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3.2 g. Fe3+ content of eclogite diamond inclusions (C. McCammon, in collaboration with I. Chinn and J.J. Gurney/Cape Town and M.E. McCallum/Colorado)

Inclusions in diamond provide chemical and mineralogical information about the source region and the genesis of diamond. Many analytical tools are available to obtain detailed information on major and trace element chemical compositions of inclusions, but few are able to distinguish between different oxidation states, for example between Fe2+ and Fe3+, which is crucial for the determination of redox conditions during mineral formation. Mössbauer spectroscopy is an ideal method to distinguish between Fe2+ and Fe3+, and can provide an estimate of  their relative abundance in different phases. Normally experiments are performed on samples approximately one cm in diameter, but a technique developed at the Bayerisches Geoinstitut (see Annual Report 1993) now enables routine Mössbauer measurements to be performed on samples as small as 100 µm in diameter.

The George Creek diamonds were taken from the K1 kimberlite located in the Colorado Wyoming province. Inclusions are dominantly of the eclogitic paragenesis, and contain (in order of abundance) clinopyroxene, garnet, rutile and ilmenite, and rare moissanite and quartz/coesite. Temperatures are estimated to be 1071 - 1178 °C at 5 GPa using non-touching pairs of garnet-clinopyroxene, while touching pairs of garnet-clinopyroxene and clinopyroxene-orthopyroxene give lower temperatures of 912 - 977 °C. The garnet inclusions studied have compositions Al37-40Py32-38Gr25-31 while the pyroxene inclusions are close to the Jd-(Di+Hd+Ts) join with jadeite compositions varying from 8 to 46 %.

Mössbauer measurements show that both phases contain relatively small amounts of Fe3+: values for Fe3+/Σ Fe in garnet range from 0 to 7 %, while values for clinopyroxene vary from 0 to 6 %. Values are positively correlated for coexisiting mineral pairs. The bimineralic eclogite system lacks sufficient constraints to calculate oxygen fugacity directly, but some generalisations can be made. The inclusions likely equilibrated within the fO2 stability field of diamond, which ranges from below iron-wüstite to above quartz-fayalite-magnetite at 5 GPa. Since many samples are known with significantly higher amounts of Fe3+ and there is no crystal chemical barrier to the incorporation of large amounts of Fe3+ in these garnet and clinopyroxene compositions, the low amount of Fe3+ in these inclusions would imply that they formed in conditions of relatively low oxygen fugacity.

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