Premium
Diamond in metasedimentary crustal rocks from Pohorje, Eastern Alps: a window to deep continental subduction
Author(s) -
Janák M.,
Froitzheim N.,
Yoshida K.,
Sasinková V.,
Nosko M.,
Kobayashi T.,
Hirajima T.,
Vrabec M.
Publication year - 2015
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/jmg.12130
Subject(s) - geology , kimberlite , diamond , subduction , eclogite , geochemistry , metamorphism , terrane , metamorphic rock , mantle (geology) , paleontology , materials science , tectonics , composite material
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 1330 cm −1 , with a full width at half maximum between 3 and 5 cm −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), CO 2 and CH 4 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 ≥3.5 GP a and 800–850 °C, similar to eclogites and garnet peridotites. We argue that diamond formed when carbonaceous sediment underwent UHP metamorphism at mantle depth exceeding 100 km during continental subduction in the Late Cretaceous ( c . 95–92 Ma). The finding of diamond confirms UHP metamorphism in the Pohorje Mountains, the most deeply subducted part of Austroalpine units.