Petrologic consequences of non-hydrostatic stress during subduction zone metamorphism

Yale University, New Haven CT

Investigators

Abstract

Rocks deep below Earth’s surface are subjected to high temperatures and intense pressures. The chemical compositions of the minerals in rocks can record the temperature–pressure conditions under which the rocks form. Different geologic environments are characterized by distinct temperature–pressure regimes. Consequently, geoscientists can use the chemical histories preserved in minerals to reveal geologic environments of rock formation, such as within a subduction zone or mountain belt. The traditional approach to reading the temperature–pressure record is to assume that the pressure on a rock was equal in all directions when it formed. However, emerging research demonstrates that pressure (or stress) on a rock may vary considerably with direction. For example, earthquakes and the existence of mountains that stand high above sea level are settings where directional stresses play an essential role by exerting fundamental controls on the reactions that take place among minerals. These include reactions that release water (dehydration reactions). This water can weaken rocks and facilitate seismic activity, or drive the rock melting that produces dangerous volcanoes such as Mt. Rainier as well as many of the world’s great ore deposits. The natural hazards are particularly acute in and around Earth’s subduction zones, regions where one tectonic plate descends beneath another (for example, in Oregon, Washington State, and Alaska). In summary, to obtain a better understanding of the rock record, we must be able to quantitatively assess the impact of unequal stresses on geologic processes. This study will entail Ph.D. and undergraduate student research as well as public outreach programs through the Yale University Peabody Museum of Natural History. Four key interrelated concepts relevant for metamorphic rocks in subduction zones will be tested. (1) Solid solution mineral compositions and crystallographic orientations record stress fields. (2) Stress variations at the grain scale can produce compositional zoning in minerals. (3) Diffusion within minerals can generate stresses that affect diffusion profile shapes and thus diffusion-based estimates for the timescales of metamorphic and tectonic processes. (4) Normal stress can trigger dehydration reactions, rock weakening, and potential seismic moment release. These tests will involve theoretical development of the thermodynamic and kinetic equations needed to quantitatively describe unequal stress in minerals, together with applications to natural samples from exhumed subduction complexes in the Alps, Germany, Greece, and Norway. The landmark contributions of materials scientists F. C. Larché and J. W. Cahn will be the basis for the theoretical development. Field work and sample collection will be done in the Bergen Arcs, Norway; samples from the other localities are already in the petrologic collections of Yale University. A wide array of laboratory techniques will be used, including thin section petrography, electron backscatter diffraction analysis of mineral crystallographic orientations, and electron-probe microanalysis of mineral chemistry. Ultimately, the research will evaluate the impact of non-hy¬drostatic stress on mineral interface stability, solid solution mineral compositions, and intracrystalline diffusion rates. This, in turn, will have broad implications for diffusion-based time¬scale estimation, reconstructing deformation, cooling, and exhumation histories, and fluid generation in subduction zones. 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.