Dehydration of chlorite explains anomalously high electrical conductivity in the mantle wedges
Résumé
Mantle wedge regions in subduction zone settings show anomalously high electrical conductivity (~1 S/m) that has
often been attributed to the presence of aqueous fluids released by slab dehydration. Laboratory-based measurements
of the electrical conductivity of hydrous phases and aqueous fluids are significantly lower and cannot readily
explain the geophysically observed anomalously high electrical conductivity. The released aqueous fluid also rehydrates
the mantle wedge and stabilizes a suite of hydrous phases, including serpentine and chlorite. In this present
study, we have measured the electrical conductivity of a natural chlorite at pressures and temperatures relevant for
the subduction zone setting. In our experiment, we observe two distinct conductivity enhancements when chlorite
is heated to temperatures beyond its thermodynamic stability field. The initial increase in electrical conductivity to
~3 × 10−3 S/m can be attributed to chlorite dehydration and the release of aqueous fluids. This is followed by a
unique, subsequent enhancement of electrical conductivity of up to 7 × 10−1 S/m. This is related to the growth of an
interconnected network of a highly conductive and chemically impure magnetite mineral phase. Thus, the dehydration
of chlorite and associated processes are likely to be crucial in explaining the anomalously high electrical
conductivity observed in mantle wedges. Chlorite dehydration in the mantle wedge provides an additional source
of aqueous fluid above the slab and could also be responsible for the fixed depth (120 ± 40 km) of melting at the
top of the subducting slab beneath the subduction-related volcanic arc front.
Domaines
Pétrographie
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