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

Abstract

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, log fO 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 ∼ 6 ∗ m Cl, D Zn ∼ 8 ∗ m Cl, D Ag ∼ 4 ∗ m Cl, D Fe ∼ 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 ∼ 20, D B ∼ 15, D As ∼ 13, D Sb ∼ 8, D Bi ∼ 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.