Diamond exploration and mantle structure imaging using PIXE microanalysis

Abstract

The compositions of garnets, sampled throughout the earth's upper mantle and carried to the surface in volcanic eruptions, show PIXE trace element signatures that reveal details of the chemistry, thermal structure, and history of fluid alteration, of their source rocks. Trace Ni concentration (5–150 ppm) is strongly temperature dependent, providing a useful geothermometer (T_Ni). Cr concentration can be used to estimate pressure (P_Cr) in Cr-saturated rocks. Trace Zn in chromite also provides a measure of temperature (T_Zn), and shows the extent of Cr-saturation (chromite present). For those garnets from a particular volcanic pipe originating in Cr-saturated rocks in the upper-mantle, T_Ni, P_Cr maps out the paleogeotherm (the Garnet Geotherm), that is the relationship between T and P (and depth) prevailing at the time of eruption. Ti and trace Zr, Y, and Ga show the effects of metasomatic fluids and deep mantle-derived melts, and show clearly the depth extent of depleted (Y < 10 ppm) rocks which helps to define the effective base of the lithosphere, the earth's solid outer shell. The Garnet Geotherm and the base of the lithosphere then define the range of depths within the diamond stability field traversed by the erupting magma, and the histogram of T_Ni provides a direct measure of the sampling in this range and hence the diamond potential of the pipe. The result is both a powerful diamond exploration tool and a method to map the composition and evolution of the earth's mantle.