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

Abstract

Isotopic analyses of mineral, leachate, and whole rock fractions from the Martian shergottite meteorite QUE 94201 yield RbSr and SmNd crystallization ages of 327 ± 12 and 327 ± 19 Ma, respectively. These ages are concordant, although the isochrons are defined by different fractions within the meteorite. Comparison of isotope dilution Sm and Nd data for the various QUE 94201 fractions with in situ ion microprobe data for QUE 94201 minerals from the literature demonstrate the presence of a leachable crustal component in the meteorite. This component is likely to have been added to QUE 94201 by secondary alteration processes on Mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractions. Initial 87Sr/ 86Sr of 0.701298 ± 14, ϵ Nd 143 of +47.6 ± 1.7, and whole rock ϵ Nd 142 of +0.92 ± 0.11 indicate that QUE 94201 was derived from a source that was strongly depleted in 87Rb/ 86Sr and enriched in 147Sm/ 144Nd early in its history. Modeling demonstrates that the SmNd isotopic compositions of QUE 94201 can be produced by either four episodes of melting at 327 Ma of cumulates crystallized from a magma ocean at 4.525 Ga or five episodes of melting of an initially solid Mars at 4.525 Ga and 327 Ma. The neodymium isotopic systematics of QUE 94201 are not consistent with significant melting between 4.525 Ga and 327 Ma. The estimated timing of these events is based on initial neodymium isotopic ratios and is independent of differentiation of the QUE 94201 parental magma. Rb-Sr-based partial melting models are unable to reproduce the composition of QUE 94201 using the same model parameters employed in the SmNd-based models, implying a decoupling of RbSr and SmNd isotopic systems. The initial decoupling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fractionate Rb from Sr compared to Sm from Nd. The fact that all Martian meteorites analyzed so far define a RbSr whole rock isochron age of 4.5 Ga suggests that virtually all Rb was partitioned out of their mantle source regions and into either fractionated residual liquids trapped in the cumulate pile or into the crust at that time. Thus, the Martian mantle cumulates and restites are not expected to evolve past 87 Sr 86 Sr of 0.700 and could not have been significantly enriched in incompatible elements by crustal recycling processes. All Martian meteorites have initial 87 Sr 86 Sr values that are higher than ∼0.700 and are, therefore, likely to be produced by mixing between evolved crustal-like and depleted mantle reservoirs. The absence of crustal recycling processes on Mars may preserve the geochemical evidence for decoupling of the RbSr and SmNd isotopic systems, underscoring one of the fundamental differences between geologic processes on Mars and the Earth.