An experimental study of trace element mobility during dehydration of lawsonite blueschist along different P-T paths: Implications for geochemical heterogeneity of Earth's mantle

Abstract

Dehydration in subduction zones is an important process that causes portioning of elements between downgoing slab and overlying mantle wedge. When subducted into Earth's deep mantle, dehydrated oceanic crust may generates critical geochemical heterogeneity. Trace elements in oceanic crust are liberated from subducting slab by dehydration. However, trace elements mobilities may vary along different subduction P-T paths. In order to constrain the mobility of trace elements during dehydration along different subduction P-T paths, we carried out three groups of high-pressure experiments on a natural lawsonite blueschist at 4 and 5.5 GPa with T ranging from 650 to 900 °C. We found that large ion lithophile elements (LILEs) are significantly released during dehydration along both hot and cold subductions. However, some water-bearing residual minerals (e.g. lawsonite and Al-10 Å phase) are stable along a cold subduction P-T path and can carry significant amounts of Rb, Sr, Th, U and light rare earth elements (LREEs) into deep mantle. The mobilities of radioactive parent and radiogenic daughter (e.g. U-Pb, Th-Pb, Rb-Sr and Sm-Nd) are rather different during dehydration along different subduction P-T paths. Along both hot and cold subduction paths, Pb shows significantly greater mobility than U and Th. However, the mobile behavior of Rb-Sr and Sm-Nd totally opposite along cold and hot subduction P-T paths. We conclude that recycling of oceanic crust in mantle through cold subduction can lead to a geochemically heterogeneous mantle more efficiently than along a hot subduction.