Petrography and geochemistry of the enriched basaltic shergottite Northwest Africa 2975

Abstract

Abstract-We present a study of the petrology and geochemistry of basaltic shergottite Northwest Africa 2975 (NWA 2975). NWA 2975 is a medium-grained basalt with subophitic to granular texture. Electron microprobe (EMP) analyses show two distinct pyroxene compositional trends and patchy compositional zoning patterns distinct from those observed in other meteorites such as Shergotty or QUE 94201. As no bulk sample was available to us for whole rock measurements, we characterized the fusion crust and its variability by secondary ion mass spectrometer (SIMS) measurements and laser ablation inductively coupled plasma spectroscopy (LA-ICP-MS) analyses as a best-available proxy for the bulk rock composition. The fusion crust major element composition is comparable to the bulk composition of other enriched basaltic shergottites, placing NWA 2975 within that sample group. The CI-normalized REE (rare earth element) patterns are flat and also parallel to those of other enriched basaltic shergottites. Merrillite is the major REE carrier and has a flat REE pattern with slight depletion of Eu, parallel to REE patterns of merrillites from other basaltic shergottites. The oxidation state of NWA 2975 calculated from Fe-Ti oxide pairs is NNO-1.86, close to the QFM buffer. NWA 2975 represents a sample from the oxidized and enriched shergottite group, and our measurements and constraints on its origin are consistent with the hypothesis of two distinct Martian mantle reservoirs: a reduced, LREE-depleted reservoir and an oxidized, LREE-enriched reservoir. Stishovite, possibly seifertite, and dense SiO 2 glass were also identified in the meteorite, allowing us to infer that NWA 2975 experienced a realistic shock pressure of~30 GPa.

Validity of the Apatite/Merrillite Relationship in Evaluating the Water Content in the Martian Mantle: Implications from Shergottite Northwest Africa (NWA) 2975

“…The petrogenesis of NWA 2975 is related to two distinct mantle magma reservoirs: LREE-enriched and LREE-depleted [41]. Ancient mantle heterogeneity and progressive crystallization of ascending magmas derived from the mantle may produce zoned pyroxene.…”

“…Crustal contamination cannot be excluded, as isotope data on the rock sample are unavailable. However, crystallization of the highly evolved magmatic liquid of NWA 2975 [29,30,41] should result in merrillite with a Fe-rich composition. The lower Fe content may point to simultaneous Fe-rich pyroxene and merrillite crystallization and to an Fe-rich competition between them.…”

“…It is difficult to relate the anomaly to oxygen fugacity because it was defined as changeable by He et al [30] during NWA 2975 basalt crystallization. The investigated F-rich apatite from NWA 2975 demonstrates a negative Eu anomaly, which most likely resulted from prior plagioclase fractionation.…”

Determination of volatile concentrations in fluorapatite of Martian shergottite NWA 2975 by combining synchrotron FTIR, Raman spectroscopy, EMPA, and TEM, and inferences on the volatile budget of the apatite host‐magma

“…; He et al. ), along with zoned pyroxene, maskelynite (a glass phase with plagioclase composition), other accessory minerals, and glass veins/pockets. Wittke et al.…”

“…He et al. () reported the REE patterns of merrillite but not of apatite. Genetically, the phosphates are considered as crystallization products of residual melts.…”