Measurement of whole-rock trace-element composition of Cr-rich rocks on fused glass by LA-ICP-MS: data reliability of chromite-bearing fused glass

Information from whole-rock trace-element compositions is significant for petrological studies. LA-ICP-MS technique can rapidly determine whole-rock trace-element compositions by fused glasses prepared by homogenization of rock powders. As already reported, several trace-element abundance (Sc, Ti, V, Cr, Co, Ni, Rb, Sr, Y, Zr, Nb, Ba, Th and REEs) of basaltic and andesitic rocks can be determined with high accuracy by the routine measurement on the flux-free fused glass using LA-ICP-MS in our laboratory. In this study, based on the previous report, whole-rock trace-element compositions of some Cr-rich rocks, mantle diopsidite and related rocks from the Oman ophiolite, were determined by the routine work. The mantle diopsidite is locally rich in chromite and its fused glass prepared includes quench crystals and residual grains of chromite. Here, we assessed the reliability and uncertainty of our data obtained from such incompletely homogenized fused glasses. High precision of 3-spot analyses supports that quench crystals of chromite were homogeneously distributed in the fused glass. This suggests that the analytical data obtained from spots unrelated to residual chromite grains are reliable for whole-rock trace-element compositions. The heterogeneity caused by residual chromite grains can be ignored in the measurement of incompatible elements (Sr, Y, Zr, and REEs). Our analytical results of 6 fused glass beads prepared from the diopsidite samples indicate that heterogeneity of the mantle diopsidite in terms of the abundance of chromite is scarcely reflected by incompatible-element compositions while the heterogeneity is clearly characterized by Cr abundance.