Hydrothermal experiments on serpentinization at crust/mantle boundary

Serpentinization commonly proceeds in seafloor hydrothermal systems at mid-ocean ridges, along the bending faults, and at the boundary of wedge mantle and subducting plate. Silica activity are key factors in controlling reaction paths and the rate of serpentinization (e.g., Frost and Beard, 2007; Klein et al., 2009; Ogasawara et al.,2013). However, most of the previous experimental studies focused on bulk solid materials and solutions within the reaction vessel, and local changes of products reaction rate in response to concentration gradient have not been clarified. Ogasawara et al. (2013) conducted hydrothermal experiments in Ol-Opx-H2O system, and modeled the progress of serpentinization by coupled reactions and silica diffusion. In their experiment, reaction product is only serpentine and no talc or brucite were found. In this study, we conducted hydrothermal experiments in olivine (Ol)-quartz (Qtz)-H2O and Ol-plagioclase (Pl)-H2O systems as the analogue of crust/mantle boundary. The condition was 250 degreeC and at a vapor-saturated pressure. Composite powders (composed of Qtz/Ol zone, or Pl/Ol zone) were set in tube-in-tube vessels, and then loaded into autocrave with fluid ( NaOHaq, pH = 13.8 at 25 degreeC ). Runnig time is up to 25 days and maximum water content in the products is 12 wt% H2O. After the experiments, solution chemistry and the extent of serpentinization were analyzed in detail. In the Ol-Qtz-H2O experiments, we observed systematic changes of reaction products in the Ol zone. Smectite and serpentine was formed at Ol-Qtz boundary due to high Na concentration although talc is expected to form in MgO-SiO2-H2O system at Ol-Qtz boundary. Mg/Si ratio of products from EDS analyze shows high Si gradient near the boundary indicate that amount of smectite decreased with increasing distance from the Ol-Qtz boundary and only serpentine zone was observed at ~10mm. At >10mm away from Ol-Qtz boundary, serpentine ( chrysotile nano tubes) and brucite was observed. Extent of hydration within the reaction tube is low (TG loss = 3.5 wt% H2O) at ~10mm (smectite zone), and it increases (TG loss =12 wt% H2O) to 40mm (the top of inner tube).Brucite and serpentine zone retreated from the boundary, by probably dehydration reaction due to silica input. In Ol-Pl-H2O experiment, Al-rich lizardite was only formed near the Ol-Pl boundary whreas serpentine (chrysotile nano tubes) + brucite was formed away from Ol-Qtz boundary. Our results imply that input of silica from the crust decelerate the overall hydration within the boundary because it inhibit the brucite formation and enhance formation the less-hydrous minerals (talc/smectite). Reference Klein, F., Bach, W., Jöns, et al (2009). Geochimica et Cosmochimica Acta, 73, 6868-6893. Frost, B. R., & Beard, J. S. (2007). Journal of Petology 48, 1351-1368. Ogasawara et al. (2013), Geochimica et Cosmochimica Acta, 119, 212-230.