, 305(5685), 800-806.
771
Brimhall, G. H., O. A. Chadwick, C. J. Lewis, W. Compston, I. S. Williams, K. J. Danti,
772
W. E. Dietrich, M. E. Power, D. Hendricks, and J. Bratt (1992), Deformational mass-
773
transport and invasive processes in soil evolution, Science, 255(5045), 695-702.
774
Buddhue, J. D. (1942), The compressive strength of meteorites, Contributions of the
775
776
Society for Research on Meteorites, 3(8), 390-391.
Burns, E. R., D. S. Morgan, R. S. Peavler, and S. C. Kahle (2010), Three-Dimensional
777
Digital Geomodel of the Columbia Plateau Regional Aquifer System, Idaho, Oregon,
778
and Washington, U.S. Geological Survey Scientific Investigations Report, 2010-
779
5246, USGS, Reston, VA.
780
Bush, J. H., J. D. Kauffman, and K. L. Schmidt (2004), Geologic Map of the Craigmont
781
Quadrangle, Lewis and Idaho Counties, Idaho, 1:24,000, Idaho Geologic Survey,
782
Moscow, Idaho.
35 783
Camp, V. E., and P. R. Hooper (1981), Geologic studies of the Columbia Plateau: Part 1.
784
Late Cenozoic evolution of the southeast part of the Columbia River Basalt Province,
785
Geological Society of America Bulletin, 92(9), 659-668.
786
Chavdarian, G. V., and D. Y. Sumner (2006), Cracks and fins in sulfate sand: Evidence
787
for recent mineral-atmospheric water cycling in Meridiani Planum outcrops?,
788
Geology, 34(4), 229-232, doi:10.1130/G22101.1.
789
Christensen, P. R., J. L. Bandfield, M. D. Smith, V. E. Hamilton, and R. N. Clark (2000),
790
Identification of a basaltic component on the Martian surface from Thermal Emission
791
Spectrometer data, J. Geophys. Res., 105(E4), 9609-9621.
792
Christensen, P. R., S. W. Ruff, R. L. Fergason, A. T. Knudson, S. Anwar, R. E. Arvidson,
793
J. L. Bandfield, D. L. Blaney, C. Budney, W. M. Calvin, T. D. Glotch, M. P.
794
Golombek, N. Gorelick, T. G. Graff, V. E. Hamilton, A. Hayes, J. R. Johnson, H. Y.
795
McSween, G. L. Mehall, L. K. Mehall, J. E. Moersch, R. V. Morris, A. D. Rogers, M.
796
D. Smith, S. W. Squyres, M. J. Wolff, and M. B. Wyatt (2004), Initial Results from
797
the Mini-TES Experiment in Gusev Crater from the Spirit Rover, Science, 305(5685),
798
837-842.
799
Clark, B. C., A. K. Baird, R. J. Weldon, D. M. Tsusaki, L. Schnabel, and M. P.
800
Candelaria (1982), Chemical composition of Martian fines, J. Geophys. Res., 87,
801
10059-10067.
802
803
Eggleton, R. A., C. Foudoulis, and D. Varkevisser (1987), Weathering of basalt: Changes
in rock chemistry and mineralogy, Clays and Clay Minerals, 35(3), 161-169.
36 804
Elwood Madden, M. E., R. J. Bodnar, and J. D. Rimstidt (2004), Jarosite as an indicator
805
of water-limited chemical weathering on Mars, Nature, 431, 821-823,
806
doi:10.1038/nature02971.
807
French, B. M. (1998), Traces of catastrophe: A handbook of shock-metamorphic effects
808
in terrestrial meteorite impact structures, 120 pp., Lunar and Planetary Institute,
809
Houston.
810
Gellert, R., R. Rieder, R. C. Anderson, J. Bruckner, B. C. Clark, G. Dreibus, T.
811
Economou, G. Klingelhoefer, G. W. Lugmair, D. W. Ming, S. W. Squyres, C.
812
d'Uston, H. Wanke, A. Yen, and J. Zipfel (2004), Chemistry of rocks and soils in
813
Gusev Crater from the Alpha Particle X-ray Spectrometer, Science, 305, 829-833.
814
Gorevan, S. P., T. Myrick, K. Davis, J. J. Chau, P. Bartlett, S. Mukherjee, R. Anderson,
815
S. W. Squyres, R. E. Arvidson, M. B. Madsen, P. Bertelsen, W. Goetz, C. S. Binau,
816
and L. Richter (2003), Rock Abrasion Tool: Mars Exploration Rover mission,
817
Journal of Geophysical Research, 108(E12), 8068, doi:10.1029/2003JE002061.
818
Grotzinger, J. P., J. Crisp, A. R. Vasavada, R. C. Anderson, C. J. Baker, R. Barry, D. F.
819
Blake, P. Conrad, K. S. Edgett, B. Ferdowski, R. Gellert, J. B. Gilbert, M. Golombek,
820
J. Gómez-Elvira, D. M. Hassler, L. Jandura, M. Litvak, P. Mahaffy, J. Maki, M.
821
Meyer, M. C. Malin, I. Mitrofanov, J. J. Simmonds, D. Vaniman, R. V. Welch, and R.
822
C. Wiens (2012), Mars Science Laboratory Mission and Science Investigation, Space
823
Science Reviews, 170, 5-56.
824
Hausrath, E. M., A. K. Navarre-Sitchler, P. B. Sak, C. I. Steefel, and S. L. Brantley
825
(2008), Basalt weathering rates on Earth and the duration of liquid water on the plains
826
of Gusev Crater, Mars, Geology, 36(1), 67-70.
37 827
Herkenhoff, K. E., S. W. Squyres, R. Arvidson, D. S. Bass, J. F. Bell, P. Bertelsen, N. A.
