Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction

We report the first finding of diamond and moissanite in metasedimentary crustal rocks of Pohorje Mountains (Slovenia) in the Austroalpine ultrahigh-pressure (UHP) metamorphic terrane of the Eastern Alps. Microscopic observations and Raman spectroscopy show that diamond occurs in situ as inclusions in garnet, being heterogeneously distributed. Under the optical microscope, diamond-bearing inclusions are of cuboidal to rounded shape and of pinkish, yellow to brownish colour. The Raman spectra of the investigated diamond show a sharp, first order peak of sp(3)-bonded carbon, in most cases centred between 1332 and 1330cm(-1), with a full width at half maximum between 3 and 5cm(-1). Several spectra show Raman bands typical for disordered graphitic (sp(2)-bonded) carbon. Detailed observations show that diamond occurs either as a monomineralic, single-crystal inclusion or it is associated with SiC (moissanite), CO2 and CH4 in polyphase inclusions(.) This rare record of diamond occurring with moissanite as fluid-inclusion daughter minerals implies the crystallization of diamond and moissanite from a supercritical fluid at reducing conditions. Thermodynamic modelling suggests that diamond-bearing gneisses attained P-T conditions of 3.5GPa and 800-850 degrees C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100km during continental subduction in the Late Cretaceous (c. 95-92Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.