Geochemistry of abyssal peridotites from ODP Hole 209-1274A

DOI

Aqueous dihydrogen (H2,aq) is produced in copious amounts when seawater interacts with peridotite and H2O oxidizes ferrous iron in olivine to ferric iron in secondary magnetite and serpentine. Poorly understood in this process is the partitioning of iron and its oxidation state in serpentine, although both impose an important control on dihydrogen production. We present results of detailed petrographic, mineral chemical, magnetic and Mößbauer analyses of partially to fully serpentinized peridotites from the Ocean Drilling Program (ODP) Leg 209, Mid-Atlantic Ridge (MAR) 15°N area. These results are used to constrain the fate of iron during serpentinization and are compared with phase equilibria considerations and peridotite-seawater reaction path models. In samples from Hole 1274A, mesh-rims reveal a distinct in-to-out zoning from brucite at the interface with primary olivine, followed by a zone of serpentine + brucite ± magnetite and finally serpentine + magnetite in the outermost mesh-rim. The compositions of coexisting serpentine (Mg# 95) and brucite (Mg# 80) vary little throughout the core. About 30-50% of the iron in serpentine/brucite mesh-rims is trivalent, irrespective of subbasement depth and protolith (harzburgite versus dunite). Model calculations suggest that both partitioning and oxidation state of iron are very sensitive to temperature and water-to-rock ratio during serpentinization. At temperatures above 330 °C the dissolution of olivine and coeval formation of serpentine, magnetite and dihydrogen depends on the availability of an external silica source. At these temperatures the extent of olivine serpentinization is insufficient to produce much hydrogen, hence conditions are not reducing enough to form awaruite. At T < 330 °C, hydrogen generation is facilitated by the formation of brucite, as dissolution of olivine to form serpentine, magnetite and brucite requires no addition of silica. The model calculations suggest that the iron distribution observed in serpentine and brucite is consistent with formation temperatures ranging from brucite -> serpentine + brucite, or reflects metastable olivine-brucite equilibria developing in the strong gradient in silica activity between orthopyroxene (talc-serpentine) and olivine (serpentine-brucite).

Supplement to: Klein, Frieder; Bach, Wolfgang; Jöns, Niels; McCollom, Thomas M; Moskowitz, Bruce; Berquo, Thelma (2009): Iron Partitioning and Hydrogen Generation During Serpentinization of Abyssal Perdotites from 15°N on the Mid-Atlantic Ridge. Geochimica et Cosmochimica Acta, 73(22), 6868-6893

Identifier
DOI https://doi.org/10.1594/PANGAEA.783396
Related Identifier https://doi.org/10.1016/j.gca.2009.08.021
Metadata Access https://ws.pangaea.de/oai/provider?verb=GetRecord&metadataPrefix=datacite4&identifier=oai:pangaea.de:doi:10.1594/PANGAEA.783396
Provenance
Creator Klein, Frieder; Bach, Wolfgang ORCID logo; Jöns, Niels ORCID logo; McCollom, Thomas M; Moskowitz, Bruce; Berquo, Thelma ORCID logo
Publisher PANGAEA
Publication Year 2009
Rights Creative Commons Attribution 3.0 Unported; https://creativecommons.org/licenses/by/3.0/
OpenAccess true
Representation
Language English
Resource Type Supplementary Publication Series of Datasets; Collection
Format application/zip
Size 2 datasets
Discipline Earth System Research
Spatial Coverage (-46.676 LON, 15.648 LAT); South Atlantic Ocean
Temporal Coverage Begin 2003-06-15T00:30:00Z
Temporal Coverage End 2003-06-18T21:45:00Z