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

Isotopic and trace element compositions of Martian meteorites show that ear ly differentiation of Mars produced complementary crustal and mantle reserv oirs that were sampled by later magmatic events. This paper describes a mas s balance model that estimates the rare earth element (REE) content and thi ckness of the crust of Mars from the compositions of shergottites. The dive rse REE and Nd isotopic compositions of shergottites are most easily explai ned by variable addition of light rare earth element (LREE)-enriched crust to basaltic magmas derived from LREE-depleted mantle source regions. Antarc tic shergottites EET 79001, ALH 77005, LEW 88516, and QUE 94201 all have st rongly LREE-depleted patterns and positive initial epsilon(143)Nd isotopic compositions, which is consistent with the generation of these magmas from depleted mantle sources and little or no interaction with enriched crust. I n contrast, Shergotty and Zagami have negative initial epsilon(143)Nd isoto pic compositions and less pronounced depletions of the LREE, which have bee n explained by incorporation of enriched crustal components into mantle-der ived magmas (Jones, 1989; Longhi, 1991; Borg er al., 1997). The mass balance model presented here derives the REE composition of the cr ustal component in Shergotty by assuming it represents a mixture between a mantle-derived magma similar in composition to EET 79001A and a LREE-enrich ed crustal component. The amount of crust in Shergotty is constrained by mi xing relations based on Nd-isotopic compositions, which allows the REE patt ern of the crustal component to be calculated by mass balance. The effectiv eness 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 similar to 10-30% of LREE-enriched crust wi th >10 ppm Nd. This crustal component would have moderately enriched LREE ( Sm/Nd = 0.25-0.27; Sm-147/Nd-144 = 0.15-0.17; La/Yb = 2.7-3.8), relatively unfractionated heavy rare earth elements (HREE), and no Eu anomaly. Crust w ith these characteristics can be produced from a primitive lherzolitic Mart ian mantle by modest amounts (2-8%) of partial melting, and it would have a globally averaged thickness of less than or equal to 45 km, which is consi stent with geophysical estimates. Mars may serve as a laboratory to investi gate planetary differentiation by extraction of a primary basaltic crust.