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The competing effects of sulfide saturation versus degassing on the behavior of the chalcophile elements during the differentiation of hydrous melts

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https://ir.library.oregonstate.edu/concern/articles/5999n5407

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  • There is a lack of consensus regarding the roles of sulfide saturation versus volatile degassing on the partitioning of Cu and Ag during differentiation and eruption of convergent margin magmas. Because of their oxidized character, volatile-rich magmas from the Eastern Manus Back-arc Basin (EMBB) only reach sulfide saturation following magnetite-driven reduction of the melt: the so-called “magnetite crisis.” If sulfide saturation typically precedes volatile saturation, the magnetite crisis will limit the proportion of Cu and Ag that can partition from the melt into an exsolving volatile-rich phase, which may contribute to the sporadic occurrence of magmatic-hydrothermal ore deposits at convergent margins. However, it is unclear whether the magnetite crisis is a common or rare event during differentiation of volatile-rich magmas. We report major and trace element data for submarine volcanic glasses from the Tonga arc-proximal Valu Fa Ridge (VFR; SW Pacific). Cu-Se-Ag systematics of samples erupting at the southern VFR suggest magnetite fractionation-triggered sulfide saturation. The similarity in chalcophile element systematics of the southern VFR and EMBB samples is unlikely to be coincidental, and may indicate that the magnetite crisis is a common event during differentiation of hydrous melts. However, unlike many convergent margin magmas, it is unlikely that the evolving VFR and EMBB were saturated in a S-bearing volatile phase prior to magnetite fractionation. Hence, the metal-depleting magnetite crisis may be restricted to back-arc basin magmas that do not degas volatiles prior to magnetite fractionation and potentially convergent margin magmas fractionating at high pressures in the continental crust.
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  • Jenner, F. E., Hauri, E. H., Bullock, E. S., König, S., Arculus, R. J., Mavrogenes, J. A., Mikkelson, N., and Goddard, C. ( 2015), The competing effects of sulfide saturation versus degassing on the behavior of the chalcophile elements during the differentiation of hydrous melts, Geochem. Geophys. Geosyst., 16, 1490– 1507, doi:10.1002/2014GC005670
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  • 16
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  • 5
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  • The postvoyage analytical costs for analyses undertaken at the RSES were supported by an Australian Research Council Discovery Grant to Richard Arculus and John Mavrogenes. Frances Jenner and Erik Hauri acknowledge funding from the Deep Carbon Observatory and the DTM for analyses undertaken at the DTM. We thank Jianhua Wang, Tim Mock, and Charlotte Allen for expert maintenance and assistance in the DTM SIMS lab and LA-ICPMS facility at the RSES, ANU, respectively
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