A ‘hidden’ 18O-enriched reservoir in the sub-arc mantle

Chuan-Zhou Liu,Wei-Qiang Ji,Qiu-Li Li,Wei-Dong Sun,Sun-Lin Chung,Fu-Yuan Wu

doi:10.1038/srep04232

Abstract

Plate subduction continuously transports crustal materials with high-δ18O values down to the mantle wedge, where mantle peridotites are expected to achieve the high-δ18O features. Elevated δ18O values relative to the upper mantle value have been reported for magmas from some subduction zones. However, peridotites with δ18O values significantly higher than the well-defined upper mantle values have never been observed from modern subduction zones. Here we present in-situ oxygen isotope data of olivine crystals in Sailipu mantle xenoliths from South Tibet, which have been subjected to a long history of Tethyan subduction before the India-Asia collision. Our data identify for the first time a metasomatized mantle that, interpreted as the sub-arc lithospheric mantle, shows anomalously enriched oxygen isotopes (δ18O = +8.03 ± 0.28 ‰). Such a high-δ18O mantle commonly does not contribute significantly to typical island arc basalts. However, partial melting or contamination of such a high-δ18O mantle is feasible to account for the high-δ18O signatures in arc basalts.

Highlights

Plate subduction continuously transports crustal materials with high-d18O values down to the mantle wedge, where mantle peridotites are expected to achieve the high-d18O features
The fluids/melts enrich the mantle wedge in elements concentrated in altered oceanic crust and its sedimentary cover, which eventually contribute these enriched signatures to arc magmas[1,4,5,7]
Previous studies[14,19] have shown that silica-rich glass/melt inclusions entrained in peridotite xenoliths from island arcs have d18O values significantly higher than the upper mantle, which have been interpreted as low-degree melts from refractory mantle metasomatized by slab-derived fluids/melts

Summary

Introduction

Plate subduction continuously transports crustal materials with high-d18O values down to the mantle wedge, where mantle peridotites are expected to achieve the high-d18O features. Oxygen isotope evidence for melts directly derived from subducted slabs is rare[16], both whole-rock and olivine phenocrysts of some arc magmas have elevated d18O values relative to the upper mantle[13,17,18]. Previous studies[14,19] have shown that silica-rich glass/melt inclusions entrained in peridotite xenoliths from island arcs have d18O values (ranging from 18.8% to 111.3%) significantly higher than the upper mantle, which have been interpreted as low-degree melts from refractory mantle metasomatized by slab-derived fluids/melts. Peridotite xenoliths from modern subduction zones have never been reported to have d18O values higher than the upper mantle It seems that the 18O-enriched reservoirs in mantle wedges are not represented by peridotite xenoliths trapped by arc-related magmas. Olivines from nine Sailipu peridotite xenoliths have been analyzed for oxygen isotopes by secondary ion mass spectrometry (SIMS)

Methods

Results

Conclusion