Sievwright, R HWilkinson, JamieO’Neill, Hugh St. C.Berry, Andrew2020-04-222020-04-220010-7999http://hdl.handle.net/1885/203374Titanomagnetite-melt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO(2)) and temperature (T) in an andesitic-dacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetite-melt partitioning of the divalent cations (Mg2+, Mn2+, Co2+, Ni2+, Zn2+) and Cu2+/Cu+ with increasing fO(2) between 0.2 and 3.7 log units above the fayalite-magnetite-quartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineral-melt partitioning of divalent cations, a more rigorous justification of magnetite-melt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO(2) implies a reciprocal relationship between a(Fe2+O) and a(Fe3+O1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite-melt partitioning of divalent cations with fO(2) in magnetite- rich spinel. As a result of this, titanomagnetite-melt partitioning of divalent cations is indirectly sensitive to changes in fO(2) in silicic, but less so in mafic bulk systems.This work was fnancially supported by the Imperial PhD Scholarship Scheme.application/pdfen-AU© 2017 The Author(s)http://creativecommons.org/licenses/by/4.0/Magnetite · Trace-element partitioning · Andesite · Dacite · Oxygen fugacity · Petrogenetic indicator201710.1007/s00410-017-1385-62019-11-25Creative Commons License (Attribution 4.0 International)