JOURNAL
Abundance
OF GEOPHYSICAL
RESEARCH, VOL. 92, NO. B10, PAGES 10,376-10,390, SEPTEMBER
and Distribution
of Ultramafic
Microbreccia
10, 1987
in Moses Rock Dike'
QuantitativeApplicationof Mapping Spectroscopy
JOHN F. MUSTARD
AND CARLE M.
PIETERS
Departmentof GeologicalSciences,Brown University,Providence,RhodeIsland
Moses Rock dike is a Tertiary diatremecontainingserpentinizedultramaficmicrobreccia(SUM) located
on the ColoradoPlateauin Utah. Field evidenceindicatesthat SUM wasemplacedfirstfollowedby breccias
derivedfrom the Permianstrataexposedin the walls of the diatremeand finally by complexbrecciascontaining basementand mantle-derivedrocks. SUM is primarily found dispersedthroughoutthe matrix of the
diatreme.We examinedthe natureof SUM abundance
and spatialdistributionusingdatacollectedremotely
by the Airborne Imaging Spectometer(AIS). The mineralsserpentine,gypsum,and illite as well as desert
varnishand the lithologiesSUM and varioussandstones
were identifiedfrom the AIS data. Six end-members
(SUM, two typesof sandstone,
gypsiferous
soil, clay-richsoil, desertvarnish)werechosento representthe
dominantlithologiesof the surfacein MosesRock dike region. Spectraof theseend-memberswere usedin
an intimate mixing model to deconvolvethe AIS spectraldata into surfaceabundancecoefficientsfor each
componentusing a nonnegativeleast squaresinverse algorithm. The resultsof this calculationof surface
compositionare consistentwith field observations
and investigations.
SUM distributionand abundancein the
matfix of the diatremewere examinedin detail, and two distinctstylesof SUM dispersionwere observed.
One style is characterized
by high to moderateSUM abundancesurrounded
by halosof lesserSUM abundance.The dispersion
halosgradesteeplyinto regionsof the matrixwith little to no SUM. The secondstyle
is characterized
by moderateSUM abundances
and broaddispersionhaloswith SUM distributedacrossthe
entire width of the diatreme.These dispersionstylesare consistentwith eraplacementof the dike as a
fluidizedsolid-volatilesystemwhereSUM, which is emplacedearly, becomesreincorporated
into the turbulent flow of the diatremeby abrasionand comminutionby particulatemattercarriedalongin the eruption.
This erodedmaterialis thendispersed
throughout
the matrixby eddydiffusion.The first stylerepresents
an
early arrestedphaseof the eruptionsequence,
while the secondstylerepresents
a morematurephase.Distributionsof the secondstyle probablyindicatethe locationof channelswhereflow was concentrated
during
later stagesof eruption.Since both stylesare observedat the samelevel of erosion,this indicatesthat the
durationof eruptionvaried alongstrikeat MosesRock dike.
INTRODUCTION
element systematicstypical of kimberlite, and evidence of ever
being a true magma [Mitchel, 1986]. Roden [1981] has sugMoses Rock dike is one of severalTertiary diatremescontain- gested serpentinizedultramafic microbreccia(SUM) as a more
ing mantle-dedhved
matedhallocatedin the Navajo volcanicfield appropriateterm to describethe ultramafictuffs of the Navajo
on the Colorado Plateau. Although these diatremesare minor volcanicfield, and we have adoptedthis usagefor MosesRock
geologic featuresvolumetricallyand in extent of outcrop, they dike.
At Moses Rock dike, SUM is found throughoutthe dike and
contain abundantgeologic information as they are believed to
erupt from volatile-richregionsdeep in the lithosphere.During is thoughtto be involvedin all stagesof the eruption.Although
the rapid ascentof these gas-chargedmagmas,mantle-derived massiveSUM is found as dikes and sills injected into the host
and deep crustalrocksare commonlyentrainedalong with upper rocks, the majority of this component(>90%) is found dispersed throughoutthe matrix of the dike [McGetchin, 1968].
crustal rocks in the diatreme
breccias.
The xenoliths
and
ultramaficmantle-derivedcomponentsprovide insight into the The SUM became dispersedin the diatreme during eruption,
compositionof otherwiseinaccessibleregionsof the lithosphere. and thereforethe natureof this dispersionmay provideinsight
The inferred sequenceof eventsfor the emplacementof the into mechanismsfor materialredistributionin the vent of exploplateau diatremesinvolves the propagationof a dike from the sive, volatile-richeruptions.The best approachto determining
sourceregion and eruptionof a complexmultiphasemedium. At the nature of SUM dispersionis to map accuratelythe abun-
Moses Rock dike the flow is believedto have eventuallystabi-
dance and distribution
lized into a few channels which
dike. However,the detailedmappingof sucha componentusing
classicalfield techniqueswould be impracticaland an inefficient
were manifested
at the surface
of SUM
in the matrix
of Moses
Rock
as maar-type eruption craters [Shoemaker, 1962; McGetchin,
use of resources.
1968]. The highly comminutedand brecciatedtexturesof the
Recentadvancesin remote sensingtechnologyhave led to the
blocks and particles in diatremes are an indication of the
development
of imaging or mappingspectrometry[Goetz et al.,
extremelyviolent natureof their formation, and mixing between
the wide rangeof components
in the dike, from upperlevel sed- 1985] which can be used to map regional mineralogicalvaria-
imentary lithologies to ultramafic mantle rocks, occurs on all
scales [McGetchin and Silver, 1972]. The primary mantle-derived componentin Moses Rock dike was originally described
as kimberlite by McGetchin [1968]. However this ultramafic
material lacks the mineralsperovskiteand phlogopite,rare earth
Copyfight1987by the AmericanGeophysicalUnion.
tions. Mapping spectrometers
measureboth spatialand spectral
informationsimultaneously
with sufficientwavelengthresolution
such that a near-infraredreflectancespectrumis measuredfor
each pictureelement(pixel) in the image. Direct mineralogical
informationis derivedfrom the position,strength,and shapeof
absorptionbandsin the spectra.The absorptionsare primarily
due to electronic processesinvolving transition metal ions (
Paper number7B 1012.
Fe3+,Fe
2+, Cr2+, etc), andvibrational
processes
involving
0148-0227/87/007B-
H20, OH-, and CO3-, where the position, strength,and shape
1012505.00
10,376
MUSTARDAND PIETERS:ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
10,377
I
coI
Qs- Quaternarydeposits
Mrd-
Moses Rock Dike
Pcd- De ChellySandstone
Pco- OrganRock Member
Pcm-
Cedar Mesa Sandstone
Cutler
Formation
Pcha- HalgiroMember
Contact, dashed where concealed
Limit of quaternarydeposits
Feeler
dike
o
Fig. 1. Generalizedgeologicmap showingthe contactbetweenMoses Rock dike and the sedimentsof the PermianCutler formation which it intrudes [after O'Sullivan, 1965]. The position of the diateme on the Colorado Plateau is shown in the
locationmap in the upperright.
is sharp and the vent walls are steep. At the present level of
exposure, approximately1500 m below the surfaceat the time
of eruption [McGetchin, 1968], the dike is in contactwith three
membersof the Cutler Formation: the Organ Rock Tongue, the
Cedar Mesa Sandstone, and the Halgito Tongue. The Organ
Rock member consists of dark reddish-brown siltstone, sandy
siltstone,and very fine grained sandstone.The lithologiesof the
Cedar Mesa Sandstonein this region include variegatedsiltstone
and shale, buff sandstone,gray limestone, and gypsum. The
on the surface.
