Serpentinized ultramafic rocks constitute a major part of the oceanic lithosphere. They form when water interacts with olivine and pyroxene—the primary minerals of Earth’s mantle—to form serpentine, magnetite, and brucite. Serpentinites are relatively widespread: They are commonly found along slow to ultra-slow spreading mid-ocean ridges, where tectonic unroofing exposes sections of the mantle to the ocean floor inducing mantle hydration (e.g., Cannat et al., 1992; Bach and Früh-Green, 2010), in deep fractures in the bend of the subducting lithospheric slab (e.g., Ranero et al., 2003; Ivandic et al., 2010), within the mantle wedge of subduction zones as a response to dehydration reactions in the downgoing slab (Hyndman and Peacock, 2003; Scambelluri and Tonarini, 2012), and in ultramafic rocks emplaced on the continent that are serpentinized upon interaction with meteoric waters (e.g., Barnes et al., 1967; Barnes and O’Neil, 1969). Serpentinization has implications on many global-scale processes and impacts, for example the chemical budget of the oceans (Snow and Dick, 1995; Früh-Green et al., 2004), and regulates the chemical flux into Earth’s mantle (Scambelluri et al., 1995; Ulmer and Trommsdorff, 1995). In addition, serpentinization can support diverse microbial communities (Kelley et al., 2005; Brazelton et al., 2006) and has even been hypothesized to play a major role in the origin of life on Earth and other planetary bodies (e.g., Martin and Russell, 2007; Martin et al., 2008).
However, despite the intense study of serpentinization in diverse aspects of geochemistry, geophysics, and biology, one aspect of the serpentinization process has remained relatively poorly understood; serpentinization should be a self-limiting process because the mineral transformation is accompanied by a significant volume increase of 25–50%. Such a volume expansion should result in the closure of fluid pathways and reduce fluid migration, preventing further hydration of the primary minerals (Coleman and Keith, 1971; Macdonald and Fyfe, 1985; O’Hanley, 1992; Plümper et al., 2012). In this issue of Geology, Tutolo et al. (2016, p. 103) use neutron scattering techniques to look for nano-scale pore spaces that can serve as fluid pathways. In fact, they find that growth of new phases such as serpentine generates more porosity than is present in the primary minerals. This finding is pioneering as it provides a mechanism for fluid migration to pervasively hydrate ultramafic rocks.
Previous studies have focused on different aspects of fluid flow within ultramafic lithologies. In mid-ocean ridge settings protracted serpentinization of ultramafic rocks is partly explained by the creation of fluid pathways through tectonic processes; the generation of fractures caused by changes in the stress regime during lithospheric uplift, formation of faults during extensional unroofing of the mantle rock, and mechanical weathering all can provide new pathways for fluids (e.g., Kelley et al., 2001; Boschi et al., 2006; Andreani et al., 2007). In the absence of external forces, progressive serpentinization has been more challenging to explain and recently focus has been mostly on a process described as reaction-driven cracking (Jamtveit et al., 2009; Kelemen and Hirth, 2012; Plümper et al., 2012). On the micrometer scale, the conversion of olivine to serpentine is a dissolution-precipitation process along mineral grain boundaries. At this scale, the volume expansion during serpentine formation may increase the stress sufficiently to crack the host rock and to allow intense self-propagating fracturing (Macdonald and Fyfe, 1985; O’Hanley, 1992; Kelemen and Hirth, 2012). This is reflected by the formation of a network of micro-fractures in adjacent minerals; i.e., new porosity, permeability, and reactive surface area is produced. However, Tutolo et al. argue that because serpentinization is relatively rapid, serpentine should immediately form in the newly opened spaces substantially reducing both porosity and permeability and thus limit fluid migration. Additionally, the formation of a serpentine + brucite rind may limit further interaction of olivine with fluid. As an alternative, Tutolo et al. suggest that fluid-flow and the generation of porosity happen at a far finer scale. Using neutron scattering they determine the porosity of variably serpentinized peridotites from the Atlantis Massif (Mid-Atlantic Ridge) and the Duluth Complex in the USA (see Tutolo et al. for references). Their results imply that during formation of serpentine from olivine, the porosity as represented by nano-scale pores increases! In other words, formation of serpentine as well as other phyllosilicates produces new fluid pathways. Tutolo et al. suggest that the pore space is present as crystal internal porosity and as porosity as a result of mineral internal defects—pores in the submicrometer scale are generated within the minerals.
This nano-scale porosity would allow continued fluid migration within a partly hydrated peridotite and has direct implications for vein internal fluid flow. For instance, growth in serpentine vein width is commonly reported as a result of increased olivine hydration (e.g., Beard et al., 2009). Within serpentine veins, porosity on the submicron scale could promote continuous fluid migration toward the surface of the primary minerals and allow their continued replacement and an increase in vein width. Similarly, Schwarzenbach et al. (2015) describe the formation of chemical gradients (e.g., gradients in water and silica activity) between the center of serpentine veins and the olivine grain boundaries. These gradients may be mediated by an interconnected pore space between vein center and vein rim (i.e., between the parent olivine grain and the fluid-rich vein core). The study by Tutolo et al. provides evidence that such an interconnected porosity may exist within the newly formed serpentine. Alternatively, fluid pathways may exist along grain boundaries of finely intergrown serpentine-brucite, which is often observed in serpentine veins replacing olivine, producing a fine-grained texture on the submicron scale (e.g., Beard et al., 2009; Schwarzenbach et al., 2015). Such a submicron intergrowth possibly also provides pathways for fluids to migrate at a lack of mineral internal porosity.
In addition, a nano-scale porosity may also have implications on the sequestration of CO2. Especially in recent years, serpentinization has increasingly gained interest due to the potential of ultramafic lithologies to sequester CO2 (Kelemen and Matter, 2008; Boschi et al., 2009; Schwarzenbach et al., 2013) and thus may be a possible way to mitigate the ever-increasing atmospheric CO2 levels. However, during CO2 sequestration (or carbonation) of ultramafic rocks the same problems are encountered as during their hydration; formation of carbonate minerals in peridotite will seal fluid pathways. In this case, reaction-driven cracking has been suggested to play a critical role to continue fluid flow (e.g., Jamtveit et al., 2008; Jamtveit et al., 2009; Kelemen and Hirth, 2012), though the results from Tutolo et al. may provide an additional explanation for fluid migration during carbonation.
Considering the importance and potential utility of serpentinization reactions, the study of microtextures on the submicrometer scale sheds new light on how serpentinization reactions proceed and how ultramafic rocks can be hydrated, even without substantial external (e.g., tectonic) forces. The study of Tutolo et al. is a first step toward a new research focus on processes that occur in these systems on the nano scale. However, many more questions are left to answer: How efficient is fluid transfer on the nano scale? Can fluid migration take place along grain boundaries of serpentine-brucite intergrowths? Is nano-scale porosity controlled by the type of serpentine mineral (e.g., lizardite vs chrysotile)? And what is the influence of temperature? Specifically, at low temperatures, nano-scale porosity may influence the extent of hydrogen formation that may be used by microbes living in continental serpentinization environments. Thus, further research is needed to clarify submicron-scale processes in order to understand how these processes affect serpentinization environments or what role they play during fluid-solid interaction within Earth’s lithosphere.
I thank Mark Caddick, Benjamin Gill, Timm John, and James Spotila for their valuable comments and input.