Subduction Zone Magnetism: The Influence of Metamorphism and Serpentinization in the Mantle Wedge

Abstract

Abstract Subduction zone magnetic anomalies have previously been used to infer their thermal structure assuming a uniformly serpentinized mantle carries a homogeneous, isotropic magnetization. However, seismic tomography, geological observations and numerical modeling provide increasing evidence for a non‐uniformly serpentinized mantle wedge that may carry heterogeneous magnetization. In this study, we characterize the rock magnetic properties in variably serpentinized samples from Santa Catalina Island/Pimu'nga in California, USA, and uncover at least two populations of magnetite formed during metamorphism and serpentinization 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 serpentinization reactions, which have been invoked for mantle wedge magnetism in the past. Heavily serpentinized samples contain stoichiometric magnetite associated with serpentinization reactions under wedge conditions. We propose that the observed magnetic anomalies above subduction zones are primarily controlled by two factors: (a) the extent of fluids available for metamorphism of the mantle wedge and the transformation of Cr‐spinel to Cr‐magnetite and (b) the extent of subsequent serpentinization which may generate a layer of heavily serpentinized 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 characterization of fluid availability for both metamorphism and serpentinization, and its implications on wedge magnetic mineralogy. Plain Language Summary The mantle wedge is a triangular prism of rock that lies between an overlying tectonic plate and underlying subducting slab in a subduction zone. Rocks from the mantle wedge have been brought up from great depth and exposed on the surface of Santa Catalina Island/Pimu'nga, USA. Samples collected from the island provide a rare opportunity to directly study an ancient hydrated mantle wedge. We are particularly interested in the magnetic properties of these samples, which affect surface magnetic measurements. We find at least two compositionally distinct groups of magnetic minerals formed through different processes: (a) metamorphic change and (b) hydration reactions. The two magnetic types of magnetic mineral possess different magnetic properties, and so affect surface magnetic measurements in different ways. Surface magnetic measurements can be used to infer subduction zone thermal structure, which is greatly important for assessing their potential as seismic and volcanic hazards. As such, a better understanding of the magnetic properties of mantle rock can facilitate improvements in understanding the natural hazards associated with subduction zones. Key Points Variably serpentinized mantle wedge rocks proximal to an early Cretaceous paleosubduction interface are exposed on Santa Catalina Island Samples all contain Cr‐magnetite, which is genetically unrelated to stoichiometric magnetite found only in heavily serpentinized samples Cr‐magnetite has different magnetic properties to the stoichiometric magnetite that's typically used to interpret magnetic anomalies