Forearc diamond from Japan

Abstract

Convergent margins are not generally considered to be suitable places for the formation of diamond and its transport to Earth's surface. Microdiamonds found in xenoliths within a lamprophyre dike in southwest Japan show that this assumption is incorrect and, furthermore, that diamond occurs in a wider range of geological settings than previously realized. Petrological constraints show that these diamond-bearing minerals rose from depths of around 160 km (∼5.5. GPa) and cooled from temperatures of ∼1500 °C. The location of the diamond-bearing rocks in the forearc and close to the subducting plate requires the existence of mantle up-flow, which brought the diamond to shallow mantle levels before traveling 100-km-scale horizontal distances. If the dimensions of this flow are large, it can help explain both forearc magmatism and perhaps the development of subduction zones hot enough to melt sediments.