DIAMOND FORMATION BY CARBON SATURATION IN C-O-H FLUIDS DURING COLD SUBDUCTION OF OCEANIC LITHOSPHERE

摘要

The origin of microdiamonds in ultrahigh pressurernmetamorphic (UHPM) terranes recrystallized atrnsubduction-zone depths≥100 km is a significantrngeological question, because it relates to the waysrnin which crustal carbon is recycled in the Earth.rnThe most widely accepted hypothesis on the originrnof metamorphic diamonds is crystallization fromrnmobile C-bearing C-O-H fluids or melts (cf.,rnDobrzhinetskaya, 2012), although solid-staterntransformation from graphite has also beenrnproposed. Recently, microdiamonds have beenrndescribed in UHP ocean floor metasediments in thernLago di Cignana Unit (LCU) in the western Alpsrn(Frezzotti et al., 2011). This new diamondrndiscovery has several characteristics that are wellrnsuited to study carbon transport and redox gradientsrnduring deep subduction (Frezzotti et al., 2014): (a)rnhost metamorphic rocks are of oceanic rather thanrncontinental affinity, (b) diamond formationrnoccurred at low T (about 600℃) and at P consistentrnwith subarc depths (≥ 3.2 Gpa), and © diamondrnprecipitated from an oxidized, carbonate-bearingrnH_2O-rich fluid buffered by mineral equilibria.