A New Analytical Technique for Measuring Element Partitioning between Experimental Vapour and Melt

Geoffrey T. Nichols,Trevor H. Green, Norm Pearson and Ashwini Sharma

GEMOC, School of Earth Sciences, Macquarie University NSW 2109

Most recent petrogenetic models for magma genesis in subduction zone environments attribute the characteristic geochemical properties of these magmas to the involvement of a water-rich fluid derived from the subducted slab, with or without an additional sediment derived silicate-melt. In order to constrain the geochemical attributes of such a water-rich fluid it is necessary to determine mineral-vapour and vapour-melt trace-element partitioning behaviour at high pressure. Usually mineral-melt partition coefficients and mineral-vapour values have been obtained independently - either derived experimentally or from natural rocks - and the desired melt-vapour partition values are produced by calculation. Here we describe experiments and a new analytical technique that enable the direct measurement of melt-vapour partition values.

A series of experiments was performed over a temperature range of 1020-1200°C, at 25 kbar, on a basanite+H2O starting composition to determine melt-vapour partitioning values. The basanite glass was enriched in La, Ce, Nd, Sm, Lu, Ba, Sr, Y, Zr, Hf, Nb, Ta, Th, U, Cs, Sn, Rb and Zn, totalling 1.045 wt%. Experiments were conducted using a 12.7 mm apparatus with AgPd capsules. Capsules were packed with an equal weight of H2O and basanite, and run in talc-Pyrex pressure cells. Experiments produced glass with minor clinopyroxene (depending on temperature), vapour and fluid. The vapour was analysed using LAM-ICPMS (Laser Ablation Microprobe-Inductively Coupled Mass Spectrometry). The LAM consists of a frequency quadrupled Nd-YAG laser, operating at 266 nm (UV). The laser beam is focussed through a petrographic microscope, producing a spot size of ~30 µm at a power of ~1 mJ/pulse. A video camera allows viewing of the ablation process. The laser was focussed on to the polished capsule-tail housed within a sealed sample-chamber, and an argon carrier gas transported emitted matter through to a Perkin-Elmer 5100 (ICPMS); the laser was used to drill through the AgPd capsule to release the vapour + liquid, at which point the laser was turned off. Data acquisition was monitored in a real-time graphics display, and the data were reduced by subtracting background counts from peak, and accounting for machine drift using four glass standard analyses. The vapour signals were transient, typically with peaks lasting ²40 seconds. Experimental glasses, exposed in polished sections of capsules, were analysed under the same conditions and were standardized using an internal standard measured by an electron-microprobe to give absolute concentrations. As standardisation procedures for vapours are extraordinarily difficult we have not yet been successful in obtaining absolute concentrations of elements in the vapour. In order to compare our results with other data we divide the vapour composition by the bulk, and normalize to Primitive Mantle (PM) values of Sr or Lu (Figure 1). Experimental vapour appears to selectively transport Rb, Ba, Nb, Sr and Zr while La behaves incompatibly with respect to the vapour. Thus this vapour could cause significant changes to Nb/Ta and Zr/Hf in both melt residues, and to mantle regions trapping this metasomatic vapour.

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