Earth and Planetary Science Letters 403 (2014) 144–156

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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 a b s t r a c t

Article history:Received 19 October 2013Received in revised form 23 April 2014Accepted 20 June 2014Available online xxxxEditor: C. Sotin

Keywords:spinelMoonMg-suiteplutoniclunar crust

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 MgAl2O4spinel, 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 dom-inated 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 MgAl2O4 spinel (minor

* Corresponding author.E-mail address: tabb_prissel@brown.edu (T.C. Prissel).

1 Tel.: +401 863 1932; fax: +401 863 2058.

http://dx.doi.org/10.1016/j.epsl.2014.06.0270012-821X/© 2014 Elsevier B.V. All rights reserved.

FeO, Cr2O3, hereafter referred to simply as spinel) and 2) an un-usually 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 sug-gest that spinel is in much greater modal abundance (near mono-mineralic 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 detec-tions 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 con-sistently detecting only the most mafic-free, pure spinel litholo-gies. Identifications typically occur in crater rims and walls, cen-tral peaks, and the inner rings of larger impact basins. Most lo-cations share a similar geological context: originally deep-seated lithologies that were uplifted to the surface during impact excava-tion (Fig. 1) (Pieters et al., 2011; Dhingra et al., 2011). However, it is still unclear whether PSA formed during endogenic or exo-genic 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 petroge-netic 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 dur-ing the interaction between lunar basaltic melts and anorthositic crust. While our interpretations of the results are centered on magma–wallrock interactions, the compositional constraints re-ported herein can be applied to both exogenic and endogenic pet-rogenetic models.

1.1. Lunar spinels

Compared to the spinel detected by M3, most lunar spinels trend from chromites (Fe2+Cr2O4) 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 cat-egorized as chromian-pleonaste ([Mg,Fe2+][Al,Cr]2O4), 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 (MgAl2O4) 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 lu-nar 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 lu-nar 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# ∼ 2respectively) matching PSA (Fig. 2b). However, every known spinel-bearing sample within the lunar collection (including those men-tioned above) contain significant proportions of olivine ± pyrox-ene (>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 low-pressure 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 pre-cipitate 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, Mon-tana, 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 crystalliza-tion 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 re-quired to reproduce the spinels observed by M3. Likewise, a strong correlation between the Al-content of parental melts and the Al-content of crystallizing spinel has been observed in several ter-restrial 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 com-ponent (high Al/Cr) to explain the low Cr-content of spinels char-acterized by M3 (Williams et al., 2012; Prissel et al., 2014).

Several petrogenetic models have been proposed suggest-ing 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 dur-ing 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 mate-rial 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, hy-pothesis 3) faces the extreme obstacle of consistently preserving large-scale, high-proportions of spinel through a diversity of im-pacts. 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 pro-cess 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 bi-modality observed in the pristine non-mare rocks. While exten-sive 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 dia-gram (Fig. 3, middle ternary). Simple mixing in a closed system would produce liquid bulk compositions along a straight line be-tween 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 + anor-thite in the absence of a mafic phase. Additionally, the re-melting and subsequent crystallization of pre-existing anorthositic trocto-lites 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 spinel-anorthite–forsterite ternary peritectic (PST assemblage). This point is monoresorptional (meaning forsterite and anorthite are pro-duced at the expense of spinel). In this case, the liquid compo-sition 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 crys-tallization (AFC) taking place (DePaolo, 1981a, 1981b). Similar to layered mafic intrusions, AFC processes on the Moon could pro-duce 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 terres-trial 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 in-stantaneous melt–wallrock ratio is low. Pyroxene dissolution and olivine precipitation will persist if several pulses of magma in-trude the wallrock, increasing both the matrix permeability and

148 T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156

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 ophio-lites, dunite is observed both as veins (tens of millimeters wide) and large tabular-like bodies (∼100 m wide) up to several kilome-ters 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 anal-ogous to the dunite channels in peridotites (where preferential dissolution of plagioclase gives rise to spinel production). It is pos-sible 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 atmo-spheric 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 pro-jection line delineates basaltic compositions that are capable of producing liquids near the spinel-anorthite cotectic when mix-ing 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 inter-acting 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 compo-sitions 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 inter-actions 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 com-position of mantle material mixing with anorthositic crust during impacts).

