Oxidising agents in sub-arc mantle melts link slab devolatilisation and arc magmas

Antoine Bénard,Kevin Klimm,Camille Rivard,Dmitri A Ionov,Roman E Botcharnikov,Richard J Arculus,Max Wilke,Nobumichi Shimizu,Oliver Nebel,Alan B Woodland

doi:10.1038/s41467-018-05804-2

Abstract

Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Here we provide direct evidence for contributions of oxidising slab agents to melts trapped in the sub-arc mantle. Measurements of sulfur (S) valence state in sub-arc mantle peridotites identify sulfate, both as crystalline anhydrite (CaSO4) and dissolved SO42− in spinel-hosted glass (formerly melt) inclusions. Copper-rich sulfide precipitates in the inclusions and increased Fe3+/∑Fe in spinel record a S6+–Fe2+ redox coupling during melt percolation through the sub-arc mantle. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δ34S (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas.

Highlights

Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges
As originally described in detail in Ionov et al.[28] and Bénard et al.[29], Cr-bearing spinel in Kamchatka xenoliths contains glass MIs with compositions spanning andesite-dacite (‘LT inclusions’, where LT stands for low temperature), magnesian andesite and dacite-rhyolite of boninitic affinity (‘vein MIs’)
High concentrations of slab agents in the inclusion parental melts were inferred from lithophile trace element signatures and very high volatile abundances in the glasses[28,29,30,31]; the second compositional feature indicates that melt entrapment occurred at high pressure in the sub-arc mantle lithosphere (Fig. 1b)

Summary

Introduction

Subduction zone magmas are more oxidised on eruption than those at mid-ocean ridges. This is attributed either to oxidising components, derived from subducted lithosphere (slab) and added to the mantle wedge, or to oxidation processes occurring during magma ascent via differentiation. Sulfate-rich glass inclusions exhibit high U/Th, Pb/Ce, Sr/Nd and δ34S (+ 7 to + 11‰), indicating the involvement of dehydration products of serpentinised slab rocks in their parental melt sources. These observations provide a link between liberated slab components and oxidised arc magmas. SerSlpaebntisnureface (Supplementary Table 12). b A subduction zone cross-section showing the original position of the studied peridotite xenoliths (‘sub-arc mantle peridotites’)

Methods

Results

Conclusion