Chemical systematics of the shergotty meteorite and the composition of its parent body (Mars)

Abstract

We report chemical data for 60 elements by INAA and RNAA in two bulk samples, for 30 elements in various mineral separates of Shergotty, and results of leaching experiments with 1M HCl on powdered aliquots of Shergotty and BETA 79001, lithologies A and B. Shergotty is homogeneous in major element composition but heterogeneous with respect to LIL trace elements (~20%). The heterogeneity is even greater for volatile and siderophile trace elements. The mineral data, including three clinopyroxene fractions with variable FeO contents, maskelynite and minor phases (Ti-magnetite, ilmenite, quartz, K-rich phase), show that major minerals do not account for the rare earth elements (REE) in the bulk meteorite. Instead, the REE are to a large extent concentrated in accessory whitlockite and apatite (shown by leaching with 1M HCl): together with the majority of REE (La, 96%, Yb 70%), Cl and Br are quantitatively dissolved by leaching. The REE patterns of the leachate of Shergotty and EETA 79001 are different. The Shergotty leachate may consist of two components. Component l is similar to that of EETA 79001 leachate (whitlockite), component 2 is enriched in light REE and may be responsible for the higher LREE contents of Shergotty in comparison to the other shergottites. There is some evidence that Shergotty was an open system and component 2 was introduced after crystallization.The REE patterns of the residues of Shergotty and EETA 79001 are identical indicating that the parent magmas of both meteorites are compositionally similar. Based on cpx separates with the lowest REE content, the REE pattern in the Shergotty parent magma was calculated. It is enriched in LREE and has a subchondritic NdSm ratio. The negative Eu anomaly in the phosphates indicates that at least some plagioclase crystallized before phosphate.Based on several element correlations in SNC meteorites, it was suggested (Dreibus and Wänke, 1984) that both the Shergotty parent body (SPB, very probably Mars), and the Earth accreted from the same two chemically different components: component A, highly reduced and devoid of volatile elements and an oxidized component B containing also volatile elements. The SPB (Mars) mantle is 2–4 times richer in volatile and moderately siderophile elements than the Earth, indicating a higher portion of component B in the SPB. The concentrations of chalcophile elements in the SPB mantle are low, reflecting equilibration with a sulfide phase and subsequent segregation of sulfide into the core. Unlike the Earth (Wänke, 1981), the SPB (Mars) may therefore have accreted almost homogeneously.