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NASA NTRS1993

Fragments of ancient lunar crust: Ferroan noritic anorthosites from the descartes region of the Moon

Noritic anorthosite clasts from breccia 67016 have bulk compositions similar to that of the upper crust of the Moon and petrogenetic affinities with pristine ferroan anorthosites. Rb-Sr and Sm-Nd isotopic compositions of mineral separates from one of these clasts suggest very old (greater than or = 4.4 Ga) ages, but interpretation of these data is complicated by the multi-stage history of the clasts which involved magmatic crystallization, brecciation, subsolidus recrystallization, and sulfide metasomatism. These clasts record some of the earliest events on the Moon, including early crust formation, accretionary bombardment, and degassing of the lunar interior. Modal analyses of these clasts show they are now composed of about 70 percent plagioclase, 28 percent pyroxene, 2 percent troilite, and minor amounts of ilmenite and chromite. No metallic iron, phosphates, or other trace phases were observed. Olivine is very rare, occurring only as relicts within secondary troilite+pyroxene intergrowths which may reflect reaction of olivine with sulfurous vapors. PIXE proton microprobe analyses of the sulfides show that the metasomatism was accompanied by enrichments of Cu, Zn, Ni, Se, and Sb. The clasts have been only mildly shocked since the observed texture was established. Major and minor element mineral compositions are very homogeneous and strikingly similar to those of pristine ferroan anorthosites. Pyroxene compositions indicate equilibration temperatures of 850-900 C. Except for the sulfide and chalcophile element metasomatism, these clasts appear to be essentially monomict and probably represent a noritic member of the ferroan anorthosite suite. Their low Ni contents and Ni/Co ratios are consistent with the interpretation of these clasts as igneous rocks which have escaped mixing with meteoritic material.

Norman, M. D.↗

NASA NTRS1977

Trace element distribution in mineral separates of the Allende inclusions and their genetic implications

Concentrations of the rare earth elements (REE), Sc, Co, Fe, Zn, Ir, Na, and Cr were determined for mineral separates of the coarseand fine-grained types (group I and II) of the Allende inclusions. These data in combination with other data suggest that the minerals in the coarse-grained inclusions (group I) crystallized in a closed system with respect to refractory elements although a totally molten stage is precluded. The data also indicate that fine-grained (group II) inclusions were formed by condensation from a super-cooled nebular gas; REE-rich clinopyroxene and spinel were formed earlier than REE-poor sodalite and nepheline. In addition, pre-existing Mg isotope anomalies in the coarse-grained inclusions must have been erased during the heating stage.

Nagasawa, H.↗

NASA NTRS1994

He, Xe and Ne isotopes in a steady-state mass transport model and implications about terrestrial volatiles

We present a model of the steady-state transport assuming three reservoirs: a lower mantle (P) with a relatively undepleted inventory of U, Th, Pu, I, He, Ne, Xe, Ar; an upper mantle that has been extensively outgassed (D); and the atmosphere. There is mass transport at a rate M(sub PD) by plumes from the lower mantle, a fraction of which is outgassed directly into the atmosphere, while the remainder feeds matter and associated nuclei into D. D is well outgassed at spreading centers and has material containing atmospheric gases added to it by subduction. In the case of He, there is no subduction component. The approach follows the treatment of Kellogg and Wasserburg. A summary of the pertinent equations and constraints was reported earlier. The U, Th and Pu in P are estimated for Earth models from refractory element abundances in meteorites. In this model the inventory of rare gases in D is governed by the simple mixing of components from P (both radiogenic and original) with distinctive atmospheric components. In addition, alpha decay and spontaneous fission of U, and (alpha, n) reaction on oxygen from energetic alpha particles produce radiogenic/nuclear daughter products in D. These include (4)He, (136)Xe and (21)Ne. (40)K in D generates excess radiogenic (40)Ar.

