Microdiamonds in garnet of graphite-free ultrahigh pressure metamorphic (UHPM) rocks from Lago di Cignana (western Alps, Italy) represent the first occurrence of diamond in a low-temperature subduction complex of oceanic origin (T=∼ 600° C; P⩾ 3.2 GPa). The presence of diamonds in fluid inclusions provides evidence for carbon transport and precipitation in an oxidized H 2 O-rich C–O–H crustal fluid buffered by mineral equilibria at sub-arc mantle depths. The structural state of carbon in fluid-precipitated diamonds was analyzed with 514 nm excitation source confocal Raman microspectroscopy. The first order peak of sp 3-bonded carbon in crystalline diamonds lies at 1331 (±2) cm− 1, similar to diamonds in other UHPM terranes. The analysis of the spectra shows additional Raman features due to sp 2 carbon phases indicating the presence of both hydrogenated carbon (assigned to trans-polyacetylene segments) in grain boundaries, and graphite-like amorphous carbon in the bulk, ie showing a structural disorder much greater than that found in graphite of other UHPM rocks. In one rock sample, disordered microdiamonds are recognized inside fluid inclusions by the presence of a weaker and broader Raman band, downshifted from 1332 to 1328 cm− 1. The association of sp 3-with sp 2-bonded carbon indicates variable kinetics during diamond precipitation. We suggest that precipitation of disordered sp 2 carbon acted as a precursor for diamond formation outside the thermodynamic stability field of crystalline graphite. Diamond formation started when the H 2 O-rich fluid reached the excess concentration of C required for the spontaneous nucleation of diamond. The interplay of rock buffered f O 2 and the prograde P–T path at high pressures controlled carbon saturation. Thermodynamic modeling confirms that the C–O–H fluids from which diamond precipitated must have been water rich (0.992< X H 2 O< 0.997), assuming that f O 2 is fixed by the EMOD equilibrium.