James J. McGee

Ion and electron microprobe study of troctolites, norite, and anorthosites from Apollo 14: evidence for urKREEP assimilation during petrogenesis of Apollo 14 Mg-suite rocks

Abstract Most of the Moon’s highland crust formed during the period 4.65–4.45 Ga ago from a vast magma ocean up to 800 km deep (Hess and Parmentier, 1995) . This early lunar crust comprises Fe-rich anorthosites with calcic plagioclase compositions. Subsequent evolution of the highland crust was dominated by troctolites, anorthosites, and norites of the Mg-suite. This plutonic series is characterized by calcic plagioclase, and mafic minerals with high mg# (=100∗Mg/[Mg + Fe]). These rocks evidently formed by partial melting of ultramafic rocks of the lunar mantle, but their bulk rock incompatible element characteristics are too enriched to represent such a primitive source. Previous studies have suggested that this enrichment in incompatible trace elements is the result of metasomatism of the crust by fluids rich in REE and P. The products of this suggested metasomatic event are REE-rich phosphates (typically whitlockite) deposited interstitially. Alternatively, the incompatible element-rich nature of these plutonic rocks may represent a characteristic of their parent magma, acquired prior to crystallization of the plutons. In an effort to distinguish the origin of this important lunar rock series, we have analyzed the REE content of primary cumulus phases in ten Mg-suite cumulates using SIMS, along with their major and minor element compositions by electron microprobe analysis. Nine of these samples have high mg#s, consistent with their formation from the most primitive parent melts of the Mg-suite. The data presented here show that Mg-suite troctolites and anorthosites preserve major and trace element characteristics acquired during their formation as igneous cumulate rocks and that these characteristics can be used to reconstruct related aspects of the parent magma composition. Our data show that primitive cumulates of the Mg-suite crystallized from magmas with REE contents similar to high-K KREEP in both concentration and relative abundance. The highly enriched nature of this parent magma contrasts with its primitive major element characteristics, as pointed out by previous workers. This enigma is best explained by the mixing of residual magma ocean urKREEP melts with ultramagnesian komatiitic partial melts from the deep lunar interior. The data do not support earlier models that invoke crustal metasomatism to enrich the Mg-suite cumulates after formation, or models which call for a superKREEP parent for the troctolites and anorthosites.

KREEP cumulates in the western lunar highlands; ion and electron microprobe study of alkali-suite anorthosites and norites from Apollo 12 and 14

Abstract Textural features combined with mineral chemistry preserved in metamorphic rocks provide insights into metamorphic reaction mechanisms as well as open vs. closed system processes. Prograde tourmaline-rich muscovite pseudomorphs after staurolite develop in sillimanite zone metapelites adjacent to peraluminous granitoid intrusives in NW Maine. Tounnalines occur in discrete domains restricted to central regions of muscovite-rich, quartz-poor pseudomorphs with biotite-rich margins. These tourmaline grains are relatively large (≤ 1.0 nun), lack detrital cores and exhibit only minor compositional zoning, in sharp contrast to matrix tourmaline from other samples. These features suggest fluid-infiltration as the causative mechanism for the fonnation of these tounnaline-rich mica pseudomorphs after staurolite. Irreversible thermodynamic models of local reactions and material transport in combination with mineral chemistry allow evaluation of reaction mechanisms that produced these pseudomorphs. Thermodynamic models in the NKCMTFASHOB system mimic the observed textural features if a three- stage process is used. Stage 1: Stamolite replacement is initiated by infiltration of an aqueous phase that adds K-Na-H2O to the rock with the concomitant removal of Al+Fe. Because the system is initially undersaturated with respect to tourmaline, a pseudomorph containing muscovite with minor biotite develops at the expense of staurolite. Stage 2: With continued infiltration, concentration of B increases, tourmaline saturation is exceeded, tourmaline nucleates and grows. Local material transport constiaints mandate that tourmaline precipitation be spatially restricted to regions of staurolite dissolution. Consequently, tourmaline forms in clusters at sites containing the last vestiges of staurolite in the pseudomorph core, also evidenced by staurolite inclusions within several tourmaline grains. Resultant domains of staurolite replacement during this stage contain about equal amounts of muscovite and tourmaline. Typical staurolite poikiloblast pseudomorphing reactions require silica transport. matrix quartz dissolves from the surrounding host resulting in a local enrichment of biotite and plagioclase at the pseudomorph margin. Stage 3: Small amounts of sillimanite nucleate and grow throughout the rock. Late-stage aqueous fluids from the adjacent monzonitic intrusive are likely to be the primary B source. Theoretical, textural, and compositional modeling combined with observational data indicate that boron must have been derived externally from the rock, that the modal amount of tourmaline is very sensitive to the B content of the fluid, that tourmaline is stable throughout the sillimanite zone depending on other cation activities and pH of the fluid, and that these pseudomorphs provide insight into B contents of metamorphic fluids and the timing of the B influx. The outer geographic extent of the tourmaline-bearing pseudomorphs marks the boundary of a reactive geochemical front, and thus defines an advective isograd. Inteipretation of subtle textural features preserved in the rock in conjunction with irreversible textural modeling provides a powerful tool with which to understand the chemical evolution of metamorphic rocks and the fluids involved.