Wasserburg, G. J.↗

NASA NTRS1993

A recent meteorite shower in Antarctica with an unusual orbital history

The Antarctic meteorite collection has proved to be a source of many important discoveries, including a number of previously unknown or very rare meteorite types. A thermoluminescence (TL) survey of meteorite samples recovered by the 1988/89 European expedition and pre-1988 American expeditions to the Allan Hills Main blue ice field resulted in the discovery of 15 meteorites with very high TL levels (greater than 100 krad at 250 C in the glow curve). It is likely that these samples are fragments of a single meteoroid body which: (1) fell very recently and (2) experienced a decrease in orbital perihelia from greater than or equal to 1.1 AU to 1 AU within the last 10(exp 5) yr. Carbon-14 data for two of the samples confirm their young terrestrial age compared to most Antarctic meteorites. Studies of the cosmogenic isotopes in at least one non-Antarctic meteorite which also has very high natural TL, Jilin, indicate that the meteorite experienced a multi-stage irradiation history, the most recent stage being 0.4 Ma in duration following a major break-up of the object. These meteorites, and the few equivalent modern falls, are the only documented samples from bodies which were recently in Earth-approaching (Amor) orbits (i.e., with perihelion greater than 1.0 AU), as opposed to the Earth-crossing (Apollo) orbits which are the source of most other meteorites. Their rarity indicates that such rapid orbit changes are unusual for meteoroid bodies and may be the result of isolated, large break-up events.

Benoit, P. H.↗

NASA NTRS1978

Chemical and Sr-isotopic characteristics of the Luna 24 samples

The chemical and Sr isotopic characteristics of Luna 24 bulk soil samples are determined and interpreted within the framework of lunar mare basalt evolution. Major and trace element compositions lead to the suggestion of candidate rock types consisting of a basalt/gabbro with very low TiO2 and MgO content (52%), a very low TiO2 basalt with 10% MgO (23%), olivine vitrophyre (20%), low K Fra Mauro basalt (4%) and anorthositic gabbro (1%). The proposed compositions are supported by the agreement of mixing models based on the proposed compositions with observed soil compositions. Sr ratios for plagioclase samples imply a lower Sr-87/Sr-86 value for low Mg soils than for other mare basalts and higher Rb/Sr and a more evolved Sr ratio in high Mg basalts. Rb, Sr and rare earth element compositions for low Mg basalts fit a model of partial remelting of cumulates containing small amounts of plagioclase.

Nyquist, L. E.↗

NASA NTRS1993

The diatom record from beneath the West Antarctic Ice Sheet and the global proxy perspective

Recent glaciological evaluation and modeling of the marine-based West Antarctic Ice Sheet (WAIS) support the possibility that the WAIS disintegrated during one or more Pleistocene interglacial period(s). The magnitude of sea level and oxygen isotope variation during certain late-Pleistocene interglacial periods is also consistent with the possibility of major retreat of the WAIS. Although oxygen isotopes from deep-sea sediments provide the best available proxy record for global ice volume (despite the ambiguities in the record), the source of ice volume changes must be hypothesized. Based on the intensity of interglacial isotopic shifts recorded in Southern Ocean marine sedimentary records, stage 11 (400,000 years ago) is the strongest candidate for WAIS collapse, but the records for stages 9, 7, and 5.5 are all consistent with the possibility of multiple late-Pleistocene collapses. Seismic reflection studies through the WAIS have revealed thick successions of strata with seismic characteristics comparable to upper Tertiary marine sediments. Small samples of glacial diamictons from beneath the ice sheet have been collected via hot-water drilled access holes. These sediments include mixed diatom assemblages of varying ages. Late-Miocene diatoms dominate many samples, probably reflecting marine deposition in West Antarctic basins prior to development of a dominantly glacial phase in West Antarctica. In addition to late-Miocene diatoms, samples from Upstream B (1988/89) contain rare post-Miocene diatoms, many of which imply deposition in the West Antarctic interior during one or more Pleistocene deglaciation periods. Age-diagnostic fossils in glacial sediments beneath ice sheets provide relatively coarse chronostratigraphic control, but they do contain direct evidence of regional deglaciation. Thus, sub-glacial till samples provide the evidence regarding the source of ice sheet variability seen in well-dated proxy records. Combined, these independent data sets can provide a more comprehensive and less speculative interpretation of the history of past glacial minima in currently glaciated polar regions.

