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

Uranium-lead ratios (commonly represented as U-238/Pb-204 = mu) calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of mu values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range (Pb-206/Pb-204 = 11.16-11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in Pb-206/Pb-204-Pb-207/Pb-204-Pb-208/Pb-204 compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial Pb. This terrestrial Pb contamination generated a Pb-206-Pb-207 array in the QUE fractions that appears to represent an ancient age, which contrasts with a much younger crystallization age of 327 +/- 10 Ma derived from Rb-Sr and Sm-Nd isochrons (Borg L. E., Nyquist L. E., Taylor L. A., Wiesmann H. and Shih C. -Y. (1997) Constraints on Martian differentiation processes from Rb-Sr and Sm-Nd isotopic analyses of the basaltic shergottite QUE 94201. Geochim. Cosmochim. Acta 61, 4915-4931). Despite the contamination, and accepting 327 +/- 10 Ma as the crystallization age, we use the U-Pb data to determine the initial Pb-206/Pb-204 of QUE 94201 to be 11.086 +/- 0.008 and to calculate the mu value of its mantle source to be 1.82 +/- 0.01. The mu value calculated for the QUE 94201 source is the lowest determined for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that mu values in martian source reservoirs vary by at least a factor of two. Additionally, the range of source mu values indicates that the mu value of bulk silicate Mars is approximately three. The amount of variation in the mu values of the mantle sources (mu similar to 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate greater extents of U-Pb fractionation during formation of the mantle sources of martian basalts. (C) 2007 Elsevier Ltd. All rights reserved.