Transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles

Abstract : Emissions of sulphur 1,2 and transition metals 3,4 from magmas in the shallow crust have global impacts on human society by affecting climate after volcanic eruptions 5,6 and by forming giant copper and gold ore deposits, which are massive anomalies of sulphur as well as of the ore metals themselves 3,4,7,8. These emissions vastly exceed the amounts that could have been derived from the associated magmas themselves 2. Intrusion of denser, hotter, vapour-and sulphide-saturated magma beneath the cooler and more buoyant bodies that are finally erupted is the most probable source of the excess sulphur and metals in eruptive or hydrothermal systems 1,4,9,10. However, the mechanism for the selective upward transfer of sulphur and metals remains poorly understood, because their main carrier phase, sulphide melt, is dense and is generally assumed to settle to the bottoms of magma bodies. Here we show, by experimental observation and by gas speciation and mass balance models, that such transfers may be achieved by the flotation of compound drops 11 comprising droplets of sulphide melt attached to vapour bubbles. By proposing and demonstrating the feasibility of a novel physical mechanism for upward mobility of sulphide liquids, our work provides a mechanism for massive atmospheric releases of sulphur and contradicts widely held assumptions that dense sulphide liquids rich in sulphur, copper, and gold will remain sequestered in the crust.
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James.E. Mungall, James M. Brenan, B Godel, S.J. Barnes, Fabrice Gaillard. Transport of metals and sulphur in magmas by flotation of sulphide melt on vapour bubbles. Nature Geoscience, Nature Publishing Group, 2015, 8, pp.216-219. ⟨10.1038/ngeo2373⟩. ⟨hal-01128839⟩

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