Scherer, Reed P.↗

NASA NTRSDate not supplied

Extraterrestrial Amino Acids in the C2 Ungrouped Carbonaceous Chondrite Tarda: A Unique Distribution

Introduction: Meteorites provide a record of the chemical processes that occurred in the early solar sys-tem. The delivery of organic matter by carbonaceous chondrites to the early Earth could have been an im-portant source of amino acids and other prebiotic or-ganic molecules required for the emergence of life [1]. To date, 96 amino acids have been named in meteor-ites, most of which are rare or absent in the biosphere [2]. The variability in amino acid concentrations and isomer distributions measured in carbonaceous chon-drites can be explained by differences in parent body chemistry and alteration conditions [3]. In addition, aqueous alteration was likely important in the amplifi-cation of some left-handed amino acids over their right-handed forms (L-excesses up to ~60%) in the Tagish Lake meteorite [3], suggesting that the origin of life on Earth and possibly elsewhere in the solar sys-tem was biased towards L-amino acids. On August 25, 2020, a fireball was witnessed in southern Morocco and the first pieces of the meteorite fall were recovered the following day near the town of Tarda [4]. Thousands of individual fragments were recovered from the strewn field totaling ~4 kg [4]. Based on petrographic observations, bulk mineralogy, and chemical and O-isotopic analyses, Tarda has been classified as a C2 ungrouped carbonaceous chondrite with similarities to Tagish Lake (though more primi-tive), as well as CI, CY, and CR chondrites [4,5]. The fall and rapid recovery of the Tarda stones provide an important opportunity to investigate a C-rich meteorite using the state-of-the-art techniques that will also be used to study the samples returned from asteroids Ryugu and Bennu by the Hayabusa2 and OSIRIS-REx missions, respectively. Here, we report the first amino acid analyses of the Tarda meteorite. The total concentrations, enantio-meric ratios and relative distributions of amino acids were determined using ultrahigh performance liquid chromatography with UV fluorescence and time-of-flight mass spectrometry (LC-FD/ToF-MS) at NASA GSFC. This analytical technique was employed on extracts from two different pre-rain Tarda meteorite fragments and a sample of sand collected from the Tarda fall site for the purpose of assessing potential terrestrial contamination. Bulk H, C, and N elemental and isotopic analyses were also conducted at the Car-negie Institution for Science (CIS) to assist with petro-logic type classification using elemental analysis iso-tope ratio mass spectrometry (EA-IRMS).

D P Glavin↗

NASA NTRSDate not supplied

Salty Bennu: Halite and Sylvite in Fines Returned by OSIRIS-REx

Salt phases such as halite (NaCl) and sylvite (KCl) are important hosts of halogens (e.g., Br and I) and volatile alkali metals (i.e., Na, K) and are very rare in extra-terrestrial materials. Halite and sylvite have been discovered in carbonaceous chondrite clasts in ordinary chondrite meteorite falls Zag H3-6 regolith breccia (Zag), Monahans (1998) breccia (Monahans), and Sidi El Habib H5 chondrite, as well as on the surface of particles from asteroid Itokawa. Yet, it is uncertain whether the evaporites are indigenous or exogenous to their respective parent bodies, and their origin is debated. Analysis of the extinct 129I/129Xe decay system and 40Ar/39Ar show that halite in Zag may have formed within the first 8 Myr of the solar system, possibly from aqueous fluids during local and episodic low-temperature alteration on the parent body. Halite and sylvite in Zag are characterized by light δ37Cl values of −1.7 ‰. Halite in Zag could have been transported from the outer solar system, perhaps a P- or D-class asteroid, as indicated by O, H, N, and C isotopes. An alternative is a cometary or interstellar ice origin due to their deuterium-rich and 16O-poor composition.

