Determination of fluid/melt partition coefficients by LA-ICPMS analysis of co-existing fluid and silicate melt inclusions: Controls on element partitioning

Analyses of co-existing silicate melt and fluid inclusions, entrapped in quartz crystals in volatile saturated magmatic systems, allowed direct quantitative determination of fluid/melt partition coefficients. Investigations of various granitic systems (peralkaline to peraluminous in composition, logfO(2) = NNO-1.7 to NNO+4.5) exsolving fluids with various chlorinities (1-14 mol/kg) allowed us to assess the effect of these variables on the fluid/melt partition coefficients (D). Partition coefficients for Pb, Zn, Ag and Fe show a nearly linear increase with the chlorinity of these fluid (D-Pb similar to 6 * m(Cl), D-Zn similar to 8 * m(Cl), D-Ag similar to 4 * m(Cl), D-Fe similar to 1.4 * m(Cl), where m(Cl) is the molinity of Cl). This suggests that these metals are dissolved primarily as Cl-complexes and neither oxygen fugacity nor the composition of the melt affects significantly their fluid/melt partitioning. By contrast, partition coefficients for Mo, B, As, Sb and Bi are highest in low salinity (1-2 mol/kg Cl) fluids with maximum values of D-Mo similar to 20, D-B similar to 15, D-As similar to 13, D-Sb similar to 8, D-Bi similar to 15 indicating dissolution as non-chloride (e.g., hydroxy) complexes. Fluid/melt partition coefficients of copper are highly variable, but highest between vapor like fluids and silicate melt (D-Cu <= 2700), indicating an important role for ligands other than Cl. Partition coefficients for W generally increase with increasing chlorinity, but are exceptionally low in some of the studied brines which may indicate an effect of other parameters. Fluid/melt partition coefficients of Sn show a high variability but likely increase with the chlorinity of the fluid (D-Sn = 0.3-42, D-W = 0.8-60), and decrease with decreasing oxygen fugacity or melt peraluminosity. (c) 2008 Elsevier Ltd. All rights reserved.