Collaborative Research: Deformation-induced Hydration of Peridotite Mylonites in Nature and Experiments

Studies of oceanic fault zones have shown that fluid circulation may drive weakening and seismic failure in the shallow brittle part of these faults. It is generally assumed that fluid circulation ends when rocks transition from brittle failure (fault) to plastic flow (mylonite). However, recent analysis of oceanic peridotite mylonites has revealed the ubiquitous presence of hydrous mineral phases. This project assesses the degree to which water circulates though the brittle-ductile transition in oceanic fault zones, where fracture propagation during seismic events can send water downward into mylonite zones. In this project, peridotite mylonites are analyzed from shear zones in two complementary settings, the St. Paul?s Rocks on the Mid-Atlantic Ridge and the New Caledonia ophiolite. Methods used in this research include fieldwork, microstructural analysis, stable isotope measurements, and mineral thermometry. In addition, laboratory rock deformation experiments are used to investigate the behavior of fluid inclusions during deformation, the chemical environment in which water reacts to form hydrous minerals, and the influence these hydrous phases have in weakening mantle rocks and therefore localizing deformation.

Plate tectonics cycles water from the hydrosphere to the deep mantle. Subducted oceanic plates drive water to several hundred kilometers depth, and volcanism brings it back to the Earth?s surface. This research focuses on how water is incorporated into rocks beneath the ocean floor at mid-ocean ridges, where extension and transform faulting combine with heating from shallow magma to drive seawater circulation. Faulting creates pathways for water flow: during a seismic cycle, fractures propagate to the deeper part of fault zones, allowing water to penetrate the region where rocks normally deform plastically. These fractures heal over time and the fluids are trapped as numerous fluid inclusions. This project investigates the fate of these fluid inclusions during long-term deformation and chemical reaction with mantle rocks to form hydrous minerals, which act as capsules of water-rich material. When the plate eventually dives into a subduction zone, this water will be available for global circulation. Results of this research are relevant to disciplines that are concerned with the role of water in Earth processes, including seismology, petrology and geochemistry, tectonics, and geodynamics. The proposed study would advance desired societal outcomes through: 1) full participation of women in STEM; 2) development of a competitive STEM workforce through training of graduate students and in-reach efforts to undergraduate students; and 3) increased international partnerships through collaboration with New Caledonian geologists and public outreach to New Caledonian communities.