Constraints on the U-Pb isotopic systematics of Mars inferred from a combined U-Pb, Rb-Sr, and Sm-Nd isotopic study of the Martian meteorite Zagami

Uranium-lead, Rb-Sr, and Sm-Nd isotopic analyses have been performed on the same whole-rock, mineral, and leachate fractions of the basaltic martian meteorite Zagami to better constrain the U-Pb isotopic systematics of martian materials. Although the Rb-Sr and Sm-Nd systems define concordant crystallization ages of 166 ± 6 Ma and 166 ± 12 Ma, respectively, the U-Pb isotopic system is disturbed. Nevertheless, an age of 156 ± 6 Ma is derived from the ²³⁸U-²⁰⁶Pb isotopic system from the purest mineral fractions (maskelynite and pyroxene). The concordance of these three ages suggest that the ²³⁸U-²⁰⁶Pb systematics of the purest Zagami mineral fractions have been minimally disturbed by alteration and impact processes, and can therefore be used to constrain the behavior of U and Pb in the Zagami source region. The μ value of the Zagami source region can be estimated, with some confidence from the ²³⁸U-²⁰⁶Pb isochron, to be 3.96 ± 0.02. Disturbance of the U-Pb isotopic systems means that this represents a minimum value. The μ value of the Zagami source is significantly lower than the μ values estimated for most basaltic magma sources from Earth and the Moon. This is surprising given the high initial ⁸⁷Sr/⁸⁶Sr ratio (0.721566 ± 82) and low initial ε_Nd value (−7.23 ± 0.17) determined for Zagami that indicate that this sample is derived from one of the most highly fractionated reservoirs from any known planetary body. This suggests that Mars is characterized by a low bulk planet U/Pb ratio, a feature that is consistent with its relatively volatile-rich nature.The leachates contain terrestrial common Pb that was probably added to the meteorite during handling, curation, or sawing. The mineral fractions, particularly those with significant amounts of impact melt glass, contain a second contaminant. The presence of this contaminant results in Pb-Pb ages that are older than the crystallization age of Zagami, indicating that the contaminant is characterized by a high ²⁰⁷Pb/²⁰⁶Pb ratio. Such a contaminant could be produced by removal of single-stage Pb from a relatively high μ martian reservoir before ∼1.8 Ga, and therefore could be an ancient manifestation of hydrous alteration of martian surface material.