Can nanodiamonds grow in serpentinite-hosted hydrothermal systems? A theoretical modelling study

Abstract

Nanodiamonds can be synthesized hydrothermally in the laboratory by using a C-O-H fluid in the graphite stability field, in which the graphite/nanodiamond transition depends on the crystal size as a function of temperature. In nature, the hydrothermal circulation of seawater in serpentinites plays an important role in the carbon speciation in the oceanic crust and exposed mantle, in which hydrocarbons (mainly CH4) of abiogenic origin (via Fischer-Tropsch-type reaction) and occasionally graphite particles are detected. Can nanodiamonds nucleate and grow in serpentinite-hosted hydrothermal systems? To answer this question, a theoretical modelling study which compares the physico-chemical conditions in hydrothermal synthesis with those observed in modern and fossil serpentinite-hosted hydrothermal systems is proposed. Nanodiamonds are predicted to precipitate from a C-O-H fluid, consisting of CH4-CO2-H2O at 350 – 400°C andP<0.2 GPa near the FMQ buffer (Fayalite-Magnetite-Quartz), which are conditions compatible with those existing in serpentinite-hosted hydrothermal systems. In these environments, carbon-supersaturated fluids can be derived from water consumption (serpentine formation) under low water/rock ratios, which may promote the growth of nanodiamonds. This theoretical approach sheds light on the intriguing problem of carbon speciation in abyssal-type hydrothermal systems, suggesting that serpentinites may host nanodiamond deposits, even though none have been found yet.

Nanodiamond Finding in the Hyblean Shallow Mantle Xenoliths

“…Our results confirm that nanodiamonds can form during serpentinization in the stability field of graphite, from an organic-water system (ref. 12), similar to that occurring in some meteoritic parent bodies (ref. 7,24).…”

“…These temperatures were estimated by using the size-temperature equation extracted from Manuella (ref. 12). Similar values derive from some petrologic evidence: (i) the presence of lizardite (ref.…”

“…Accordingly, Manuella12 inferred that modern and fossil hydrothermal systems hosted in serpentinites, where organic matter and water coexist at a wide range of temperatures, can be considered as potential sites for the nucleation and growth of nanodiamonds.…”

Natural Graphite Cuboids

“…Moderately oxidizing conditions are favorable for diamond crystallization from COH fluids [51], while in reducing fluids, only graphite precipitates, even under conditions of the diamond stability field [129]. Based on theoretical constraints, Manuella [130] and Manuella et al [131] argued that nanodiamond and other carbon compounds can originate hydrothermally from COH fluids in serpentinite-hosted hydrothermal systems. The crucial role in the nanodiamond stability was attributed to a crystal size as a function of temperature (see Figure 2 in [130]).…”

Diamond synthesis in aluminum matrix in molten alkali-halide at ambient pressure

“…The metal or alloys used at the diamond's synthesis are conventionally assumed to provide a good wetting of diamond and graphite in order to dissolve carbon and to have low melting temperatures. The intermediate metal solvent carbides formation was described as the factor that facilitates cubic diamonds formation [16]. Otherwise, the synthesis temperature and the applied pressure would be high.…”