N E Timms↗

NASA NTRS1987

Rare earth patterns in shergottite phosphates and residues

Leaching experiments with 1M HCl on ALHA 77005 powder show that rare earth elements (REE) are concentrated in accessory phosphate phases (whitlockite, apatite) that govern the REE patterns of bulk shergottites. The REE patterns of whitlockite are typically light REE-depleted with a negative Eu anomaly and show a hump at the heavy REE side, while the REE pattern of apatite (in Shergotty) is light REE-enriched. Parent magmas are calculated from the modal compositions of residues of ALHA 77005, Shergotty, and EETA 79001. The parent magmas lack a Eu anomaly, indicating that plagioclase was a late-stage crystallizing phase and that it probably crystallized before the phosphates. The parent magmas of ALHA 77005 and Shergotty have similar REE patterns, with a subchondritic Nd/Sm ratio. However, the Sm/Nd isotopoics require a light REE-depleted source for ALHA 77005 (if the crystallization age is less than 600 Myr) and a light REE-enriched source for Shergotty. Distant Nd and Sr isotopic signatures may suggest different source regions for shergottites.

Laul, J. C.↗

NASA NTRSDate not supplied

The Aguas Zarcas Breccia - Similarities to Surface Features of C-Complex Asteroids Ryugu and Bennu

Knowledge of the materials originally present in the proto-solar nebula is one of the most significant constraints for models concerning the formation of our solar system. The physicochemical properties of these original materials can be determined through two distinct and complementary approaches: astronomical observations of nascent stellar systems and the dense clouds from which they originated; and laboratory studies of available astromaterials such as meteorites, interplanetary dust particles, comets, as well as returned mission samples (e.g., from Hayabusa -1 -2;OSIRIS -Rex). Laboratory study of these extraterrestrial materials provides valuable information about the first solids portion of the early solar system and their evolution. Here, we present petrographic and mineralogical characteristics of the polymic beccias Aguas Zarcas based on the research results of [1].On April 23, 2019, at 21:07 local time, a meteorite fall occurred in Aguas Zarcas, San Carlos County, Alajuela province, Costa Rica. The rapid recovery of this brecciated carbonaceous chondrite after its fall provides an opportunity to study a freshly fallen, relatively uncontaminated, and highly brecciated meteorite to compare with samples returned by theHayabusa2 and OSIRIS-REx spacecraft from C-group asteroids. The study includes several pre-rain fragments [1].The X-ray Computed Tomography (XCT) results show many different lithologies. We describe the petrography and mineralogy of five different lithologies of the Aguas Zarcas meteorite. We also present data on bulk oxygen isotopes. We describe all fragments in detail and attempt to classify each lithology in order to understand the origin and formation history of the Aguas Zarcas parent body. Our results show that some lithologies of Aguas Zarcas are similar to those in CM chondrites, but others are unique. The different lithologies [1] also represent different degrees of hydration and heating, which are good analogues for the types of materials returned by the Bennu and Ryugu asteroids. Spectroscopic observations of the Ryugu and Bennu asteroids compared to laboratory measurements of meteorites suggest that the asteroids have some similarities to heated CM, heated CI, or CI chondrites[2-5]. Both asteroids are thought to be composed of materials altered by aqueous alteration (e.g., [5]) and formed by reaccretion after destruction by impacts and brecciation (e.g., [6-7]). Considering the different lithologies in Aguas Zarcas [2] and other CM chondrites [8-9], these types of carbonaceous chondrites can be considered good analogues for samples from the Ryugu and Bennu asteroids. The presence of unique and rare lithologies in the Aguas Zarcas, different from the typical CM chondrite lithologies, suggests complex mixing of different materials in a highly dynamic environment.