828
Cabrol, L. Gaddis, A. G. Hayes, S. F. Hviid, J. R. Johnson, K. M. Kinch, M. B.
829
Madsen, J. N. Maki, S. M. McLennan, H. Y. McSween, J. W. Rice, M. Sims, P. H.
830
Smith, L. A. Soderblom, N. Spanovich, R. Sullivan, and A. Wang (2004), Textures of
831
the soils and rocks at Gusev Crater from Spirit's Microscopic Imager, Science,
832
305(5685), 824-827, doi:10.1126/science.3050824.
833
Herkenhoff, K. E., S. W. Squyres, R. Anderson, B. A. Archinal, R. E. Arvidson, J. M.
834
Barrett, K. J. Becker, J. F. Bell, C. Budney, N. A. Cabrol, M. G. Chapman, D. Cook,
835
B. L. Ehlmann, J. Farmer, B. Franklin, L. R. Gaddis, D. M. Galuszka, P. A. Garcia, T.
836
M. Hare, E. Howington-Kraus, J. R. Johnson, S. Johnson, K. Kinch, R. L. Kirk, E. M.
837
Lee, C. Leff, M. Lemmon, M. B. Madsen, J. N. Maki, K. F. Mullins, B. L. Redding,
838
L. Richter, M. R. Rosiek, M. H. Sims, L. A. Soderblom, N. Spanovich, R. Springer,
839
R. M. Sucharski, T. Sucharski, R. Sullivan, J. M. Torson, and A. Yen (2006),
840
Overview of the Microscopic Imager Investigation during Spirit's first 450 sols in
841
Gusev crater, Journal of Geophysical Research (Planets), 111(E02), E02S04,
842
doi:10.1029/2005JE002574.
843
Herkenhoff, K. E., M. P. Golombek, E. A. Guinness, J. B. Johnson, A. Kusack, L.
844
Richter, R. J. Sullivan, and S. Gorevan (2009), In situ observations of the physical
845
properties of the Martian surface, in The Martian Surface, edited by J. Bell, pp. 451-
846
467, Cambridge Planetary Science.
847
848
Hobbs, K. M. (2010), Global climate change in the Miocene recorded in Columbia River
Basalt-hosted paleosols, MS thesis, 74 pp, Univ. of Idaho.
38 849
850
851
852
853
854
Hoek, E., and E. T. Brown (1997), Practical estimates of rock mass strength,
International Journal of Rock Mechanics and Mining Sciences, 34(8), 1165-1186.
Hoffman, E. L. (1992), Instrumental neutron activation in geoanalysis, Journal of
Geochemical Exploration, 44(1-3), 297-319.
Horz, F. (1977), Impact cratering and regolith dynamics, Physics and Chemistry of Earth,
10, 3-15.
855
Hurowitz, J. A., and S. M. McLennan (2007), A ~3.5 Ga record of water-limited, acidic
856
weathering conditions on Mars, Earth and Planetary Science Letters, 260(3-4), 432-
857
443.
858
Hurowitz, J. A., W. Fischer, N. J. Tosca, and R. E. Milliken (2010), Origin of acidic
859
surface waters and the evolution of atmospheric chemistry on early Mars, Nature
860
Geoscience, 3(5), 323-326, doi:10.1038/ngeo831.
861
Hurowitz, J. A., and W. W. Fischer (2014), Contrasting styles of water–rock interaction
862
at the Mars Exploration Rover landing sites, Geochimica et Cosmochimica Acta, 127,
863
25-38.
864
Jakobsson, S. P., and J. G. Moore (1986), Hydrothermal minerals and alteration rates at
865
Surtsey Volcano, Iceland, Geological Society of America Bulletin, 97(5), 648-659.
866
Kauffman, J. D. (2004), Geologic Map of the Gifford Quadrangle, Nez Perce County,
867
868
Idaho, 1:24,000, Idaho Geological Survey, Moscow, Idaho.
Kauffman, J. D. (2009), Revised stratigraphy for several Saddle Mountains and
869
Wanapum Basalt units, Columbia River Basalt Group, paper presented at Geological
870
Society of America Annual meeting, GSA, Portland, OR, 18-21 October 2009.
39 871
Kristmannsdottir, H. (1979), Alteration of basaltic rocks by hydrothermal activity at 100-
872
300°C, in Developments in Sedimentology, edited by M. M. Mortland and V. C.
873
Farmer, pp. 359-367, Elsevier B. V.
874
Lewis, R. S., J. H. Bush, R. F. Burmester, J. D. Kauffman, D. L. Garwood, P. E. Myers,
875
and K. L. Othberg (2005), Geologic map of the Potlatch 30 x 60 minute quadrangle,
876
Idaho., 1:100,000, Idaho Geological Survey, Moscow, ID.
877
Macke, R. J., D. T. Britt, and G. J. Consolmagno (2010), Analysis of systematic error in
878
“bead method” measurements of meteorite bulk volume and density, Planet. Space
879
Sci., 58(3), 421-426.
880
Macke, R. J., D. T. Britt, and G. J. Consolmagno (2011), Density, porosity, and magnetic
881
susceptibility of achondritic meteorites, Meteoritics & Planetary Science, 46(2), 311-
882
326.
883
884
885
Malin, M. C. (1974), Salt weathering on Mars, Journal of Geophysical Research, 79(26),
3888-3894.
McGlynn, I. O., C. M. Fedo, and H. Y. McSween, Jr. (2011), Origin of basaltic soils at
886
Gusev crater, Mars, by aeolian modification of impact-generated sediment, Journal of
887
Geophysical Research, 116, E00F22, doi:10.1029/2010JE003712.
