Diamonds and other ultrahigh pressure (UHP) minerals have been reported previously from the Luobusa ophiolite of Tibet, but these minerals have thus far been found only as individual grains. Here we report the occurrence of diamond as an inclusion in OsIr alloy and coesite as part of a silicate assemblage rimming a grain of FeTi alloy, both of which were recovered from chromitite. These occurrences confi rm the presence of UHP minerals in the Luobusa chromitite requiring minimum pressures of ~2.8-4 GPa. Individual coesite "crystals" have external form similar to that of stishovite and are polycrystalline, suggesting pseudo morphic replacement and implying a pressure >9 GPa. We propose that the UHP minerals were incorporated into the chromitites in the deep upper mantle or that they have an impact origin; the preponderance of evidence favors the former.

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Eclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027-1427°C, confining pressures of 2.5-3.5 GPa, and strain rates of 1 · 10 )5 s )1 to 5 · 10 )4 s )1 . We obtained a power-law creep for the high temperature and pressure deformation of a ÔdryÕ eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A ¼ 10 3.3 ± 1.0 , n ¼ 3.5 ± 0.4, DE ¼ 403 ± 30 KJ mol )1 and DV ¼ 27.2 cm 3 mol )1 . The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back-scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear-strain conditions, yielding a random LPO similar to that of non-deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110}[001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature.