Electrical investigation of natural lawsonite and application to subduction contexts

Abstract

We report an experimental investigation of the electrical properties of natural polycrystalline lawsonite from Reed Station, CA. Lawsonite represents a particularly efficient water reservoir in subduction contexts, as it can carry about 12 wt % water and is stable over a wide pressure range. Experiments were performed from 300 to about 1325 °C and under pressure from 1 to 10 GPa using a multi‐anvil apparatus. We observe that temperature increases lawsonite conductivity until fluids escape the cell after dehydration occurs. At a fixed temperature of 500 °C, conductivity measurements during compression indicate electrical transitions at about 4.0 and 9.7 GPa that are consistent with crystallographic transitions from orthorhombic C to P and from orthorhombic to monoclinic systems, respectively. Comparison with lawsonite structure studies indicates an insignificant temperature dependence of these crystallographic transitions. We suggest that lawsonite dehydration could contribute to (but not solely explain) high conductivity anomalies observed in the Cascades by releasing aqueous fluid at a depth (~50 km) consistent with the basalt‐eclogite transition. In subduction settings where the incoming plate is older and cooler (e.g., Japan), lawsonite remains stable to great depth. In these cooler settings, lawsonite could represent a vehicle for deep water transport and the subsequent triggering of melt that would appear electrically conductive, though it is difficult to uniquely identify the contributions from lawsonite on field electrical profiles in these more deep‐seated domains.