U-pb Isotope Systematics of Shergottite Queen Alexandra Range 94201: Seeing through Terrestrial Lead Contamination to Identify an Even Lower-mu Source on Mars

Introduction: Basaltic shergottite Queen Alexandra Range 94201 originated from an extremely depleted mantle source, and has the highest initial εNd (+ 47.6 ± 1.7) and the lowest initial Sr/Sr (0.701298 ± 14) measured for any martian meteorite to date [1]. Rb-Sr and Sm-Nd isochrons for this sample yield concordant crystallization ages of 327 ± 12 Ma and 327 ± 19 Ma, respectively [1]. Knowledge of the crystallization age provides a context in which to discuss new evidence for a very low U/Pb (‘μ’) source in the martian mantle, as well as the effects of terrestrial contamination on U-Pb and Pb-Pb isotope systematics in martian meteorites. Analytical techniques: We were allocated a 325 mg fraction of QUE 94201, a martian meteorite collected in Antarctica. Initial sample preparation and leaching were completed at University of New Mexico, and sample digestion, column chemistry and mass spectrometry were completed at The University of Texas. The sample was crushed in a sapphire mortar and pestle, and the coarsest (74-150 μm) grain size fraction was separated into pure (>99%) mineral separates and reject mineral fractions by a combination of magnetic separation and hand-picking. All fractions were washed with 0.5 M acetic acid followed by QD water. All fractions were leached for 10 minutes at room temperature in HCl (2N for the silicates and 1N for the oxides), and rinsed with QD water. The HCl leachates + final water rinse were combined, processed and analyzed for U and Pb isotopic compositions. The mineral fractions were dissolved with HF+HNO3+HCl. Pb was purified on anion resin columns using HBr and HCl. U was purified with UTEVA U-specific resin and HNO3. Samples were run on a Finnegan 261 thermal ionization mass spectrometer, in static mode and using all Faraday cups. Procedural blanks for dissolution + column chemistry are less than 2.5 pg Pb, and sample-blank ratios range from 135 to 3260 for the mineral fractions. Total uncertainties on the Pb isotopic compositions reflect the combination of uncertainties determined for the internal analytical precision, and corrections for mass fractionation and laboratory blank, and are typically less than 0.1% for Pb/Pb and Pb/Pb, and are less than 0.05% for Pb/Pb. Results: The Pb isotopic compositions of the leached silicate fractions are very unradiogenic and define a narrow range (Pb/Pb = 11.16-11.61, Pb/Pb = 11.47-11.72). In contrast, the oxide mineral fractions and leachates have Pb isotopic compositions significantly more radiogenic and variable than the leached silicate mineral fractions. On a conventional uranogenic Pb-Pb diagram (Fig. 1), all the data define a single linear array, the slope of which corresponds to an age of 4325 ± 19 Ma. The data also define a single linear array in Pb/PbPb/Pb space. However, the composition of Pb in Antarctic ice falls on both these arrays. We interpret these arrays to be the result of contamination by Antarctic ice, and therefore assign no age significance to the uranogenic Pb-Pb array. The mineral fractions have U/Pb compositions that are both very low and quite limited (1.2 – 4.3), so that minimal quantities of Pb and Pb have been produced by radioactive decay since the rock crystallized at 327 Ma. Thus, the mineral fractions effectively represent a single point in Pb-Pb space. Variable amounts of contamination of these mineral fractions by terrestrial Pb results in a 2-component mixing line. Furthermore, this sample contains exceedingly low abundances of Pb (WR(R) + WR(L) has 0.16 ppm Pb), and thus is particularly susceptible to contamination. Simple binary mixing models are consistent with 2-7 % Antarctic contamination in the pure silicate fractions, and 6-87 % contamination in the other mineral fractions and leachates.