Rare earth element variations resulting from inversion of pigeonite and subsolidus reequilibration in lunar ferroan anorthosites

We present results of a secondary ion mass spectrometry study of the rare earth elements (REEs) in the minerals of two samples of lunar ferroan anorthosite, and the results are applicable to studies of REEs in all igneous rocks, no matter what their planet of origin. Our pyroxene analyses are used to determine solid-solid REE distribution coefficients (D = CREE in low-Ca pyroxene/CREE in augite) in orthopyroxene-augite pairs derived by inversion of pigeonite. Our data and predictions from crystal-chemical considerations indicate that as primary pigeonite inverts to orthopyroxene plus augite and subsolidus reequilibration proceeds, the solid-solid Ds for orthopyroxene-augite pairs progressively decrease for all REEs; the decrease is greatest for the LREEs. The REE pattern of solid-solid Ds for inversion-derived pyroxene pairs is close to a straight line for Sm-Lu and turns upward for REEs lighter than Sm; the shape of this pattern is predicted by the shapes of the REE patterns for the individual minerals. Equilibrium liquids calculated for one sample from the compositions of primary phases, using measured or experimentally determined solid-liquid Ds, have chondrite-normalized REE patterns that are very slightly enriched in LREEs. The plagioclase equilibrium liquid is overall less rich in REEs than pyroxene equilibrium liquids, and the discrepancy probably arises because the calculated plagioclase equilibrium liquid represents a liquid earlier in the fractionation sequence than the pyroxene equilibrium liquids. “Equilibrium” liquids calculated from the compositions of inversion-derived pyroxenes or orthopyroxene derived by reaction of olivine are LREE depleted (in some cases substantially) in comparison with equilibrium liquids calculated from the compositions of primary phases. These discrepancies arise because the inversion-derived and reaction-derived pyroxenes did not crystallize directly from liquid, and the use of solid-liquid Ds is inappropriate. The LREE depletion of the calculated liquids is a relic of formation of these phases from primary LREE-depleted minerals. Thus, if one attempts to calculate the compositions of equilibrium liquids from pyroxene compositions, it is important to establish that the pyroxenes are primary. In addition, our data suggest that experimental studies have underestimated solid-liquid Ds for REEs in pigeonite and that REE contents of liquids calculated using these Ds are overestimates. Our results have implications for Sm-Nd age studies. Our work shows that if pigeonite inversion and/or subsolidus reequilibration between augite and orthopyroxene occurred significantly after crystallization, and if pyroxene separates isolated for Sm-Nd studies do not have the bulk composition of the primary pyroxenes, then the Sm-Nd isochron age and eNd will be in error.

Petrology of the Western Highland Province: Ancient crust formation at the Apollo 14 site

Plutonic rocks found at the Apollo 14 site comprise four lithologic suites: the magnesian suite, the alkali suite, evolved lithologies, and the ferroan anorthosite suite (FAN). Rocks of the magnesian suite include troctolite, anorthosite, norite, dunite, and harzburgite; they are charaterized by plagioclase ≈An95 and mafic minerals with mg#s 82–92. Alkali suite rocks and evolved rocks generally have plagioclase ≈An90 to ≈An40 and mafic minerals with mg#s 82–40. Lithologies include anorthosite, norite, quartz monzodiorite, granite, and felsite. Ferroan anorthosites have plagioclase ≈An96 and mafic minerals with mg#s 45–70. Whole rock geochemical data show that most magnesian suite samples and all alkali anorthosites are cumulates with little or no trapped liquid component. Norites may contain significant trapped liquid component, and some alkali norites may represent cumulate-enriched, near-liquid compositions, similar to KREEP basalt 15386. Evolved lithologies include evolved partial cumulates related to alkali suite fractionation (quartz monzodiorite), immiscible melts derived from these evolved magmas (granites), and impact melts of preexisting granite (felsite). Plots of whole rock mg# versus whole rock Ca/(Ca + Na + K) show a distinct gap between rocks of the magnesian suite and rocks of the alkali suite, suggesting either distinct parent magmas or distinct physical processes of formation. Chondrite-normalized rare earth element (REE) patterns show that rocks of both the magnesian suite and alkali suite have similar ranges, despite the large difference in major element chemistry. Current models for the origin of the magnesian suite call for a komatiitic parent magma derived from early magma ocean cumulates; these melts must assimilate plagiophile elements to form troctolites at low pressures and must assimilate a highly enriched KREEP component so that the resulting mixture has REE concentrations similar to high-K KREEP. There are as yet no plausible scenarios that can explain these unusual requirements. We propose that partial melting of a primitive lunar interior and buffering of these melts by ultramagnesian early magma ocean cumulates provides a more reasonable pathway to form magnesian troctolites. Alkali anorthosites and norites formed by crystallization of a parent magma with major element compositions similar to KREEP basalt 15386. If the parent magma of the alkali suite and evolved rocks is related to the magnesian suite, then that magma must have evolved through combined assimilation-fractional crystallization processes to form the alkali suite cumulates.