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Constraints on mantle geochemistry and planetary differentiation derived from Fe isotopes

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Recent advances in mass spectrometry (specifically multi-collector inductively coupled plasma mass spectrometry; MC-ICPMS) allow for precise measurements of the stable isotope compositions of “heavy” elements such as the transition metals. In this talk I will present variations in the iron isotope compositions of igneous rocks, meteorites and high-pressure experiments and discuss the significance of these results with respect to mantle geochemistry and the differentiation of the terrestrial planets.

Part i) Iron isotopes in mantle peridotites and eclogites

Large variations exist in the iron isotope compositions (δ57/54Fe, deviation in parts per 1000 with respect to the IRMM -14 pure Fe standard) of mantle peridotites from different tectonic settings (0.9‰) although the range in the δ57/54Fe values of MORB and OIB is comparatively small (0.1‰; average MORB 0 .14 ±0.06 ‰). Mineral separates prepared from the same mantle peridotites and pyroxenites show a surprisingly large range in δ57/54Fe (olivines 0.6‰, clinopyroxenes 0.9‰ and orthopyroxenes 0.8‰.), with spinels showing the greatest total variation of 1.7‰. There are positive correlations between the δ57/54Fe values of coexisting orthopyroxene, clinopyroxene and olivine, strongly suggesting that the δ57/54Fe values of these minerals reflect intra-sample mineral-mineral isotopic equilibrium. Bulk-rock, clinopyroxene and spinel δ57/54Fe values correlate with chemical indices of both melt extraction and oxidation. Coupled with simple models, these data suggest that iron isotope fractionation takes place during spinel-facies partial melting, with the residue becoming isotopically light relative to the melt and to the initial source region.

The extent to which Fe isotopes might fractionate during partial melting and melt-rock reaction in the garnet stability field was investigated in a follow-up study of fresh eclogite xenoliths originating from South Africa. Surprisingly large isotopic variations were observed. The δ57/54Fe values of the eclogite garnets analysed range between –0.41 and 0.61 ‰ whereas those of co-existing pyroxenes range from –0.21 to 0.57 ‰. Calculated bulk rock δ57/54Fe values range from -0.59 to 0.33 ‰, and encompass a total variation of 0.92 ‰. Oxygen isotope measurements were also carried out on these samples and positive correlations between both garnet and bulk sample δ18O and δ57Fe are present. These correlations suggest that both Fe and O isotopes are fractionated by the same underlying process in this sample suite. Comparisons of these data with published data for altered and fresh MORB and for samples from the sheeted dike part of the ODP 504B drill core suggest that these co-variations in Fe and O isotopes were not inherited from oceanic crust protoliths. However, we observed that the samples with the isotopically lightest δ18O and δ57Fe values are also enriched in Sc and Cr, elements which become concentrated in the residues of melting. We therefore suggest that Fe and O isotopes are fractionated by either partial melting or by melt-rock reaction processes, which could take place either in the lithospheric mantle or in the downgoing slab.

Part ii) Iron isotopes in iron meteorites and high pressure experiments

Magmatic iron meteorites are considered to be remnants of the metallic cores of differentiated asteroids, and may be used as analogues of planetary core formation. Sulphur is believed to be a significant light element in the cores of iron meteorite parent bodies and the terrestrial planets, although its exact abundance remains unknown. We have found evidence for significant equilibrium Fe isotope fractionation (0.26 ‰/amu) between metal and troilite (FeS) in iron meteorites. Coupled with published data for pallasites, which constrains the magnitude of metal-silicate Fe isotope fractionation, Fe isotopes may be used to provide information about the S contents of cores of the iron meteorite parent bodies and the terrestrial planets. However, it is not yet clear if the observed fractionations can be extrapolated to the pressure and temperature conditions of planetary core formation. In order to test this we are currently investigating Fe isotope fractionation between silicate melt and liquid Fe-S alloys and between liquid iron and basaltic melt using high-pressure experiments.

Weyer, S. Anbar, A.,G.P. Brey, G., Munker, C, Mezger, K. and Woodland, A.B. Iron isotope fractionation during planetary differentiation, Earth and Planetary Science Letters 240(2), 251-264, 2005.

Williams, H.M., Nielsen, S.G., Renac, C, Griffin, W.L., O’Reilly, S.Y., McCammon, C., Pearson, N., Viljoen F, Alt, J.C. (2009) Fractionation of oxygen and iron isotopes in the mantle: implications for crustal recycling and the source regions of oceanic basalts. Earth and Planetary Science Letters, 283 (1-4): 156-166. Williams, H. M., Markowski, A., Quitté, G, Halliday, A. N., Teutch, N. and Levasseur, S. (2006): Fe isotope fractionation in iron meteorites: New insights into metal-sulphide segregation and planetary accretion. Earth andPlanetary Science Letters, 250, 486–500 Williams, H. M., Peslier, A., McCammon, C., Halliday, A.N. Teutch, N., Levasseur, S., and Burg, J.-P. (2005):Iron isotope fractionation in mantle minerals, partial melting and mantle oxygen fugacity. Earth and PlanetaryScience Letters, 235, 435-452 Williams, H. M., McCammon, C., Peslier, A., Halliday, A.N. Teutsch, N. Levasseur, S., and Burg, J.-P. (2004):Iron Isotope Fractionation and the Oxygen Fugacity of the Mantle. Science, 304, 1656-1659.

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