Evolution of chalcophile elements in the magmas of the Bonin Islands

Abstract

Melt inclusions and submarine volcanic glasses of boninitic composition are well preserved in beach sands from the islands of Chichijima and Mukojima along the Bonin Ridge. Volcanic glass as well as orthopyroxene- and spinel hosted melt inclusions in these sands span almost the entirety of boninite series melt compositions, ranging from >18 wt% to <0.5 wt% MgO. Copper, V and other trace metals were tracked over the entirety of the boninite range and when compared to other suprasubduction melts from the Manus and Lau basins, highlight the variability of these elements across different arc magmas. Boninite series melts show similar behaviour of Cu and V to melts from the Manus and Lau basins, suggestive of magnetite (saturation)-triggered sulfide saturation (a ‘magnetite crisis’). Furthermore, a direct link of these geochemical trends to physical sulfides and magnetite in sample material has been established for the first time. Comparisons of incompatible trace elements suggests that sulfide saturation in the boninite series melts occurred after less fractionation (earlier) than either the Manus or Lau basin melts. This is surprising, given secondary melts such as boninites are generated from melt-depleted mantle sources and are thought to contain very little S. Indeed S contents of melt inclusions of primitive boninitic composition are less than half that of the primitive Manus Basin melts. While the mechanism of sulfur enrichment in the boninite series melts remains a mystery, the oxidised nature of the Bonin Ridge boninites suggests that magnetite-triggered melt reduction is necessary to stabilise sulfide. Finally the timing of magnetite crises generally may be mostly controlled by magmatic oxidation state and total iron contents.