LASER-ABLATION ICP-MS ANALYSIS OF MELT INCLUSIONS: ANALYTICAL APPROACH AND DATA EVALUATION

A new quantification technique for the chemical analysis of entire silicate and sulfide melt inclusions by excimer laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICPMS) has been applied to determine melt compositions in pyroxene, amphibole, plagioclase and quartz form intrusive and extrusive rocks. Details of the analytical approach and the quantification procedure are presented and the results are evaluated to test the accuracy of the technique and the validity of the interpretation of analytical signals. Various steps in the de-convolution of the analytical signal are presented and discussed. Uncertainties are calculated using appropriate uncertainty propagation and the accuracy was tested by comparison with other analytical techniques.

Results show similar compositions of silicate melt inclusions trapped in co-precipitating mineral pairs, demonstrating that the quantification approach through an internal standard is valid. This correspondence also indicates that melt inclusions are not influenced by the boundary layer around a growing crystal or by post-entrapment modifications. Post-entrapment diffusive re-equilibration only affected the Fe and Mg content of melt inclusions in mafic phases. Thus, melt inclusions are representative samples of the melt from which the host mineral crystallized with regard to most major and trace elements.

Sulfide melt inclusions (mainly Cu and Au-rich iron sulfides) were analyzed for their bulk Fe, Cu and Au content, and the abundance of these elements was quantified using a silicate glass as external standard. The validity of this calibration was tested by comparing electron microprobe analyses of Fe, Cu, Ni and Co in homogeneous sulfide minerals with LA-ICPMS results. Identical results within calculated uncertainties (one standard deviation of five to nine analyses) demonstrate that for these elements, measured element ratios are independent of the matrix using our analytical setup.

These results show, that entire silicate and sulfide melt inclusions can be reliably analyzed without prior homogenization and exposure to the sample surface using LA-ICPMS.