Physics and Chemistry of Minerals

, Volume 34, Issue 9, pp 647–656 | Cite as

Decompression and unmixing of crystals included in diamonds from the mantle transition zone

Original Paper

Abstract

A suite of exceptional mineral inclusions in diamonds from the São Luiz river, Juina province, Brazil, shows a wide range of garnet/majorite mineral compositions co-existing with clinopyroxene; the overall bulk compositions are eclogitic. The inclusions have a wide variety of textural arrangements, but crystallographic data obtained by EBSD shows that each inclusion consists of a single garnet with constant crystallographic orientation whilst clinopyroxene grains have preferred orientation with relation to garnet {110} and <111>. This suggests that the inclusions were originally single phase majoritic garnets, and that they preserve various states of progressive unmixing (exsolution) into lower pressure garnet and clinopyroxene compositions during transport of the host diamonds towards the Earth’s surface. On the basis of high pressure–temperature experimental data some of the original majoritic garnets must have come from depths of 450 km or more, and therefore resided in the transition zone and asthenospheric upper mantle. Particularly extensive re-equilibration of many inclusions took place at depths of ca 180–200 km (probably close to the base of the continental lithosphere). The partially unmixed state of the inclusions provides a unique opportunity for using mineral diffusion data to roughly estimate the rate of transport through the asthenospheric upper mantle, and within error this rate is found to be broadly compatible with expected transport rates by upper mantle convection or plume flow.

Keywords

Majorite Exsolution Eclogite Garnetite EBSD 

Notes

Acknowledgments

We thank Jeff Harris and De Beers Consolidated Mines for access to these remarkable inclusions. The help of colleagues, John Craven, Mike Hall and Peter Hill in EMMAC (Edinburgh Materials and Micro-analysis Centre) in analysis and sample preparation is gratefully acknowledged. BH wishes to thank David Prior (Liverpool University) for introducing him to the EBSD technique. Maya Kopylova and Thomas Stachel are thanked for constructive reviews. The work was done under NERC Grants GR3/8562 AND NER/H/S/1998/00008.

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Copyright information

© Springer-Verlag 2007

Authors and Affiliations

  1. 1.Grant Institute of Earth Science, School of GeoSciencesUniversity of EdinburghEdinburghUK

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