Imene Kerraouch↗

NASA NTRS2020

Condensation, Partial Melting and Evaporation Processes Influence the Bulk Compositions of Spinel-Cored Spherules in the CO3.1 Chondrite Miller Range 90019

Here we focus on spinel-cored spherule calcium-aluminum rich inclusions (CAI), dominantly ~75-80 microns in diameter in the CO3.1 chondrite Miller Range 90019, which make up ~ 12 % of the fine-grained CAIs in one thin section. Their mineralogical content ranges from rare grossite- and hibonite-bearing varieties, through perovskite-melilitebearing, to fassaite-bearing and finally anorthitebearing. Non-spherical CAIs have been divided into 4 other groups, defined based on mineralogical abundances. We also characterized a group of AOAs from this sample. No glass has been recognized in any inclusions. Some relatively evolved members (anorthite-, spinel- + fassaite-bearing) among the spherules are found engulfed in AOAs. We characterized the bulk compositions of ~145 CAIs and AOAs in this meteorite, derived from EDS-x-ray mapping of the inclusions. We determined bulk compositions both with and without Wark-Lovering rims (when present), which are largely composed of diopside  forsterite. The balance of the inclusions appear to have not been melted or partially melted, but rather they have textures that indicate they are condensates, often modified by extensive reaction with nebular gases. This presents the opportunity to examine effects on the bulk compositions of spherules resulting potentially from melting plus evaporation. Other aspects of this suite of refractory inclusions have been discussed in these abstracts. Oxygen isotope variations in one spherule were presented in [4]. The latter study showed a complex history of reaction with nebular gases possessing a variety of Oisotope compositions. Additional O isotopic studies of inclusions in this work are included in Mane et al.

Ross, D. K.↗

NASA NTRS1992

On the isotopic signature of recent solar-wind nitrogen

One of the most intriguing discoveries yielded by the Apollo samples was evidence pointing towards a significant long-term change in the composition of the sun. Such a change, of the size inferred from the lunar sample data, is inconsistent with present theories of solar evolution. Consequently, there is much interest in exploring this phenomenon as closely as possible, to determine exactly what compositional changes have taken place and whether those changes really did take place in the sun, or whether the cause lies elsewhere. The reason why we can use the moon to analyze the elements in the sun is that the sun emits a stream of ions, known as the solar wind, whose composition, on average, is believed to be the same as that in the surface regions of the sun. When the solar-wind ions hit the surface of the moon, many of them penetrate a short distance into the dust grains lying on the lunar surface. Thus, after a grain has sat on the lunar surface for a while, it has a rim of material that is partly lunar and partly solar in composition. For most chemical elements, the difference between lunar and solar composition is so sufficiently small that the solar elements cannot be detected, but for a handful of elements that are missing from the moon, their solar 'signature' can be observed in samples of lunar soil brought back by the astronauts. Among those elements is nitrogen, the most common element in the air we breathe, but very rare indeed on the moon. Our analytical techniques are not sophisticated enough yet to enable us to analyze individual lunar soil grains for nitrogen, much less to zero in on just the nitrogen in the surface of such a grain. Consequently we are forced to analyze samples consisting of many different grains, each of which could have experienced its own individual history. This makes it difficult to identify the nitrogen implanted in grain surfaces, and also to define the age of a sample.

Kim, Y.↗

NASA NTRSDate not supplied

The Aguas Zarcas Breccia - Similarities to Surface Features of Asteroids Ryugu and Bennu

