Atomic-scale mechanism of carbon nucleation from a deep crustal fluid by replica exchange reactive molecular dynamics simulation
- Université de Bordeaux (France); Centre National de la Recherche Scientifique (CNRS) (France)
- Université de Lausanne (Switzerland); Swiss Federal Institute of Technology, Zurich (Switzerland)
- Aix-Marseille Univ., Marseille (France). MultiScale Materials Science for Energy and Environment (MSe2); Centre National de la Recherche Scientifique (CNRS) (France); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States)
- Aix-Marseille Univ., Marseille (France). MultiScale Materials Science for Energy and Environment (MSe2); Centre National de la Recherche Scientifique (CNRS) (France); Massachusetts Inst. of Technology (MIT), Cambridge, MA (United States); George Washington Univ., Washington, DC (United States). EpiDaPo Lab
- Pennsylvania State Univ., University Park, PA (United States)
Here we present a mechanistic model of carbon nucleation and growth from a fluid at elevated temperature (T) and pressure conditions, typical of those found in the shallow Earth’s lithosphere. Our model uses a replica exchange reactive molecular dynamics framework in which molecular configurations are swapped between adjacent T replica at regular intervals according to underlying statistical mechanics. This framework allows predicting complex molecular structures and thermodynamics while remaining computationally efficient. Here we simulate the reactivity of an unstable mixture of CO2 and CH4 at 1000 K and 1 GPa. We find that the path to thermodynamic equilibrium is initially entropy-driven, producing a diversity of short-lived species, including various alcohols with intermediate carbon oxidation states. Cyclic and polycyclic radicals that are sometimes resonance-stabilized form next and set the stage for carbon nucleation. The carbon exsolution process releases abundant water, is exothermic and starts with the nucleation of a large aggregate of hydrogenated graphene flakes from covalently bonded polycyclic units. The carbon backbone of this nucleus subsequently grows into a hydrogen-depleted fullerene-like structure, before evolving toward a partially bilayered graphene layer. Overall, our results show that the mechanism of graphitic C formation is certainly not bimolecular, and that it may involve a combination of key condensation and radical chain reactions. This will help understand the isotopic, and reactive characteristics of carbon-bearing fluids during their upward transit through the Earth’s mantle and crust. Moreover, the mechanistic insights outlined here present intriguing similarities with the process of soot and interstellar dust formation, which suggests that the widespread distribution of abiotic polyaromatic and graphitic material on Earth and beyond may reflect the prevalence of a fundamental chemical pathway.
- Research Organization:
- Energy Frontier Research Centers (EFRC) (United States). Multi-scale Fluid-Solid Interactions in Architected and Natural Materials (MUSE)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES)
- OSTI ID:
- 1981627
- Journal Information:
- Geochimica et Cosmochimica Acta, Vol. 329, Issue C; ISSN 0016-7037
- Publisher:
- Elsevier; The Geochemical Society; The Meteoritical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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