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.
Highlights
At subduction zones, most diamonds form by carbon saturation in hydrous fluids released from lithospheric plates on equilibration with mantle rocks
The results show that significant amounts of carboxylic acids are dissolved in aqueous fluids released from deeply subducting slabs, and how they contribute to the nucleation and growth of diamonds
Diamond crystallization occurred by precipitation from an oxidized hydrous fluid buffered by redox equilibrium with surrounding slab rocks[9,20], which is preserved in fluid inclusions
Summary
A few nano-sized diamonds coated by conspicuous disordered and amorphous carbon in the sp[2] and sp[3] configurations (Fig. 2a–d) show Raman modes of attached functional groups, whose width and featureless nature suggest organic compounds (refer to Supplementary Note 2; Fig. 3; Table 2). In newly formed nano-sized diamonds, the high reactivity of surface carbon atoms could have induced the conversion of defective sp3-bonded carbon into graphitic or sp2–sp[3] amorphous carbon surface domains[39,40,41,45,48] (Fig. 3c) These processes, which reduce the total surface energy, might have inhibited physical or chemical interaction with solvent hydrous fluids at lower temperature[37,45]
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