Experimental constraints on fluid-rock reactions during incipient serpentinization of harzburgite

Abstract

24The exposure of mantle peridotite to water at crustal levels leads to a 25 cascade of interconnected dissolution-precipitation and reduction-oxidation 26 reactions -a process referred to as serpentinization. These reactions have major 27 implications for microbial life through the provision of hydrogen (H2). To simulate 28 incipient serpentinization under well-constrained conditions, we reacted cm-sized 29 pieces of uncrushed harzburgite with chemically modified seawater at 300°C and 35 30 MPa for ca. 1.5 years (13441 hours), monitored changes in fluid chemistry over 31 time, and examined the secondary mineralogy at the termination of the experiment. 32Approximately 4 mol % of the protolith underwent alteration forming serpentine, 33 accessory magnetite, chlorite, and traces of calcite and heazlewoodite. Alteration 34 textures bear remarkable similarities to those found in partially serpentinized 35 abyssal peridotites. Neither brucite nor talc precipitated during the experiment. 36Given that the starting material contained ~4 times more olivine than 37 orthopyroxene on a molar basis, mass balance requires that dissolution of 38 orthopyroxene was significantly faster than dissolution of olivine. Coupled mass 39 transfer of dissolved Si, Mg, and H + between olivine and orthopyroxene reaction 40 fronts was driven by steep activity gradients and facilitated the precipitation of 41 serpentine. Hydrogen was released in significant amounts throughout the entire 42 experiment; however, the H2 release rate decreased with time. Serpentinization 43 consumed water but did not release significant amounts of dissolved species (other 44 than H2) suggesting that incipient hydration reactions involved a volume increase of 45 ~40%. The reduced access of water to fresh olivine surfaces due to filling of 46 This is a preprint, the final version is subject to change, of the American Mineralogist (MSA) Cite as Authors (Year) Title. American Mineralogist, in press. (DOI will not work until issue is live.) DOI: http://dx.doi. org/10.2138/am-2015-5112 10/20Always consult and cite the final, published document.

Evolution of fracture permeability of ultramafic rocks undergoing serpentinization at hydrothermal conditions: An experimental study

“…Klein et al . [] also did not observe brucite in their experiments, as a result of simultaneous dissolution of olivine and orthopyroxene.…”

Experimental constraints on the serpentinization rate of fore‐arc peridotites: Implications for the upwelling condition of the slab‐derived fluid

“…A decrease in the amount of reacted olivine resulted in a lower magnesium/silicon ratio of the effective composition involved in the reaction, producing more talc rather than serpentine at high temperature and low pressure (Figures a and b). A faster opx reaction in an olivine‐opx‐H 2 O system was also reported in previous low‐pressure experiments at T ≥ 300°C [ Allen and Seyfried , ; Klein et al ., ].…”

“…To constrain the hydration processes mainly in the ocean floor, previous hydration experiments in ultramafic systems have been conducted at relatively low pressures (P) and temperatures (T) of up to 0.3 GPa and ca. 400°C, respectively, with focus on the extent and rate of the hydration reaction [ Martin and Fyfe , ; Wegner and Ernst , ; Okamoto et al ., ; Lafay et al ., ; Malvoisin et al ., ; Andreani et al ., ; Malvoisin and Brunet , ], the evolution of fluid chemistry during serpentinization [ Janecky and Seyfried , ; Allen and Seyfried , ], and hydrogen production during the reaction [ Marcaillou et al ., ; Klein et al ., ]. However, no peridotite hydration experiment has been conducted under the conditions of the fore‐arc mantle wedge, where antigorite, the high‐temperature variety of serpentine, is stable.…”

Serpentinite with and without brucite: A reaction pathway analysis of a natural serpentinite in the Josephine ophiolite, California

“…Experimental reactions of peridotite with seawater and seawater-derived solutions at 200-300°C, 350-500 bars, Figure 8. Alteration mineralogy predicted by CHIM-XPT models over a range of water-to-rock ratios for (a) harzburgite composition H3 (Table 2) reacted with water at 200°C and 20 bars and (b) harzburgite composition H3 with 10 vol% added orthopyroxene (Table 1) and 10-30 water-to-rock ratios showed that enstatite and olivine dissolution rates are similar over the course of the experiments, but that enstatite reacts out faster than olivine during the initial steps of serpentinization due to its higher chemical affinity in the initial seawater composition (Janecky and Seyfried, 1986;Klein et al, 2015). These experiments also showed that preferential dissolution of orthopyroxene gives brucite-free alteration assemblages.…”