Partitioning of copper between olivine, orthopyroxene, clinopyroxene, spinel, garnet and silicate melts at upper mantle conditions

摘要

Previously published Cu partition coefficients (D_(Cu)) between silicate minerals and melts cover a wide range and have resulted in large uncertainties in model calculations of Cu behavior during mantle melting. In order to obtain true D_(Cu)~(mineral=melt) values, this study used Pt_(95)Cu_(05) alloy capsules as the source of Cu to experimentally determine the D_(Cu) between olivine (ol), orthopyroxene (opx), clinopyroxene (cpx), spinel (spl), garnet (grt) and hydrous silicate melts at upper mantle conditions. Three synthetic silicate compositions, a Komatiite, a MORB and a Di_(70)An_(30), were used to produce these minerals and melts. The experiments were conducted in piston cylinder presses at 1.0–3.5 GPa, 1150–1300 ℃ and oxygen fugacities (fO_2) of from ~2 log units below to ~5 log units above fayalite–magnetite–quartz (FMQ). The compositions of minerals and quenched melts in the run products were measured with EMP and LA-ICP-MS. Attainment of equilibrium is verified by reproducible D_(Cu) values obtained at similar experimental conditions but different durations. The results show that D_(Cu) for ol/, opx/, spl/ and possibly cpx/melt increase with increasing fO_2 when fO_2 > FMQ + 1.2, while D_(Cu) for cpx/ and spl/melt also increase with increasing Na_2O in cpx and Fe_2O_3 in spinel, respectively. In the investigated P–T–fO_2 conditions, the D_(Cu)~(mineral=melt) values are 0.04–0.14 for ol, 0.04–0.09 for opx, 0.02–0.23 for cpx, 0.19–0.77 for spl and 0.03–0.05 for grt. These results confirm that Cu is highly incompatible (D_(Cu) < ~0.2) in all the silicate minerals and oxides of the upper mantle with the exception of the high-Fe spinel, in which Cu is moderately incompatible (D_(Cu) = 0.4–0.8) and thus Cu will be enriched in the derived melts during mantle partial melting and magmatic differentiation if sulfide is absent. These experimental D_(Cu) values are used to assess the controls on Cu behavior during mantle melting. The model results suggest that MORBs and most arc basalts must form by sulfide-present melting at relatively reduced conditions, while high Cu (>70 ppm) arc basalts may form at oxidized, sulfide-absent conditions, which is consistent with the possibility of some high fO_2 regions present in the arc mantle.