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 U-Pb isotopic system from the purest mineral fractions (maskelynite and pyroxene). The concordance of these three ages suggest that the U-Pb 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 value of the Zagami source region can be estimated, with some confidence from the U-Pb isochron, to be 3.96 0.02. Disturbance of the U-Pb isotopic systems means that this represents a minimum value. The value of the Zagami source is significantly lower than the values estimated for most basaltic magma sources from Earth and the Moon. This is surprising given the high initial Sr/Sr ratio (0.721566 82) and low initial 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 Pb/Pb ratio. Such a contaminant could be produced by removal of single-stage Pb from a relatively high martian reservoir before 1.8 Ga, and therefore could be an ancient manifestation of hydrous alteration of martian 0016-7037/05 $30.00 .00 surface material. Copyright © 2005 Elsevier Ltd