888
McGlynn, I. O., C. M. Fedo, and H. Y. McSween, Jr. (2012), Soil mineralogy at the Mars
889
Exploration Rover landing sites: An assessment of the competing roles of physical
890
sorting and chemical weathering, Journal of Geophysical Research (Planets), 117,
891
E01006, doi:10.1029/2011JE003861.
892
893
McSween, H. Y., R. E. Arvidson, J. F. Bell, D. Blaney, N. A. Cabrol, P. R. Christensen,
B. C. Clark, J. A. Crisp, L. S. Crumpler, D. J. Des Marais, J. D. Farmer, R. Gellert, A.
40 894
Ghosh, S. Gorevan, T. Graff, J. Grant, L. A. Haskin, K. E. Herkenhoff, J. R. Johnson,
895
B. L. Jolliff, G. Klingelhoefer, A. T. Knudson, S. McLennan, K. A. Milam, J. E.
896
Moersch, R. V. Morris, R. Rieder, S. W. Ruff, P. A. de Souza, S. W. Squyres, H.
897
Wanke, A. Wang, M. B. Wyatt, A. Yen, and J. Zipfel (2004), Basaltic rocks analyzed
898
by the Spirit Rover in Gusev Crater, Science, 305, 842-845.
899
McSween, H. Y., M. B. Wyatt, R. Gellert, J. F. Bell, R. V. Morris, K. E. Herkenhoff, L.
900
S. Crumpler, K. A. Milam, K. R. Stockstill, L. L. Tornabene, R. E. Arvidson, P.
901
Bartlett, D. Blaney, N. A. Cabrol, P. R. Christensen, B. C. Clark, J. A. Crisp, D. J.
902
Des Marais, T. Economou, J. D. Farmer, W. Farrand, A. Ghosh, M. Golombek, S.
903
Gorevan, R. Greeley, V. E. Hamilton, J. R. Johnson, B. L. Joliff, G. Klingelhöfer, A.
904
T. Knudson, S. McLennan, D. Ming, J. E. Moersch, R. Rieder, S. W. Ruff, C.
905
Schröder, P. A. de Souza, S. W. Squyres, H. Wänke, A. Wang, A. Yen, and J. Zipfel
906
(2006), Characterization and petrologic interpretation of olivine-rich basalts at Gusev
907
Crater, Mars, Journal of Geophysical Research, 111(E02), E02S10,
908
doi:10.1029/2005JE002477.
909
910
911
912
913
McSween, H. Y., G. J. Taylor, and M. B. Wyatt (2009), Elemental composition of the
martian crust, Science, 324(5928), 736-739, doi:10.1126/science.1165871.
McSween, H. Y., Jr. (1994), What we have learned about Mars from SNC meteorites,
Meteoritics, 29, 757-779.
Milam, K. A., K. R. Stockstill, J. E. Moersch, H. Y. McSween, L. L. Tornabene, A.
914
Ghosh, M. B. Wyatt, and P. R. Christensen (2003), THEMIS characterization of the
915
MER Gusev crater landing site, Journal of Geophysical Research (Planets),
916
108(E12).
41 917
Ming, D. W., D. W. Mittlefehldt, R. V. Morris, D. C. Golden, R. Gellert, A. Yen, B. C.
918
Clark, S. W. Squyres, W. H. Farrand, S. W. Ruff, R. E. Arvidson, G. Klingelhöfer, H.
919
Y. McSween, D. S. Rodionov, C. Schröder, P. A. de Souza, and A. Wang (2006),
920
Geochemical and mineralogical indicators for aqueous processes in the Columbia
921
Hills of Gusev crater, Mars, Journal of Geophysical Research, 111(E02), E02S12,
922
doi:10.1029/2005JE002560.
923
Moon, V., and J. Jayawardane (2004), Geomechanical and geochemical changes duriing
924
early stages of weathering of Karamu Basalt, New Zealand, Engineering Geology, 74,
925
57-72.
926
927
928
Mustard, J. F., S. Murchie, S. Erard, and J. Sunshine (1997), In situ compositions of
Martian volcanics: Implications for the mantle, J. Geophys. Res., 102, 25605-25616.
Myrick, T. M., L. Carlson, C. J. Chau, J. Powderly, P. Bartlett, K. Davis, and S. Gorevan
929
(2004), The RAT as a Mars rock physical properties tool, paper presented at AIAA
930
Space 2004 Conference & Exhibit, AIAA-2004-6096, AIAA, San Diego, CA.
931
932
933
934
Nesbitt, H. W., and R. E. Wilson (1992), Recent chemical weathering of basalts,
Americal Journal of Science, 292(10), 740-777.
Newsom, H. E. (1980), Hydrothermal alteration of impact melt sheets with implications
for Mars, Icarus, 44(1), 207-216.
935
Niles, P. B., and J. Michalski (2009), Meridiani Planum sediments on Mars formed
936
through weathering in massive ice deposits, Nature Geosciences, 2, 215-220.
937
Nisbet, E. G. (1982), The tectonic setting and petrogenesis of komatiites, in Komatiites,
938
edited by N. T. Arndt and E. G. Nisbet, pp. 501-520, George Allen & Unwin,
939
London.
42 940
941
Petrovic, J. J. (2001), Review mechanical properties of meteorites and their constituents,
Journal of Materials Science, 36(7), 1579-1583.
942
Popova, O., J. Borovicka, W. K. Hartmann, P. Spurny, E. Gnos, I. Nemtchinov, and J. M.
943
Trigo-Rodiguez (2011), Very low strengths of interplanetary meteoroids and small
944
asteroids, Meteoritics & Planetary Science, 46(10), 1525-1550, doi:10.1111/j.1945-
945
5100.2011.01247.x.
