Trace Element Partitioning between H2O-Fluids and Silicate Melts

J. Adam T.H. Green and S.H. Sie

(GEMOC, School of Earth Sciences, Macquarie University, Sydney, N.S.W. 2109, Australia)

A proton microprobe was used to measure concentrations of Rb, Sr, Ba, Y, Zr, Hf, Nb, Ta, La, Lu, Th and U in solutes from H2O-fluids, and in co-existing trondhjemitic and basaltic melts (glasses) equilibrated at 900-1100oC, and 2.0 GPa. Total solute concentrations in the H2O-fluids were between 30 and 50 wt.%. H2O-fluids co-existing with trondhjemitic melts are moderately enriched in Rb and K, whereas other elements partition neutrally between fluid and melt. Excepting alkalis, trace element concentrations in H2O-fluids are reduced by the addition of Cl. In contrast, F has little effect of fluid/melt partitioning. H2O-fluid in equilibrium with basaltic melt is moderately enriched in Rb and K but depleted in other trace elements. In no case are H2O-fluids depleted in HFSE (Ti, Zr, Hf, Nb and Ta) relative to alkaline earths, REE, Th and U. Consequently, HFSE depletions and elevated alkaline earths/REE in arc magmas cannot be due to the relative solubilities of these elements in H2O-fluids. Crystal-chemical controls may play a more important role.