Earth and Planetary Science Letters 403 (2014) 144–156
Contents lists available at ScienceDirect
Earth and Planetary Science Letters
www.elsevier.com/locate/epsl
Pink Moon: The petrogenesis of pink spinel anorthosites and
implications concerning Mg-suite magmatism
T.C. Prissel ∗,1 , S.W. Parman, C.R.M. Jackson, M.J. Rutherford, P.C. Hess, J.W. Head, L. Cheek,
D. Dhingra, C.M. Pieters
Department of Earth, Environmental & Planetary Sciences, Brown University, Providence, RI 02912, USA
a r t i c l e
i n f o
Article history:
Received 19 October 2013
Received in revised form 23 April 2014
Accepted 20 June 2014
Available online xxxx
Editor: C. Sotin
Keywords:
spinel
Moon
Mg-suite
plutonic
lunar crust
a b s t r a c t
NASA’s Moon Mineralogy Mapper (M3 ) has identified and characterized a new lunar rock type termed
pink spinel anorthosite (PSA) (Pieters et al., 2011). Dominated by anorthitic feldspar and rich in MgAl2 O4
spinel, PSA appears to have an unusually low modal abundance of mafic silicates, distinguishing it from
known lunar spinel-bearing samples. The interaction between basaltic melts and the lunar crust and/or
assimilation of anorthitic plagioclase have been proposed as a possible mechanism for PSA formation
(Gross and Treiman, 2011; Prissel et al., 2012). To test these hypotheses, we have performed laboratory
experiments exploring magma–wallrock interactions within the lunar crust. Lunar basaltic melts were
reacted with anorthite at 1400 ◦ C and pressures between 0.05–1.05 GPa. Results indicate that PSA spinel
compositions are best explained via the interaction between Mg-suite parental melts and anorthositic
crust. Mare basalts and picritic lunar glasses produce spinels too rich in Fe and Cr to be consistent with
the M3 observations.
The experiments suggest that PSA represents a new member of the plutonic Mg-suite. If true, PSA can
be used as a proxy for spectrally identifying areas of Mg-suite magmatism on the Moon. Moreover,
the presence of PSA on both the lunar nearside and farside (Pieters et al., in press) indicates Mg-suite
magmatism may have occurred on a global scale. In turn, this implies that KREEP is not required for
Mg-suite petrogenesis (as KREEP is constrained to the nearside of the Moon) and is only necessary to
explain the chemical make-up of nearside Mg-suite samples.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
High-resolution mineralogical data acquired by NASA’s M3
(Moon Mineralogy Mapper) experiment aboard the Chandrayaan-1
spacecraft has identified a potentially new lunar rock type (Pieters
et al., 2011; Dhingra et al., 2011). The lithology appears to be dominated by anorthitic feldspar and is rich in “pink” spinel. As such,
the term “pink spinel anorthosite” (PSA) has been adopted (Prissel
et al., 2013; Taylor and Pieters, 2013) paying homage to the “pink
spinel troctolites” (PST) of the magnesian-suite (Mg-suite) lunar
samples. Unlike PST however, there is no spectral evidence for
a significant amount of mafic phases (olivine and/or pyroxene)
within PSA (Pieters et al., 2011).
Near-infrared observations (Pieters et al., 2011) have defined
general petrological characteristics of the lithology, which suggest
PSA contains 1) nearly pure endmember MgAl2 O4 spinel (minor
*
1
Corresponding author.
E-mail address: [email protected] (T.C. Prissel).
Tel.: +401 863 1932; fax: +401 863 2058.
http://dx.doi.org/10.1016/j.epsl.2014.06.027
0012-821X/© 2014 Elsevier B.V. All rights reserved.
FeO, Cr2 O3 , hereafter referred to simply as spinel) and 2) an unusually low modal abundance of mafic minerals with no pyroxene
or olivine detected. Although this current evaluation of PSA is a
reasonable working hypothesis, the exact physical/chemical nature
of the lithology is still in question. Present interpretations suggest that spinel is in much greater modal abundance (near monomineralic spectral signature) relative to mafic silicates (Pieters et
al., 2011). Olivine ± pyroxene abundances are estimated to be no
more than 5 vol.%. The remainder of the lithology is inferred to be
anorthitic plagioclase (relatively featureless in the near-infrared)
(Cloutis et al., 2004; Pieters et al., 2011). The spectral signature
of PSA is consistent with spinels having Mg# > 90 (Mg/[Mg +
Fe] × 100) and Cr# < 5 (Cr/[Cr + Al] × 100) (Pieters et al., 2011;
Williams et al., 2012; Jackson et al., in press). Grain size, extent of
space weathering, cooling rate, modal mixing and a host of other
factors will influence these estimates (e.g. Cheek and Pieters, in
press; Jackson et al., in press). However, the general characteristics
outlined above appear to be robust. For the purpose of discussion
we adopt them and explore the implications.
T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
145
Fig. 1. The first two spinel detections from M3 on the lunar farside and nearside (pink filled circles). Spinel-rich detections are found on both the nearside and farside
of the Moon (Pieters et al., 2011; Dhingra et al., 2011; Pieters et al., in press). PSA occurs in central peaks, crater rims, and basin rings indicating a deep crustal origin
and transported to the surface during impact excavation. Top (Lunar Farside): Spinel-rich lithology detected on the inner-ring of Moscoviense basin (Pieters et al., 2011).
Perspective view outlined in blue modified from Pieters et al. (2011) where green and red patches represent olivine and orthopyroxene. Spinel-rich lithologies are indicated
by pink stars. Bottom (Lunar Nearside): Central peak of Theophilus crater within the Nectaris Basin (Dhingra et al., 2011). Perspective view outlined in blue modified from
Dhingra et al. (2011), where the pink star represents the approximate location of spinel-rich lithologies. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
Several groups have focused on the spectral identification of
lunar PSA by remote sensing (Pieters et al., 2011; Dhingra et al.,
2011; Bhattacharya et al., 2012; Kaur et al., 2012, 2013b, 2013a;
Lal et al., 2012; Srivastava and Gupta, 2012, 2013; Pieters et al.,
2013; Sun et al., 2013; Yamamoto et al., 2013). Confirmed detections of PSA (∼20) are located on both the lunar nearside and
farside (Pieters et al., in press). Note that increased Fe-content
within the spinel and/or the addition of abundant mafics could
mask the PSA spinel signature. Thus, the data are biased in consistently detecting only the most mafic-free, pure spinel lithologies. Identifications typically occur in crater rims and walls, central peaks, and the inner rings of larger impact basins. Most locations share a similar geological context: originally deep-seated
lithologies that were uplifted to the surface during impact excavation (Fig. 1) (Pieters et al., 2011; Dhingra et al., 2011). However,
it is still unclear whether PSA formed during endogenic or exogenic processes (e.g. Pieters et al., 2011; Gross and Treiman, 2011;
Prissel et al., 2012; Vaughan et al., 2013; Yue et al., 2013; Gross et
al., in press).