946
Reidel, S. P., T. L. Tolan, P. R. Hooper, M. H. Beeson, K. R. Fecht, R. D. Bentley, and J.
947
L. Anderson (1989), The Grande Ronde Basalt, Columbia River Basalt Group;
948
Stratigraphic descriptions and correlations in Washington, Oregon, and Idaho, in
949
Volcanism and Tectonism in the Columbia River Flood-Basalt Province, edited by S.
950
P. Reidel and P. A. Hooper, pp. 21-54, Geological Society of America, Boulder, CO.
951
Retallack, G. J. (1991), Untangling the effects of burial alteration and ancient soil
952
formation, Annual Review of Earth and Planetary Sciences, 19, 183-206.
953
Rodriguez-Navarro, C., and E. Doehne (1999), Salt weathering: influence of evaporation
954
rate, supersaturation and crystallization pattern, Earth Surface Processes and
955
Landforms, 24, 191-209.
956
Sheldon, N. D. (2003), Pedogenesis and geochemical alteration of the Picture Gorge
957
subgroup, Columbia River basalt, Oregon, Geological Society of America Bulletin,
958
115(11), 1377-1387, doi:10.1130/B25223.1.
959
Squyres, S. W., R. E. Arvidson, E. T. Baumgartner, J. F. Bell, P. R. Christensen, S.
960
Gorevan, K. E. Herkenhoff, G. Klingelhöfer, M. B. Madsen, R. V. Morris, R. Rieder,
961
and R. A. Romero (2003), Athena Mars rover science investigation, Journal of
962
Geophysical Research, 108, 8062, doi:10.1029/2003JE002121.
43 963
Squyres, S. W., R. E. Arvidson, D. L. Blaney, B. C. Clark, L. Crumpler, W. H. Farrand,
964
S. Gorevan, K. E. Herkenhoff, J. Hurowitz, A. Kusack, H. Y. McSween, D. W. Ming,
965
R. V. Morris, S. W. Ruff, A. Wang, and A. Yen (2006), Rocks of the Columbia Hills,
966
Journal of Geophysical Research (Planets), 111(E02), E02S11,
967
doi:10.1029/2005JE002562.
968
Stewart, D. B., G. W. Walker, T. L. Wright, and J. J. Fahey (1966), Physical properties of
969
calcic labradorite from Lake County, Oregon, American Mineralogist, 51, 177-197.
970
Swanson, D. A., T. L. Wright, P. R. Hooper, and R. D. Bentley (1979), Revisions in
971
stratigraphic nomenclature of the Columbia River Basalt Group, U. S. Geological
972
Survey Bulletin, 1457-G, 1-59.
973
974
975
976
977
Takeda, H. (1972), Structural studies of rim augite and core pigeonite from lunar rock
12052, Earth and Planetary Science Letters, 15(1), 65-71.
Teale, R. (1965), The concept of specific energy in rock drilling, International Journal of
Rock Mechanics and Mining Sciences & Geomechanics Abstracts, 2(1), 57-73.
Thomson, B. J., N. T. Bridges, J. Cohen, J. Hurowitz, and A. Lennon (2011), Estimating
978
rock strength parameters from Rock Abrasion Tool (RAT) grinds, Lunar and
979
Planetary Science Conference, 42, abstract #2567.
980
Thomson, B. J., N. T. Bridges, J. Cohen, J. A. Hurowitz, A. Lennon, G. Paulsen, and K.
981
Zacny (2013), Estimating rock compressive strength from Rock Abrasion Tool
982
(RAT) grinds, Journal of Geophysical Research, accepted,
983
doi:10.1029/2012JE004240.
44 984
Thuro, K., R. Plinninger, S. Zäh, and S. Schütz (2001), Scale effects in rock strength
985
properties. Part 1: Unconfined compressive test and Brazilian test, paper presented at
986
ISRM regional symposium, EUROCK, 169-174.
987
Tolan, T. L., S. P. Reidel, M. H. Beeson, J. L. Anderson, K. R. Fecht, and D. A. Swanson
988
(1989), Revisions to the estimates of the areal extent and volume of the Columbia
989
River Basalt Group, in Volcanism and Tectonism in the Columbia River Flood-Basalt
990
Province, edited by S. P. Reidel and P. A. Hooper, pp. 1-20, Geological Society of
991
America, Boulder, CO.
992
Tosca, N. J., S. M. McLennan, D. H. Lindsley, and M. A. A. Schoonen (2004), Acid-
993
sulfate weathering of synthetic Martian basalt: The acid fog model revisited, Journal
994
of Geophysical Research (Planets), 109(E5), 5003, doi:10.1029/2003JE002218.
995
Wang, A., R. L. Korotev, B. L. Jolliff, L. A. Haskin, L. Crumpler, W. H. Farrand, K. E.
996
Herkenhoff, P. de Souza, A. G. Kusack, J. A. Hurowitz, and N. J. Tosca (2006),
997
Evidence of phyllosilicates in Wooly Patch, an altered rock encountered at West
998
Spur, Columbia Hills, by the Spirit rover in Gusev crater, Mars, Journal of
999
Geophysical Research, 111(E2), E02S16, doi:10.1029/2005JE002516.
1000
1001
Wechsler, B. A., and C. T. Prewitt (1984), Crystal structure of ilmenite (FeTiO3) at high
temperature and at high pressure, American Mineralogist, 69(1-2), 176-185.
1002
1003
1004
45 1005
Figure Captions
1006
Figure 1. Map of the northwestern United States with the extent of Columbia
1007
River Basalt (CRB) given in orange [from Burns et al., 2010] and sample localities
1008
marked with black circles. The flows themselves are more extensive [e.g., Reidel et al.,
1009
1989], but the outline covers the main central area of thickest CRB.
