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.