Diamonds from the Alps reveal carbon mobility in subduction zones

Abstract

Microdiamonds in garnetites of graphite-free ultrahigh pressure metamorphic rocks (T = 600 °C; P = 3.2 GPa) from Lago di Cignana (western Alps, Italy) represent the first occurrence of diamond in a low-temperature subduction complex of oceanic origin. The presence of diamonds in fluid inclusions provides evidence for carbon dissolution, transport, and precipitation in oxidized C–O–H crustal fluid buffered by mineral equilibria at sub-arc mantle depths.