3. Experimental methods

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# lu-nar basaltic compositions to react with anorthite: 1) an Apollo 15C green glass (A15C), representing the most MgO-rich compo-sition of the picritic glasses with Mg# ∼ 67 (Delano, 1986) and2) a theoretical Mg-suite parental liquid (MSPL) with Mg# ∼ 87(Longhi et al., 2010). The melt compositions (Table 1) were syn-thesized from reagent grade oxide powders and then conditioned at the iron–wüstite (Fe–FeO, denoted ‘IW’) buffer inside a hori-zontal 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 sin-tered at 1400 ◦C for 16 h within a Deltech box furnace.

Low-pressure experiments were performed in a Harwoodinternally-heated pressure vessel (IHPV) with Ar as the pressure

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T.C. Prissel et al. / Earth and Planetary Science Letters 403 (2014) 144–156 149

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 re-gions (triangles), liq-only regions (circles), and ol ± chromite regions (squares). 2σstandard 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 experi-ments (Prissel et al., 2012) where thicker-walled graphite crucibles are believed to have prevented the sample pressure from equal-ing the confining pressure during low-P runs. The experimental charge was then completely surrounded by carbon–graphite pow-der (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–Pt90Rh10 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 esti-mates reported here are maxima (since C powder is always present in the experiments). Each experiment was drop quenched by tilt-ing 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 us-ing 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 O2for 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-

like profiles of Al-content (i.e. anorthite contamination) are ob-served in the melt as in previous studies (e.g. Finnila et al., 1994;Morgan et al., 2006), though advection also appears to have oc-curred (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 mea-surements both indicate Al-rich rims. Cr-cored spinels crystallized early in the experiment, where diffusion and/or advection later car-ried 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% norma-tive 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 crys-talline phases appears to positively correlate with pressure. This may be due to the higher solubility of volatiles at greater pres-sures, 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 experi-ment, no single homogeneous melt composition exists for calculat-ing mineral-melt KD (Fe–Mg cation fraction exchange coefficient; [XFe/Xmg]xtal × [XMg/XFe]liq). Therefore, we calculate a “local” KDby 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 produc-tion 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 exper-iment to produce an assemblage of ∼15 vol.% spinel (poikilitically enclosing anorthite) and <5 vol.% mafics (estimated from the in-terstitial melt observed with BSE images) all within the anorthite-layer (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. Orthopyrox-ene (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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Table 2Summary of lunar melt–rock interaction experiments.

Run # Devicea P(GPa)

T(◦C)

Duration (min)

Melt comp. (+An)b

Productsc f O2d

1 PC 0.8 1400 180 A15C gl, sp, ol, opx IW + 1.715 IHPV 0.1 1400 180 A15C gl, sp, ol IW − 0.421 PC 1.05 1400 180 MSPL gl, sp IW + 222 IHPV 0.1 1400 180 MSPL gl, sp IW − 0.423 IHPV 0.2 1400 180 MSPL gl, sp, ol, chrm IW + 0.326 IHPV 0.05 1400 180 MSPL gl, sp IW − 1.127 IHPV 0.05 1400 180 A15C gl, sp, ol IW − 1.1

a PC = Piston Cylinder; IHPV = Internally Heated Pressure Vessel.b Melt composition juxtaposed against sintered pure anorthite.c gl = glass; sp = spinel; ol = olivine; opx = orthopyroxene; chrm = chromite.d 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 plagioclase-contaminated 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 aver-age 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 ex-periment 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. Cr-cored spinels with Al-rich rims are observed with olivine away from the MRI (Fig. 5). In addition to higher Cr#s (∼31 ± 5), Cr-cored 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 compo-sition), where subsequent plagioclase contamination of the melt resulted in the Al-rich rims.

