Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..372j&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 372
Other
Earth, Early, Osmium Isotopes, Snc Meteorites
Scientific paper
We are measuring Os isotopes in SNC meteorites, which may represent samples from the planet Mars, and comparing this with the general isotopic systematics observed on Earth. The moderately siderophile elements are considerably more variable in their abundances on Mars. The Ni/Co ratio is about 5-6 on Mars compared with 21 in the Earth's mantle. Cobalt is slightly depleted and Ni is strongly depleted in the martian mantle compared to Earth. It appears that Zn, Cu, and In have also been depleted in the martian mantle. Highly siderophile elements are also not in chondritic abundances on Mars. Chassigny, an olivine-rich SNC meteorite, has a Re/Os ratio that is about half the chondritic ratio, while the Os isotopic composition of Chassigny is higher than the general range of the chondritic Os isotopes. It is evident that the Os isotopes are not supported by Re in Chassigny. An Os evolution model for Chassigny may be embedded in a general isotopic model for the SNC meteorites. The Rb-Sr isotopic systematic as well as the Pb-Pb, Sm-Nd, and U-Pb systematics of SNC meteorites clearly show that complicated multistage models can be excluded. In the Rb-Sr system the SNC meteorites plot close to the 4.55-Ga isochron. The Pb isotopes measured in maskelinite plot close to the geochron. In a concordia diagram the trend of the data also suggests that a two-stage model--an early differentiation and a late differentiation, where the Rb/Sr ratio should not be changed drastically--might comply with the data. To model the Os evolution, we therefore use a two-stage evolution. At 1.3 Ga the internal evolution of Chassigny stopped and a straight line leads to the initial differentiation of Mars. The interesting result of our investigation is the slope of this line. This slope tells us with good reason that the Re/Os ratios on Mars were 30% higher than in the Earth's mantle. The chondritic Re-Os ratio might lie somewhere in between those of Mars and Earth. The sources of Chassigny and ALHA 77005 seem to be distinct and seem to have formed at the early differentiation of Mars. The martian noble metal pattern seems to be the fingerprint of the core formation. In contrast, the noble metal content of the Earth's mantle might be dominated by a late veneer that does not seem to be incorporated into the martian mantle. This may indicate that Mars was not homogenized after the accretional differentiation, which is in line with results from the other isotopic systems. Figure 1 appears here in the hard copy.
Jagoutz Emil
Luck Mck. J.
Wanke Heinrich
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