The composition and thickness of the crust of Mars estimated from rare earth elements and neodymium-isotopic compositions of Martian meteorites

Abstract

Isotopic and trace element compositions of Martian meteorites show that early differentiation of Mars produced complementary crustal and mantle reservoirs that were sampled by later magmatic events. This paper describes a mass balance model that estimates the rare earth element (REE) content and thickness of the crust of Mars from the compositions of shergottites. The diverse REE and Nd isotopic compositions of shergottites are most easily explained by variable addition of light rare earth element (LREE)-enriched crust to basaltic magmas derived from LREE-depleted mantle source regions. Antarctic shergottites EET 79001, ALH 77005, LEW 88516, and QUE 94201 all have strongly LREE-depleted patterns and positive initial ε143Nd isotopic compositions, which is consistent with the generation of these magmas from depleted mantle sources and little or no interaction with enriched crust. In contrast, Shergotty and Zagami have negative initial ε143Nd isotopic compositions and less pronounced depletions of the LREE, which have been explained by incorporation of enriched crustal components into mantle-derived magmas (Jones, 1989; Longhi, 1991; Borg et al., 1997). The mass balance model presented here derives the REE composition of the crustal component in Shergotty by assuming it represents a mixture between a mantle-derived magma similar in composition to EET 79001A and a LREE-enriched crustal component. The amount of crust in Shergotty is constrained by mixing relations based on Nd-isotopic compositions, which allows the REE pattern of the crustal component to be calculated by mass balance. The effectiveness of this model is demonstrated by the successful recovery of important characteristics of the Earth's continental crust from terrestrial Columbia River basalts. Self-consistent results for Nd-isotopic compositions and REE abundances are obtained if Shergotty contains ~10-30% of LREE-enriched crust with >10 ppm Nd. This crustal component would have moderately enriched LREE (Sm/Nd = 0.25-0.27; 147Sm/144Nd = 0.15-0.17; La/Yb = 2.7-3.8), relatively unfractionated heavy rare earth elements (HREE), and no Eu anomaly. Crust with these characteristics can be produced from a primitive lherzolitic Martian mantle by modest amounts (2-8%) of partial melting, and it would have a globally averaged thickness of ≤45 km, which is consistent with geophysical estimates. Mars may serve as a laboratory to investigate planetary differentiation by extraction of a primary basaltic crust.