APPLICATION OF PROTON-MICROPROBE DATA TO TRACE-ELEMENT PARTITIONING IN VOLCANIC-ROCKS

In situ proton microprobe analyses are used to calculate partition coe fficients (D) for Mn, Fe, Cu, Zr, Ga, Rb, Sr, Y, Zr, Ba and Pb, and mo re restricted data for Ni, Ge, Nb and As, in pyroxenes, olivines, biot ites, hornblendes, feldspars and Fe-Ti-oxides, from 32 volcanic rocks (all with glassy matrices). These range from leucitite and picritic ba salt to high-silica and peralkaline rhyolites. Data for pigeonite, aen igmatite, allanite and leucite are included, where these occur as addi tional phases. Coefficients have also been Rayleigh fractionation corr ected, this correction being significant for those elements with high coefficients in specific mineral phases, and where pronounced crystall isation has occurred. Partition coefficients are presented as plots vs . NBO/T ratios of coexisting glasses (=melts). Negative correlations a re observed, based on log-transformed data, for Mn, Zn, Fe and Y (pyro xenes), Mn, Zn and Fe (olivines), Mn and Zn (Fe-Ti-oxides),Nb, Y, Sr a nd Zr (hornblendes), Sr (plagioclase),and Be, Cu, Zn and Fe (feldspars ). These element partition coefficients are not correlated with bulk m ineral compositions. Zr and Ga are not correlated with either glass or mineral compositions (except Zr in hornblende). The correlations supp ort the role of melt polymerisation in controlling element partitionin g, at least for those elements listed, although possible effects of te mperature cannot be uniquely isolated in the present data set. The coe fficients presented here are generally consistent with published value s for equivalent rock compositions, one difference being that some of the higher published coefficients (e.g., Sr in plagioclase) are not ma tched in the present data set. From a practical viewpoint, the relativ e regularity of the coefficient data presented does facilitate choice of relevant coefficient values that are required for petrogenetic mode lling procedures.