Molybdenum systematics of subducted crust record reactive fluid flow from underlying slab serpentine dehydration

Shuo Chen,Ben Bryan,Yaoling Niu,Remco C Hin,Richard Brooker,Timm John,Tim Elliott

doi:10.1038/s41467-019-12696-3

Abstract

Fluids liberated from subducting slabs are critical in global geochemical cycles. We investigate the behaviour of Mo during slab dehydration using two suites of exhumed fragments of subducted, oceanic lithosphere. Our samples display a positive correlation of δ98/95MoNIST 3134 with Mo/Ce, from compositions close to typical mantle (−0.2‰ and 0.03, respectively) to very low values of both δ98/95MoNIST 3134 (−1‰) and Mo/Ce (0.002). Together with new, experimental data, we show that molybdenum isotopic fractionation is driven by preference of heavier Mo isotopes for a fluid phase over rutile, the dominant mineral host of Mo in eclogites. Moreover, the strongly perturbed δ98/95MoNIST 3134 and Mo/Ce of our samples requires that they experienced a large flux of oxidised fluid. This is consistent with channelised, reactive fluid flow through the subducted crust, following dehydration of the underlying, serpentinised slab mantle. The high δ98/95MoNIST 3134 of some arc lavas is the complement to this process.

Highlights

Fluids liberated from subducting slabs are critical in global geochemical cycles
Mo isotope systematics can identify the style of serpentine involvement in arc magmatism, other interpretations of high δ98/95MoNIST 3134 in arc lavas have been proposed[16,19]
By analysing material complementary to arc magmatism, namely exhumed fragments of subducted oceanic lithosphere, we provide a fresh perspective on the cause of Mo isotopic fractionation during subduction

Summary

Introduction

Fluids liberated from subducting slabs are critical in global geochemical cycles. We investigate the behaviour of Mo during slab dehydration using two suites of exhumed fragments of subducted, oceanic lithosphere. The strongly perturbed δ98/95MoNIST 3134 and Mo/Ce of our samples requires that they experienced a large flux of oxidised fluid This is consistent with channelised, reactive fluid flow through the subducted crust, following dehydration of the underlying, serpentinised slab mantle. Serpentine forms above the slab, trapping fluids released from the crust in the shallow subduction zone, and is down-dragged with the descending plate. The high δ98/95MoNIST 3134 in depleted arc lavas has been inferred to reflect Mo isotopic fractionation caused in its partitioning into fluids traversing the subducting crust from underlying, serpentine dehydration[17,18]. We further report low δ98/95MoNIST 3134 and Mo/Ce in bulk eclogite and blueschist samples that reflect the control of residual rutile during fractional loss of Mo caused by a large, oxidised, reactive-fluid flux

Methods

Results

Conclusion