The mechanisms of fluid penetration through the gabbroic lower crust are important for the hydration of oceanic lithosphere. In the Oman ophiolite, which preserves an entire sequence of oceanic lithosphere formed at a fast-spreading ridge, the layered gabbros and dunites are extensively serpentinized. In this paper, we describe the characteristic textures of serpentinized troctolite and olivine gabbros recovered from the CM1A site of the Oman Drilling Project. In the troctolite, an olivine mesh texture is pervasively developed and is characterized by two types of veins: early lizardite + brucite + magnetite and late Al-rich lizardite + magnetite. These veins suggest the initiation of serpentinization at <350°C and a supply of Si and Al from plagioclase during the later stages of serpentinization. Plagioclase surrounding serpentinized olivine grains commonly shows radial fracturing. Numerical simulations using the discrete element method applied to coupled fluid flow, reaction, and fracturing reveal that volume expansion of olivine grains during serpentinization results in the simultaneous fracturing of olivine and surrounding plagioclase, and that the thermal stress during cooling of oceanic lithosphere might also cause preferential olivine fracturing prior to serpentinization. The simulations also predict a self-organizing fracture network that connects the olivine grains and passes through both olivine-rich and olivine-poor layers, resulting in permeability enhancement during serpentinization. Our results suggest that reaction-induced fracturing plays an essential role in the infiltration of seawater through the lower crust and into the mantle within oceanic lithosphere. Plain Language Summary The Oman ophiolite preserves evidence of pervasive serpentinization of the lower crust, Moho transition zone, and uppermost mantle. In this study, we investigated a troctolite and olivine gabbros from the CM1A site of the Oman Drilling Project, and compared fracture patterns in these rocks with those predicted by discrete element method (DEM) numerical simulations. In the troctolite, olivine grains are serpentinized to form a mesh texture, and radial fractures are developed in plagioclase and clinopyroxene adjacent to olivine. The olivine mesh texture consists of two types of veins: early serpentine + brucite + magnetite and late Al-rich serpentine + magnetite. Numerical simulations of coupled fracturing, reaction, and fluid flow reproduced the observed fracture patterns and indicate that reaction-induced fracturing plays a key role in permeability enhancement within the lower oceanic crust during low-temperature alteration.

More than one teramole of carbon per year is subducted as carbonate or carbonaceous material. However, the influence of carbonation/decarbonation reactions on seismic activity within subduction zones is poorly understood. Here we present field and microstructural observations, including stable isotope analyses, of carbonate veins within the Higuchi serpentinite body, Japan. We find that the carbon and oxygen isotope compositions of carbonate veins indicate that carbonic fluids originated from organic materials in metasediments. Thermodynamic calculations reveal that carbonation of serpentinite was accompanied by a solid volume decrease, dehydration, and high magnesium mobility. We propose that carbonation of the mantle wedge occurs episodically in a self-promoting way and is controlled by a solid volume contraction and fluid overpressure. In our conceptual model, brittle fracturing and carbonate precipitation were followed by ductile flow of carbonates and hydrous minerals; this might explain the occurrence of episodic tremor and slip in the serpentinized mantle wedge.

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