In this study we begin by comparing PSA spinel to spinels from
the lunar sample collection. We show that the lack of a mafic
spectral signature within PSA presents serious hurdles for petrogenetic models relying on normal crystal fractionation from basaltic
melt. Among the alternative petrogenetic models presented, this
paper focuses on testing the hypothesis of PSA production during the interaction between lunar basaltic melts and anorthositic
crust. While our interpretations of the results are centered on
magma–wallrock interactions, the compositional constraints reported herein can be applied to both exogenic and endogenic petrogenetic models.
1.1. Lunar spinels
Compared to the spinel detected by M3 , most lunar spinels
trend from chromites (Fe2+ Cr2 O4 ) to ulvöspinel (Fe2 2+ TiO4 ),
which are the expected products of crystal fractionation from
mare basalts (Fig. 2a) (e.g. Haggerty, 1971). Spinels from Apollo
14 samples are less Cr-rich than mare basalt spinels and categorized as chromian-pleonaste ([Mg,Fe2+ ][Al,Cr]2 O4 ), crystalliz-
146
T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
Fig. 2. Compositional variation of lunar spinels. Solid blue box shows M3 compositional estimates in both plots (Mg# > 90, Cr# < 5). Compositional characterizations are
discussed further in the text. a) Modified multicomponent spinel prism after Haggerty (1973). Spinel (MgAl2 O4 ) is rare among the lunar sample collection. The majority
of lunar spinels are Fe-rich chromites trending to ulvöspinels (indicated by transparent gray volume) and plot along or in between the mare basalt and Luna 16 trends
shown (Haggerty, 1971, 1972, 1973, 1977). A few chromian-pleonaste spinels are found within the Apollo 14 collection (indicated by the dashed oval) and are interpreted to
be the crystallization products from Al-rich mare basalts (e.g. Steele, 1972). A fractionation trend within the Luna 20 spinels moves from spinel to Fe-rich chromites. Only
spinels from PST (pink spinel troctolites) of the Mg-suite rocks plot near the compositional estimates from M3 . b) Bottom plane of the spinel prism: Cr# vs. Mg# of common
spinel compositions found in mare basalts (black filled triangles; Papike et al., 1976) and PST (pink-filled triangles; Keil et al., 1970; Anderson, 1973; Ridley et al., 1973;
Prinz et al., 1973; Baker and Herzberg, 1980; Marvin et al., 1988; Snyder et al., 1998; Daubar et al., 2002; Gross and Treiman, 2011) relative to compositional characterizations
from M3 . Samples ALHA 81005, 10019, and ST2003 are discussed in the text. (For interpretation of the references to color in this figure legend, the reader is referred to the
web version of this article.)
ing from aluminous picritic melts (Fig. 2a) (e.g. Haggerty, 1971;
Steele, 1972).
Pink spinel troctolite (PST) spinels most closely resemble the
spinel compositions inferred from M3 data, but in general are
slightly more Fe and Cr-rich (Fig. 2b). The PSTs, like many lunar rock types, are found as clasts within breccias. A PST clast
within lunar sample 67435 contains coarse-grained spinel (grain
sizes 0.1–0.7 mm in diameter, Mg# ∼85, Cr# ∼ 8) and a cumulate
texture, suggesting it is plutonic in origin (Prinz et al., 1973). The
Luna 20 suite forms a continuous trend from spinel to chromite
(Fig. 2a) (Haggerty, 1973). A small (350 × 150 μm) PST clast in lunar meteorite ALHA 81005 contains ∼30 vol.% spinel. Because of
the small clast size however, the modal proportions may not be
representative of the whole rock (Gross and Treiman, 2011). To our
knowledge, a plagioclase-rich lithic fragment from regolith breccia
10019 (Keil et al., 1970) and spinel troctolite 2003 from Luna 20
(Snyder et al., 1999; Cohen et al., 2001) are the only two samples
with nearly pure spinel (Mg# ∼ 93, Cr# ∼ 2; Mg# ∼ 91, Cr# ∼ 2
respectively) matching PSA (Fig. 2b). However, every known spinelbearing sample within the lunar collection (including those mentioned above) contain significant proportions of olivine ± pyroxene (>8 vol.%), inconsistent with an approximately mafic-free PSA
lithology.
2. On the petrogenesis of pink spinel anorthosites
How then might a mafic-poor PSA lithology form on the Moon?
Many lunar rock types, including the Mg-suite (dunites, PST,
troctolites, norites, gabbronorites), are consistent with the lowpressure crystallization sequence from basaltic melts (e.g. Walker
et al., 1976). However, this process has difficulty explaining the
markedly low mafic abundance within PSA (Fig. 3). In order to precipitate spinel in the absence of olivine ± pyroxene during crystal
fractionation, parental melt compositions must be near the spinel
+ plagioclase cotectic.
PSA has been observed at Moscoviense Basin (Fig. 1) with
nearby olivine-rich and orthopyroxene-rich lithologies, similar to
terrestrial layered mafic intrusions (e.g. Stillwater Complex, Montana, USA; McCallum et al., 1980). Crystal settling within a mare
basaltic intrusion could explain the spinel-rich (and mafic-poor)
lithology of PSA. However, spinels produced during crystallization of mare basalts and lunar picritic melts are expected to be
Fe and Cr-rich (Fig. 2a) (e.g. Haggerty, 1971; Steele, 1972). So,
while crystal settling may reproduce the high spinel and low mafic
modes, melts more MgO-rich than mare basalts would be required to reproduce the spinels observed by M3 . Likewise, a strong
correlation between the Al-content of parental melts and the Alcontent of crystallizing spinel has been observed in several terrestrial volcanic and magmatic systems including MORB-type lavas
and abyssal peridotites (Dick and Bullen, 1984; Allan et al., 1988;
Kamenetsky et al., 2001). Thus, in addition to a high Mg#, PSA
parental melts must also have a high normative plagioclase component (high Al/Cr) to explain the low Cr-content of spinels characterized by M3 (Williams et al., 2012; Prissel et al., 2014).
