Inclusions in diamond and the mineral chemistry of the upper mantle

Abstract

During the last decade significant progress regarding our understanding of the mineralogy and geochemistry of the earth's upper mantle has been achieved through studies of the ultramafic and eclogitic xenoliths in kimberlite, and of mineral inclusions in diamond. The ultramafic and eclogitic xenoliths in kimberlite represent fragments of various horizons of the mantle that have been rapidly transported to the surface by the mechanism of kimberlite eruption. Textural differences in these mineralogically simple ultramafic xenoliths (lherzolite, harzburgite, pyroxenite) are in part reflected in chemical differences between the constituent minerals. Thus many xenoliths which display a “fluidal” or “mosaic” texture indicative of intense deformation are much less depleted in Ti, Al, Ca, Fe and Na than xenoliths in which granular or coarse tabular textures predominate. Interestingly, there is generally a contrast in chemistry between all the mineral constituents of the xenoliths and similar ones that occur as inclusions in diamonds. Typically, inclusions in diamond are monominerallic and homogeneous. Olivine and orthopyroxene inclusions have very small compositional differences in spite of the host diamonds being markedly different in geologic age and geographic locality. Characteristically, two distinct, but always separate, suites of minerals occur as inclusions. One suite resembles the minerals that are present in ultramafic xenoliths, whereas the second is similar to minerals in eclogitic xenoliths. Thus it is likely that natural diamond can form in at least two distinct chemical environments in the mantle. Much of the above data has been combined with the results of experimental phase equilibria and theoretical studies with the conclusion that it is now possible to suggest conditions of pressure and temperature for the genesis of these xenoliths as well as diamond. All the pressure and temperature values are consistent with final equilibration at depths of the order of 150–200 km in the mantle.