The age of Dar al Gani 476 and the differentiation history of the martian meteorites inferred from their radiogenic isotopic systematics

Abstract

Samarium–neodymium isotopic analysis of the martian meteorite Dar al Gani 476 yields a crystallization age of 474 ± 11 Ma and an initial ε Nd 143 value of +36.6 ± 0.8. Although the Rb-Sr isotopic system has been disturbed by terrestrial weathering, and therefore yields no age information, an initial 87Sr/ 86Sr ratio of 0.701249 ± 33 has been estimated using the Rb-Sr isotopic composition of the maskelynite mineral fraction and the Sm-Nd age. The Sr and Nd isotopic systematics of Dar al Gani 476, like those of the basaltic shergottite QUE94201, are consistent with derivation from a source region that was strongly depleted in incompatible elements early in the history of the solar system. Nevertheless, Dar al Gani 476 is derived from a source region that has a slightly greater incompatible enrichment than the QUE94201 source region. This is not consistent with the fact that the parental magma of Dar al Gani 476 is significantly more mafic than the parental magma of QUE94201, and underscores a decoupling between the major element and trace element-isotopic systematics observed in the martian meteorite suite. Combining the ε Nd 142-ε Nd 143 isotopic systematics of the martian meteorites yields a model age for planetary differentiation of 4.513 +0.033 −0.027 Ga. Using this age, the parent/daughter ratios of martian mantle sources are calculated assuming a two-stage evolutionary history. The calculated sources have very large ranges of parent/daughter ratios ( 87Rb/ 86Sr = 0.037–0.374; 147Sm/ 144Nd = 0.182–0.285; 176Lu/ 177Hf = 0.028–0.048). These ranges exceed the ranges estimated for terrestrial basalt source regions, but are very similar to those estimated for the sources of lunar mare basalts. In fact, the range of parent/daughter ratios calculated for the martian meteorite sources can be produced by mixing between end-members with compositions similar to lunar mare basalt sources. Two of the sources have compositions that are similar to olivine and pyroxene-rich mafic cumulates with variable proportions of a Rb-enriched phase, such as amphibole, whereas the third source has the composition of liquid trapped in the cumulate pile (i.e. similar to KREEP) after ∼99% crystallization. Correlation between the proportion of trapped liquid in the meteorite source regions and estimates of f O 2 , suggest that the KREEP-like component may be hydrous. The success of these models in reproducing the martian meteorite source compositions suggests that the variations in trace element and isotopic compositions observed in the martian meteorites primarily reflect melting of the crystallization products of an ancient magma ocean, and that assimilation of evolved crust by mantle derived magmas is not required. Furthermore, the decoupling of major element and trace element-isotopic systematics in the martian meteorite suite may reflect the fact that trace element and isotopic systematics are inherited from the magma source regions, whereas the major element abundances are limited by eutectic melting processes at the time of magma formation. Differences in major element abundances of parental magma, therefore, result primarily from fractional crystallization after leaving their source regions.