Several petrogenetic models have been proposed suggesting PSA: 1) formed by magma–wallrock interactions within the
lunar crust including assimilation and fractional crystallization
(Gross and Treiman, 2011; Prissel et al., 2012), 2) is produced during basin-forming impacts, which may result in melt mixtures
of anorthositic crust and mantle material (an exogenic equiva-
T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
147
Fig. 3. Potential PSA formation processes illustrated using the Fo–An–Qtz pseudo ternary phase diagram. Solid black lines and dashed black lines represent phase boundaries
at 1 atm and 10 kbar pressure respectively (Morse, 1980). Left ternary: Solid red lines represent the low-pressure liquid line of descent (LLD) of a forsterite-normative
basalt (black-filled circle). The crystallization sequence is highlighted within the inset and modal abundances for each phase at the points indicated are listed in the table
below. Note that no olivine-poor, spinel-rich lithology is produced (see table inset). Furthermore, the composition of spinels produced during crystal fractionation of natural
basalts will be Fe and Cr-rich (see text). Middle ternary: Alternative examples capable of producing an approximately mafic-free spinel-bearing lithology: 1) the re-melting
and subsequent crystallization of a pre-existing anorthite-rich troctolite (half-filled circle with LLD shown in red) or 2) the mixing of forsterite normative basalts with an
anorthositic wallrock (graded and dashed mixing line between filled and open circle). In case 2) both the wallrock and basalt can act as a contaminate to the other, mixing
to form melts near the Sp–An cotectic. Right ternary: Apollo 15 green glass, yellow glass, and red glass compositions (green, yellow, and red filled circles respectively,
Delano, 1986) have been projected onto the pseudo ternary along with a theoretical Mg-suite parental liquid (light gray filled circle, Longhi et al., 2010). Transparent
blue line (pressure projection) drawn from An to the tip of the spinel field delineates melt compositions capable of producing spinel during case 2) at 1 atm. Here, only
melt compositions to the left of this line can mix with anorthite to form spinel + anorthite lithologies. However, the spinel stability field increases with pressure. Higher
pressures would therefore allow more Fe-rich melt compositions to produce spinel during magma–rock reactions on the Moon (e.g. Prissel et al., 2012). (For interpretation
of the references to color in this figure legend, the reader is referred to the web version of this article.)
lent to magma–wallrock interactions) (e.g. Vaughan et al., 2013),
or 3) are not produced from material inherent to the Moon, but
are projectile remnants from meteorite impacts (Yue et al., 2013).
Hypothesis 2) may suffer from wholesale melting of target material during impacts, where mixing between ferroan anorthositic
crust and mantle material would likely produce ferroan-spinel
saturated melts (inconsistent with PSA) (Vaughan et al., 2013;
Vaughan and Head, 2014). In addition to wholesale melting, hypothesis 3) faces the extreme obstacle of consistently preserving
large-scale, high-proportions of spinel through a diversity of impacts. Given the abundance of both anorthositic crust and basaltic
melts on the Moon (e.g. Hiesinger and Head, 2006), we focus on
testing hypothesis 1).
The experimental study of Morgan et al. (2006) found that
spinel was produced during the dissolution of plagioclase into
picritic melts. Spinel compositions were not reported, but the
study demonstrates that melt–anorthite interaction is a viable process for spinel production on the Moon. In fact, Warren (1986)
proposed that assimilation of anorthositic wallrock by Mg-suite
magmas was a significant factor in the origin of the Fe/Mg bimodality observed in the pristine non-mare rocks. While extensive assimilation of the lunar crust may have been minor on
the basis of thermodynamic limitations, the process is likely to
have occurred at some scale (Finnila et al., 1994; Hess, 1994;
Gross and Treiman, 2011). Dissolution of plagioclase has also been
proposed as a possible source for Al-variability within mare basalts
(Morgan et al., 2006).
Magma–wallrock interactions on the Moon can be visualized
using the forsterite–anorthite–quartz pseudo-ternary phase diagram (Fig. 3, middle ternary). Simple mixing in a closed system
would produce liquid bulk compositions along a straight line between the melt and the wallrock. Shown in the middle ternary
of Fig. 3 is the melt–rock mixing line between an MgO-rich melt
and anorthositic wallrock. Melt mixtures near the spinel-anorthite
divariant assemblage could fractionate to produce spinel + anorthite in the absence of a mafic phase. Additionally, the re-melting
and subsequent crystallization of pre-existing anorthositic troctolites could also produce melts that would fractionate along the
spinel-anorthite cotectic, potentially producing PSA (Fig. 3, middle
ternary).
At low pressures, the liquid line of descent would evolve
along the spinel-anorthite cotectic until reaching the spinelanorthite–forsterite ternary peritectic (PST assemblage). This point
is monoresorptional (meaning forsterite and anorthite are produced at the expense of spinel). In this case, the liquid composition will fractionate away from the peritectic point (along the
anorthite–forsterite cotectic) only after all of the spinel has been
consumed. In nature, however, mixing will likely behave as an
open system process with both assimilation and fractional crystallization (AFC) taking place (DePaolo, 1981a, 1981b). Similar to
layered mafic intrusions, AFC processes on the Moon could produce mafic-free, spinel-rich lithologies occurring in close spatial
relation to mafic-rich lithologies as seen at Moscoviense Basin (e.g.
Gross and Treiman, 2011).
The wallrock itself could also become contaminated during
the reactive porous flow (RPF) of basaltic melts through the
anorthositic crust. RPF has been extensively studied in terrestrial magmatic systems, particularly on the formation of dunite
channels during porous flow of basaltic melts through peridotite
(Daines and Kohlstedt, 1994; Kelemen et al., 1995; Kelemen and
Dick, 1995; Morgan and Liang, 2003). The process begins with
the preferential dissolution of pyroxene (at the grain scale) as
olivine-saturated melts percolate through peridotite. Here, the instantaneous melt–wallrock ratio is low. Pyroxene dissolution and
olivine precipitation will persist if several pulses of magma intrude the wallrock, increasing both the matrix permeability and
3
# of analyses. Compositions were glassed and then analyzed by EMPA.
s.d. denotes 2 sigma standard deviation on the last significant digit reported.
T
FeO = total Iron.
Mg# = cation fraction of [Mg/(Mg + Fe)] × 100.
% An = initial % normative anorthite in the melt with respect to forsterite–anorthite–quartz pseudo ternary space (see Table A2 for worked example).
A15C modeled after Apollo 15C green glass (Delano, 1986). MSPL modeled after theoretical Mg-suite parental liquid composition reported in Longhi et al. (2010).
6
a
% An
s.d.
2
2
67.0
86.8
Mg#
s.d.
7
4
100.0
99.5
Total
s.d.
K2 O
s.d.
___
0.33
___
0.35
Na2 O
s.d.
3
1
8.6
12.0
CaO
s.d.
1
2
18.5
17.6
MgO
s.d.
3
3
0.17
0.15
MnO
s.d.
2
1
16.2
4.8
FeOT
s.d.
4
5
0.42
0.31
Cr2 O3
s.d.
2
3
7.7
17.2
Al2 O3
s.d.
3
5
0.25
0.88
TiO2
s.d.
4
3
48.1
46.0
SiO2
na
We have conducted a series of experiments exploring magma–
wallrock interactions between lunar basaltic melts and anorthite
similar to those of Morgan et al. (2006), but at lunar crustal
pressures. We have selected and synthesized two high Mg# lunar basaltic compositions to react with anorthite: 1) an Apollo
15C green glass (A15C), representing the most MgO-rich composition of the picritic glasses with Mg# ∼ 67 (Delano, 1986) and
2) a theoretical Mg-suite parental liquid (MSPL) with Mg# ∼ 87
(Longhi et al., 2010). The melt compositions (Table 1) were synthesized from reagent grade oxide powders and then conditioned
at the iron–wüstite (Fe–FeO, denoted ‘IW’) buffer inside a horizontal gas-mixing furnace (H2 + CO2 continuous flow) at 900 ◦ C
for three hours. Stoichiometric anorthite glass powder was pressed
into porous plugs with ethanol as a binding agent, and then sintered at 1400 ◦ C for 16 h within a Deltech box furnace.
