Subduction Zone Magnetism: The influence of metamorphism and serpentinisation in the mantle wedge

Abstract

Subduction zone magnetic anomalies have previously been used to infer their thermal structure assuming a uniformly serpentinised mantle carries a homogeneous, isotropic magnetisation. However, seismic tomography, geological observations and numerical modelling provide increasing evidence for a non-uniformly serpentinised mantle wedge that may carry heterogeneous magnetisation. In this study, we characterise the rock magnetic properties in variably serpentinised samples from Santa Catalina Island/Pimu'nga in California, USA, and uncover at least two populations of magnetite formed during metamorphism and serpentinisation of the mantle wedge. All samples contain Cr-magnetite, which is believed to form through the amphibolite facies metamorphism of Cr-spinel under high fluid:rock ratios, as opposed to serpentinisation reactions, which have been invoked for mantle wedge magnetism in the past. Heavily serpentinised samples contain stoichiometric magnetite associated with serpentinisation reactions under wedge conditions. We propose that the observed magnetic anomalies above subduction zones are primarily controlled by two factors: (1) the extent of fluids available for metamorphism of the mantle wedge and the transformation of Cr-spinel to Cr-magnetite and (2) the extent of subsequent serpentinisation which may generate a layer of heavily serpentinised rock containing a mixture of Cr- and stoichiometric magnetite at the base of the mantle wedge. This proposal precludes the direct connection between subduction zone magnetic anomalies and thermal structure without prior characterisation of fluid availability for both metamorphism and serpentinisation, and its implications on wedge magnetic mineralogy.