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Plastic deformation of lower mantle diamonds by inclusion phase transformations

¹ School of GeoSciences, University of Edinburgh, West Mains Road, Edinburgh EH9 3JW, UK
² Geochemical Laboratory, Graduate School of Science, University of Tokyo, Tokyo 113-0033, Japan

^* Corresponding author, e-mail: ben.harte{at}ed.ac.uk

The changes in pressure (P) and temperature (T) conditions experienced by a natural diamond, as it rises to shallower depths and is eventually erupted at the Earth’s surface, give rise to stresses between the diamond and any inclusions it contains as a consequence of their differing changes in volume cell parameters in response to P and T. In the case of diamonds with lower-mantle silicate inclusions such effects will be markedly enhanced by phase transformations by which MgSi-perovskite and CaSi-perovskite transform to pyroxene, pyroxenoid and other phases. Using micro-Raman and SEM-EBSD techniques, we have carried out investigations of the diamond immediately adjacent to inclusions with lower mantle perovskite chemical compositions, and compared them with studies of lower-mantle ferropericlase and upper-mantle inclusions.

In the Raman studies evidence of shifts in frequency and width in the diamond Raman spectral band at 1333 cm^–1 were sought and found to be greater for perovskite than ferropericlase. However, even for MgSi-perovskite the effects only indicated stored residual pressures of ca 0.35 GPa, which have also been found for typical upper mantle inclusions. The EBSD studies showed a marked contrast between diamond adjacent to former perovskite (now transformed to pyroxene and other phases) and ferropericlase. EBSD mapping shows lattice distortions in diamond adjacent to former MgSi-perovskite and CaSi-perovskite of 4 to 7°; but adjacent to ferropericlase the distortion was only 1–2°. The results show that the large strain changes accompanying decompression and phase transformation of the perovskite inclusions were largely accommodated by plastic strain within the diamonds at high temperatures, and only small residual pressures were retained in the diamonds.

Key-words: diamonds, lower mantle, deformation haloes, residual pressure, inclusion, Raman.