Low-pressure experiments were performed in a Harwood
internally-heated pressure vessel (IHPV) with Ar as the pressure
Table 1
Experimental starting materials reported in wt% oxides.
3. Experimental methods
6
15
overall melt–wallrock ratios (Kelemen et al., 1997; Morgan and
Liang, 2003, 2005). Hence, dunite channels are formed (as olivine
is the primary reaction product). Within terrestrial mantle ophiolites, dunite is observed both as veins (tens of millimeters wide)
and large tabular-like bodies (∼100 m wide) up to several kilometers in length (Boudier and Nicolas, 1985; Kelemen et al., 2000;
Morgan and Liang, 2005). If RPF has occurred within the lunar
crust, spinel channels may have been produced in a manner analogous to the dunite channels in peridotites (where preferential
dissolution of plagioclase gives rise to spinel production). It is possible that both AFC and RPF mechanisms have formed spinel and
PSA lithologies on the Moon.
Furthermore, the spinel stability field is pressure dependent
(Fig. 3, right ternary), which may aid in determining depths of
formation for PSA. In the right-hand ternary of Fig. 3, a line has
been projected from pure anorthite running tangent to the tip of
the spinel stability field (pressure projection line) at both atmospheric and 1 GPa pressure. The 1 GPa phase diagram is used to
fully demonstrate pressure effects on the phase boundaries, though
recent estimates of the lunar crust place the thickest regions at
∼60 km (∼0.3 GPa) (Wieczorek et al., 2013). The pressure projection line delineates basaltic compositions that are capable of
producing liquids near the spinel-anorthite cotectic when mixing with anorthite. As the spinel field expands with increasing
pressure, less MgO-rich melts become capable of producing PSA
assemblages during mixing. As a corollary, Mg-suite melts interacting with the anorthositic crust can produce spinel at lower
pressures (shallower depths) than mare basaltic compositions and
the picritic glasses (Fig. 3, right ternary). Thus, for magma compositions plotting between the 1 atm and 1 GPa pressure projections
(i.e. picritic glasses), the presence/absence of spinel could be used
as a geobarometer for PSA formation during magma–rock interactions on the Moon (see Fig. 8). MgO-rich magma compositions
plotting to the left of the 1 atm pressure projection (i.e. Mg-suite
parental melts) could produce liquid bulk compositions near the
spinel-anorthite divariant assemblage when mixing with anorthite
at all pressures.
Therefore, the aim of this study is to experimentally constrain
the physical and chemical conditions necessary during magma–
rock interactions to reproduce PSA spinel compositions consistent
with the spectral characterization from M3 . Though our discussion
will focus on the process of plagioclase assimilation by basaltic
melts, the compositional constraints resulting from this study can
be used in alternative models for PSA formation (e.g. the composition of mantle material mixing with anorthositic crust during
impacts).
23.8
49.4
T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
A15C
MSPL
148
T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
Fig. 4. The % normative anorthite component in the melt measured along transects
(distance from the magma–rock interface). Initial % An values are shown for both
the A15C and MSPL basalt compositions (dashed green and gray lines respectively).
Gray-filled symbols are from MSPL + An runs whereas green-filled symbols are
from A15C + An runs (experimental pressures in GPa are reported next to each
transect). Along the transect, melt measurements were taken within sp + liq regions (triangles), liq-only regions (circles), and ol ± chromite regions (squares). 2σ
standard deviation of % normative anorthite are within each symbol. The red-dotted
transect and symbols are from run #23 (see Section 4.2). (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of
this article.)
medium. For these experiments, thin (∼0.5 mm outer diameter,
∼3.3 mm total diameter) graphite sleeves with a fixed bottom
and open top were machined to house the IW-conditioned basaltic
powders juxtaposed against the sintered anorthite plugs (Fig. A1).
The thin graphite sleeves are a modification from previous experiments (Prissel et al., 2012) where thicker-walled graphite crucibles
are believed to have prevented the sample pressure from equaling the confining pressure during low-P runs. The experimental
charge was then completely surrounded by carbon–graphite powder (fixing f O2 at or below the C–CO buffer) and sealed within an
outer Pt capsule (Fig. A1). Experimental capsules were then placed
at the hot spot within hand wound La-doped Mo wire furnaces.
Temperatures were measured with a Pt–Pt90 Rh10 thermocouple.
Experimental pressures spanned 0.05–0.2 GPa. All experiments
were run for three hours at 1400 ◦ C. Because the C–CO buffer is
highly pressure dependent, f O2 estimates range from IW − 1.1
to IW + 0.3 log units at 0.05–0.2 GPa respectively (e.g. Fogel and
Rutherford, 1995; Nicholis and Rutherford, 2009). The f O2 estimates reported here are maxima (since C powder is always present
in the experiments). Each experiment was drop quenched by tilting the IHPV to a vertical position allowing the charge to fall into
the cold end of the furnace. Cooling rates are estimated to be
∼60–100 ◦ C/s.
High-pressure experiments (0.8–1.05 GPa) were carried out using a piston cylinder apparatus following the procedures of Morgan
et al. (2006). Capped graphite crucibles housed a similar setup
in Fig. 4 (with basalt powder below anorthite plug). The f O2
for high-P experiments ranged from IW + 1.7 to IW + 2 log
units at 0.8–1.05 GPa respectively (e.g. Fogel and Rutherford, 1995;
Nicholis and Rutherford, 2009). Piston cylinder runs served as a
proof of concept (since experimental pressures exceed those found
in the lunar crust), providing ideal conditions for spinel formation
(Fig. 3, right ternary).
See Appendix A for analytical summary.
4. Results
Experiments reacting high-Mg# basalts and anorthite produced
spinel over a wide range of pressures (0.05–1.05 GPa). Diffusion-
149
like profiles of Al-content (i.e. anorthite contamination) are observed in the melt as in previous studies (e.g. Finnila et al., 1994;
Morgan et al., 2006), though advection also appears to have occurred (Fig. 4). Euhedral spinel grains are typically found within
50 μm of the melt–rock interface (MRI). Some spinels are found
further than 50 μm from the MRI, but were presumably carried
there by advection (Table A1).
Spinels with Cr-rich cores are observed in two experiments, Run
#1 (Section 4.1) and Run #23 (Section 4.2). Cr-cored spinels are
too small to obtain reliable core-rim profiles, but back-scattered
electron (BSE) images and energy-dispersive spectrometry measurements both indicate Al-rich rims. Cr-cored spinels crystallized
early in the experiment, where diffusion and/or advection later carried Al to the region, resulting in Al-rich rims.
Olivine (± orthopyroxene at high pressure) is observed in a
few runs, but was always restricted to melts with <42% normative anorthite (calculated with respect to forsterite and quartz in
pseudo-ternary space, see Table A2 for a worked example) in all
but Run #23 (Fig. 4). Similar to the Cr-cored spinels, olivine and
orthopyroxene are products of normal crystal fractionation from
the least contaminated regions of the melt. Grain size for all crystalline phases appears to positively correlate with pressure. This
may be due to the higher solubility of volatiles at greater pressures, which would increase elemental diffusion rates, enhancing
grain growth.