1010
Figure 2. Plot of unconfined compressive strength (UCS) test results ordered by
1011
decreasing mean value. KG = Kendrick Grade samples; LC = Lawyer Canyon samples;
1012
and SH = Shumaker Grade samples. Mean values are given by green horizontal bars.
1013
Figure 3. Plot of Rock Abrasion Tool (RAT) grinding test results given as
1014
specific grind energy (SGE, units J/mm3) ordered by decreasing mean value. KG =
1015
Kendrick Grade samples; LC = Lawyer Canyon samples; and SH = Shumaker Grade
1016
samples. Mean values are given by green horizontal bars.
1017
Figure 4. Mean compressive strength values (in MPa) plotted against mean
1018
specific grind energy (SGE) values (in J/mm3). Left-hand figure is a duplicate of right-
1019
hand figure with error bars omitted for clarity. Error bars on right-hand figure are ± one
1020
standard deviation of the mean value. Dashed and dot-dashed lines give power law and
1021
linear fits to the calibration data of Thomson et al. [2013]. Shaded field on right-hand plot
1022
is measured SGE values of Adirondack-class basalts in Gusev crater measured by the
1023
MER Spirit; concomitant compressive strength values calculated from fits to calibration
1024
data.
1025
1026
Figure 5. (a) Chemical index of alteration versus compressive strength. (b)
Chemical index of alteration versus grind energy.
46 1027
1028
1029
1030
1031
Figure 6. (a) Compressive strength versus bulk density. (b) Grind energy versus
bulk density. (c) Chemical index of alteration versus bulk density.
Figure 7. Major oxides versus depth below top of paleosol surface for Lawyer
Canyon samples.
Figure 8. Mobility ratio calculated according to Eq. 3 for Lawyer Canyon (LC)
1032
samples indicating cation gain or loss compared to least weathered material (LC-01)
1033
arranged in order of sample depth below top of paleosol surface. Index element (assumed
1034
to be immobile) is Zr.
1035
Figure 9. Ternary Al2O3, (CaO+Na2O+K2O), and (FeOT+MgO) diagram, data
1036
plotted in mole percent. Solid black arrow labeled ‘A1’ indicates weathering trend in of
1037
Lawyer Canyon samples LC-01 to LC-07 consistent with dissolution of clinopyroxene
1038
phenocrysts or groundmass olivine. Dashed back arrow labeled ‘A2’ indicates weathering
1039
trend required to reach Lawyer Canyon paleosol composition that is consistent with
1040
feldspar removal and clay deposition. Dashed black arrow labeled ‘B1’ is weathering
1041
trend displayed by Shumaker grade samples; no evidence of an early weathering phase
1042
dominated by loss of ferromagnesian minerals is evident.
1043
Figure 10. Mobility ratio calculated according to Eq. 3 for Lawyer Canyon (LC)
1044
samples indicating cation gain or loss compared to least weathered material (LC-01).
1045
Index element (assumed to be immobile) is Ti. Gray shaded field represents maximal
1046
cation gain or loss (excluding K2O) of Adirondack-class basalts comparing brushed
1047
surfaces to those abraded by the RAT. Specifically, the samples considered are
1048
Adirondack brush and RAT, Humphrey brush and RAT1, and Humphrey brush at RAT2.
1049
All data are normalized to 100% to a LOI-free basis for Lawyer Canyon, and SO3- and
47 1050
Cl-free basis for the Adirondack-class basalt. Vertical dashed lines on right mark positive
1051
mobility ratio or Tau values for K2O.
1052
48 1053
Table 1. Compressive strength test results
1054
1055
Sample
ID
Length
(mm)
Diameter
(mm)
LC-01a
LC-01b
LC-02a
LC-02b
LC-03a
LC-03b
LC-03c
LC-04a
LC-04b
LC-05a
LC-05b
LC-06a
LC-06b
LC-06c
LC-07a
LC-07b
SH-01a
SH-01b
SH-02a
SH-02b
SH-03
KG-01b
KG-01a
KG-02a
KG-02b
38.20
39.19
42.61
37.91
23.15
36.13
26.33
24.36
32.28
38.32
28.51
44.30
39.69
37.25
36.11
24.78
41.82
46.04
32.32
22.15
n/a
28.40
40.54
21.95
21.62
24.52
24.77
24.67
24.75
24.65
24.64
24.64
24.71
24.53
24.77
24.53
24.71
24.68
24.50
24.64
24.70
24.64
24.72
24.34
24.66
n/a
24.72
24.74
24.57
24.71
Peak
Load
(kN)
99.4
95.0
108.0
108.0
109.0
81.2
98.5
60.1
71.0
88.1
51.7
62.7
53.3
33.8
55.4
73.0
107.0
102.0
31.4
35.1
n/a
94.3
64.7
86.8
119.0
Stress
area
(mm2)
472.2
482.0
477.9
481.0
477.1
476.9
476.8
479.6
472.5
482.0
472.8
479.6
478.4
471.4
476.9
479.1
476.9
480.0
465.4
477.7
n/a
480.0
480.7
474.1
479.6
Peak
stress
(MPa)
210.5
197.1
226.0
224.5
228.5
170.3
206.6
125.3
150.3
182.8
109.4
130.7
111.4
71.7
116.2
152.4
224.4
212.5
67.5
73.5
n/a
196.4
134.6
183.1
248.1
.
49 Failure mode
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
?