Halgito Tongue is composeddominantlyof reddish-brownsiltIn this paper, data from the Airborne Imaging Spectrometer stone and very fine grained silty sandstonewith thin beds of
(AIS) are used to map the distributionand abundanceof the gray limestone [O'Sullivan, 1965]. The dike was mapped in
SUM componentin MosesRock dike. The geologicsettingand detail by McGetchin [1968], a summaryof which appearsin the
compositionof Moses Rock dike are reviewed, followed by a work by McGetchin and Silver [1970, 1972]. The following
discussionof the nature of the surface materials (composition, accountof the geology and eruption of the diatreme is derived
texture, and relationshipto bedrock). These observationsalong from these sources.
with laboratoryspectroscopicdata are used to interpret surface
The breccia fragments filling the dike range in size from
mineralogyof the dike and surroundingregionsfrom the imag- microscopicparticlesto blocks up to a hundredmeters across.
ing spectrometer
data. The spatialdistributionand abundanceof The large blocks, derived almost exclusively from the units
primary surface componentsare calculatedusing a nonlinear exposedin the vent walls, constitutethe majority of the breccia.
model for the mixing of spectrafrom multicomponentsurfaces. The interstices between the blocks are filled with a matrix conThe nature of the derived distribution and abundance of SUM is
taining fragmentsfrom the large blocks, limestonefragments,
examinedin detail and supportsMcGetchin's [1968] model for crystalline rock fragments, and mafic and ultramafic constitutheemplacementof MosesRockdike asa fluidizedsystem.
ents. Basement-derivedigneous, altered igneous, and foliated
metamorphicrocks compriseabout 3% of the dike, while dense
GEOLOGIC SETTING
ultramafic mantle-derivedfragmentsof eclogite, lherzolite, and
of the absorptionbands are controlledby the particular crystal
structurein which the absorbingspeciesare contained [Burns,
1970; Hunt and Salisbury, 1970; Adams, 1975]. Although most
natural surfacesare complex mixturesof soil, rock, and vegetation, reflectancespectraof mixturesare systematiccombinations
of the reflectancespectraof the surfacecomponents.By deconvolving reflectancespectrainto componentabundances,the high
spatialresolutionin imaging spectrometers
can be used to map
the distribution and abundanceof specific mineral components
websterite
Moses
Rock
dike
is located
on the Colorado
Plateau
in the
extreme southeasterncorner of Utah (Figure 1). It occurs in a
cluster of diatremeseast of the Monument Uplift along the axis
of Comb Ridge Monocline. The dike is hook-shapedin plan,
about 6 km long and 100-300 m in width. It intrudes undeformed and unmetamorphosedsedimentsof the Permian Cutler
Formation, and the contact between the dike and the sediments
are rare.
Outcropsof SUM constituteabout 1% of the dike and are
found primarily as small dikes and sills in the wall rocks and
large breccia blocks in the dike. SUM is also found as small
bodiesplasteredagainstthe walls of the dike and as podswithin
the dike. Mineralogically, SUM is a highly serpentinized
ultramafic microbrecciacontaining remnantsof olivine, pyroxene, garnet, and spinel set in a fine-grainedserpentinematrix.
10,378
MUSTARDANDPIETERS:ULTRAMAfiC
MICROBRECCIA
IN MOSESROCKDIKE
Petrologicevidenceindicatesthat the sourceregion for the SUM
was between 50 and 110 kin, but there was no appreciablemelt
component associatedwith this eruption. Instead, the minerals
in SUM are thought to be derived from the dissagregationof
garnet- and spinel-lherzolite [McGetchin, 1968; Hunter and
Smith, 1981]. Dissagregationhas also been proposedas a possible mechanismfor deriving the ultramafic material in the Red
Mesa and Garnet Ridge diatremeswhich are related texturally
and petrologically to Moses Rock dike [Hunter and Smith,
1981]. The fact that the microbreccias of the Colorado Plateau
never had an appreciablemelt componentimplies that models
for formation of kimberlite diatremes, which have well-docu-
mented melt components,may not be appropriatefor the diatremes of the Colorado
Plateau.
Although massive SUM constitutesonly 1% of the dike, a
further 12% of SUM derived material is found dispersedin varying concentrationsthroughoutthe matrix of the dike. The contribution
of SUM
in the matrix
varies from
>90%
in zones of
diluted SUM to almost zero in regions consistingprimarily of
Cutler rubble. Complex breccias, identified by the presenceof
basement- and mantle-derived xenoliths, contain SUM concen-
trationswhich vary from 50% to 10%.
The following observationsmade by McGetchin [1968] constrain the mechanismof dike eraplacement:(1) the absenceof
metamorphism and deformation in the host rocks indicate that
the dike was eraplaced rapidly, (2) no evidence of a silicate
melt componentin the SUM is observed, (3) the breccias are
particulateon all scaleswith particle size distributionslike those
producedby comminution, (4) the brecciasare intricately mixed
on all scales, and (5) stratigraphicand superpositionrelationships between the breccia units in the dike indicate that SUM
dikes were eraplaced early, followed by rubble consistingof
large blocks of Cutler rocks and later by units of complex breccias containing progressivelylarger crystalline rock fragments.
These field observations and hydrodynamic modeling led
McGetchin [1968] to conclude that the diatreme was formed as
a fluidized solid-volatile system probably driven by H20 and
CO2.
5% and averagesaround2.5%. Also, of the total vegetation,
50% is unfoliated.Two ancillary broadbandspectraldata sets
for this region(infraredcolorphotographs
and thematicmapper
simulator) chosen to emphasize vegetation indicate that
significantvegetationis not detectablewith resolutionsof 2-10
m exceptin somewashchannels
andin thevicinityof springs.
Outcropsof massivebedrockare sparseand restrictedmainly
to cliff exposuresand local topographichighs. More commonly
the surfaceis composedof the local productsof weathering,and
in this region the dominant weatheringprocessesare mechanical, namely, exfoliation, abrasion,and crystalgrowth. Chemical
erosionand alteration, due primarily to infrequentrainfalls and
morningdews, are less important.The primary productsof this
weatheringare fragmentsof the original lithologies, and these
generallyaccumulatein situ on the surface.Particle sizesrange
from platey fragments3-7 cm acrossto microscopicmineral
grains and rock fragments.The dominantparticle size in any
given area is loosely correlatedto lithology. For example, limestonebeds commonly weather to platey fragments,while sandstone and siltstonelithologies typically weather to fine-grained
particles. Transport of the weatheringproductsis primarily by
wind andrare rainfallssufficientto generatesurfacerunoff.
Surfacesof unalteredcrystallinerock fragments,found in the
dike, are commonlycoveredwith a dark shiny mineralcoating
referredto as desertvarnish.The coatingis only rarelyobserved
on sedimentaryrock surfacesand soft, friable lithologiessuch
as the microbrecciated
SUM. It has been shownthat the physical propertiesof varnish from widely separatedlocalities are
consistent,and the compositionis independentof the host rock
[Perry and Adams, 1978]. It is typicallyup to 100 [tm thick and
consistsof ferro-manganeseoxides depositedwithin a clay
matrix [Potterand Rossman,1977]. Due to the stronglyabsorbing natureof this coatingit, masksthe compositionof the host
rock. Varnish has preferentially developed on the crystalline
rock fragmentssuchas meta-basaltand gabbrobecausethe surfacesof theselithologiesare stablein the local weatheringenvironment.
The nature of the surfacematerials (i.e., particle size, texture), relationshipsto the underlyinggeologic formations,and
SURFACE CHARACTERISTICS
the stratigraphyof the upper 10-15 cm were examinedat many
locations both in the dike and sedimentaryunits. In general,
The near-infraredreflectancespectrameasuredby AIS contain thereis a stratigraphyconsistingof threelayers.The top layer is
two primary componentsof radiation scatteredby the surface: composedof erosionallag depositsof locally derived resistant
reflection and volume scattering. Diagnostic absorptions are rock fragments0.5-5 cm across.This layer is most commonly
present in the volume-scatteredcomponent of this radiation observedon the tail sectionof the dike and on exposedlimecausedby the absorptionof photonsduring transmissionthrough stonebedswhere the fragmentsare large (>3 cm) and can commineral phases.Typically, the penetrationof this componentof prise greater than 60% of the surface. Elsewherethe fragments
radiation into the surfaceis limited to the outermost50-100 [tm of the lag depositsare sparseor absent.The middle layer is
composedof medium to very fine grained particles predomi[Buckinghamand Sommer, 1983] but could extend up to a millimeter [Pieters, 1983]. Therefore, in order to understand and nantly derivedfrom the local substratum
or the fragmentslying
interpret geologically the remote observationsof a surface, a on the surface. It is not uncommon to observe a vesicular texthorough characterizationof the propertiesof the topmost layer, ture in the upper 1-2 cm of the middle layer. The vesicules
of water in clays
the relationshipof the surfaceto the underlyingbedrock, and an form from periodicabsorptionand desiccation
such as montmorillionite
and illite. The vesicular textured midassessment
of the vegetationcoverare essential.
