Abstract
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 <150 to 250 °C and bulk water-to-rock ratios between 0.1 and 5. These conditions coincide with peak hydrogen fugacities during serpentinization and are conducive to awaruite formation during main stage serpentinization. The development of the common brucite rims around olivine is either due to an arrested reaction olivine → 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).
Original language | English (US) |
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Pages (from-to) | 6868-6893 |
Number of pages | 26 |
Journal | Geochimica et Cosmochimica Acta |
Volume | 73 |
Issue number | 22 |
DOIs | |
State | Published - Nov 15 2009 |
Bibliographical note
Funding Information:The authors thank Michael Hentscher for his help in setting up the thermodynamic database. Many thanks go to Ron Frost for inspiring and stimulating discussions. James Beard, Dionysis Foustoukos and Ivan Savov are thanked for helpful and critical reviews. Jeff Alt is thanked for the editorial handling. We thank Barbara Mader and Peter Appel for their assistance with the electron microprobe analyses. Carlos Garrido and Holger Paulick provided sample material and thin sections. We used samples supplied by the Ocean Drilling Program (ODP). ODP is sponsored by the US National Science Foundation (NSF) and participating countries under management of Joint Oceanographic Institutions (JOI), Inc. This work was supported with funds from the Special Priority Program 1144 of the German Science Foundation ( BA 1605/1-1 and BA 1605/1-2 ) and by the DFG-Research Center/Excellence Cluster ‘The Ocean in the Earth System’ . This is publication No. 40 of the Priority Program 1144 “From Mantle to Ocean: Energy-, Material- and Life-cycles at Spreading Axes”. The Institute for Rock Magnetism (IRM) is funded by the Instrumentation and Facilities program of the Earth Science Division of NSF ( NSF/EAR0732473 ), the W. M. Keck Foundation and University of Minnesota. This is IRM publication No. 0901.
Copyright:
Copyright 2011 Elsevier B.V., All rights reserved.