Diamond growth from organic compounds in hydrous fluids deep within the Earth

Abstract

At subduction zones, most diamonds form by carbon saturation in hydrous fluids released from lithospheric plates on equilibration with mantle rocks. Although organic molecules are predicted among dissolved species which are the source for carbon in diamonds, their occurrence is not demonstrated in nature, and the physical model for crustal diamond formation is debated. Here, using Raman microspectroscopy, I determine the structure of carbon-based phases inside fluid inclusions in diamond-bearing rocks from the Alps. The results provide direct evidence that diamond surfaces are coated by sp2-, and sp3-bonded amorphous carbon and functional groups of carboxylic acids (e.g., carboxyl, carboxylate, methyl, and methylene), indicating the geosynthesis of organic compounds in deep hydrous fluids. Moreover, this study suggests diamond nucleation via metastable molecular precursors. As a possible scenario, with carbon saturation by reduction of carboxylate groups, I consider tetrahedral H-terminated C groups as templates for the growth of sp3-structured carbon.

Dissolution susceptibility of glass-like carbon versus crystalline graphite in high-pressure aqueous fluids and implications for the behavior of organic matter in subduction zones

“…The stability of glass-like carbon over diamond, although predicted by the suggested model in the 3.7-4.2 GPa pressure window, relies on thermodynamic parameters that are affected by uncertainties that are currently unquantifiable. The occurrence of nano-crystalline disordered graphitic carbon and amorphous sp 2 and sp 3 carbon has been indeed reported in micro-and nano-sized diamonds from Cignana Lake in the Western Alps (Frezzotti et al, 2014;Frezzotti, 2019) and glass-like X-ray amorphous carbon has been obtained experimentally at diamond-stable conditions (7.7 GPa and 1000°C;Yamaoka et al, 2002). However, more investigations are required to confirm this hypothesis.…”

Along-dip variations of subduction fluids: The 30–80 km depth traverse of the Schistes Lustrés complex (Queyras-Monviso, W. Alps)

“…Saturated hydrocarbons are formed in HP experiments by carbonate reduction (Tao et al, 2018) or transformation of dissolved organic species (Huang et al, 2017;Li, 2017). (Frezzotti et al, 2011;Frezzotti, 2019), could also contribute to form hydrocarbons at BS-facies conditions.…”

Role of Magnetite, Rutile and H2S in C-O-H Fluids’ Speciation: Implications For Localized Reduction Zones in the Shallow Upper Mantle

“…Two shoulder peaks at 2895 and 2913 cm −1 give this band a broader appearance and these peaks correspond to ethane and methane respectively 32 . The second three component band with maxima at 2969 cm −1 marks the presence of propane with characteristic signature of methyl group 32,34 . Many of these monophase inclusions (Fig.…”