UCS (mean ±
std dev)
204±
9
225±
1
202±
29
138±
18
146±
52
105±
30
134±
26
218±
8
70±
4
166±
44
216±
46
1056
Table 2. RAT grinding results over the last 0.25 mm of grind
Mean
Grind
Voltage
[V]
Mean
Grind
Current
[mA]
Duration
[min]
Rotate
Energy
[J]
Z
Grind
Depth
[mm]
Material
Removed
[mm3]
SGE
[J/mm3]
SH-01-a-1
SH-01-a-2
26
287
289
157
40
37759
10825
1.48
0.77
232
292
163
37
26
SH-01-a-3
26
364
85
34792
0.82
293
119
SH-01-a-4
26
297
120
44948
0.56
308
146
SH-01-a-5
26
304
120
46417
0.41
308
151
SH-01-a-6
26
304
60
22852
0.36
308
74
SH-01-a-7
26
293
47
17209
0.36
308
56
SH-03-a-1
26
255
105
20427
1.74
361
57
SH-03-a-2
26
282
120
14095
1.58
330
43
SH-03-a-3
26
259
106
10112
2.04
320
32
KG-01-a-1
26
270
120
34382
1.33
388
89
KG-01-a-2
26
294
120
45766
0.51
388
118
KG-01-a-3
26
301
120
47587
0.31
388
123
KG-01-a-4
26
287
120
43882
0.56
388
113
SH-02-a-1
26
268
120
34104
1.22
358
95
SH-02-a-2
26
300
120
44222
0.61
358
124
SH-02-a-3
26
292
120
45383
0.51
358
127
KG-02-a-1
26
299
180
71383
0.46
317
225
KG-02-a-2
26
274
120
39039
0.97
395
99
KG-02-a-3
26
278
120
39745
0.87
395
101
LC-01-a-1
26
276
120
42227
1.02
325
130
LC-01-a-2
23
257
60
14728
0.82
357
41
LC-01-a-3
26
271
60
34556
0.66
397
87
LC-02-a-1
26
268
120
41062
1.48
154
267
LC-02-a-2
26
278
60
18857
0.77
225
84
LC-02-a-3
26
280
60
18444
0.71
256
72
LC-02-a-4
26
257
44
10811
0.82
288
38
LC-03-a-1
26
304
120
41511
0.97
395
105
LC-03-a-2
26
284
60
19071
0.72
394
48
LC-03-a-3
26
289
60
24268
0.51
394
62
LC-04-a-1
26
282
120
37599
1.12
393
96
LC-04-a-2
26
291
79
27619
0.61
392
71
LC-04-a-3
26
294
60
20798
0.56
393
53
LC-05-a-1
26
279
120
39623
1.07
380
104
Target ID
SGE
(mean ±
std dev)
107±
51
44±
13
111±
15
115±
18
142±
72
86±
45
115±
103
72±
30
73±
22
63±
50 LC-05-a-2
26
248
60
14987
0.92
387
39
LC-05-a-3
26
263
60
17257
0.72
387
45
LC-06-a-1
26
276
120
40109
1.28
324
124
LC-06-a-2
26
278
60
18589
0.52
392
47
LC-06-a-3
26
273
60
17739
0.77
391
45
LC-07-a-1
26
273
120
35615
1.23
383
93
LC-07-a-2
26
269
60
18282
0.71
382
48
LC-07-a-3
26
261
60
15956
0.87
383
42
36
72±
45
61±
28
1057
1058
51 1059
Table 3. Comparison of grind depth values recorded by Z-axis motor versus
1060
independently.
Z Grind
Independent
Depth (mm)
Depth (mm)a
SH-01-a-1
1.479
0.762
SH-03-a-2
1.581
0.762
SH-03-a-3
2.040
1.549
KG-01-a-1
1.327
0.432
KG-01-a-2
0.510
0.076
KG-01-a-3
0.306
0.075
KG-01-a-4
0.561
0.001
SH-02-a-2
0.612
0.508
SH-02-a-3
0.510
0.152
KG-02-a-1
0.458
0.127
KG-02-a-2
0.969
0.127
KG-02-a-3
0.868
0.178
LC-01-a-1
1.021
0.203
LC-01-a-2
0.816
0.178
LC-01-a-3
0.663
0.025
LC-02-a-1
1.478
0.762
LC-02-a-2
0.765
0.203
LC-02-a-4
0.817
0.152
LC-03-a-1
0.970
0.254
LC-03-a-2
0.715
0.127
LC-03-a-3
0.510
0.025
LC-04-a-1
1.123
0.356
LC-04-a-2
0.612
0.050
LC-04-a-3
0.561
0.051
LC-05-a-1
1.073
0.457
LC-05-a-2
0.917
0.025
LC-05-a-3
0.715
0.051
LC-06-a-1
1.276
0.508
LC-06-a-2
0.515
0.203
LC-06-a-3
0.765
0.127
LC-07-a-1
1.225
0.533
LC-07-a-2
0.714
0.127
LC-07-a-3
0.867
0.127
a
Gray shaded fields indicate grind tests where a depth of 0.25 mm was not attained.
