Experimental constraints on fluid-rock reactions during incipient serpentinization of harzburgite
Grozeva, Niya G.
Seewald, Jeffrey S.
McCollom, Thomas M.
Humphris, Susan E.
Berquo, Thelma S.
MetadataShow full item record
KeywordSerpentinization; Phase relations; Reaction rates; Reaction pathways; Fracturing; Hydrogen; Hydrothermal experiment
The exposure of mantle peridotite to water at crustal levels leads to a cascade of interconnected dissolution-precipitation and reduction-oxidation reactions—a process referred to as serpentinization. These reactions have major implications for microbial life through the provision of hydrogen (H2). To simulate incipient serpentinization under well-constrained conditions, we reacted centimeter-sized pieces of uncrushed harzburgite with chemically modified seawater at 300 ºC and 35 MPa for ca. 1.5 yr (13 441 h), monitored changes in fluid chemistry over time, and examined the secondary mineralogy at the termination of the experiment. Approximately 4 mol% of the protolith underwent alteration forming serpentine, accessory magnetite, chlorite, and traces of calcite and heazlewoodite. Alteration textures bear remarkable similarities to those found in partially serpentinized abyssal peridotites. Neither brucite nor talc precipitated during the experiment. Given that the starting material contained ~4 times more olivine than orthopyroxene on a molar basis, mass balance requires that dissolution of orthopyroxene was significantly faster than dissolution of olivine. Coupled mass transfer of dissolved Si, Mg, and H+ between olivine and orthopyroxene reaction fronts was driven by steep activity gradients and facilitated the precipitation of serpentine. Hydrogen was released in significant amounts throughout the entire experiment; however, the H2 release rate decreased with time. Serpentinization consumed water but did not release significant amounts of dissolved species (other than H2) suggesting that incipient hydration reactions involved a volume increase of ~40%. The reduced access of water to fresh olivine surfaces due to filling of fractures and coating of primary minerals with alteration products led to decreased rates of serpentinization and H2 release. While this concept might seem at odds with completely serpentinized seafloor peridotites, reaction-driven fracturing offers an intriguing solution to the seemingly self-limiting nature of serpentinization. Indeed, the reacted sample revealed several textural features diagnostic of incipient reaction-driven fracturing. We conclude that fracturing must have far reaching impacts on the rates of serpentinization and H2 release in peridotite-hosted hydrothermal systems.
Author Posting. © The Author(s), 2014. This is the author's version of the work. It is posted here by permission of Mineralogical Society of America for personal use, not for redistribution. The definitive version was published in American Mineralogist 100 (2015): 991-1002, doi:10.2138/am-2015-5112.
Suggested CitationPreprint: Klein, Frieder, Grozeva, Niya G., Seewald, Jeffrey S., McCollom, Thomas M., Humphris, Susan E., Moskowitz, Bruce, Berquo, Thelma S., Kahl, Wolf-Achim, "Experimental constraints on fluid-rock reactions during incipient serpentinization of harzburgite", 2014-10-20, https://doi.org/10.2138/am-2015-5112, https://hdl.handle.net/1912/7269
Showing items related by title, author, creator and subject.
Kinetics of H2–O2–H2O redox equilibria and formation of metastable H2O2 under low temperature hydrothermal conditions Foustoukos, Dionysis I.; Houghton, Jennifer L.; Seyfried, William E.; Sievert, Stefan M.; Cody, George D. (2010-07-23)Hydrothermal experiments were conducted to evaluate the kinetics of H2(aq) oxidation in the homogeneous H2-O2-H2O system at conditions reflecting subsurface/near-seafloor hydrothermal environments (55-250 oC and 242-497 ...
Ciazela, Jakub; Dick, Henry J. B.; Koepke, Juergen; Pieterek, Bartosz; Muszynski, Andrzej; Botcharnikov, Roman; Kuhn, Thomas (2017-07)Gabbroic veins enclosed in mantle peridotite from ocean core complexes next to oceanic transform faults demonstrate sub-crustal crystallization of silicate minerals from a MORB-like melt. Cooler lithosphere there may ...
Modeling sulfate reduction in methane hydrate-bearing continental margin sediments : does a sulfate-methane transition require anaerobic oxidation of methane? Malinverno, Alberto; Pohlman, John W. (American Geophysical Union, 2011-07-12)The sulfate-methane transition (SMT), a biogeochemical zone where sulfate and methane are metabolized, is commonly observed at shallow depths (1–30 mbsf) in methane-bearing marine sediments. Two processes consume sulfate ...