CAREER: Rates and Timescales of Metamorphic Reactions at Convergent Plate Boundaries

Trustees Of Boston University, Boston

Investigators

Abstract

Modern research and teaching in the earth sciences must include an increased awareness of the rates and timescales of geologic processes. Frontier research in plate boundary processes involves modeling of the production, consumption, and transport of fluids, as well as the evolving physical properties of the solid earth. Metamorphic reactions are at the roots of these processes. Model predictions of subduction zone processes require quantification of the rate and timescale over which metamorphic reactions in the crust and mantle proceed. However, in light of field-based and lab-based studies which differ in their kinetic predictions by many orders of magnitude, a lack of consensus exists regarding the rates most applicable to natural systems. Intellectual Merit: This proposal will address this fundamental need for field-based quantification of metamorphic reaction rates and timescales, with an emphasis on mineral transformations and volatile release in subduction zones. The approach involves the integration of three existing methodologies: 1) precise Sm/Nd geochronology of zoned garnets, 2) thermodynamic and modal analysis of zoned garnets, 3) modeling of reactive transport in layered rock systems to evaluate relative rates of reaction and transport. The main focus will be on garnet-forming dehydration reactions which occur in a wide variety of tectonic and lithologic settings. Isotopic (i.e. age) microsampling within zoned garnets will be provided using the recent joint NSF-Boston University TIMS/MicroMill Facility at BU under the directorship of the PI. The analytical mission of this facility, to push the limits of analytical precision and small sample size, is perfectly suited to the needs of the proposed research. The combined methodology will be applied to well-studied garnets from New England representing a regional metamorphic setting. Garnets from subduction zone settings (i.e. blueschist or low-T eclogite) will be sampled and analyzed similarly with the goal of developing a rate law for subduction zone mineral reactions and volatile release. Broader Impacts: It is important to infuse high school and undergraduate experiences with an appreciation of the exciting quantitative aspects of Earth Science, exemplified by the research described here. The education plan centers around a community outreach program called 'Rocks Beneath Our Toes' or RoBOT, integrated with undergraduate teaching and research. RoBOT engages Boston area high school students in the selection and sampling of rock outcrops in their own communities, which should enhance their level of interest and sense of connection to the RoBOT program. Undergraduate mineralogy students will complete semester-long research projects on these and other samples from their own field trip. The RoBOT program culminates with a visit by high school students to BU to tour the TIMS Facility, use the microscopes in small breakout groups led by undergraduates, and hear the undergraduates' final reports. This outreach program gives all students the chance to share in the excitement of discovery-based learning while gaining an appreciation of frontier research in geochemistry. Outstanding undergraduates coming from the RoBOT experience would be tapped each year to help establish and maintain a synergistic research team. The research plan is rich with opportunities for undergraduates including field experience in New England or abroad within the context of the broader research goals. The proposal will support 2 graduate students who will be responsible for conducting the bulk of the research activities, while mentoring the undergraduates.