At 21:07 local time on 23rd April 2019, a meteorite fall occurred in Aguas Zarcas, San Carlos County, Alajuela province, Costa Rica. The rapid recovery of this brecciated carbonaceous chondrite after its fall provides an opportunity to investigate a freshly-fallen, relatively uncontaminated and highly-brecciated meteorite for comparison to the samples returned from C-group asteroids by the Hayabusa2 and OSIRIS-REx spacecraft. The study includes several pre-rain fragments. Our X-ray Computed Tomography(XCT) results show many different lithologies. In this study we describe the petrography and mineralogy of five different lithologies of the Aguas Zarcas meteorite. We also present bulk oxygen isotope data of some lithologies and results concerning the organic matter. We describe all the fragments in detail and attempt a classification of each lithology in order to understand the origin and the history of formation of the Aguas Zarcas' parent body. Our results show some lithologies of Aguas Zarcas similar to those in CM chondrites, but others are unique. The different lithologies [1] represent different levels of hydration and heating as well, which are good analogues for the types of materials returned from asteroids Bennu and Ryugu. Spectroscopic observations of the Ryugu and Bennu asteroids compared to laboratory measurements of meteorites suggest that the asteroids show some similarities to heated CM, heated CI or, CI chondrites [2-5]. Both asteroids are regarded to consist of materials altered by aqueous alteration (e.g., [5]) and formed by re-accretion after impact destruction and brecciation (e.g., [6-7]). Considering the various different lithologies in Aguas Zarcas [2] and other CM chondrites [8-9] these types of carbonaceous chondrites may be regarded as good analogues for samples from the Ryugu and Bennu asteroids. The presence of unique and rare lithologies in the Aguas Zarcas, that are distinct from typical CM chondrite lithologies, indicates a complex mixing of various materials in a highly dynamic environment.

Imene Kerraouch↗

NASA NTRS2004

Shock Effects in the Metal-rich Chondrites QUE 94411, Hammadah al Hamra 237 and Bencubbin

We have studied the metal-rich chondrites QUE94411 (QUE), Hammadah al Hamra 237 (HH237) and Bencubbin with an emphasis on the petrographical and mineralogical effects of the shock processing that these meteorite assemblages have undergone. Iron-nickel metal and chondrule silicates are the main components in these meteorites. These high-temperature components are held together by shock melts consisting of droplets of dendritically intergrown FeNi metal/sulfide embedded in ferrous silicate glass. The silicate glass component of the shock melt is substantially more FeO-rich (FeO: 30 to 40 wt%) than the prevailing chondrule silicates (FeO less than 5 wt%). Fine-grained matrix material, which is a major component in most other chondritic assemblages, is extremely scarce in QUE4411 and HH237; it has not been observed in Bencubbin. This material occurs as rare, hydrated, and fine-grained matrix lumps with major and minor element abundances roughly similar to the ferrous silicate shock melts (and CI). We infer that hydrated, fine-grained material, compositionally similar to these matrix lumps, was originally present between the FeNi metal grains and chondrules, but was preferentially heated by the shock wave and melted. Other shock-related features in QUE94411, HH237, and Bencubbin include a distinct alignment and occasionally strong plastic deformation of metal and chondrule fragments. However, the existence of chemically zoned and metastable FeNi metal condensates condensates in direct contact with shock melts indicates that the shock did not substantially increase the average temperature of the aggregate. TEM analyses of the FeNi metal droplets embedded in the shock melts show martensitic crystal structures, indicating rapid cooling, essentially quenching, of the shock melts. These findings indicate that the preshock temperature of the assemblage was low. Because porphyritic olivine-pyroxene chondrules are absent in QUE94411, HH237, and Bencubbin, it is difficult to determine the precise shock stage of these meteorites, but the shock was probably relatively light (S2- S3; 5-20 GPa) consistent with a bulk temperature increase of the assemblages of less than 200 C. The higher pressures within this range (i.e. 15-20 GPa) are consistent with the findings of shock produced diamonds in Bencubbin. The many mineralogical, petrological and isotopic (e.g. O and N) similarities between Bencubbin, Weatherford, Gujba and QUE/HH237 have been used to argue for a common origin of these meteorites on a single asteroidal parent body. The findings of this study, which indicate similar shock processing, lend support to this view.

