Seismicity of peridotite alteration

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

The Earth's mantle is a layer of silicate rock found in between the crust and the outer core that represents ~67% of its mass and ~84% of its volume. Mantle rocks are rarely observed on the Earth's surface because they are covered by the crust. Yet, the upper part of the mantle, which is composed of peridotite, is commonly exposed on the seafloor, and occasionally exposed on land in geological features called ophiolites. When peridotite is exposed to, and reacts with, water, it undergoes chemical reactions that change the composition of both the rocks and the water. This process, known as alteration, results in carbon being transferred from the water to the rocks. Peridotite alteration is thus a natural process that sequesters carbon in the Earth's interior. This project uses data from a unique seismic experiment - conducted at the world's largest ophiolite in Oman - to determine how fast this natural process sequesters carbon. The project provides support to a post-doctoral associate at Woods Hole Oceanographic Institution. It fosters international collaboration with the Sultana of Oman, Czech Republic, and the UK. Its outcomes are directly relevant to applications in engineered CO2 capture and enhanced geothermal systems. Peridotite alteration is a fundamentally important process for a spectrum of geoscience topics. These include arc volcanism, earthquake processes, the carbon cycle, and chemosynthetic biological communities. Although fully altered peridotites (serpentinites, listvenites, soapstones) are commonly observed in outcrops, it is unclear how this can happen because alteration reactions are typically self-limiting. In most geological systems alteration products armor reactive surfaces and reduce pore space, which eventually halts the alteration process. It has been proposed that reaction-driven cracking - due to volume increase during serpentinization and carbonation of mantle rocks - is the key process that perpetuates alteration by creating and/or maintaining permeable pathways for continued fluid flow. In principle, this process should generate very small cracking events with volume change (isotropic) focal mechanism components. But such events have never been observed in situ. Here the team acquired a unique seismic dataset with short inter-element spacings (order meters) and 1 kHz sampling at a site of ongoing peridotite alteration to study its seismogenic character. They determine if the very small earthquakes detected by the seismic network represent cracking from peridotite alteration. They interpret the results in the context of the multi-disciplinary studies (hydrology, petrology, microbiology) conducted at the study site. The results provide new insight into the complex set of processes that occur when water interacts with peridotite. These processes occur over vast expanses of the Earth's solid surface. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.