A summary of the experimental conditions and results is given
in Table 2. The average compositions of the glass and crystalline
phases for each run are reported in Table A1. Only those crystalline
phase analyses showing oxide totals of 100 ± 1.5 wt.% and good
stoichiometry ([ideal cation total]/[# oxygen] ± 0.01) are accepted.
Glass analyses are accepted using the same standard for wt.% oxide
totals.
Because both diffusion and advection occur during each experiment, no single homogeneous melt composition exists for calculating mineral-melt KD (Fe–Mg cation fraction exchange coefficient;
[XFe /Xmg ]xtal × [XMg /XFe ]liq ). Therefore, we calculate a “local” KD
by measuring the glass within 15 microns for a single mineral
grain of interest. We report the average of these KD ’s for a given
mineral population with 2-sigma standard deviation (Table A1). No
major variance was observed in KD over the wide range in f O2 ,
suggesting high f O2 was not a significant factor for spinel production in the experiments reported herein (Fig. A2).
4.1. Very low-Ti green glass (A15C) + anorthite
Spinel grains were produced by interaction of composition
A15C (green glass) with anorthite over the entire experimental
pressure range (0.05–0.8 GPa). Spinels are found only in regions
of the melt with >52% normative anorthite (Fig. 4). Spinels have
Mg#’s ∼ 77 ± 5 (2 sigma standard deviations are reported herein)
and show a wide range in Cr#’s ∼3–20 (Table A1). The average
spinel-melt KD is ∼0.54 ± 0.09. Run #1 (0.8 GPa) is the only experiment to produce an assemblage of ∼15 vol.% spinel (poikilitically
enclosing anorthite) and <5 vol.% mafics (estimated from the interstitial melt observed with BSE images) all within the anorthitelayer (Fig. 5). The compositions of the spinels included in the
anorthite-layer (Mg# ∼77.8 ± 0.7; Cr# ∼ 3 ± 1) are within error
to those found near the MRI (Mg# ∼ 78.0 ± 0.3; Cr# ∼ 3.8 ± 0.9)
(Table A1).
Olivine is present in each run, but observed only in regions of
the melt with <42% normative anorthite. Olivine has an average
KD value of ∼0.33 ± 0.05 and average Mg# ∼ 83 ± 3. Orthopyroxene (Run #1) has an Mg# = 84.4 ± 0.5 and a KD of 0.36 ± 0.03.
Additionally, Run #1 is the only A15C + An experiment containing
a few spinels with Cr-rich cores. The chromites, which are found
∼1–1.7 mm from the MRI and within the ol + opx + liq field, are
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T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
Table 2
Summary of lunar melt–rock interaction experiments.
Run #
Devicea
P
(GPa)
T
(◦ C)
Duration
(min)
Melt comp.
(+An)b
Productsc
f O2 d
1
15
21
22
23
26
27
PC
IHPV
PC
IHPV
IHPV
IHPV
IHPV
0.8
0.1
1.05
0.1
0.2
0.05
0.05
1400
1400
1400
1400
1400
1400
1400
180
180
180
180
180
180
180
A15C
A15C
MSPL
MSPL
MSPL
MSPL
A15C
gl,
gl,
gl,
gl,
gl,
gl,
gl,
IW
IW
IW
IW
IW
IW
IW
a
b
c
d
sp,
sp,
sp
sp
sp,
sp
sp,
ol, opx
ol
ol, chrm
ol
+
−
+
−
+
−
−
1.7
0.4
2
0.4
0.3
1.1
1.1
PC = Piston Cylinder; IHPV = Internally Heated Pressure Vessel.
Melt composition juxtaposed against sintered pure anorthite.
gl = glass; sp = spinel; ol = olivine; opx = orthopyroxene; chrm = chromite.
Estimated from C–CO buffer (e.g. Fogel and Rutherford, 1995).
Fig. 5. Back-scattered electron images of experimental results. a) Run #15; showing typical post-run conditions. The melt shows diffusion-like profiles with Al-content
increasing toward the melt–rock interface creating separate regions of different mineral stability. Shown here are the anorthite (An), spinel + liquid field (± anorthite), and
liquid only field. b) Run #23; the only experiment to contain an olivine + chromian-pleonaste + liq field ∼300 μm away from the magma–rock interface. This experiment
also shows an increase in Al-content moving away from the melt–rock interface (discussed in text). c) Run #1; spinel was found poikilitically enclosing anorthite (within the
anorthite layer) at high pressure in addition to displaying typical melt-zoning and mineral fields as shown in a). d) Inset from c). Experimental results and phase compositions
are listed in Tables 2 and A1 respectively.
likely products of crystal fractionation from the less plagioclasecontaminated portion of the melt. In Run #1, Cr-cored spinels have
slightly lower Mg#s (∼74.94 ± 0.05) and significantly higher Cr#s
(∼17 ± 2) than spinel produced at the MRI.
4.2. Mg-suite parental liquid (MSPL) + anorthite
Experiments reacting MSPL (Mg-suite parental liquid) with
anorthite show morphologies similar to the A15C runs, where
spinels were also produced over the entire experimental pressure
range (0.05–1.05 GPa). Spinels are typically found near the MRI
in regions of the melt with >62% normative anorthite (Fig. 4).
Spinel compositions measured in the MSPL + An runs have average Mg#’s ∼93 ± 2, little variability in Cr#’s ∼0.74–5.53, and a
KD value of ∼0.46 ± 0.04. No orthopyroxene is observed in any of
the MSPL runs and olivine (Mg# ∼94.9 ± 0.4; KD ∼ 0.23 ± 0.02)
is present only in Run #23 (0.2 GPa). Run #23 is the only experiment where the melt increases in Al-content (i.e. anorthite
contamination) away from the MRI. This observation is interpreted
to reflect advection due to the crystallization of dense spinel. Crcored spinels with Al-rich rims are observed with olivine away
from the MRI (Fig. 5). In addition to higher Cr#s (∼31 ± 5), Crcored spinels have significantly lower Mg#s (∼75 ± 2) than spinels
produced at the MRI. As mentioned above, we believe the Cr-cored
spinels crystallized early (from a less-contaminated melt composition), where subsequent plagioclase contamination of the melt
resulted in the Al-rich rims.
5. Discussion
Two factors affect the application of our experiments in understanding PSA (pink spinel anorthosite) petrogenesis on the Moon.
