Hydrothermal serpentinization of peridotite within the oceanic crust: Experimental investigations of mineralogy and major element chemistry |
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| Institution: | 1. Institute of Geochemistry and Petrology, ETH Zurich, Clausiusstrasse 25, 8092 Zürich, Switzerland;2. Bigelow Laboratory for Ocean Sciences, East Boothbay, ME, USA;3. University of Washington, School of Oceanography, Seattle, WA 98195, USA;4. Japan Agency for Marine-Earth Science and Technology, Kochi Institute for Core Sample Research, Kochi, Japan;5. ECORD Science Operator, British Geological Survey, Edinburgh EH14 4AP, UK;6. Institut de Physique du Globe de Paris, CNRS, 1 rue Jussieu, 75238 Paris, Cedex 05, France;7. University of Wyoming, Laramie, WY 82071, USA;8. University of Leeds, School of Earth and Environment, Leeds LS2 9JT, United Kingdom;9. Institute of Geological Sciences, Freie Universität Berlin, Malteserstrasse 74-100, 12249 Berlin, Germany;10. The Australian National University, Department Research School of Earth Sciences, Mills Rd, Acton, Australia;11. Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, United Kingdom;12. University of South Carolina, 701 Sumter St. EWS 617, Columbia, SC 29208, USA;13. Michigan State University, 288 Farm Lane, East Lansing, MI 48824,USA;14. University of Utah, 257 South 1400 East, Salt Lake City, UT 84112-0840, USA;15. Kanazawa University, Department of Earth Sciences, Kakuma Kanazawa, Ishikawa 920-1192, Japan;p. Institute of Geosciences and Earth Resources – CNR, Via Moruzzi 1, 56124 Pisa, Italy;q. Physics of Geological Processes, University of Oslo, Sem Sælands vei 24, 0371 Oslo, Norway;r. Mediterranean Institute of Oceanography (MIO), Aix Marseille Univ., Université de Toulon, CNRS, IRD, MIO UM 110, 13288 Marseille, France;s. Korea University, Department Earth and Environmental Sciences, Seoul 136-701, Republic of Korea;t. University of Colorado – Boulder, 2200 Colorado Ave., Boulder, CO 80466, USA;u. University of Southampton, National Oceanography Centre, European Way, Southampton SO14 3ZH, United Kingdom;v. GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, 24148 Kiel, Germany;w. University of Hawaii at Manoa, SOEST-Hawaii Institute of Geophysics and Planetology, 1680 East West Road, Honolulu 96822, USA;x. GeoZentrum Nordbayern, Friedrich-Alexander-University Erlangen-Nuremberg, Schlossgarten 5a, 91054 Erlangen, Germany;y. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Key Laboratory of Marginal Sea Geology, 511 Kehua Street, Tianhe, Guangzhou 510640, China;z. Shimane University, Faculty of Science and Engineering, 1060 Nishikawatsu Matsue, Shimane 690-8504, Japan;11. University of British Columbia, Earth, Ocean and Atmospheric Sciences, 2020-2207 Main Mall, Vancouver BC V6T-1Z4, Canada;12. Centre for Geobiology, University of Bergen, Allegaten 4, 5007 Bergen, Norway |
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| Abstract: | Mantle derived ultramafic rocks form a significant portion of lithosphere created at slow-spreading mid-ocean ridges. These rocks are ubiquitously serpentinized, due at least in part to interaction with seawater, at temperatures below approximately 500°C. To evaluate reaction pathways, primary mineral reaction rates, major element exchange between rock and solution, and alteration mineral formation, interaction of equigranular peridotites with seawater and seawater derived solutions has been investigated experimentally at 200°C and 300°C, 500 bars.Seawater chemistry changed greatly during the experiments. Initially, the concentrations of Mg, Ca, and SO4 decreased, as did pH. During Iherzolite experiments, however, the trend of dissolved Ca concentrations reversed with time, first decreasing, then increasing. pH also increased during the latter part of the experiments. Mg, Ca, SiO2, Fe, Cl and ΣCO2 decreased as pH increased FeII oxidation is shown to be affected by solution pH, being greatly enhanced under alkaline conditions. Resulting solution composition and reaction pathway are dependent on initial solution composition, particularly initial concentrations of Mg in solution. Consistent with changes in solution chemistry, the peridotites were significantly altered. Substantial amounts of olivine, relatively minor amounts of diopside and all the enstatite dissolved. Alteration products included serpentine + anhydrite ± magnesium hydroxide sulfate hydrate ± magnetite ± brucite ± tremolite-actinolite or truscottite.From the changes in solution chemistry and examination of the alteration products, the reaction rates (moles per unit time) of olivine to enstatite to diopside during 300°C Iherzolite-seawater experiments are estimated to be approximately 1.0/1.0/0.1. These rates correspond to constant surface area rates of 1.5:5:1 (moles per unit time per unit surface area), which are consistent with experimental data on the dissolution kinetics of these minerals and emphasize the importance of initial rock texture on reaction rates. |
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