Moses Rock dike is located in the arid central region of the dle layer is found in regions of mature soil profiles and in
Colorado Plateau at about 1600 m elevation. The vegetationin regionsof the dike composedof very fine grainedmaterial.The
this area consistsprimarily of small shrubs and bushes with third layer is composedof angularfragmentsderived from bedradii of 10-30 cm and heights of less than 0.5 m. Vegetation rock which typically lies 5-15 cm below the surface.
counts from several regions around Moses Rock dike were
Although there is no major mixing between geologic units in
obtained for representative20 by 20 m areasover the sedimen- the weatheringprocess,locally significantdepositsof foreign,
tary units and the diatreme in July 1985. Resultsof thesemeas- dominantlywind transported,material accumulate.Thesedeposurements show that the total vegetation cover does not exceed its are generally composedof very fine grained (<40 [tm) parti-
MUSTARDAND PIETERS:ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
10,379
toward longer wavelengthscaused by decreasing solar irradiance. In order to identify mineral absorptionfeaturesit is necessary to correct not only for the atmosphericand solar effects
shown in Figure 2 but also for overall detectorand instrumental
response. Currently, the most efficient and satisfactory
techniquetoward this purposeis to calibrate the data using an
0.40
internal standardsuch as a homogeneouslow-relief region contained within the AIS scene. For optimum results the standard
area must either have known spectralreflectancecharacteristics
or a featurelessspectrum.
0.20
The area selected for standardizingthe Moses Rock data is
locatedsouthof the dike in quaternarysanddeposits(Qs in Figure 1). These consistof wind-depositedsandof uniform texture
I
,
,
I
,
,
I
,
,
I
0.00
and color with an averageparticle size of 150 [tm. The surface
1.1 o
1.40
1.70
2.00
2.30
of the depositis characterizedby low undulatingswells and holWavelength In Microns
lows with a local relief of less than a meter. Twenty continuous
Fig. 2. Unprocessed
raw spectroscopic
data measuredby AIS for a sur- lines of data were selectedfrom a homogeneousregion in the
face covered by sandstonenear Moses Rock dike. Note the overall data and averaged in the along-track direction only. This was
decreasein reflectancetoward longerwavelengthscausedby decreasing done to preserve systematicacross-trackdetector variations in
0.60
[
Uncalibrated
AIS Data
_
solar irradianceand the prominent atmosphericabsorptionbands centerednear 1.4 and 1.9 gm
cles derived
from the reddish-brown
silt and sandstones of the
Cutler Formation. The depositsoccur in wind-depositedsediment traps suchas in the lee of small ridgesand, on a smaller
scale, in the intersticesbetween large surface rock fragments.
Examinationof the stratigraphyin the areasof interstitialdeposition showsthat mixing between the foreign and local componentsoccursin the upper 1-2 cm and contaminationby this foreigncomponentcanbe asmuchas 25-30%.
In summary, surfacecomponentsare primarily derived from
local geologic units. Since the dominant weathering processes
are mechanical, there is little or no chemical erosion and altera-
tion. Particlessizeson the surfacerangefrom silt to cobble, and
the dominant size varies with the lithology from which it was
derived. Vegetationcover is sparseand homogeneously
distributed. To first order, the surfacecomponentsimaged by AIS are
an accuratereflection of local bedrock geology, although contaminationby foreign, wind-blown material is a factor in some
localities. These variablesneed to be consideredwhen interpreting the spectrageologicallyandwhenapplyingmixing models.
the
calibration
data.
Each
line
in
the
as 8-bit data in a 16-bit word.
To correct for these features, the AIS relative reflectance data
are multiplied by the laboratoryspectrumof the standardmaterial obtained under viewing conditionssimilar to that for AIS
0.70
The
,
I
I
'
AIS Coverage
'1 I
I
spectralresolutionof 9.3 nm/channeland a groundresolutionof
about 10 m/pixel. The swath width is therefore approximately
320 m. The AIS data were collected in late July 1984 under
clear and cloudless conditions.
/,
Water
".'.'.L.v..'..'.'-.t.
' '
Bands
'l.':..V.'•:.:.'..'::¾.il
' '
r.-;?..'-..'-.'.-'.:•
r;::'.'.:;i'-':'.:i?;i'-'.:.
i
E:.::'::
:..
--:.
ßß
ß:..'..:..'..:..':
:::..'::..'::'.'::
0.60
I?.:.2..'.:;..'-.'..'.:½•
t.':.:Z
;':"':':"':':':':1
0.50
':½.'5:...
0.40
0.30
particular configurationof gratings used in the flights over
Moses Rock dike recordeddata from 1.155 to 2.337 [xm with a
then
wavelength region of the AIS data there is an overall small
increase in reflectance toward longer wavelengths, OH_ and
H20 overtone bands at 1.4 and 1.9 •tm, and several small but
distinct absorptionbandslongward of 2.1 [tm in the laboratory
spectrum.Thin sectionand XRD analysisof the standardindicatesthat it is composedprimarily of roundedquartz grainswith
thin iron oxide coatings, 10% clay, and less than 1% opaques.
Standard
states and is then recorded
was
The results from this first-order calibration technique were
sufficientto identify severalmineral species[Mustardand Pieters, 1985] that were later verified by field investigations.However, a laboratoryreflectancespectrumof a surfacesamplefrom
the standardarea, shown in Figure 3, revealed several spectral
featureswhich bias the relative reflectanceAIS spectra.In the
Almospherm
Spectroscopic
data were acquiredover Moses Rock dike with
the Airborne Imaging Spectrometer(AIS). AIS is an infrared
mappingspectrometer
that images32 cross-trackpixels simultaneously in 128 contiguousspectralbands [Goetz et al., 1985].
The detector for this instrument is a 32 by 32 element array
which is steppedthrough four grating positionsto obtain the
128 spectral channels. In order to maintain signal dynamic
range, the analog data are digitized with a 12-bit analogueto
digital converter (ADC). The ADC switchesamong four gain
scene
referred to as relative reflectance.
•
DATA ACQUISITION AND CALIBRATION
AIS
divided by the along-track average for the 20 lines from the
internal standard.Reflectancedata processedin this manner are
0.20
0.60
'
'
•
0.90
'
'
I
::'.'.'::'.':::5
,''"'
:' .'.::i!l
I
i"::'i:i.il
I
'........
."'...
':'
..ß
'.':::('"
:'-'::?-'"::1
i'-...-':',..-':'..
I
•.•o
•.•o
•.ao
z•o
2.40
Wavelength In Microns
Fig. 3. Laboratory reflectancespectrumof soil from the standardarea
which consistsof quartz grainswith Fe-oxide coatingsand clay. The
The uncorrecteddata for AIS, expressedas intensityin Figure bandsin the visibleandat 0.9 [tm are dueto Fe3+ andFe2+, while
2, are dominatedby two broad atmosphericwater absorption the OH- bandsnear 1.4 and longwardof 2.0 [tm and the H20 bandsat
bands centered at 1.4 and 1.9 [xm and a falloff in intensity 1.9 [tm are characteristicof the clay illite.
10,380
MUSTARDANDPIETERS:ULTRAMAfiC
MICROBRECCIA
IN MOSESROCKDIKE
,•
AIS Coverage
Atmospherm
Water
ß
July 1985, an apparentlywetter seasonthan July 1984. IRIS is
a dual-beam spectroradiometerwhich simultaneouslymeasures
the reflectancespectraof a sample and the standardused for
calibration. The instrumenthas a spectralresolutionof 2-4 nm
Bands
0.80
0.70
andsamples
a surface
areaof approximately
10cm2 Thestandard used in the field for these measurements
(:v 0.60
o
is Fiberfax.
The
raw reflectancedata are initially calibratedby dividing by the
simultaneouslymeasured spectrum of Fiberfax. To maintain
consistencyin our measurements,a sample of Fiberfax was
0.50
measured in RELAB,
rT- 0.40
0.30
0.20
0.60
0.90
1.20
1.50
1.80
2.10
2.40
and then the IRIS relative reflectance
spectra were multiplied by the RELAB spectrumof Fiberfax.
This then calibratesthe laboratory, field, and AIS spectraldata
to the same standard.Although no independentwavelengthcalibration was available for the IRIS data, comparisionsto laboratory data indicate that a 0.012-0.014 rim correctionis required
in the IRIS data for wavelengthslonger than about 1.6 rim.
However, this does not affect the results discussedhere.
WavelengthIn Microns
Fig. 4. Laboratoryspectra(solid lines) of illitc and the standardtogether
with AIS spectra (asterisks)of soils interpretedto contain illitc. The
wavelength interval covered by AIS is indicatedas well as regions of
poor data quality in the atmosphericwater bands.See captionfor Figure
3 for bandassignments.