Target ID
1061
1062
52 1063
Table 4. Elements analyzed by ICP-OES following fusion digestion
Oxide /
Element
SiO2
TiO2
Al2O3
Fe2O3(T)
MnO
MgO
CaO
Na2O
K 2O
P 2O 5
LOI
Total
1064
1065
1066
Ba
Be
Sr
V
Y
Zr
LC-01
LC-02
LC-03
LC-04
LC-05
LC-06
LC-07
LC-PS*
KG-01
KG-02
SH-01
SH-02
SH-03
SH-05
SH-PS*
54.38
2.313
13.23
14.11
0.207
3.26
7.12
3.19
1.73
0.45
ND
99.7
54.95
2.299
13.06
13.34
0.192
3.17
6.92
3.15
1.74
0.41
0.07
99.32
54.29
2.347
13
12.96
0.195
3.15
7.05
2.9
1.87
0.46
0.57
98.8
54.18
2.458
14.41
12.79
0.132
2.41
6.18
3.04
1.77
0.39
2.56
100.3
52.73
2.401
13.52
13.77
0.118
1.83
5.82
2.77
1.65
0.45
4.33
99.39
55.92
2.399
14.01
11.26
0.124
1.97
5.92
2.99
1.98
0.52
2.97
100.1
55.32
2.516
14.34
10.73
0.089
1.85
5.82
3.04
1.86
0.52
3.04
99.13
40.28
2.278
26.28
12.69
0.059
0.33
0.66
0.08
0.05
0.13
17
99.85
52.8
2.268
14.34
12.9
0.191
3.85
7.86
2.88
1.14
0.43
2.11
100.8
53.26
2.126
13.14
12.66
0.197
3.75
7.7
2.77
1.51
0.42
1.18
98.71
49.78
1.251
14.49
11.42
0.161
6.44
10.62
2.58
0.3
0.3
3.25
100.6
47.23
1.252
14.29
12.63
0.263
4.62
10.66
2.19
0.21
0.27
6.02
99.63
41.88
1.491
17.52
14.83
0.172
3.32
4.85
0.99
0.2
0.27
13.3
98.83
41.55
1.492
17.16
13.2
0.184
2.54
4.14
0.83
0.2
0.7
16.1
98.09
35.59
1.529
19.26
15.32
0.049
1.13
1.45
0.06
0.11
0.21
24.68
99.38
741
757
784
820
780
870
853
439
748
726
285
377
442
515
1071
2
2
2
2
2
2
2
2
1
1
ND
ND
ND
ND
1
324
329
332
367
344
362
375
72
367
339
357
328
156
139
96
396
387
402
400
409
413
417
254
414
408
339
333
292
358
306
33
36
33
38
33
46
38
21
36
33
25
23
26
38
15
174
196
177
204
184
214
223
465
185
176
97
86
102
119
150
Notes: *Sample designator ‘PS’ = paleosol samples. ND = Not Detected. Oxide values are reported in %; elemental values are in ppm. Detection limits are 0.01%
for SiO2, Al2O3, Fe2O3(T), MgO, CaO, Na2O, K2O, and P2O5; 0.001% for TiO2 and MnO; 1 ppm for Ba, Be, and Y; 2 ppm for Sr and Zr; and 5 ppm for V.
53 1067
1068
1069
1070
1071
Table 5. Elements analyzed by INAA
Analyte
LC-01
LC-02
LC-03
LC-04
LC-05
LC-06
LC-07 LC-PS* KG-01 KG-02 SH-01
SH-02
SH-03
SH-05 SH-PS*
Symbol
Au
ND
ND
ND
ND
ND
10
ND
2
ND
ND
ND
ND
ND
3
ND
As
ND
1
ND
ND
ND
3
ND
4
ND
2
ND
ND
ND
ND
2
Br
ND
ND
ND
ND
ND
ND
ND
ND
1.9
ND
ND
ND
ND
ND
ND
Co
36.1
36.7
39.2
33.8
30
43.6
41.9
20.7
41.7
38.3
41.3
41.3
49.7
41.9
29.8
Cr
8.3
ND
5.7
4.5
ND
ND
ND
10.4
11.8
14.1
218
230
436
382
141
Cs
1.2
1.6
2.9
2
2.4
2.1
2.8
ND
ND
2
ND
1.2
ND
ND
1.1
Hf
5.4
5.2
5.2
5.5
5.8
5.8
5.7
11.3
4.8
4.6
2.4
2.3
3
3.1
3.9
Hg
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Ir
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Rb
60
40
50
50
50
70
50
ND
30
40
ND
ND
ND
10
ND
Sb
ND
ND
0.3
ND
0.3
3.5
ND
0.9
0.2
ND
ND
ND
ND
ND
ND
Sc
34
33.9
33.4
33.9
33
35.9
35.5
33.9
38.9
36.7
44.7
43.7
53.6
52.3
48.4
Se
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Ta
1.2
1
1
ND
0.9
ND
1
1.5
0.9
ND
ND
ND
0.4
0.5
0.5
Th
5.5
5.8
5.3
6.6
6.5
6.7
6.5
11.4
4.6
4.3
0.7
0.6
0.9
0.7
2
U
1
2
1.9
1.8
1.3
1.8
2
2
1
ND
ND
ND
ND
ND
0.9
W
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
La
30.8
30.4
30.1
32
33.1
35.1
34.5
35.3
25
24.2
13.2
12.8
13.1
16
12.2
Ce
56
56
55
56
63
65
61
87
46
44
26
27
30
33
29
Nd
25
23
25
29
31
32
31
35
19
21
15
14
18
22
15
Sm
7.12
7.15
6.97
7.28
7.78
8.26
8.12
9.72
6.72
6.38
4.29
4.14
5.2
5.57
3.71
Eu
2.33
2.4
2.34
2.5
2.68
2.88
2.77
2.79
2.43
2.19
1.58
1.53
1.85
2.05
1.22
Tb
1.2
1.4
1.4
1.2
1.3
1.6
1.8
ND
1.3
1.2
ND
0.8
1
1.1
0.8
Yb
3.47
3.53
3.3
3.61
3.5
4.7
3.75
2.89
3.5
3.44
2.68
2.82
3.4
3.98
3.44
Lu
0.55
0.57
0.53
0.58
0.52
0.78
0.57
0.44
0.56
0.53
0.44
0.41
0.51
0.65
0.56
Au
ND
ND
ND
ND
ND
10
ND
2
ND
ND
ND
ND
ND
3
ND
Notes: *Sample designator ‘PS’ = paleosol samples. ND = Not Detected. Elements Au and Ir are reported in ppb; remainder are in ppm. Detection limits are 10
ppm for Rb; 1 ppm for As, Ce, Hg, Nd, and W; 0.5 ppm for Br, Cr, and Se; 0.3 ppm for Ta; 0.2 ppm for Cs and Hf; and 0.1 ppm for Co, Sb, Tb, Th, and U; 0.05
ppm for Eu, La, and Yb; 0.01 ppm for Lu, Sc, and Sm; and 1 ppb for Au and Ir.
