7.5.4: Serpentinization
- Page ID
- 131859
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)Serpentinization is the hydration and oxidation of ultramafic rocks to produce serpentinites, which are composed of serpentine group minerals with varying amounts of brucite, magnetite, and/or ironnickel alloys (Sleep et al., 2004). Ultramafic rocks are mainly composed of olivine and pyroxene. Alteration of these minerals creates strongly reducing conditions that can generate dihydrogen, methane, and other low-molecular weight organic compounds, which can be used as energy sources by chemotrophs (Proskurowski et al., 2008; Sleep et al., 2004).
The overall process of serpentinization can be illustrated by the following reaction (McCollom and Seewald, 2013): \[\text{ Olivine } + \text{ pyroxene } + \text{ H}_{2}\text{O} \longleftrightarrow \text{ serpentine } +/- \text{ brucite } +/- \text{ magnetite } +/- \text{ H}_{2}\]
The reaction is largely restricted to environments with temperatures below \(330\) to \(400^{\circ} \mathrm{C}\) because at higher temperatures, olivine is stable in the presence of water (McCollom and Bach, 2009). During serpentinization, dihydrogen is produced because the olivine and pyroxene contain ferrous iron, which can oxidize to ferric iron and reduce water as illustrated in the following reaction (McCollom and Seewald, 2013): \[3 \ \left(\text{Fe}^{2+}\text{O}\right)_{\text{rock}} + \text{H}_{2}\text{O} \longleftrightarrow \left(\left(\text{Fe}^{2+}\right) \left(\text{Fe}^{3+}\right)_{2} \text{O}_{4}\right)_{\text{rock}} + \text{H}_{2}\]
Magnetite can serve as a sink for the oxidized iron. The iron can also reside in the serpentine minerals. The fate of the iron is important, from the perspective of microbiology, because it helps determine the amount of dihydrogen that is produced during serpentinization (McCollom and Bach, 2009).
You may be wondering how organic compounds are produced? The dihydrogen produced from serpentinization can in turn form methane and other organics through abiotic reactions with carbon dioxide: \[\text{CO}_{2} + 4 \ \text{H}_{2} \longleftrightarrow \text{CH}_{4} + 2 \ \text{H}_{2}\text{O}\]
This reaction is equivalent to that catalyzed by hydrogenotrophic methanogens (reaction \(5.1.4.2\)). In a system hosting serpentinization, the reaction can occur abiotically because of the elevated temperatures and strongly reducing conditions. However, the reaction becomes slow without catalysis at temperatures below about \(350^{\circ}\mathrm{C}\) (Seewald et al., 2006). Therefore, the temperature influences the extent to which serpentinization generates potential organic electron donors.
Microbial communities can only be supported by serpentinization where ultramafic rocks are exposed to aqueous alteration. This includes a significant portion of the seafloor (Brazelton et al., 2013). Formation and exposure of ultramafic rocks is most common along the axes of slow spreading ocean ridges where the mantle is too cool to form basalts (Dick et al., 2003; Sleep et al., 2004). About 10% of the oceanic crust forms along these locations (Dick et al., 2003). In addition to marine environments, serpentinization also occurs on portions of each continent (Brazelton et al., 2013). Thus, serpentinization has the potential to support microbial communities across a wide range of settings.