Thermodynamic controls on element partitioning between titanomagnetite and andesitic–dacitic silicate melts

• Published in: Contributions to mineralogy and petrology 2017-08, Vol.172 (8), p.1, Article 62
• Main Authors: Sievwright, R. H., Wilkinson, J. J., O’Neill, H. St. C., Berry, A. J.
• Format: Article
• Language: English
• Subjects: Aluminum; Buffers; Carbon dioxide; Carbon monoxide; Cations; Chemical composition; Cobalt; Copper; Crystallization; Divalent cations; Earth and Environmental Science; Earth Sciences; Fayalite; Fugacity; Geochemistry; Geology; Magnesium; Magnetite; Manganese; Melts; Mineral Resources; Mineral-melt partitioning; Mineralogy; Minerals; Molybdenum; Nickel; Niobium; Original Paper; Partitioning; Petrology; Quartz; Silicates; Spinel; Tantalum; Temperature; Thermodynamics; Titanium; Zinc; Zinc compounds; Zirconium
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-017-1385-6

Titanomagnetite–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 ( f O 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 (Mg 2+ , Mn 2+ , Co 2+ , Ni 2+ , Zn 2+ ) and Cu 2+ /Cu + with increasing f O 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 f O 2 implies a reciprocal relationship between a (Fe 2+ O) and a (Fe 3+ O 1.5 ) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetite–melt partitioning of divalent cations with f O 2 in magnetite-rich spinel. As a result of this, titanomagnetite–melt partitioning of divalent cations is indirectly sensitive to changes in f O 2 in silicic, but less so in mafic bulk systems.