Constraints on Martian differentiation processes from RbSr and SmNd isotopic analyses of the basaltic shergottite QUE 94201

Uranium–lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201

“…This presumably reflects the contribution of phosphate to the leachates. In the 238 U-206 Pb system, the line with a slope corresponding to the age closest to the crystallization age of 327 Ma (Borg et al, 1997 Pb/ 204 Pb plot can reflect either radiogenic ingrowth such that the array has chronometric significance, or variable mixtures of two components, in which case the array has no chronometric significance. The array defined by the QUE fractions is likely to reflect various mixtures of martian Pb and modern terrestrial Pb because: 1) all fractions analyzed in this study define a single linear array, the extension of which passes through the isotopic composition of modern terrestrial Pb (including Pb in Antarctic ice, Fig.…”

“…In fact, QUE represents the end-member in the compositional continuum of martian meteorites. It has the lowest initial 87 Sr/ 86 Sr ratio, the highest initial ε Nd value (Borg et al, 1997), and the most light rare earth element (LREE)-depleted REE pattern (e.g., Lodders, 1998) of all martian meteorites. The other end of the continuum is defined by meteorites such as Zagami.…”

“…In contrast to QUE, Zagami has the highest initial 87 Sr/ 86 Sr ratios, lowest initial ε Nd value , and one of the most LREE-enriched REE patterns. The enormous range of incompatible trace element abundances and Sr and Nd isotopic compositions observed in the martian meteorite suite has been attributed to extreme elemental fractionation in their source regions as a result of solidification of a martian magma ocean (e.g., Borg et al, 1997;Blichert-Toft et al, 1999;Brandon et al, 2000;Borg and Draper, 2003;Borg et al, 2003). In this hypothesis, the meteorites with trace element and isotopic signatures indicative of derivation from the most depleted source regions represent differentiates of melts of early-formed magma-ocean cumulates consisting primarily of olivine and pyroxene.…”

“…Thus, the only way to determine the Pb isotopic compositions of the source regions is to evaluate the U-Pb isotope systematics of the meteorites in the context of their crystallization ages determined with other age dating techniques. The crystallization age of QUE is well-defined to be 327 ± 10 Ma (weighted average of Rb-Sr and Sm-Nd ages determined by Borg et al, 1997) and belongs to a growing group of ~330 Ma martian basalts that includes NWA 1195 (Symes et al, 2005) and NWA 1460 (Nyquist et al, 2004). Thus, even the partially disturbed U-Pb isotope systematics of QUE will provide constraints on the U-Pb isotopic evolution of the QUE source region.…”

Isotopic links between atmospheric chemistry and the deep sulphur cycle on Mars

“…The degree to which Martian magmas may have assimilated crustal material, thus altering the geochemical signatures acquired from their mantle sources, is unclear 1 . This issue features prominently in efforts to understand whether the source of light rare-earth elements in enriched shergottites lies in crustal material incorporated into melts 1,2 or in mixing between enriched and depleted mantle reservoirs 3 . Sulphur isotope systematics offer insight into some aspects of crustal assimilation.…”