Low‐Temperature Hydrogen Formation During Aqueous Alteration of Serpentinized Peridotite in the Samail Ophiolite

Abstract

Reactions that occur during the hydration and aqueous alteration of ultramafic rocks (collectively, serpentinization reactions) are important where fluids circulate in seafloor oceanic crust and uplifted mantle rocks on Earth. Evidence of serpentinization has also been found in carbonaceous chondrite meteorites (Velbel et al., 2012), and serpentine minerals have been detected spectrally on a variety of rocky bodies in the solar system (Mars,

Initial Results From the Oman Drilling Project Multi‐Borehole Observatory: Petrogenesis and Ongoing Alteration of Mantle Peridotite in the Weathering Horizon

“…On the one hand, this kind of reaction might not be an important source of SiO 2 in the MBO, since many investigators have concluded that formation of magnetite during oxidative, hydrous alteration of peridotite is uncommon below ∼200°C (e.g., Klein et al., 2009, 2014; McCollom & Bach, 2009; McCollom et al., 2016; Streit et al., 2012). On the other hand, however, whole core magnetic susceptibility data (Kelemen et al., 2021d, 2021f, 2021g; Templeton et al., 2021) indicate an average abundance of less than 1.5 wt% magnetite throughout most of the core (e.g., Figure 1 and related calculations in Ellison et al., 2021). Alternatively, Streit et al.…”

“…Thus, though small amounts of Mg removal may have occurred, it is likely that the pervasively low Mg/Si in MBO harzburgites and dunites has been produced mainly by SiO 2 addition. In particular, many workers have proposed that serpentine can form, in part, via addition of dissolved SiO 2 to brucite, in general (Bach et al., 2004, 2006; Beard et al., 2009; Frost et al., 2013; Klein & Bach, 2009; Klein et al., 2009; Tutolo et al., 2018) and related to specific observations in the MBO (Ellison et al., 2021; Mayhew et al., 2018; Miller et al., 2016; Templeton & Ellison, 2020; Templeton et al., 2021). Evaluating the likely effect of water/rock reaction, using observed fluid compositions from the MBO and elsewhere in Oman, provides substantial support for the SiO 2 addition hypothesis.…”

“…In turn, Mg‐brucite‐Mg‐chrysotile equilibrium in this SiO 2 activity range constrains pH to be ≥ 10. Mg‐brucite and Mg‐chrysotile in combination with andradite, which is commonly intergrown with serpentine in core samples from the MBO (Ellison et al., 2021; Kelemen et al., 2021d, 2021f, 2021g; Templeton et al., 2021), constrain pH ≤ 11.5. Though pH in borehole waters increases downward near the top of most holes, it is variable at depth, and does not show the asymptotic patterns that are evident in the fO 2 data.…”

“…FeO contents of shipboard rock powders from core from Hole BA1A were measured at the Geoscience Laboratories of the Ontario Geological Survey, using methods described online at https://www.mndm.gov.on.ca/sites/default/files/2021_geo_labs_brochure.pdf. It is possible that some redox‐sensitive minerals became oxidized prior to analysis (e.g., brucite may become oxidized in air within a matter of days, Ellison et al., 2021). Fe 2 O 3 contents of our samples were then estimated by subtraction of these FeO contents (recalculated as Fe 2 O 3 from total iron measured as Fe 2 O 3 onboard DV Chikyu.…”

“…First, there has been widespread recognition that highly reducing environments and large redox gradients produced during serpentinization (e.g., Frost, 1985) can stabilize hydrocarbons and provide energy for microbial chemosynthesis, leading to an explosion of hypotheses concerning the subsurface biosphere, abiotic hydrocarbon genesis, and the origin of life on this and other planets (e.g., Berndt et al., 1996; Boulart, et al., 2013; Ehlmann et al., 2009, 2010; Etiope et al., 2013; Fisk & Giovannoni, 1999; Foustoukos & Seyfried, 2004; Frost & Beard, 2007; Horita & Berndt, 1999; Martin et al., 2008; Martin & Russell, 2007; McCollom, 1999, 2007; Russell et al., 2010; Schulte et al., 2006; Shock, 1997; Shock et al., 1995; Sleep et al., 2004, 2011; Varnes et al., 2003). There has been a substantial amount of work on the Samail ophiolite related to this topic (Boyd et al., 2020; Canovas et al., 2017; de Obeso & Kelemen, 2018, 2020; Ellison et al., 2021; Fones et al., 2019, 2021; Glombitza et al., 2021; Kraus et al., 2021; Leong, Howells, et al., 2021; Leong, Nielson, McQueen, McGillis, & Kelemen, 2021; Leong & Shock, 2020; Mayhew et al., 2018; Miller et al., 2016; Newman et al., 2020; Nothaft, Templeton, Boyd, et al., 2021; Nothaft, Templeton, Rhim, et al., 2021; Rempfert et al., 2017; Sano et al., 1993; Templeton et al., 2021; Vacquand et al., 2018).…”