Constraints on the Formation of Metal and Sulphide in Iron Meteorites as Inferred From Nickel Isotopes

Details Constraints on the Formation of Metal and Sulphide in Iron Meteorites as Inferred From Nickel Isotopes Constraints on the Formation of Metal and Sulphide in Iron Meteorites as Inferred From Nickel Isotopes

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.p41e..02q&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #P41E-02

Mathematics

Logic

0454 Isotopic Composition And Chemistry (1041, 4870), 1028 Composition Of Meteorites (3662, 6240), 1155 Extinct Radionuclide Geochronology, 1160 Planetary And Lunar Geochronology, 6240 Meteorites And Tektites (1028, 3662)

Scientific paper

Iron meteorites formed early in solar system history, some of them being apparently even as old as the calcium aluminium rich inclusions (CAIs) found in chondrites and generally considered the oldest objects in the solar system. Short-lived chronometers are particularly well suited for studying the exact timescale for metal-silicate differentiation and core formation in planetary bodies. Among them, the 60Fe radionuclide that decays to 60Ni is of special interest. First, its short half-life (t1/2 = 1.49 Myr) provides precise chronological constraints on the crystallization of iron meteorites. Second, the amount of live 60Fe, a potential heat source for planetary melting, can be estimated at the time of metal-silicate differentiation. Here we present nickel isotopic data for more than 30 iron meteorites. All metals have the same isotopic composition as the standard within error. No variations in radiogenic 60Ni are detected in the metal phase indicating that iron meteorites crystallized more than 4.3 Myrs after the start of the solar system, in good agreement with other isotopic studies. Ten sulphides have also been analysed. Several of them show an excess of 61Ni that correlates with a deficit of 60Ni, despite very high Fe/Ni ratios. The 61Ni/58Ni vs. 60Ni/58Ni correlation is interpreted as a mixing between a pure s-process component and terrestrial-like nickel. Thus, sulphides carry a nucleosynthetic anomaly that is probably too diluted in metal to be detected. Nickel stable isotopes provide insights into the formation processes of metal and sulphide. Generally speaking, sulphides and metals plot on the same mass dependant fractionation line: the solar system formed from well mixed material on average even if some sulphides show the presence of admixed abnormal material. In most cases, the sulphide is heavier than the corresponding metal. In the case of IIICD irons, however, sulphides are lighter than metals supporting the idea that these non magmatic irons formed in a different way, possibly by impact at the surface of the parent body. Nickel-poor kamacite and Ni-rich taenite do not display the same isotopic composition. The isotopic composition of our bulk iron samples may therefore depend on their mineralogy. This observation makes the interpretation of the stable isotope data in the framework of crystallization models difficult.

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