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.
Li et al. (Sun,) studied this question.