Building and Destroying Continental Mantle

Abstract

Continents, especially their Archean cores, are underlain by thick thermal boundary layers that have been largely isolated from the convecting mantle over billion-year timescales, far exceeding the life span of oceanic thermal boundary layers. This longevity is promoted by the fact that continents are underlain by highly melt-depleted peridotites, which result in a chemically distinct boundary layer that is intrinsically buoyant and strong (owing to dehydration). This chemical boundary layer counteracts the destabilizing effect of the cold thermal state of continents. The compositions of cratonic peridotites require formation at shallower depths than they currently reside, suggesting that the building blocks of continents formed in oceanic or arc environments and became “continental” after significant thickening or underthrusting. Continents are difficult to destroy, but refertilization and rehydration of continental mantle by the passage of melts can nullify the unique stabilizing composition of continents.