54 1072
1073
1074
Table 6. Elements analyzed by ICP-OES following acid digestion
Analyte
LC-01
LC-02
LC-03
LC-04
LC-05
LC-06
LC-07 LC-PS* KG-01 KG-02 SH-01
SH-02
SH-03
SH-05 SH-PS*
Symbol
Ag
ND
ND
ND
0.5
ND
ND
ND
0.6
ND
ND
ND
ND
ND
ND
ND
Bi
ND
ND
ND
7
ND
ND
ND
ND
ND
ND
ND
ND
ND
4
ND
Cd
ND
ND
ND
ND
ND
ND
ND
ND
0.6
ND
ND
ND
ND
ND
ND
Cu
62
26
27
30
24
40
24
50
23
20
87
76
93
77
79
Mo
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
ND
Ni
12
12
14
11
11
10
10
31
12
12
50
50
50
38
83
Pb
9
10
9
6
11
10
11
29
10
ND
ND
ND
ND
ND
5
S
0.058
0.035
0.034
0.027
0.025
0.036
0.028
0.004
0.036
0.041
0.044
0.04
0.02
0.016
0.032
Zn
120
114
121
131
146
179
147
92
122
111
76
81
94
80
101
Notes: *Sample designator ‘PS’ = paleosol samples. ND = Not Detected. Elements S is reported in %; remainder are in ppm. Detection limits are 0.001% for S; 5
ppm for Pb; 2 ppm for Bi and Mo; 1 ppm for Cu, Ni, and Zn; and 0.5 ppm for Ag and Cd.
55 Figure 1. Map of the northwestern United States with the extent of Columbia River
Basalt (CRB) given in orange [from Burns et al., 2010] and sample localities marked
with black circles. The flows themselves are more extensive [e.g., Reidel et al., 1989],
but the outline covers the main central area of thickest CRB.
Figure 2. Plot of unconfined compressive strength (UCS) test results ordered by
decreasing mean value. KG = Kendrick Grade samples; LC = Lawyer Canyon samples;
and SH = Shumaker Grade samples. Mean values are given by green horizontal bars.
Figure 3. Plot of Rock Abrasion Tool (RAT) grinding test results given as specific grind
energy (SGE, units J/mm3) ordered by decreasing mean value. KG = Kendrick Grade
samples; LC = Lawyer Canyon samples; and SH = Shumaker Grade samples. Mean
values are given by green horizontal bars.
Figure 4. Mean compressive strength values (in MPa) plotted against mean specific grind
energy (SGE) values (in J/mm3). Left-hand figure is a duplicate of right-hand figure with
error bars omitted for clarity. Error bars on right-hand figure are ± one standard deviation
of the mean value. Dashed and dot-dashed lines give power law and linear fits to the
calibration data of Thomson et al. [2013]. Shaded field on right-hand plot is measured
SGE values of Adirondack-class basalts in Gusev crater measured by the MER Spirit;
concomitant compressive strength values calculated from fits to calibration data.
Figure 5. (a) Chemical index of alteration versus compressive strength. (b) Chemical
index of alteration versus grind energy.
Figure 6. (a) Compressive strength versus bulk density. (b) Grind energy versus bulk
density. (c) Chemical index of alteration versus bulk density.
Figure 7. Major oxides versus depth below top of paleosol surface for Lawyer Canyon
samples.
Figure 8. Mobility ratio calculated according to Eq. 3 for Lawyer Canyon (LC) samples
indicating cation gain or loss compared to least weathered material (LC-01) arranged in
order of sample depth below top of paleosol surface. Index element (assumed to be
immobile) is Zr.
Figure 9. Ternary Al2O3, (CaO+Na2O+K2O), and (FeOT+MgO) diagram, data plotted in
mole percent. Solid black arrow labeled ‘A1’ indicates weathering trend in of Lawyer
Canyon samples LC-01 to LC-07 consistent with dissolution of clinopyroxene
phenocrysts or groundmass olivine. Dashed back arrow labeled ‘A2’ indicates weathering
trend required to reach Lawyer Canyon paleosol composition that is consistent with
feldspar removal and clay deposition. Dashed black arrow labeled ‘B1’ is weathering
trend displayed by Shumaker Grade samples; no evidence of an early weathering phase
dominated by loss of ferromagnesian minerals is evident.
Figure 10. Mobility ratio calculated according to Eq. 3 for Lawyer Canyon (LC) samples
indicating cation gain or loss compared to least weathered material (LC-01). Index
element (assumed to be immobile) is Ti. Gray shaded field represents maximal cation
gain or loss (excluding K2O) of Adirondack-class basalts comparing brushed surfaces to
those abraded by the RAT. Specifically, the samples considered are Adirondack brush
and RAT, Humphrey brush and RAT1, and Humphrey brush at RAT2. All data are
normalized to 100% to a LOI-free basis for Lawyer Canyon, and SO3- and Cl-free basis
for the Adirondack-class basalt. Vertical dashed lines on right mark positive mobility
ratio or Tau values for K2O.