T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
151
5.1. Associating pink spinel anorthosites with Mg-suite
Similar to mafic silicates, the Mg# of spinel will be largely controlled by the Mg# of the melt from which it crystallizes. Therefore, the MgO-rich signature of PSA spinel requires an MgO-rich
parental melt. The highest Mg# samples collected from the Moon
are the Mg-suite rocks (Warren and Wasson, 1977; James, 1980;
Warren, 1986; Shearer and Papike, 2005). Thus, it is not surprising
that interactions between MSPL and anorthite produced MgO-rich
spinels matching PSA (Fig. 6). Phase equilibria data from our analysis (spinel-melt KD ∼ 0.5) suggests melt compositions with Mg# >
82 are needed to produce spinels consistent with M3 characterizations. Changes to the Fe3+ /Fe2+ ratio of the system will affect
the mineral-melt KD , but sample analyses (e.g. Sato et al., 1973;
Sato, 1976) indicate the lunar interior is near the iron–wüstite
buffer (similar to our low-P experimental runs). Thus, the spinelmelt KD used here is appropriate for Al-rich melts. Although a melt
with Mg# > 82 is less magnesian than MSPL (Mg# ∼ 87), it is ∼15
Mg# units greater than A15C (Mg# ∼ 67). Because A15C contains
the lowest Fe/Mg of the picritic glasses, other mare compositions
would produce higher-Fe spinels, inconsistent with PSA.
Fig. 6. Similar to Fig. 2b, now with experimental spinel compositions relative to
lunar spinels from mare samples (gray field), PST samples (pink field), and spinel
compositions of PSA from M3 (dashed light blue box). Plus signs are chromites
from crystallization experiments on synthesized lunar basaltic compositions (Green
et al., 1971; Donaldson et al., 1975; Elkins-Tanton et al., 2003). Green-filled circles
are spinels from A15C + An experiments (this study). Light gray-filled circles, also
from this study, are spinels that formed during MSPL + An experiments. Light gray,
apex-down triangles are chromites that co-precipitated with olivine in Run #23.
Only spinels produced during MSPL + An experiments are compositionally consistent with M3 observations. (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
The first is whether any of the experiments produce PSA with
the characteristics observed in the remote sensing data; the second is the possible significance of scale in the experiments relative to the natural lunar environment. Considering the latter, the
spatial scale of spinel production in our experiments is on the
order of ∼0.1 mm from the melt–rock interface (MRI), an area
much smaller than PSA detections (M3 offers ∼140 m/pixel spatial
resolution). Despite this apparent scaling discrepancy, it is plausible that the actual lunar MRI is far from a simple plane as is
found in the experiments. Plutonic intrusions would present complex geometries (fracturing, injection, stoping), enhancing the surface area in which assimilation would operate and thus, the area
of spinel production. Additionally, spinel could accumulate into
>100 m sized blocks via crystal settling within a mafic intrusion (Fig. 8) and/or during reactive porous flow, which can form
tabular-like dunite channels ∼100 m in width on Earth (Boudier
and Nicolas, 1985; Kelemen et al., 2000; Morgan and Liang, 2005).
To better understand the accumulation process, future experimental work should include quantifying spinel production rates during assimilation of plagioclase in order to extrapolate to intrusive
timescales.
In examining magma–wallrock interactions on the Moon, the
MgO-rich signature of PSA serves as the basis for our investigation
of lunar MgO-rich basaltic compositions. We find that only spinels
produced during experiments reacting Mg-suite parental melts and
anorthite match the spinels in PSA (Fig. 6). Spinels produced during the Apollo 15C green glass (A15C) and anorthite experiments
are relatively more Fe and Cr-rich, inconsistent with PSA (Fig. 6).
Thus, experimental results imply the interaction between Mg-suite
parental magmas and anorthositic crust is a feasible petrogenetic
process for PSA on the Moon. Does this also imply PSA is a member of the Mg-suite?
5.1.1. Additional lines of evidence linking PSA to Mg-suite
In addition to low Fe/Mg ratios, Mg-suite samples also have
characteristically low Cr-contents (e.g. Shearer and Papike, 2005;
Wieczorek et al., 2006) similar to PSA (Mg# > 90, Cr# < 5). As
mentioned above, greater Al/Cr ratios in the parent melt should result in lower Cr# spinels (Dick and Bullen, 1984; Allan et al., 1988;
Kamenetsky et al., 2001). Such would be the case for PSA production during magma–wallrock interactions within the lunar crust,
where the Al/Cr of the melt increases with the assimilation of
anorthosite. Note also, that assimilation of plagioclase will lower
the liquidus temperature of the melt (e.g. Morgan et al., 2006).
This delays olivine crystallization, allowing for an increase in the
Al/Cr of the melt without lowering its Mg#. In this scenario, higher
Mg# spinels will precipitate relative to systems that have undergone olivine fractionation alone.
Moreover, the Mg-suite samples show cumulate textures and
coarse grain sizes, suggestive of a plutonic origin (Prinz et al.,
1973; Warren and Wasson, 1977; James, 1980; Shearer and Papike,
2005). The production of PSA via magma–wallrock interaction may
have been a contemporaneous process during the plutonic emplacement of Mg-suite parental magmas. This mechanism is also
consistent with current M3 observations, which suggest PSA is a
deep-seated lithology (presumably intrusive) that has been excavated and exposed at the lunar surface during impact processes
(e.g. Pieters et al., 2011; Dhingra et al., 2011; Lal et al., 2012;
Pieters et al., in press).
Finally, parent magmas to the Mg-suite troctolites require a significant anorthite component (∼49% normative anorthite, Longhi
et al., 2010) in order to co-precipitate forsteritic-olivine with
anorthitic-plagioclase (e.g. Warren, 1986; Ryder, 1991; Hess, 1994).
Experimental results suggest that PSA parental melts would require
∼62% normative anorthite or more (Fig. 4). High normative anorthite contents could occur during assimilation of plagioclase crust
(Warren, 1986), where greater degrees of contamination form PSA
(without a significant mafic component) and moderate degrees of
contamination produce Mg-suite troctolites. The high normative
anorthite content required for both PSA and Mg-suite troctolite
parental melts argues PSA is also an Mg-suite rock type.
5.2. Modal mineralogy of pink spinel anorthosites
Spectral estimates of the PSA lithology from mixture modeling imply the modal fraction of spinel to mafic (sp/(ol + px))
is >1 (Cheek and Pieters, in press). For reference, the estimated
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T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
melt, resulting in spinel compositions with low Cr#s (Dick and
Bullen, 1984; Allan et al., 1988; Kamenetsky et al., 2001) (Fig. 7).
Thus, crystal settling during AFC can account for the high mode of
spinel in PSA, but would still require Mg-suite parental melts (i.e.
not the picritic melts) to produce spinels compositionally consistent with M3 .
Fig. 7. The Mg# and Cr# of lunar spinels plotted against the modal fraction of spinel
to olivine + pyroxene. Symbols are the same as in Fig. 3. Estimates from M3 suggest
sp/mafic modal fractions >1 (dashed light blue box, Pieters et al., 2011, Cheek and
Pieters, in press). Dotted gray line represents an approximate “maximum” sp/mafic
ratio reached during the crystallization of basalt as shown in Fig. 3 left ternary. Solid
gray line shows sp/mafic value of 1. Modal fractions of the mare basalts were calculated by taking the opaque (illmenite, chromite, spinel etc.)/(ol + px) ratios (e.g.