Meibom, A.↗

NASA NTRS2009

Rare Earth Element Measurements of Melilite and Fassaite in Allende Cai by Nanosims

The rare earth elements (REEs) are concentrated in CAIs by approx. 20 times the chondritic average [e.g., 1]. The REEs in CAIs are important to understand processes of CAI formation including the role of volatilization, condensation, and fractional crystallization [1,2]. REE measurements are a well established application of ion microprobes [e.g., 3]. However the spatial resolution of REE measurements by ion microprobe (approx.20 m) is not adequate to resolve heterogeneous distributions of REEs among/within minerals. We have developed methods for measuring REE with the NanoSIMS 50L at smaller spatial scales. Here we present our initial measurements of REEs in melilite and fassaite in an Allende Type-A CAI with the JSC NanoSIMS 50L. We found that the key parameters for accurate REE abundance measurements differ between the NanoSIMS and conventional SIMS, in particular the oxide-to-element ratios, the relative sensitivity factors, the energy distributions, and requisite energy offset. Our REE abundance measurements of the 100 ppm REE diopside glass standards yielded good reproducibility and accuracy, 0.5-2.5 % and 5-25 %, respectively. We determined abundances and spatial distributions of REEs in core and rim within single crystals of fassaite, and adjacent melilite with 5-10 m spatial resolution. The REE abundances in fassaite core and rim are 20-100 times CI abundance but show a large negative Eu anomaly, exhibiting a well-defined Group III pattern. This is consistent with previous work [4]. On the other hand, adjacent melilite shows modified Group II pattern with no strong depletions of Eu and Yb, and no Tm positive anomaly. REE abundances (2-10 x CI) were lower than that of fassaite. These patterns suggest that fassaite crystallized first followed by a crystallization of melilite from the residual melt. In future work, we will carry out a correlated study of O and Mg isotopes and REEs of the CAI in order to better understand the nature and timescales of its formation process and subsequent metamorphic history.

Ito, M.↗

NASA NTRS2013

The Petrology and Geochemistry of Feldspathic Granulitic Breccia NWA 3163: Implications for the Lunar Crust

Lunar meteorites are crucial to understand the Moon s geological history because, being samples of the lunar crust that have been ejected by random impact events, they potentially originate from areas outside the small regions of the lunar surface sampled by the Apollo and Luna missions. The Apollo and Luna sample sites are contained within the Procellarum KREEP Terrain (PKT, Jolliff et al., 2000), where KREEP refers to potassium, rare earth element, and phosphorus-rich lithologies. The KREEP-rich rocks in the PKT are thought to be derived from late-stage residual liquids after approx.95-99% crystallization of a lunar magma ocean (LMO). These are understood to represent late-stage liquids which were enriched in incompatible trace elements (ITE) relative to older rocks (Snyder et al., 1992). As a consequence, the PKT is a significant reservoir for Th and KREEP. However, the majority of the lunar surface is likely to be significantly more depleted in ITE (84%, Jolliff et al., 2000). Lunar meteorites that are low in KREEP and Th may thus sample regions distinct from the PKT and are therefore a valuable source of information regarding the composition of KREEP-poor lunar crust. Northwest Africa (NWA) 3163 is a thermally metamorphosed ferroan, feldspathic, granulitic breccia composed of igneous clasts with a bulk anorthositic, noritic bulk composition. It is relatively mafic (approx.5.8 wt.% FeO; approx.5 wt.% MgO) and has some of the lowest concentrations of ITEs (17ppm Ba) compared to the feldspathic lunar meteorite (FLM) and Apollo sample suites (Hudgins et al., 2011). Localized plagioclase melting and incipient melting of mafic minerals require localized peak shock pressures in excess of 45 GPa (Chen and El Goresy, 2000; Hiesinger and Head, 2006). NWA 3163, and paired samples NWA 4481 and 4883, have previously been interpreted to represent an annealed micro-breccia which was produced by burial metamorphism at depth in the ancient lunar crust (Fernandes et al., 2009). This is in contrast to the interpretation of Hudgins et al. (2009) where NWA 3163 was interpreted to have formed through contact metamorphism. To further constrain its origin, we examine the petrogenesis of NWA 3163 with a particular emphasis on in-situ measurement of trace elements within constituent minerals, Sm-Nd and Rb-Sr isotopic systematics on separated mineral fractions and petrogenetic modeling.

McLeod, C. L.↗