SURFACE MINERALOGY
Several surface units with distinct compositions were
identified from the calibrated AIS data. Interpretations of
surface mineralogy were aided by field and laboratory spectra
data. This cancels the effects of the slope and absorptionfea- and from samples returned from the field. In the wavelength
tures of the standardarea and calibratesthe AIS data using the region of AIS (1.15-2.34 rim), clays and other minerals conabsolutecalibrationof laboratorystandards.
taining structuralwater and hydroxyl groupscommonly exhibit
The quality of the resulting calibratedAIS data is generally characteristicabsorptionsbetween 2.0 and 2.34 rim. Absorpgood, and absorption features can be interpreted in terms of tions associatedwith the transitionmetal ions are lessprominent
mineralogy. However, distinctline to line spatialbandingin the in this wavelength region, although the presence of these
AIS data near the broad atmosphericwater bands and a very absorptionsmay be inferred from the overall slope of the speclow signal to noise ratio in the center of thesebandsresult in a tra.
notable decreasein the precision and accuracyof the data for
Two AIS spectraof surface soils interpretedto contain clay
the wavelength regions 1.32-1.52 rim. and 1.76-2.03 rim. mineralsare shown in Figure 4 along with laboratoryspectraof
Becauseof the overall low quality of the data in these wave- illite and a samplefrom the standardarea. Both AIS spectraare
length regions, these data were not used in these analysesand characterizedby a slight increasein reflectancetoward longer
for purposesof clarity have been omitted from the figures of wavelengths(positive slope), a reflectivity maximum near 2.14
AIS spectra.
rim, a 5-15% absorptionnear 2.22 rim followed by a local
Other spectraldata importantto theseanalysesinclude labora- reflectivity maximum near 2.28 rim. The 2.14- and 2.28-rim
tory and field spectral measurements.Laboratory spectra of maxima are asymmetricabout the 2.22-rim absorptionwhere the
samplesreturnedfrom the field were measuredin the RELAB, a 2.14-rim peak is brighter. The asymmetric reflectancepeaks
high-resolution bidirectional reflectance laboratory [Pieters, about the 2.22-rim absorptionband are characteristicof several
1983], using approximately the same viewing geometry as the clays including muscovite, montmorillionite, and illite. HowAIS reflectancedata with an incidence angle of 30ø and an ever, laboratoryreflectancespectraof field samples(Figures 3
emergence or detection angle of 0ø. Most samples were and 4) show a small absorptionat 2.34 rim and a reflectance
preparedas bulk soils to simulatefield conditions.Selectedsoils peak at 2.37 rim which are more diagnosticof illire than other
were sieved to determinethe range of dominantparticle sizeson clays. The positive slope is due primarily to absorptionsassoci-
atedwithreddish-brown
ironoxidebearing
soilsor withFe3+
the surface.
Reflectancespectrameasuredwith RELAB are obtainedrela-
in the clay structure.
Field spectra and laboratory spectraof SUM are shown in
spectrallyfeaturelessin visible and near-infraredwavelengths.It Figure 5 along with a calibrated AIS spectrumof a surface
has an averagereflectanceof >96% and can be approximatedas interpretedto contain SUM. This AIS spectrumis characterized
a Lambertian reflector (scatterslight equally in all directions) by a reflectivity maximum near 2.2 rim, a 15-30% absorption
[Pieters, 1983]. Reflectance values measured relative to halon centeredat 2.33 rim, and a strong positive slope. The features
are then calibrated to the absolute reflectance of halon as deterlongwardsof 2.0 rim are due to a combinationof a MgOHmined by the National Bureau of Standards(1975 National bending mode with OH-stretching fundamentalsand overtones
Bureau of Standardstest 232.04/213908). The laboratory spec- [Hunt and Evarts, 1981]. The positive slopeis due primarily to
tive to the standard
tral measurements
halon.
Halon
is an inert fluorocarbon
and is
are to first order true or absolute reflectances.
Field spectraof soils and vegetationwere measuredwith a
Geophysical Environmental Research infrared interferometer
spectroradiometer(IRIS), a portable field spectrometer.These
spectra are an important cross reference between the AIS data
and the laboratory spectra. The field spectrawere obtained in
electronic
absorptions
in Fe2+ andFe3+ ionsat 0.72, 0.91,
and 1.1 rim observedin both the laboratory and field spectra
[Adams, 1974]. X-ray diffraction patternsof field samplescollected from the surfaceunits containingthe strongestserpentine
spectralfeaturesindicate that the primary mineral in this unit is
antigorite.
MUSTARDAND PIETERS: ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
AIS Coverage
•
•
AtmosphericWater Bands
0.80
'
'
•
'
'1 •
' :'-:.::.'..':..t
....
10,381
AIS Coverage
ß
AtmosphericWater Bands
t.':.:..':.'..'.'.::':..:[•
' '1 •
I
II ,i:ii.:i::jo:..::::i½
I] Jt
5i
r?:::.-.:'i::j.:!
SUM
I:::ø::'::;'"'=":;'::'";I
½':i'"':'"::':'='"::'::":::":ti
iD
esertV
arnishi ,:.:.;-:.--;i,'
I:;i:..'.::i:.i•
'?:.:."1
[".:i'"-:."'=
0.70
•o0.60
•00.30
"• 0.50
-•
i'h ':"'::;';
AIS
0.20
rr' 0.40
........
.ii::.':i:•
0.30
0.20
0.90
0.60
1.20
1.50
1.80
2.10
2.40
0.60
0.90
1.20
1.50
1.80
2.1 0
2.40
Wavelength In Microns
Wavelength In Microns
Fig. 5. Laboratory (solid line), field (crosses), and AIS (asterisks)
reflectancespectraof SUM-bearing soils. The wavelengthinterval covered by AIS is indicatedas well as regionsof poor data quality in the
atmospheric water bands. The 2.33-[zm absorption bands, 2.2-[zm
reflectancepeaks, and minor 2.15-[zm absorptionbandsseenin all three
spectraare due to Mg-OH stretchingmode absorptions.The AIS spectrum is offsetby 10% towardlower reflectancefor clarity.
Fig. 7. Laboratory (solid line), field (crosses) and AIS (asterisks)
spectraof surfacescontainingdesertvarnishcharacterizedby very low
albedoanda small2.33-[tm absorptionband.
tional program, including the Moses Rock flights. It is
suspectedthat this designerror causeda componentof radiation
to be included in the measurementsbetween 1.6 and 2.34 •tm
proportionalto the brightnessof the spectrumbetween 0.8 and
The AIS spectrumis from the sameregionas the field meas- 1.17 •tm. The contributionto the measuredspectrabetween 1.6
urementsand would be expectedto exhibit similar features.It is and 2.4 •tm may be of the order of several tens of percent
evident from Figure 5, however, that the AIS spectrumhas the [Vane, 1986]. Since an internal standard was used to calibrate
same overall characteristicsas the laboratory and field spectra the Moses Rock data and the standardregion has a somewhat
except that the 2.33-•tm absorptionband in the AIS spectrumis higher than average albedo for the AIS scene, the apparent
20% less strong. Also the maximum reflectance in the
strengthsof absorptionbandsin the calibrateddata are reduced,
2.04-2.34 •tm wavelengthregion of the AIS spectrumis 5-10%
and regionswhich have strongpositive slopesdisplay an apparless than is expectedfrom the lab and field spectra. The most ent drop in reflectancein the 2.04-2.34 •tm wavelength region
likely sourcefor the observeddiscrepancyis a recently detected
as illustrated by the AIS spectrum of SUM in Figure 5.
instrumentalproblem with the AIS detector concerningsecond
Although at present we are not able to correct fully for this
orderoverlap.
instrumentaleffect, the diagnosticfeaturesof surface materials
In grating spectrometers,second-orderspectraare generally are clear.
eliminated with long pass filters so that only first-orderspectra
Laboratory spectra of pure gypsum and gypsiferoussoils,
are measured.
This was not done for much of the AIS observa-
•
AIS Coverage
Atmospheric
Water
.
•
Bands
' ' I ' ' ' I ' "''"/"t*' '
'•
common in the Cedar Mesa member of the Cutler formation,
are shown in Figure 6 along with an AIS spectrumfrom a surface interpretedto be gypsiferous.The prominent spectral features in the AIS spectrumare an overall negative slope in the
spectrum, a distinct 1.68-•tm reflectancepeak and a 2.21-•tm
absorptionband centeredbetween asymmetricreflectancepeaks,
the higher one at 2.08 •tm and the lower one near 2.3 •tm. The
absorptionbands, shown more stronglyin the spectrumof pure
gypsum, are due to overtones and combination overtones of
molecular water in the crystal structureof this hydrated calcium
sulfate [Hunt et al. , 1971].
The spectrashown in Figure 6 clearly illustratethe effects of
mixing betweendifferent geologicmaterialson reflectancespectra. The strongabsorptionsshown in the spectrumof pure gypsum are much reducedwhen gypsum occurs as a componentin
reddish-brownsoils derived from local sandstoneand gypsiferous sandstone.This effect is observed both in the laboratory
spectraand AIS spectra,and the degreeof attenuationis proportional to the amount of reddish-brownsoil mixed with the gyp-
:....-::i:::ii
:...........-.:..¾.:.:....::.......,
0.80
':,..'):..'.:•;i)
.
