IN SITU TRACE-ELEMENT ANALYSIS OF INDIVIDUAL SILICATE MELT INCLUSIONS BY LASER ABLATION MICROPROBE-INDUCTIVELY COUPLED PLASMA-MASS SPECTROMETRY (LAM-ICP-MS)

Abstract

This paper reports the successful application of laser ablation microprobe-inductively coupled plasma-mass spectrometry (LAM-ICP-MS) to the in situ analysis of a diverse suite of twenty trace elements including Zr, Hf, Nb, Ta, Y, and REEs, in individual silicate melt inclusions in phenocrysts from Fantale volcano, Ethiopia. The UV laser, a frequency quadrupled Nd:YAG operating at 266 nm, significantly improves the ablation characteristics of minerals that do not absorb strongly at near-IR wavelengths (e.g., quartz and feldspar). Furthermore, it allows for a significant reduction in ablation pit size to ca. 10 μm, thereby permitting numerous applications that require high-resolution sampling. Multiple ablations in individual melt inclusions in the size range 10-50 μm demonstrate both the effectiveness of the technique and the generally homogeneous character of the inclusions. Comparison of the LAM-ICP-MS data for international reference material RGM-1 (a rhyolite), with recommended values, indicates an analytical precision of <10% for most of the trace elements determined in this study. The trace element abundances of the Fantale melt inclusions, determined by LAM-ICP-MS, are typical of those of pantellerites (i.e., peralkaline rhyolites), and are consistent with their origin as tiny volumes of melt trapped in quartz and alkali-feldspar phenocrysts during the final stage of fractional crystallization of the host peralkaline magma.