Mathematics
Scientific paper
Jan 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998lpico.957...53w&link_type=abstract
Origin of the Earth and Moon, Proceedings of the Conference held 1-3 December, 1998 in Monterey, California. LPI Contribution N
Mathematics
Earth Mantle, Mars (Planet), Planetary Geology, Planetary Mantles, Planetary Structure, Planetary Cores, Planetary Composition, Planetary Evolution, Snc Meteorites, Terrestrial Planets
Scientific paper
New bulk compositional data for six martian meteorites, including highly siderophile elements Ni, Re, Os, Ir, and Au [ I ], are utilized along with literature data for comparison with the siderophile systematics of igneous rocks from Earth, the Moon, and the HED asteroid. Queen Alexandra Range 94201 and EET 79001-B are both depleted in the "noblest" siderophiles (Os and Ir) to roughly 0.00001 x Cl chondrites. Whether the siderophile-poor composition of ALH 84001 reflects a more reducing environment on primordial Mars when this ancient rock first crystallized or secondary alteration is unclear. Among martian rocks, as in terrestrial igneous rocks, Ni, Os, and Ir (Fig. 1) show strong correlations vs. MgO. The martian MgO vs. Ni trend is displaced toward lower Ni by a large factor (S), but the Os and Ir trends are not significantly displaced from their terrestrial counterparts. Martian Re also shows a rough correlation with MgO, indicating compatible behavior, in contrast to its mildly incompatible behavior on Earth. Among martian MgO-rich rocks, Au shows an anticorrelation vs. MgO, resembling the terrestrial distribution except for a displacement toward 2-3x lower Au. The same elements (Ni, Re, Os, Ir, and Au) show similar correlations with Cr substituted for MgO. These trends are exploited to infer the compositions of the primitive 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 primitive mantle concentration of the siderophile element. Results for Earth show good agreement with earlier estimates. For Mars, the implied primitive mantle composition is remarkably similar to the Earth's, except for 5x lower Ni. The best constrained of the extremely siderophile elements, Os and Ir, are present in the martian mantle at 0.5x CI, in comparison to 0.7x CI in Earth's mantle. This similarity constitutes a key constraint of the style of core-mantle differentiation on both Mars, and Earth. successful models should predict similarly high concentrations of noble siderophile elements in both the martian and terrestrial mantles ("high" compared to the lunar and HED mantles and to models of simple partitioning at typical low pressure magmaic 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 the aftermath of a Moon-forming giant 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 inefficient on the largest terrestrial planets, because during the later stages of accretion, these bodies acquired sufficient H20 to oxidize most of the later accreting Fe metal, thus eliminating the carrier phase for segregation of siderophile elements into the core.
Kallemeyn Gregory W.
Kyte Frank T.
Warren Harry P.
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