Nuclear microprobe analysis of melt inclusions in minerals: Windows on metasomatic processes in the earth's mantle

Rare samples of melts and fluids, responsible for metasomatic change and evolution of the earth's upper mantle, and preserved as melt inclusions, pose special problems to quantitative analysis. Elements such as Zr, Nb, Th and the rare-earth elements (REE) can be highly concentrated into rare minor phases beneath the surface and easily overlooked using surface analysis techniques. A more accurate approach uses PIXE and the deep penetration of MeV protons to sample inclusion content, including minor quench phases, to ∼40 μm depth. A method has been developed using the CSIRO-GEMOC nuclear microprobe to image the minor and trace element components of melt inclusions, and integrate all contributions across and into each inclusion, using the dynamic analysis method. Examples show that this approach can quantitatively image spatial variation in component elements at 1.8 μm spatial resolution (at 8 nA) and at levels of ∼2 ppm and determine melt composition with detection sensitivities down to 0.2 p pm.