Determination of fluid/melt partition coefficients by LA-ICPMS analysis of co-existing fluid and silicate melt inclusions: Controls on element partitioning |
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| Authors: | Zoltán Zajacz Werner E Halter Marcel Guillong |
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| Institution: | a Institute of Isotope Geochemistry and Mineral Resources, Department of Earth Sciences, ETH Zürich, ETH Zentrum NW, Clausiusstrasse 25, Zürich CH-8092, Switzerland
b Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, Bern CH-3012, Switzerland |
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| 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 fO2 = 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 (DPb ∼ 6 ∗ mCl, DZn ∼ 8 ∗ mCl, DAg ∼ 4 ∗ mCl, DFe ∼ 1.4 ∗ mCl, where mCl 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 DMo ∼ 20, DB ∼ 15, DAs ∼ 13, DSb ∼ 8, DBi ∼ 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 (DCu ? 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 (DSn = 0.3-42, DW = 0.8-60), and decrease with decreasing oxygen fugacity or melt peraluminosity. |
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