CD 0.60
...-.:::::½.?;.....(:;,
(!.) 0.40
0.20
,i::i'..":ii½.'i½
i:'
.'.
."-.-.
.'.-.?,
.
0.00
0.60
0.90
1.20
1.50
, ,I, I
1.80
Wavelength In Microns
2.1 0
2.40
sum.
Fig. 6. Laboratory(solid lines) and AIS (asterisks)reflectancespectraof
AIS spectra from the tail region of Moses Rock Dike are
gypsum and gypsiferoussoils. The gypsiferouscomponentis identified
shown
in Figure 7 along with IRIS and laboratory spectraof
in the AIS spectrumby the broad 2.2-•m absorption,the prominent
1.68-[tm reflectancepeak, and an overall decreasein reflectancetoward desert varnish. As discussed earlier, desert varnish is common
longerwavelengths.
as a coating on stable surfacessuch as the crystalline rock frag-
10,382
MUSTARDANDPIETERS:ULTRAMAfiC
MICROBRECCIA
IN MOSESROCKDIKE
ments in the dike. The laboratoryand field spectraof the desert
varnish in Figure 7 are characterizedby a positive slope, low
reflectance(10-20%), a reflectivity maximum near 2.2 •tm and
a 7-15% absorptionnear 2.32 •tm. AIS spectrafrom the same
area as the field samples, however, show a reflectivity
maximum near 1.7 •tm and a 10% absorptionat 2.32 •tm, and
the reflectancesare in general >20%. The higher reflectance
relative to the spectraof pure desert varnish indicatesthat mixing betweendesertvarnishand units of the dike and Cutler Formation is occurringin pixels of AIS. The drop in reflectancein
the last grating positionof the AIS data is due to second-order
overlap.
coefficients of componentsin intimate mixtures can be accurately calculatedfrom reflectancespectrameasuredwith a fixed
viewing geometryif spectraare availablefor each componentor
end-memberin the mixture. In this approach,reflectancespectra
of the end-membercomponentsand the mixture spectraare converted to single-scattering
albedo(SSA). The proportionof each
end-memberis calculatedby a least squaresfit of the end-member SSA spectrato the mixture SSA spectrum.For each component an abundance coefficient
is calculated
that is referred
to as
the relative geometric cross-section,or F parameter. The F
parameteris definedby
F, = (M, I @,d,)I Y.(M,,/@nd,,)
APPROACH TO ABUNDANCE
ESTIMATES
where F,, M,, @,, and d, are the F parameter, mass fraction,
density, and particle diameterof componenti, respectively,and
In addition to detecting the presenceof individual mineral n is the number of componentsin the mixture. For a surface
species, spectral reflectancedata can be used to derive direct containingcomponentshaving similar effective particle size and
information about the abundanceof componentson the surface. densities,the F parameterscalculatedare roughly equivalentto
Since the AIS spectraldata contain spatial information, the spa- massËaction. This is the case for the intimately mixed matrix
tial distribution of the surface componentscan also be exam- regions of the diatreme, and in the discussionbelow, the term
ined. Reflectancespectraof mineral mixtures, however, are not abundance is used interchangeabllyfor F parameter in the
a simple linear combinationof the spectraof the mineral com- discussion of SUM distribution and abundance.
ponents in the mixture. An analytical model that accurately
The F parametersof end-membercomponentsin surface
describesthe observedspectrumof multicomponentmixtures is material at Moses Rock dike are calculated from the AIS data
required to deconvolveAIS reflectanceinto abundancesof indi- using an advancedmixture programdevelopedby the Jet Providual surface components.SUM abundanceand distributionin pulsion Laboratory as part of the Spectral Analysis Manager
Moses Rock dike can then be determinedand analyzedquantita- (SPAM) softwarepackage.The programwas modifiedat Brown
tively.
University to include the method of conversionfrom bidirecThe systematicsof spectralmixing between componentsin a tional reflectanceto single-scattering
albedodevelopedby Musmixture can be describedas belonging to one of two broad tard and Pieters [1987]. The advancedmixing program uses a
classes, macroscopicand microscopic, where these classesare constrained,nonnegativeleast squaresinverse algorithm to caldefined largely by the scale of mixing between componentsin culate F parametersfor each componentin every pixel. The
the mixture. Macroscopicmixing occurs when the components numerical results are quantitative and can be displayed as a
of the surfaceare arrangedin discreteareasor patchesthat are seriesof images, one for each end-member,showingthe abunlarge relative to the average path length of a photon. In this dance coefficient of that mineral or end-member in gray level
case the spectrumof the surface is a linear combinationof the intensities.A residual is also calculatedfor each pixel to deterspectraof the surfacecomponents[Singer and McCord, 1979]. mine the qualityof the fit of the modelspectrato AIS spectra.
Microscopic mixing refers to fine-grained,intimate mixing of
Determinationof appropriateend-memberreflectancespectra
the surface componentswhere photonsoften interact with more to be used in deconvolving the AIS data into surface
than one type of material. In this case the mixing systematics abundancesof individual componentsis critical. The end-memare nonlinear [Nash and Conel, 1974; Singer, 1981]. Different bers must be representativeof materials in the dike and be
analytical models have been developed for both macroscopic unique with respect to one another. The end-memberspectra
[e.g., Singer and McCord, 1979] and microscopic [Hapke, need not be of pure mineralsbut may be of mineral mixtures
1981] mixtures. Therefore, in order to calculateaccuratelythe [Mustard and Pieters, 1987]. Therefore common homogeneous
abundanceof surfacecomponentsfrom the AIS data, the domi- lithologies, like the reddish-brownsandstonefound throughout
nant type of mixing first needsto be determined.
the field area, can be accurately representedby a single endThe generalphysicalnatureof Moses Rock dike can be char- memberspectrum.
acterized as large blocks of locally derived wall rock set in a
The end-memberreflectancespectrausedin this analysiswere
fine-grained, fragmentedmatrix which containsthe dispersed selectedon the basis of field observations,laboratory data, and
componentof SUM. Althoughwind transportand splashtend to interactiveanalysisof the AIS images. The end-memberspectra
mix intimately and to homogenize locally the surface, at the chosenare shown in Figure 8 and include SUM, standard,dark
scale of the AIS pixels (10 m by 10 m), the type of mixing sandstonesoil, gypsiferoussoil, clay-rich soil (containsmore
between erosion surfacesdeveloped over blocks in the matrix illite than typical soils), and desertvarnish.The SUM and dark
and surfacesof the matrix constitutesmacroscopicmixing. Mixsandstonesoil spectra were carefully selectedfrom areas of
ing in the matrix betweenSUM and particlesderived from com- known compositionin the AIS data, while the rest of the endminution of xenolithsin the vent, however, is primarily micro- membersare laboratoryreflectancespectraof field samples.The
scopic. Since microscopicmixing of SUM with other breccia spectraselectedfrom the AIS data minimize the problemof seccomponents is expected to dominate the spectral reflectance ond-order overlap. Although replacing laboratory spectra of
measurementsof the matrix, a model for intimate mixing is SUM with spectra derived from the AIS data does not
usedin determiningSUM distributionon the surface.
significantly alter the derived distribution patterns discussed
Mustard and Pieters [1987] have modified Hapke's [1981] below, the calculatedF parametervalues are more accuratewith
equations for bidirectional reflectancesuch that the abundance the internally derived SUM spectrum. The SUM end-member
MUSTARDAND PIETERS' ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
0.70
Endmembers
0.60
0.50
c• 0.40 •_ 0.30 0.20
0.1 0
0.00
'
,
1.1 0
I
,
,
1.40
I
,
,
1.70
I
I
2.00
2.30
Wavelength In Microns
10,383
region. The standardcomponentis most heavily concentratedin
the Quaternary deposits at the bottom of the strip. The
sandstonecomponent exhibits higher F parameter values in
regionscontaining lower albedo sandstones.The clay-rich component (CLAY) displaysthe greatestconcentrationin thin beds
of the Organ Rock Tongue and indicate that these sandstones
contain a greater componentof clay than the other sandstones.
Field observationsand analysis of samples returned from the
area supportthese results. Reddish-brownsandstonesoils are a
pervasivecomponentof almostall surfaces,while the sediments
of the Organ Rock Tongue contain a greater than average componentof clay.