Papike et al., 1976). Therefore, mare basalt modal fractions represent an overestimate of true sp/(ol + px) values. Only ALHA 81005 has a modal fraction >1 (Gross
and Treiman, 2011), though this mode may not be representative of the whole rock
(indicated by ‘?’) and the composition of spinel does not match PSA. Crystal settling can raise the sp/mafic ratio, but unless the spinels accumulating have Mg# >
90 and Cr# < 5 this process alone cannot account for PSA petrogenesis. Additional
labeled vectors are discussed within the text.
maximum spinel/mafic produced during normal crystal fractionation is ∼0.2 (Fig. 7). As mentioned above, Fe and Cr-rich spinels
are expected to precipitate from mare basalts and picritic melts
(e.g. Haggerty, 1971; Steele, 1972). However, Fe and Cr-rich spinels
were also observed in uncontaminated regions of the experimental Mg-suite parental melt (Fig. 5). This suggests melts approaching
the olivine-plagioclase cotectic from the olivine stability field have
Al/Cr ratios too low to produce PSA spinel (and possibly also PST
spinel) (Prissel et al., 2014). Therefore, crystal fractionation alone
has difficulty explaining both the composition of PSA spinels as
well as their modal abundance relative to the mafic silicates.
To our knowledge, the only sample from the Moon containing a
spinel/mafic >1 is found within the small PST clast from ALHA
81005 (Gross and Treiman, 2011) (Fig. 7). As mentioned above,
the actual mode of the full clast is likely different. The following proceeds with this caveat, if only to highlight that it is both
the spinel/mafic ratio and composition of spinel that characterizes
PSA. Gross and Treiman (2011) argue that crystal settling during
assimilation and fractional crystallization (AFC) within the lunar
crust can produce spinel-rich lithologies with spinel/mafic >1. The
assimilation of plagioclase should also increase the Al/Cr of the
5.2.1. Sub-solidus Re-equilibration with olivine: evidence for MgO-rich
melts and low mafic contents
As coexisting phases, spinel and olivine will exchange Fe2+ and
Mg at sub-solidus temperatures (e.g. Irvine, 1965, 1967; Roeder
et al., 1979; Jamieson and Roeder, 1984; McCallum and Schwartz,
2001). If olivine is present within the PSA lithology, spinel will
progressively incorporate Fe2+ from olivine as they both cool
and equilibrate. For example, at 1200 ◦ C the olivine-spinel KD
([Mg/Fe]ol [Fe/Mg]sp ) value is ∼1.5 and increases to ∼2.6 at 700 ◦ C
for spinels with Cr# ∼ 5 (Fabries, 1979). The resulting effect will
both lower the Mg# of spinel and increase the Mg# of olivine
(Fig. 7). Two key arguments can be made from this process with
respect to PSA:
1) Because the Mg# of the spinel is expected to decrease during sub-solidus re-equilibration with olivine, the high Mg# of PSA
spinel suggests little to no re-equilibration took place (i.e. little to
no olivine was present to alter the composition of the spinel). In
this way, the high-Mg# inferred for PSA spinels are minima and
any sub-solidus re-equilibration with olivine further argues for the
presence of Mg-suite parental melts during PSA petrogenesis.
2) If present, nearly pure forsterite olivine (Fo∼100 ) could be
spectrally unidentifiable due to a lack of an Fe-absorption. However, Fo∼100 olivine could only arise from either a) an Fe-free system, or b) extensive re-equilibration with spinel. The first case is
highly unlikely in natural geologic systems. Although equally unlikely, the latter case implies spinel Mg#s were even higher to
begin with. Moreover, the efficiency of re-equilibration between
spinel and olivine is highly dependent on their respective modal
abundances (McCallum and Schwartz, 2001). In particular, olivine
will be most affected by sub-solidus re-equilibration (i.e. more
likely to reach Fo∼100 values) when it is in low modal abundance relative to spinel (spinel/mafics 1). The petrologic arguments above are consistent with spectral interpretations of a low
(<5 vol.%) mafic abundance in PSA (Pieters et al., 2011).
5.3. A compositional diversity of spinel anorthosites?
The spinels produced during A15C + An experiments do not
match PSA (Fig. 6). Note however, they are clearly distinct from
mare chromites and are more consistent in composition with pink
spinel troctolite (PST) spinels. Experimental data suggest that if
Fe-rich, pleonaste spinel (>10 wt.% FeO in [Mg,Fe2+ ]Al2 O4 ) is detected, then picritic glass and mare compositions would become
candidates for spinel formation during magma–wallrock interactions. However, spinel formation due to Fe-rich magmas interacting
with the anorthositic crust could be constrained to greater depths
relative to MgO-rich magmas (Fig. 3). Depending on the thickness
of the lunar crust (i.e. pressures at the crust–mantle interface), it
is possible that low-Ti and high-Ti glass compositions also formed
Fe-rich spinel when reacting with anorthosite (Fig. 8).
Such may be the case on the nearside of the Moon at Sinus
Aestuum, where spinel has been remotely detected among dark
mantle deposits (DMD) (Yamamoto et al., 2013). The spinels at Sinus Aestuum are similar to the spinel of PSA, but appear to be
more Fe-rich. It is possible that the Fe-rich spinels formed during the temporary emplacement of high-Ti picritic melts (likely
source of the DMD, e.g. Pieters et al., 1974; Weitz et al., 1998)
into the anorthositic crust. Here, assimilation and spinel production may have occurred prior to the eruption that resulted in the
T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156
153
Fig. 8. The potential vertical distribution of spinel anorthosite lithologies within the lunar crust as a function of magma composition. Shown are various magmas ponding at
the base of the crust, with subsequent dike propagation/intrusion. Mg# of magmas are listed within the legend boxes (Mg-suite = Mg-suite parental liquid (Longhi et al.,
2010); A15C GG = Apollo 15C green glass; A15 YG = Apollo 15 yellow glass; A15 RG = Apollo 15 red glass) (Delano, 1986). Colored dashed lines correspond to potential
minimum depths of formation of the respective magma based on spinel stability (e.g. Prissel et al., 2012 has shown that spinel production during magma–rock reactions
between A15C green glass and anorthosite may be restricted to crustal depths near 10 km or greater). Spinel formation during lunar melt–rock reactions can proceed to
shallower depths with Mg-suite parental melts. The potential for a compositional diversity of spinel anorthosites is discussed in the text. (After Head and Wilson, 1992). (For
interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
DMD (Yamamoto et al., 2013). However, it is still unclear whether
or not spinels at Sinus Aestuum contain a significant chromite
component. If so, they may simply be products of normal crystal
fractionation as assimilation of plagioclase crust is not necessary
for chromite-spinel production.
5.4. Implications concerning the addition of PSA to the lunar rock record
The compositional relationship between PSA and MgO-rich
melts with a high anorthite component (i.e. Mg-suite parental liquids) suggests PSA is a member of the Mg-suite. PSA production
by magma–wallrock interactions is consistent with the plutonic
models proposed for Mg-suite, as well as current M3 observations,
which infer PSA is a deep-seated lithology. Would the addition
of PSA to the lunar rock record, specifically the Mg-suite, modify
interpretations of lunar evolution? To answer, it is worth briefly
reviewing current models of ancient lunar magmatism.
