Measurements of trace elements in basalts and their phenocrysts by laser probe microanalysis inductively coupled plasma mass spectrometry (LPMA-ICP-MS)

Abstract

An inductively coupled plasma mass spectrometer (ICP-MS) coupled to a modified Nd:YAG laser has been used for the in situ determination of trace-element zonation and partition coefficients in olivines and clinopyroxenes from natural phenocryst-matrix systems. The laser produces ablation craters of 20–40-μm diameter when operated in Q-switched mode and can provide spatial analyses of trace elements in minerals. The ICP-MS is calibrated against a synthetic trace-element glass reference material and can be used for the rapid determination of elements at low (sub-ppm) concentrations. Partition coefficients are presented for: (1) olivine in tholeiite glass from Stapafell, Iceland; and (2) olivine and clinopyroxene in an ankaramitic basalt from Mauna Kea, Hawaii. Data are presented for 37 compatible and incompatible trace elements, including Li, B and the REE from both suites. Detection limits are typically sub-ppm, with, for example, Tm present at 7 ppb in the Icelandic olivine and Lu at 120 ppb in the Hawaiian clinopyroxene. The calculated trace-element partition coefficients differ between the two suites. Partition coefficients for olivine ( D ol/matrix) for Li are 0.435 (Iceland) and 0.915 (Hawaii); for B are 0.559 (Iceland) and 0.984 (Hawaii). Other examples include D Sc ol/matrix 0.24–0.273 (Iceland) and 0.225 (Hawaii); whilst D's for Cr are 0.524–0.975 (Iceland) and 2.563 (Hawaii). D ol/matrix for REE are all greater for the Hawaiian ankaramite than for the Icelandic tholeiite. The partition coefficients calculated in this study generally fall within published ranges.