The distributionof abundancescoefficientscalculatedfor gypsum (strip labeled GYPS in Plate 1) are for the complex endmember that contains gypsum in a mixture with fine-grained
reddish-brown
soil.
The
calculated
distribution
is
confined
almost entirely to the Cedar Mesa Sandstoneof the Cutler Formation and shows peak F parametervalues near the top of the
Cedar Mesa unit. O'Sullivan [1965] reported that gypsum is a
characteristiccomponentof the Cedar Mesa unit in this area,
while detailed stratigraphicsectionsby McGetchin [1968] show
contains>90% serpentine,the gypsiferoussoil containsapproxthe thickest gypsum beds to be near the top of the Cedar Mesa
imately 60% gypsum and 40% fine-grained quartz with iron unit. Therefore the distribution and abundances calculated for
oxide coatings, and the dark sandstonesoil is similar to the
the gypsiferoussoil are consistentwith previous field investigastandardin compositionexcept it containsa greater proportion
tionsandclearly showthe distributionpattern.
of opaquesand iron oxide materials.
Desert varnish is virtually absent as a component of these
Carbonate, although a componentof the breccias of Moses
soils, as shown in the strip labeled VARN. This is an expected
Rock dike and host rock sediments, is not included in the suite
result since desert varnish is found primarily as a coating on
of end-members because the spectra of SUM and carbonate
fragmentsof crystalline rock fragmentson the tail region of the
have similar overall albedo and absorptionband positions and
dike which is not coveredby this flight line.
strengthsin the wavelength range of the AIS data. In the least
Pixels containing SUM, shown in the strip labeled SUM, are
squaresapproachto calculating surface abundances,serpentine
confinedprimarily to the matrix regionsof the dike. Peak SUM
and carbonatecannot be distinguishedwith the present spectral
abundancevalues are greater than 95% and the location of the
range and signal to noise ratio of AIS. Given improved detector
peak values correspond with SUM mapped by McGetchin
responseand a greater spectral range, however, these compo[19681. The spatial distributionand abundanceof SUM throughnents should be readily differentiated with the more advanced out the diatreme is discussed in more detail below.
sensorsbeingdeveloped(e.g., AVIRIS, HIRIS)
Several thin bands of low "SUM"
values observed
in the
Figure 8. Endmemberreflectancespectraused to derive surface abundances from the AIS reflectancespectra.These are gypsiferoussoil
(crosses),clay rich soil (squares),standard(circles), SUM (solid line),
dark sandstone
(triangles),anddesertvarnish(invertedtriangles)
Cedar Mesa and Organ Rock sedimentaryunits are clearly not
RESULTS
due to SUM
but rather
correlate
with
surfaces
which
contain
Figure9 showstheareasfor whichabundance
coefficients
of limestone. This illustrates an ambiguity that results from the
surfacecomponents
havebeencalculated.
The abundance
and limited spectral range and signal to noise ratio of the imaging
distributionfor all end-memberswill be discussedfor the flight
line area outlined by the dashedline. Discussionwill then focus
on the SUM abundanceand distributionfor the regions of the
dike where AIS data are available.
Abundance
and
distribution
for
all
six
end-members
calculatedfor the the area outlined by the dashedline in Figure
9 are shownin Plate 1. The left strip is a black and white image
showing the surface features imaged at approximately 1.2 gm.
The strip crosses, from top to bottom, the Halgito Tongue,
Moses Rock dike, the Cedar Mesa Sandstone,and the Organ
Rock Tongue and ends in the Quaternary deposits. The seven
colored strips show the calculatedF parametersfor the six endmembers (standard, dark sandstone,clay, gypsum, desert varnish, and SUM) as well as an image showing the residual from
the least squarescalculation.The valuesof the F parametersare
color coded accordingto the scale along the bottom of Plate 1
where blue colors representlow values, yellows are intermediate, andred to purplesare high.
The calculatedF parametervalues for the standardand reddish-brown sandstonecomponents(the strips labeled STD and
SST) indicate these are the primary surfacecomponentsin this
spectrometerused in this study in differentiatingbetween SUM
and carbonate. However, since carbonate does not constitute a
significantproportion of the dike in this region, the inability to
discriminate between SUM and carbonate does not seriously
compromisetheseresults.
In the residual image the goodnessof fit between the endmember spectraand the AIS data is shown qualitatively where
pixels colored dark blue representthe best fit. In quantitative
terms, residuals representedby the light blue color are of the
order of 1-2%. The homogeneityof the residualsin the image
indicatesthat the suite of end-memberspectraused does explain
most of the spectral variation encounteredin this region. The
areas where the residualsare higher than average correlate with
drainage channelsand shadowedregions seen in the black and
white image. This is expected since viewing geometry varies
rapidly in drainage channelsand no spectral componentswere
included
in
the
suite
of
end-members
for
shadowed
areas
[Adams e! al., 1986].
Details
of SUM
abundance
and distribution
within
the dike
are shown in Figure 10 for the areas labeled a-e in Figure 9.
Figure 10a is an enlarged view of SUM abundancein the dike
10,384
LOC
MUS'•ARDANDPIE'•ERS:UL'•RAMAfiC
MICROBRECCIA
IN MOSESRocheDIKE
ST
CLAY
GYPS
VARN
u
RESIDUAL
0.5
Plate 1. F parameterabundancescalculatedfrom the AIS data for the six end-memberspectrashown in Figure 9. The strip
labeled LOC is a black and white image at 1.2 •tm for locationpurposes.The approximateboundariesof the dike are shown
by the yellow lines and the contactbetweenthe Cedar Mesa Sandstoneand Organ Rock Tongueis shownby the blue line in
the LOC strip. The strip labeledredidualshowsqualitativelythe goodnessof fit of the end-memberspectrato the AIS spectra
See text for discussionof individualstripsof end-memberconcentrationpatterns.
1.0
MUSTARDAND PIETERS: ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
10,385
•.."z::i;:':.,-':.:::,...•..
-:-•.•,
. :<'•
=====================
..:..:
ß........::..-..:..-.•...:.:...:
.......... :::•.•:::•:;.•..:•:•;...?*::•:F:•::.•::.•.•.•.•.•..:.:•:..•:•.•.*:.•.s..•:.•.•`.:
...:.....:.:.:.:.::.....::...:....,,.,.,
,•x.,.•.•...::.:::..:.:.;:....:-:.:
..:r,•,.........•.,..,.:..•...,.
...........
-.....,-,-....:-.•....
:...?.-•,;•tv•?.:•:5.:,:::,.:
.... ;**:•,.<..::.,,.
::.,w::.::,;::;. :-;.- .:
.........':?, ......
...h.
Fig. 9. Black and white aerial photographof MosesRock dike showingareasfor which AIS data have beenanalyzedto obtain
surfaceabundancesof endmembercomponents.The resultsfor the area outlined by the dashedline are shown in Plate 1,
while the resultsfor the segmentslabeleda-e are shownin Figure 10. The field of view is approximately7 km by 11 km.
of the line shown in Plate 1. For each outlined area, or inde-
In Figure 10a the large elliptical zone of moderateto high
SUM abundancecontainsvalues >90% near the top of the zone
and shows strong concentrationgradientsin the contour plot in
all directionsfrom the peak values. The location of these peak
values correlates with an outcrop of SUM mapped by
McGetchin [1968]. Bulk analysisof a samplefrom this outcrop
showsit is composedof approximately95% SUM and 5% rock
fragments [McGetchin, 1968]. The easternedge of the zone is
distribution
of these lower values do not correlate with field
defined by the walls of the dike, while the westernedge abuts
observations and are believed to be within the noise of the data.
againstseveral large blocks of Cedar Mesa Sandstoneentrained
The 0.2 contourtherefore marksthe lower limit of unambiguous in the dike (see Figure 9). The north and south limits of this
detection of SUM. The contour interval is 0.1 and the bold conelliptical zone grade into regions of the matrix where SUM
tours mark the 0.6 contour line.
abundanceis below detection. The matrix here is composedpri-
pendentsegmentof AIS data, an image of the calculatedSUM
F parametersis shown where high values are bright. For each
region a contourplot of the same data with values between0.0
and 1.0 is also shown to illustrate better the systematicsof
SUM distribution. In the contour plots the lowest contour
encloses pixels with calculated F parameters >0.2 or 20%.
