The stability of subducted glaucophane with the Earth's secular cooling

Subduction-related plate tectonics has operated since the Archean, and the secular cooling of the Earth has led to diverse geodynamical processes governed by a range of subduction geotherms. The blueschist to eclogite transition is one of the major geochemical-metamorphic processes typifying the subduction zone, which releases fluids triggering earthquakes and arc volcanism. Although glaucophane is an index hydrous mineral for the blueschist facies, its stability at mantle depths in diverse subduction regimes of contemporary and early Earth has not been experimentally determined. We have performed a suite of high-pressure and high-temperature experiments on glaucophane and show here that the maximum depth of glaucophane stability increases with decreasing thermal gradients of the subduction system. Along the cold subduction geotherm, glaucophane remains stable down to ~172 km depth conditions whereas under the warm subduction geotherm, it is dehydrated and breaks down into pyroxenes and silica at ~63 km depth. Under the Proterozoic tectonic setting with high thermal gradients, glaucophane undergoes facile dehydration and breakdown into albite and pyroxenes at depths as shallow as ~44 km conditions. Our results imply that secular cooling of the Earth has extended the subducting depth of glaucophane and consequently enabled the transportation of water into deeper interior of the Earth suppressing arc magmatism, volcanism, and seismic activities along subduction zones.