Origin of planetary cores: Evidence from highly siderophile elements in martian meteorites

We present new bulk compositional data for 6 martian meteorites, including highly siderophile elements Ni, Re, Os, Ir and Au. These and literature dat a are utilized for comparison versus the siderophile systematics of igneous rocks from Earth, the Moon, and the HED asteroid. The siderophile composit ion of ALK84001 is clearly anomalous. Whether this reflects a more reducing environment on primordial Mars when this ancient rock first crystallized, or secondary alteration, is unclear. QUE94201 shows remarkable similarity w ith EET79001-B for siderophile as well as lithophile elements; both are ext raordinarily depleted in the "noblest" siderophiles (Os and Ir), to roughly 0.00001 X CI chondrites. As in terrestrial igneous rocks, among martian ro cks Ni, Os and Ir show strong correlations vs. MgO. In the case of MgO vs. Ni, the martian trend is displaced toward lower Ni by a large factor (5), b ut the Os and Ir trends are not significantly displaced from their terrestr ial counterparts. For Mars, Re shows a rough correlation with MgO, indicati ng compatible behavior, in contrast to its mildly incompatible behavior on Earth. Among martian MgO-rich rocks, Au shows a weak anticorrelation vs. Mg O, resembling the terrestrial distribution except for a displacement toward 2-3 times lower Au. The same elements (Ni, Re, Os, Ir and Au) show similar correlations with Cr substituted for MgO. Data for lunar and HED rocks gen erally show less clear-cut trends (relatively few MgO-rich samples are avai lable). These trends are exploited to infer the compositions of the primiti ve Earth, Mars, Moon and HED mantles, by assuming that the trend intercepts the bulk MgO or Cr content of the primitive mantle at the approximate prim itive mantle concentration of the siderophile element. Results for Earth sh ow good agreement with earlier estimates. For Mars, the implied primitive m antle composition is remarkably similar to the Earth's, except for 5 times lower Ni. The best constrained of the extremely siderophile elements, Os an d Zr, are present in the martian mantle at 0.005 times CI, in comparison to 0.007 times CI in Earth's mantle. This similarity constitutes a key constr aint on the style of core-mantle differentiation in both Mars and Earth. Su ccessful models should predict similarly high concentrations of noble sider ophile elements in both the martian and terrestrial mantles ("high" compare d to the lunar and HED mantles, and to models of simple partitioning at typ ical low-pressure magmatic temperatures), but only predict high Ni for the Earth's mantle. Models that engender the noble siderophile excess in Earth' s mantle through a uniquely terrestrial process, such as a Moon-forming gia nt impact, have difficulty explaining the similarity of outcome (except for Ni) on Mars. The high Ni content of the terrestrial mantle is probably an effect traceable to Earth's size. For the more highly siderophile elements like Os and Ir, the simplest model consistent with available constraints is the veneer hypothesis. Core-mantle differentiation was notably inefficient on the largest terrestrial planets, because during the final similar to 1% of accretion these bodies acquired sufficient H2O to oxidize most of the l ater-accreting Fe-metal, thus eliminating the carrier phase for segregation of siderophile elements into the core. Copyright (C) 1999 Elsevier Science Ltd.