CONSTRAINTS ON MARTIAN DIFFERENTIATION PROCESSES FROM RB-SR AND SM-NDISOTOPIC ANALYSES OF THE BASALTIC SHERGOTTITE QUE-94201

Isotopic analyses of mineral, leachate, and whole rock fractions from the Martian shergottite meteorite QUE 94201 yield Rb-Sr and Sm-Nd crys tallization ages of 327 +/- 12 and 327 +/- 19 Ma, respectively. These ages are concordant, although the isochrons are defined by different f ractions within the meteorite. Comparison of isotope dilution Sm and N d data for the various QUE 94201 fractions with in situ ion microprobe data for QUE 94201 minerals from the literature demonstrate the prese nce of a leachable crustal component in the meteorite. This component is likely to have been added to QUE 94201 by secondary alteration proc esses on Mars and can affect the isochrons by selectively altering the isotopic systematics of the leachates and some of the mineral fractio ns. Initial Sr-87/Sr-86 Of 0.701298 +/- 14, epsilon(Nd)(143) Of +47.6 +/- 1.7, and whole rock epsilon(Nd)(142) Of +0.92 +/- 0.11 indicate th at QUE 94201 was derived from a source that was strongly depleted in R b-87/Sr-86 and enriched in Sm-147/Nd-144 early in its history. Modelin g demonstrates that the Sm-Nd isotopic compositions of QUE 94201 can b e produced by either four episodes of melting at 327 Ma of cumulates c rystallized 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 isot opic systematics of QUE 94201 are not consistent with significant melt ing between 4.525 Ga and 327 Ma. The estimated timing of these events is based on initial neodymium isotopic ratios and is independent of di fferentiation of the QUE 94201 parental magma, Rb-Sr-based partial mel ting models are unable to reproduce the composition of QUE 94201 using the same model parameters employed in the Sm-Nd-based models, implyin g a decoupling of Rb-Sr and Sm-Nd isotopic systems. The initial decoup ling of the two isotopic systems can be attributed to either cumulate or crust formation processes which are able to more efficiently fracti onate Rb from Sr compared to Sm from Nd. The fact that all Martian met eorites 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 i n the cumulate pile or into the crust at that time. Thus, the Martian mantle cumulates and restites are not expected to evolve past Sr-87/Sr -86 Of 0.700 and could not have been significantly enriched in incompa tible elements by crustal recycling processes. All Martian meteorites have initial Sr-87/Sr-86 values that are higher than similar to 0.700 and are, therefore, likely to be produced by mixing between evolved cr ustal-like and depleted mantle reservoirs. The absence of crustal recy cling processes on Mars may preserve the geochemical evidence for deco upling of the Rb-Sr and Sm-Nd isotopic systems, underscoring one of th e fundamental differences between geologic processes on Mars and the E arth. Copyright (C) 1997 Elsevier Science Ltd.