The Accretion and Differentiation of Mars: Trace element constraints

The aim of this study is to use the increasing amounts of information on the composition of the Martian surface as a means to determine the history of Martian accretion and differentiation. The approach requires an estimate of the average composition of the Martian mantle combined with experimental data on the partitioning of a wide range of elements between metallic and silicate liquids and between silicate crystals and melts. When applied to the Earth this methodology indicates that Earth started accreting as a small reduced body and progressively added more oxidised and volatile-rich material as it grew. We began by testing the Martian mantle composition proposed by Dreibus and Wänke (D&W; [1]) using experimental phase equilibrium data [2] to test whether the observed basaltic compositions can be derived from such a mantle. We find that the experimentally observed phase compositions allow calculation of a simple differentiation path, dominated by olivine and pyroxene fractionation, which explains most major and minor element compositional trends in both the Martian surface basalts (Gusev Crater and Meridiani Planum) and the SNC basaltic meteorites: in short, the Martian samples may be derived from a Martian mantle similar to that proposed by D&W (i.e. richer in FeO than the terrestrial mantle). Using the major element model as a starting point, we then calculated trace-element fractionation trends, using experimentally determined mineral-melt partition coefficients, in order to back-calculate the trace-element concentrations in the Martian mantle. In agreement with previous work, our calculated Martian mantle contains higher Cr and lower Ni and Mo than the Terrestrial mantle. Differences in Ni (and Zn) between the Martian surface samples and the SNCs are most likely due to hydrothermal alteration of the former (e.g., serpentinization in the case of increased Ni in the Martian surface samples). We also calculate that the Martian mantle has similar or lower contents of moderately volatile (e.g. K2O and Na2O) and volatile components (e.g. Ga, Pb) than those in Earth's mantle, indicating that Mars is not as volatile-rich compared to the Earth as previously thought. Overall it appears that Mars and Earth were constructed from similar compositional mixtures of planetesimals. Given the similarities in their overall planetary compositions, the compositional differences between the mantles of Mars and Earth principally reflect differences in their conditions of accretion and core formation. We used experimental metal-silicate partitioning data to model element distributions between core and mantle. These indicate that Mars accreted under conditions of lower pressure and temperature and higher oxygen fugacity than Earth. 1. Dreibus, G. and H. Wanke, Meteoritics 20, 367-381 (1985). 2. Bertka, C.M. and J.R. Holloway, Contributions To Mineralogy And Petrology 115, 323-338 (1994).