5. Discussion

Two factors affect the application of our experiments in under-standing PSA (pink spinel anorthosite) petrogenesis on the Moon.

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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 con-sistent 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 sec-ond is the possible significance of scale in the experiments rela-tive 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 plau-sible that the actual lunar MRI is far from a simple plane as is found in the experiments. Plutonic intrusions would present com-plex geometries (fracturing, injection, stoping), enhancing the sur-face 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 intru-sion (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 experimen-tal work should include quantifying spinel production rates dur-ing 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 dur-ing 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 mem-ber of the Mg-suite?

5.1. Associating pink spinel anorthosites with Mg-suite

Similar to mafic silicates, the Mg# of spinel will be largely con-trolled by the Mg# of the melt from which it crystallizes. There-fore, 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 analy-sis (spinel-melt KD ∼ 0.5) suggests melt compositions with Mg# >

82 are needed to produce spinels consistent with M3 characteri-zations. 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 spinel-melt KD used here is appropriate for Al-rich melts. Although a melt with Mg# > 82 is less magnesian than MSPL (Mg# ∼ 87), it is ∼15Mg# 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.

5.1.1. Additional lines of evidence linking PSA to Mg-suiteIn 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 re-sult in lower Cr# spinels (Dick and Bullen, 1984; Allan et al., 1988;Kamenetsky et al., 2001). Such would be the case for PSA produc-tion 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 under-gone 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 em-placement 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 exca-vated 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 sig-nificant 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 anor-thite 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 model-ing 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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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 cal-culated by taking the opaque (illmenite, chromite, spinel etc.)/(ol + px) ratios (e.g. Papike et al., 1976). Therefore, mare basalt modal fractions represent an overesti-mate 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 set-tling 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 fractiona-tion 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 experimen-tal 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 follow-ing 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

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 consis-tent with M3.

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 dur-ing 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. How-ever, Fo∼100 olivine could only arise from either a) an Fe-free sys-tem, or b) extensive re-equilibration with spinel. The first case is highly unlikely in natural geologic systems. Although equally un-likely, 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 abun-dance relative to spinel (spinel/mafics � 1). The petrologic argu-ments 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+]Al2O4) is de-tected, then picritic glass and mare compositions would become candidates for spinel formation during magma–wallrock interac-tions. 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 Si-nus Aestuum are similar to the spinel of PSA, but appear to be more Fe-rich. It is possible that the Fe-rich spinels formed dur-ing 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 produc-tion 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 liq-uids) 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 appar-ent onset of mare basaltic volcanism (∼3.9 Ga) (e.g. Nyquist and Shih, 1992; Hiesinger et al., 2000; Whitten and Head, 2013). Com-prised 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 co-existing 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 multi-step 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 Mg-suite) 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 an-cient (pre-mare).

5.4.1. PSA as a proxy for Mg-suite magmatismThe 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 MoonThe growing number of PSA detections (e.g. Pieters et al.,

in press) implies that magma–wallrock interactions were com-mon 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 dis-tributed 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 wall-rock would become more prone to assimilation because of its near-solidus temperature (e.g. Huppert and Sparks, 1988), pro-viding 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, partic-ularly the Mg-suite (Warren, 1986).

5.4.3. The role of KREEP during Mg-suite petrogenesisAs 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 near-side sampling (assuming PSA are proxies for Mg-suite). If true, the initial heat required could have come from decompression melt-ing 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 pro-duced 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 ex-plain 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 contamina-tion 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 far-side presents several implications concerning early magmatism and lunar evolution. For instance, lunar evolution models must account for the widespread distribution of PSA, and by exten-sion, 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 In-stitute grant NNA09DB34A and NASA SSERVI grant NNA14AB01A.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at http://dx.doi.org/10.1016/j.epsl.2014.06.027.

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