Although values less that 20% are calculatedin the images, the
10,386
MUSTARD
ANDPIETERS'ULTRAMAfiC
MICROBRECCIA
IN MOSES
ROCKDIKE
Fig. 10. SUM abundances
calculated
for AIS datasegments
a-e shownin Figure9. For eachsegment
a blackandwhite
imageshowing
thepixelsandabundances
in graytonesis displayed
alongwitha contourplotof thesamedatawitha contour
intervalof 0.1. The lowestcontourrepresents
an F parameter
of 0.2 andtheboldcontouris 0.6. The approximate
outlineof
the dikeis shownby the dashedline in thecontourplots.In thecontourplotin Figure10ethecross-ha.
tchedpatternshowsthe
extent of limestoneoutcrop.
MUSTARDANDPIETERS:ULTRAMAfiC
MICROBRECCIA
IN MOSESROCKDIKE
10,387
...
:.:
.
(e)
:
(f)
Fig. 10. (continued)
marily of fragmentsfrom the Cutler Formation. Note the circular depression
contournear the southwestern
edgeof the ellipti-
rock in the matrix.
A diffuse halo of lower abundance values is
observednortheastand southwestfrom the peak concentrations.
cal zone which is due to a block of wall rock in this zone of
The locationof this halo correlateswith complexbrecciamaprelativelyhighSUM abundance
(Figure9).
ped by McGetchin[1968] which containsa higherproportionof
Another region of relatively high SUM abundancein Figure matrix SUM than the Cutler breccias,althoughconcentrations
10a occursnear the westernedgeof the dike. This occurrenceis may vary in these breccias between 10 and 60% [McGetchin,
set between the western wall of the dike and a block of wall
1968]. Elsewherein the dike in Figure 10a SUM valuesof the
10,388
MUSTARDANDPIETERS:ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
matrix are low; much of the area showing no SUM abundance em regions of the dike. The highest SUM abundances(0.7)
is occupied by large blocks of wall rock in this region of the occur within the dike along the southernborderof this segment
dike.
and are associatedwith relatively broad halos of SUM. These
The region shown in Figure 10b (segment b in Figure 9)
distributions
however,lack both the uniformityobservedin Figcrossesthe lower part of the diluted SUM zone shownin Figure ure 10candthe regularhalo systematics
in Figure 10a.
10a. Although the distribution patterns vary in detail between
IMPLICATIONS FOR ERUPTION STYLE AND PROCESSES
Figures 10a and 10b, the maximum SUM abundancesin both
areas are >0.7, and the locationsof the peak values are well
correlated.
The
same
block
of wall
rock
in the diluted
SUM
noted earlier is also evident in Figure 10b. The small
differences in distribution patterns can be attributed to the fact
that the flight paths for the data acquisitionsare perpendicular
and slight variation in air speedas well as roll and pitch of the
aircraft during flight result in minor changesin the physical size
of the pixels on the ground. The calculatedabundancesof SUM
from line to line across the same areas are neverthelessvery
From this detailed examination
of SUM
abundance and distri-
bution in Moses Rock dike, two primary distributionstylesare
observed.Style A shows high peak abundancessurroundedby
tight halos of lesser abundance,steep concentrationgradients,
and regions of the matrix with SUM abundancebelow detection. In the dike this style is illustratedin Figures 10a and 10b.
Style B is characterizedby moderate peak abundancessurroundedby diffuse halos, shallow concentrationgradientsand a
consistent.
cross-dikedistributionof SUM (all regionsof the matrix contain
The southernregion of the dike, coveredby segmentc con- detectableSUM concentrations).
This style is well illustratedby
tains fewer mappableblocks of wall rock comparedto segments the abundancemapof SUM in Figure 10c.
a and b and is composedprimarily of fine-grainedcomminuted
The implications of these distribution styles for diatreme
material. Analysis of near-surfacestratigraphyin this region eruptionrelate to the mechanismby which the eruptingsystem
indicatesthat the surface material is comparableto the subsur- propagatedthroughthe crust and the nature of the eruptionin
face material in compositionand contaminationby wind-trans- the vent after the system has breached the surface. The dike
ported material is not significant. Although redistribution of
probablyrose throughthe crustin a mannersimilar to Clement's
material by rain splashand downslopemovementmay homog- [1979, 1982] model for root zone emplacementof kimberlite. In
enize the surface compositionon a fine scale, field observations this model, CO2 is exsolvedfrom the rising magma, and this
indicatethat this is not importanton the scaleof the AIS pixels. high-pressurevolatile phase penetrates fractures and joints
Taking these factors into consideration,the F parametervalues (hydraulicramming) in the wall rock. Pressurereductionsdue to
of SUM calculatedfor segmentc in Figure 10c show a funda- the opening of new pathways and fluctuationsin the rate of
mentally different distributionthan observedin Figures 10a and magma rise cause implosion and shatteringof the country rock
10b. The SUM is more homogeneouslydistributedacrossthe and the formation of a brecciation front. Continued rise of the
full width of the dike and peak values (0.6) are less than in Figsystemadvancesthe front of brecciation toward the surface. The
ures 10a and 10b. Bulk analysis of a sample taken from near situation at Moses Rock dike differs from Clement's [1979,
the center of the dike in the area of segmentc indicated a SUM
1982] model in that the volatiles were not exsolvedfrom a liqabundance of approxi•nately 50% [McGetchin, 1968], which uid magma but were most likely a fundamentalpart of the
correlateswell with the F parametervalue derived from the A|S
eruptingsolid-volatilematerialand there was probablyonly one
data. The abundancedistributionsdo not show the strong con- phase of intrusion. Nevertheless,in either case it is the highcentrationgradientsobservedin Figures 10a and b. Instead, the pressurevolatiles which do most of the work of brecciationand
concentration gradients are shallow giving a more diffuse propagation.
appearanceto the halos. The weak linear bands crossingthe
Two competingmodels that describediatreme formation after
dike evident in the black and white image are believed to be a systemhas breachedthe surfaceare fluidizationand hydrovoldue to the presenceof carbonate-bearing
sedimentscrossingthis canism. Hydrovolcanism is an explosive interaction between
magma and some external sourceof water [Sheridan and Wohsegmentof the dike.
The northernregion of the dike, covered in part by segment letz, 1983] and has been strongly advocatedby Lorenz [1973,
1975, 1986] as the fundamental mechanism for diatreme formad, is characterized by many meter sized blocks set in a
fine-grainedfragmentedmatrix. The distributionand abundance tion. In this model a hydrostaticpressurebarrierof about20-30
of SUM in this segmentshown in Figure 10d, reflect thesechar- bars controls the maximum depth of explosive interactions
acteristics in that the low to moderate abundances are distributed
between magma and groundwater. Ejection of groundwateras
in the matrix surroundingthe blocks in a meshlike pattern. The steam, and wall rock fragments,leads to a decreasein pressure
abundancesare also nonuniform and show large variations in
above the explosioninterface and thereforethe depth at which
abundancefrom pixel to pixel.
explosivemagma-waterinteractionsoccur can propagatedownThe "head" region of the dike, a portion of which is covered ward. The maximum depth of penetrationis controlledby the
by segmente, the most complex region of Moses Rock dike. It
availability of groundwaterbut may reach depthsof 2000-2500
contains a wide variety of lithologies varying from basement m. A diatreme is formed between the explosionfront and the
rock fragments to fragments derived from stratigraphiclevels surfaceand growslaterally as the front propagatesdownwardby
above those exposedin the walls of the dike. Also, this is the collapseand slumpingof wall rock [Lorenz, 1986]. If hydrovolonly region of the dike where significant,mappabledepositsof canic processeswere responsiblefor the distributionof SUM in
carbonateoccur. Regionsof carbonateas mappedby McGetchin Moses Rock dike, a central conduit of well-mixed pyroclastic
[1968] have been identified by a cross-hatchedpattern in the rocks surroundedby a collar of subsidedwall rock would be
contour plot in Figure 10e. The complexity of the dike and expected [Hearn, 1968; Lorenz, 1986]. This is not observed
presenceof carbonatemake it difficult to interpret the distribu- exceptin Figure 10c which has a centralregionof homogeneous
tion and abundanceof SUM shown in Figure 10e. Distributions SUM abundance, although it lacks a collar of subsidedwall
similar to those observedin Figure 10d can be seen in the west- rock.
MUSTARDAND PIETERS:ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
10,389
A
0
100
0
I
50
I
.'.•.'F:•:.'.'.
- -'::5,':f..:.' .'.':
z-Z-_-•_-C• Z Z-
Early
- -
Late
Fig. 11. Schematicdiagramillustratingthe conceptsof SUM erosionand dispersionby eddydiffusionduringeruptionas discussedin the text. The arrowsindicatethe directionof flow, the solidbar represents
a dike of SUM, and the dotsrepresent
erodedSUM reincorporated
into the vent flow. The numbersalongthe top illustratetypicaldistributiongradients.Figures1l a
and 1lb referto earlyandlatestagesof eruption,respectively.
