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Title: Reaction path model of the formation of abiotic immiscible hydrocarbon fluids in subducted carbonated serpentinites, Lanzo Massif (Western Italian Alps)

Journal Article · · Lithos
 [1];  [2];  [3];  [4]
  1. Johns Hopkins University, Baltimore, MD (United States); University of Idaho, Moscow, ID (United States); Carnegie Institution for Science, Washington, DC (United States); Johns Hopkins University
  2. Johns Hopkins University, Baltimore, MD (United States)
  3. Universite Claude Bernard Lyon1, Villeurbanne (France); Ecole Normale Supérieure de Lyon (France)
  4. Universit di Bologna (Italy); Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC) Sorbonne Université, Paris (France); National Research Council of Italy, Pisa (Italy)
+ Show Author Affiliations

Fluids generated from subducted slabs participate in the cycling of deep carbon in the crust and upper mantle. In these fluids, aqueous carbon species vary in oxidation state between +IV and -IV depending on whether the fluids are oxidizing or reducing, respectively. Most studies of subduction-zone fluids have focused on oxidized carbon species. However, recent studies of natural samples have demonstrated the occurrence of deep, reducing fluids, generated in both subducted oceanic upper mantle and crustal rocks. CH4-H2-rich fluid inclusions in subducted carbonated serpentinites have demonstrated the existence of abiotic, immiscible, hydrocarbon fluids at upper mantle conditions. To investigate the formation of such immiscible hydrocarbon fluids during the evolution of subducted carbonated serpentinites, we used equilibrium constants from the Deep Earth Water model to carry out predictive chemical mass transfer modeling to simulate the alteration reactions. A novel feature of the models was the inclusion of an immiscible hydrocarbon fluid containing six components (CH4,f, C2H6,f, C3H8,f, isoC4H10,f, CO2,f, H2,f). This feature enabled prediction of the formation of a separate immiscible fluid in equilibrium with aqueous species and minerals. We developed a predictive reaction path model of invasive H2,f reacting with carbonated serpentinites and interstitial aqueous fluids for comparison with the natural samples from the Lanzo Massif, western Italian Alps. Over a range of temperatures and pressures, immiscible hydrocarbon fluids formed in association with altered mineral assemblages. Reaction progress caused the transformation of carbonated serpentinites and the formation of clinopyroxene, brucite, graphite, and hydrocarbon fluids, along with changes of pH, logfO2, and aqueous species. CH4,f was the most abundant hydrocarbon species in all the models. The overall results at 2.0 GPa and 400 to 450 °C were consistent with the natural samples from the Lanzo Massif. Interestingly, large amounts of H2O formed due to oxidation of H2. More hydrocarbons and H2O formed in models with lower fluid/rock mass ratios or with more reactant H2. Models at different pressure and temperature conditions showed similar results with some variation in the relative stabilities of aragonite, graphite and olivine solid solution, and associated differences in mineral sequences, hydrocarbon fluids, values of aqueous species, and the final log fO2 and pH. As a result, our models strongly support the laboratory and field evidence that reduction of carbonated serpentinites by infiltrating H2 fluids can cause the formation of immiscible, abiotic hydrocarbon fluids in subduction zones.

Research Organization:
Johns Hopkins University, Baltimore, MD (United States)
Sponsoring Organization:
European Union's Horizon 2020 research and innovation programme; NSF Petrology and Geochemistry; USDOE Office of Science (SC), Basic Energy Sciences (BES). Materials Sciences & Engineering Division (MSE)
Grant/Contract Number:
SC0019830
OSTI ID:
2482236
Journal Information:
Lithos, Journal Name: Lithos Vol. 468-469; ISSN 0024-4937
Publisher:
ElsevierCopyright Statement
Country of Publication:
United States
Language:
English

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58 GEOSCIENCES
Abiotic hydrocarbon
Carbonated serpentinites
DEW model
Deep carbon fluids
Immiscible hydrocarbon fluids
Subduction zones
Thermodynamic modeling