Proton probe quantitative trace elemental microscopy of kyanite bearing eclogytic samples

Abstract

Bi-mineralic eclogites (clinopyroxene and garnet) commonly occur as xenoliths in kimberlites and are generally thought to be the product of subduction of oceanic crust. Less common and less well understood are the kyanite-bearing eclogites and in particular grospydites. Grospydites were first observed in Siberian kimberlite pipes from the Daldy-Akalkit kimberlite field and specifically the Zagadochnaya Pipe. The name “grospydite” is a descriptive term for Ca- and Al-rich eclogites made up of the rock-forming mineral's gro ssular, py roxene and kyanite ( di sthen) at which the grossular component of garnet is at least > 50 mol%. One of the outstanding questions regarding these rocks, concerns the distribution of trace element concentrations such as Rb and Sr in gropydite forming minerals. In particular the concentrations of these elements in kyanite are virtually unknown, although the concentrations should be in the sub-ppm range. A knowledge of their concentrations is important for future RbSr isotopic studies. A feature of the samples that make them geologically unique but difficult to analyse is that they exhibit many polymineralic phases separated by alteration material on the micron scale. The proton induced X-ray emission (PIXE) technique was used for in-situ, non-destructive quantitative mapping of Rb and Sr trace elemental distributions in the different minerals. The focused beam (15 μm spot) could be scanned over the sample covering an area of up to 2.5 × 2.5 mm 2 to generate elemental distributions as maps. In addition, the scan area could be reduced to an arbitrary line, an arbitrary shape, or simply a spot to improve statistical accuracy while following the complicated shapes of the various phases present in the mineral. The data were also collected in list-mode allowing off-line replaying of the measurement using different scan masks to generate X-ray spectra unique to a given phase. The X-ray spectra were analysed in detail by the GUPIX computer programme to deliver trace elemental concentrations appropriate to each phase. The trace elemental concentration determinations were complemented by major and minor elemental analyses using the electron probe, and this provided an internal calibration via the Fe content of the minerals.