On average, Mg-suite rocks (∼4.5–4.1 Ga) pre-date the apparent onset of mare basaltic volcanism (∼3.9 Ga) (e.g. Nyquist and
Shih, 1992; Hiesinger et al., 2000; Whitten and Head, 2013). Comprised of dunites, PST, troctolites, norites and gabbronorites, the
Mg-suite is most notably characterized by high Mg# mafic silicates
positively correlating with the An# (Ca/[Ca + Na] × 100) of coexisting plagioclase (e.g. Shearer and Papike, 2005). Despite their
primitive major element chemistry, Mg-suite samples have high
concentrations of trace elements (potassium, rare earth element,
and phosphorus, i.e. KREEP component). This has led to a multistep model for Mg-suite petrogenesis. Following the differentiation
of the LMO (lunar magma ocean) and formation of an anorthositic
crust, high-Mg# mafic cumulates are believed to have hybridized
with a residual KREEP layer during cumulate mantle overturn (e.g.
Longhi et al., 2010; Elardo et al., 2011). Partial melts from the
hybridized source region then formed plutons (and thus, the Mgsuite) within the lunar crust. Mg-suite rocks therefore represent
the earliest known post-LMO magmatism on the Moon. If PSA was
produced during Mg-suite magmatism, then PSA would also be ancient (pre-mare).
5.4.1. PSA as a proxy for Mg-suite magmatism
The distribution and extent of Mg-suite lithologies on the Moon
remains an outstanding unanswered question in lunar science (e.g.
Tompkins and Pieters, 1999; Jolliff et al., 2000; Cahill et al., 2009).
If PSA is a member of the Mg-suite, their locations can be used as a
proxy for Mg-suite magmatism on the Moon. The presence of PSA
on both the nearside and farside of the Moon (e.g. Pieters et al.,
2011; Dhingra et al., 2011; Pieters et al., in press) implies Mg-suite
magmatism was perhaps a global phenomenon. An early, global
emplacement process may be necessary to explain the presence of
PSA (and by extension, Mg-suite) on both the lunar nearside and
farside.
5.4.2. Magma–wallrock interactions in the early Moon
The growing number of PSA detections (e.g. Pieters et al.,
in press) implies that magma–wallrock interactions were common in the early Moon. Thermodynamic analyses suggest these
interactions were likely minor during mare basaltic volcanism
(<3.9 Ga) due to a cold and brittle lunar crust (Finnila et al., 1994;
Hess, 1994). However, recent observations suggest this may not
have always been the case, specifically during the formation of
Mg-suite as was proposed by Warren (1986). Andrews-Hanna et
al. (2013) have identified linear gravity anomalies globally distributed on the Moon, interpreted to be ancient igneous intrusions.
The lengths and linearity of the intrusions (all of which appear to
pre-date the onset of mare volcanism) are similar to dike swarms
observed on Earth, Venus and Mars (e.g. Ernst et al., 2001, 2003;
Wilson and Head, 2002). In contrast, the widths of the intrusions
exceed those observed on the terrestrial planets. Andrews-Hanna
et al. (2013) suggest this may be the result of a hot, ductile crust
at the time of magmatic emplacement. In this scenario, the wallrock would become more prone to assimilation because of its
near-solidus temperature (e.g. Huppert and Sparks, 1988), providing prime conditions for PSA production (Fig. 8). As discussed
above, both AFC (contamination of the melt) and reactive porous
flow (contamination of the wallrock) could have contributed to the
formation of PSA. Additionally, the presence of ancient igneous
intrusions is consistent with current plutonic models regarding
Mg-suite (e.g. Longhi et al., 2010; Elardo et al., 2011) and/or PSA
petrogenesis via Mg-suite magma–wallrock interactions (Prissel et
al., 2012). Thus, magma–wallrock interactions should be revisited
as a petrogenetic process for early rock types of the Moon, particularly the Mg-suite (Warren, 1986).
5.4.3. The role of KREEP during Mg-suite petrogenesis
As discussed, the collected Mg-suite samples contain a KREEP
chemical signature. It is possible that heat (U, Th) from KREEP
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may be required for Mg-suite formation (e.g. Longhi et al., 2010;
Elardo et al., 2011). However, KREEP appears to be confined to the
nearside of the Moon (Lawrence et al., 1998, 2000; Elphic et al.,
2000) while PSA has been observed on both the lunar nearside
(preferentially outside the Procellarum KREEP terrane, see Jolliff et
al., 2000) and farside (e.g. Pieters et al., 2011; Dhingra et al., 2011;
Pieters et al., in press). This suggests KREEP is not necessary for
the formation of Mg-suite rocks and is only a constituent of nearside sampling (assuming PSA are proxies for Mg-suite). If true, the
initial heat required could have come from decompression melting of ultramafic cumulates during mantle overturn (e.g. Hess and
Parmentier, 1995; Zhong et al., 2000; Elkins Tanton et al., 2002;
Laneuville et al., 2013).
6. Conclusion
Spinels matching the M3 characterization of new lunar rock
type, Pink Spinel Anorthosite (PSA), have been experimentally produced during the interaction of Mg-suite parental liquids (MSPL)
with anorthite. Experimental results suggest melts with Mg# > 82,
contaminated with anorthite (∼62% normative anorthite), can explain the spinel compositions of PSA as well as the apparent lack
of mafic phases within the spectrally defined lithology. Given that
the anorthositic crust is out of equilibrium with all lunar basalts,
it is not unreasonable to expect extensive interaction to take place
during magmatic and volcanic events. For instance, the contamination could occur during assimilation and fractional crystallization
(Gross and Treiman, 2011) and/or during reactive porous flow as
melts intruded the lunar crust (Prissel et al., 2012). In either case,
the compositional evidence strongly implies that PSA represents a
new member of the Mg-suite. If so, remote sensing studies can use
PSA as a proxy for Mg-suite magmatism on the Moon.
The identification of PSA on both the lunar nearside and farside presents several implications concerning early magmatism
and lunar evolution. For instance, lunar evolution models must
account for the widespread distribution of PSA, and by extension, Mg-suite as a potentially global igneous product. Thus, an
early, global emplacement process may be required to explain the
widespread distribution of both Mg-suite and PSA lithologies on
the Moon. A global distribution of Mg-suite also suggests KREEP
is not required for Mg-suite petrogenesis and is only necessary to
explain the geochemical signature of nearside samples collected
within/near the Procellarum KREEP terrane.
Acknowledgements
This study was greatly strengthened by the constructive and
thoughtful review of Paul Warren. We would also like to extend
thanks to both Joseph Bosenberg for help with the microprobe
analyses at Brown University and Christophe Sotin for handling of
the manuscript. Research supported by the NASA Lunar Science Institute grant NNA09DB34A and NASA SSERVI grant NNA14AB01A.
Appendix A. Supplementary material
Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.epsl.2014.06.027.
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