Fluidizationas a geologicprocessrefers to the circulationand
transportof solids by a high-velocitygas-solidflow [Reynolds,
1954] and has been advocated by Dawson [1962, 1971],
McGetchin [1968], and Wyllie [1980] as a principal transport
mechanismfor ultramaficmaterialfrom the mantleand as a processto explain uppercrustalstructures.In this model the erupting medium propagatesthrough the crust as a mixture of solids
and gas under high pressure.After the dike breachesthe surface
a rarefractionwave passesdownwardthroughthe vent accelerating dike materialsacrossthe wave and greatly increasingflow
velocitiesof the materials.Fragmentsof wall rock are incorporatedinto the flow by spalling,slumping,and tensilefracturing.
The turbulentflow is continuouslyfed by the solid-volatilesystem below
the accelleration
front
and xenoliths
and wall
rock
fragmentsrise or fall in the vent dependingon their densityrelativeto the apparentdensityof the system.
If fiuidization were the primary processoccurring in the
vent, material exposedin the vent would be expected to be
erodedby blocks and particlescarried by the flow. This flow is
almost certainly turbulent, and the eroded materials will be
redistributedin the vent by eddy diffusion.In a fluidized system
as complexas a diatreme,the locationsof flow regimeswould
not be restrictedto any particularpart of the vent but would follow paths of least resistance.The distributionof SUM observed
in Figures lea and 10b are consistent with distributions
resultingfrom a fluidized eruption. The elliptical zone of moderate to high SUM abundancecan be explained as erosion of
SUM
from
a coherent
source and redistribution
of the eroded
buted into the central regions of the vent by eddy diffusion in
the turbulent flow. Extended eruption or increasedflow velocities would result in a homogeneousdistributionof SUM across
the width of the vent asillustratedin Figure I lb.
In McGetchin's [1968] discussionof the eruption, he concludesthat the main flow in the diatreme was eventually stabilized into two or three primary channels and identified these
channelsbased on field evidence. Figure lea covers one of the
areas McGetchin [1968] identified as a possible channel. The
interpretation that the homogeneous distribution of SUM
observedin the remote measurementsin Figure l ec indicatesa
more mature stage of mixing (i.e., this region has been subjected to a more prolongedperiod of fragmentationand mixing
than the regions covered by Figures lea, 10b, and led) is consistent with the field evidence for a proposedchannel in the
dike.
The "head" region of the dike, coveredin part in Figure lee,
has also been identified as a late stage channel by McGetchin
[1968]. Although some aspectsof SUM distribution and abundance in this region are consistentwith this interpretation,the
chaotic nature of the head region and the presenceof carbonate
complicate interpretationsfrom the incompleteremote measurements.
Several
broad
halos
of moderate
SUM
abundance
are
neverthelessobserved, apparently set between large blocks of
wall rock in matrix. These regions exhibit peak values of SUM
close to 0.7. These distributionssuggest a relatively mature
stateof mixing that would also be consistentwith being located
in a late stagechannel.
material by turbulentflow. Also the locationof this high SUM
zone againstthe east wall is permittedin a fluidized systembut
SUMMARY
is inconsistent
with the hydrovolcanicmodel which predictsthis
AIS data obtained for Moses Rock dike in Utah contain
area shouldbe occupiedby slumpedwall rock. The systematics
observed in Figure l ec are also consistentwith the fluidized importantinformationconcerningthe distributionand abundance
model if this region has experienceda longer period of flow or of components
in the diatreme.Interactiveanalysisof the speca more intense flow.
troscopic data in conjunction with field observations and
Followingfrom the arguementsabove, the two primary distri- reflectancespectrameasuredin the field and laboratoryresulted
bution styles observedat Moses Rock dike are interpretedas in the recognitionof the primarylithologiesand mineralspecies
representingdifferent stages of erosion and redistribution of on the surface.Spectrarepresentative
of theseprimary surface
SUM in the vent by turbulentflow in a fluidizedsystem.This is units (SUM, two types of sandstone,gypsum,desertvarnish,
illustratedschematicallyin Figure 11. In Figure 1la fragments and clay-richsoil) were selectedfor use in an intimatemixing
of SUM
are eroded from a dike in the wall rock and redistri-
modelto derive surfaceabundances
of thesecomponents
from
10,390
MUSTARDANDPIETERS.'ULTRAMAfiCMICROBRECCIA
IN MOSESROCKDIKE
the AIS reflectancespectra. The calculatedabundancesand distributions for each componentare entirely consistentwith field
observationsof this studyas well as previousinvestigations.
The
distribution
and
abundance
of
SUM
in the
dike
was
examined in detail using the AIS, field, and laboratoryspectral
data. Two distinct styles of SUM concentrationand dispersion
were observedin the dike. Style A is characterizedby moderate
to high abundancesof SUM surroundedby well-definedhalos of
lesserSUM abundanceand relative steepconcentrationgradients
into regions of the matrix where SUM abundanceis below
detection. Style B is characterizedby moderateSUM abundance
surroundedby broad diffuse halosof lower SUM abundanceand
SUM
is distributed
in the matrix
across the entire width
of the
determine the degree of serpentinizationof ultramafic rocks, Geophysics, 46, 316-321, 1981.
Hunt, G. R., and J. W. Salisbury, Visible and near infrared reflectance
spectra of minerals and rocks, I, Silicate minerals, Mod. Geol., 1,
219-228,
1970.
Hunt, G. R., J. W. Salisbury, and C. J. Lenhoff, Visible and near-infrared spectra of minerals and rocks, IV, Sulphides and sulphates,
Mod. Geol., 3, 1-14, 1971.
Hunter, W. C., and D. Smith, Garnet peridotite from Colorado Plateau
ultramafic diatremes:Hydrates, carbonates,and comparativegeothermometery,Contrib. Mineral. Petrol., 76, 312-320, 1981.
Lorenz, V., On the formation of maars, Bull. Volcano!., 37, 183-204,
1973
Lorenz, V., Formation of phreatomagmaticmaar-diatreme volcanoes
and its relevance to kimberlite diatremes, Phys. Chem. Earth, 9,
17-27, 1975.
dike. These stylesare consistentwith a fluidized environmentin Lorenz, V., On the growth of maarsand diatremesand its relevanceto
the formationof tuff rings, Bull. Volcanol., 48,265-274, 1986.
the vent [McGetchin, 1968] and are interpretedto be due to eroMcGetchin, T. R., The Moses Rock dike: Geology, petrology, and
sion of SUM by abrasion and comminution in the vent of the
mode of emplacementof kimberlite-bearingbreccia dike, San Juan
erupting diatreme. The eroded material is then dispersed County, Utah, Ph.D. thesis, 405 pp., Calif. Inst. of Technol. Pasadena, 1968.
throughoutthe matrix by eddy diffusion. Regions of the dike
which
exhibit
SUM
distribution
in the matrix
similar
to the first
McGetchin, T. R., and L. T. Silver, Compositionalrelationsin minerals from kimberlite and related rocks in Moses Rock dike, San Juan
style indicatethat the eruptionwas arrestedat a relatively early
County, Utah, Am. Mineral., 55, 1737-1771, 1970.
phase, while regionsof the dike which show SUM distribution McGetchin, T. R., and L. T. Silver, A crustal-uppermantle model for
more characteristicof the secondstyle representregions of the
the Colorado Plateau based on observationsof crystalline rock fragmentsin MosesRock dike, J. Geophys.Res., 77, 7022-7037, 1972.
dike where the eruption proceededfor a longer period of time
and probably indicate the location of mature channelsin the Mitchel, R. H., Kimberlites: Mineralogy, Geochemistry,and Petrology,
442 pp., PlenumPress,New York, 1986.
eruptingdiatreme.
Mustard, J. F., and C. M. Pieters, Spectroscopyof Moses Rock dike
Acknowledgments. Many thanksto Lee Silver (California Instituteof
Technology)for leadershipand discussionin the field, to ARCO for the
use of their IRIS field spectrometer,to the U.S. Geological Survey in
Flagstaff for field support,to the Navajo Nation for permissionto conduct field work on their land, and to Paul Fisher (Brown University) for
invaluable programmingassistance.Helpful reviews by D. Smith, K
Wohletz, F. Kruse, and M. Kingston are greatly appreciatedand helped
to strengthenthis manuscript.Supportfrom NASA grantsNASW-4048.
NAGW-748, and an NSERC post graduate scholarshipis gratefully
acknowledged.
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(Received December 12, 1986;
revisedFebruary5, 1987;
acceptedMay 12, 1987.)