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.
This CAREER grant focused on the development of field-based geochronologic-petrologic approaches to measure the rate and duration of metamorphic reactions within convergent margin settings. During this funding period, we have invested heavily in the establishment of methods to date the progressive growth of garnet using the Samarium to Neodymium decay system. Garnets are a common mineral which grows in response to broad tectonic processes such as continental collision, mountain building, and subduction of oceanic crust. Akin to tree rings, garnet growth concentrically; thus garnets thus hold one of the longest continuous records of tectonic processes. We have now shown on numerous samples the ability to acquire growth ages with <1 million year age precision on small garnet samples from a variety of samples (e.g. Lancaster et al. 2008; Harvey and Baxter, 2009; Peterman et al. 2009; Pollington & Baxter 2010, 2011; Dragovic et al. 2012; Korhonen et al. 2012). Three key innovations have been realized – and integrated together – paving the way for future success in Sm-Nd geochronology of zoned garnets: 1) We developed a new method permitting TIMS analysis of very small quantities of Neodymium (such as would be extracted from small concentric garnet microsamples) at high precision (Harvey & Baxter 2009); 2) We have refined our abilities to cleanse garnets of undesirable contaminating micro-mineral inclusions (Pollington & Baxter 2011); and 3) We have established routine methods for chemically contoured microdrilling of zoned garnets for geochronology (Pollington & Baxter 2010, 2011). These methodological investments have been made available to the entire geosciences community; indeed, visitors to the lab have already published garnet ages in their respective field areas (Peterman et al. 2008; Korhonen t al. 2012). With these and other methods we have made several discoveries about the timescales of metamorphic and tectonic processes. In a spectacular 6 cm diameter garnet collected from the Austrian Alps, we produced an 8 million year continuous record of tectonic processes (Pollington & Baxter 2010). This garnet may be likened to a giant redwood recording the "tree rings" of tectonic processes. The data consist of 12 discrete chemically contoured microsampled growth rings, each dated with the Sm/Nd isotopic system at precision better than +/- 700 Kyr. These data reveal that garnet growth was punctuated by at least two pulses of accelerated growth from 28 to 20 million years ago during the waning stages of the Alpine Orogeny, which are related to pulses of fluid infiltration and related heating and/or deformation and exhumation driven by tectonics. In the subduction zone setting, we have shown that garnet growth can be linked thermodynamically to water release from subducted rocks. It is this water, liberated by metamorphic dehydration reactions, that leads to melting of the mantle and volcanism above all the worlds subdiction zones. Direct chronology of zoned subduction zone garnets from the island of Sifnos, Greece indicates short pulse(s) of accelerated garnet growth and related dehydration. These water pulses are not consistent with many existing models of subduction zones which predict water release should be slow and steady. The existence of the water pulses will help test and refine new geodynamic models of subduction zones some of which may predict periods of rapid water release. We have also documented evidence for brief pulses of metamorphic heating and mineral growth in regional mountain building events (Ague & Baxter 2007; Lancaster et al. 2008). All of this evidence suggests that metamorphism in general may be dominated by brief events, perhaps superimposed on slower longer lasting tectonically paced processes, that simply couldn’t be recognize in the past without some of the modern innovations realized in this research. Pulsed metamorphism is a new paradigm that geoscientists are seeking now to explain and integrate into their geodynamic models. This grant also supported the establishment of a high school outreach program called "RoBOT: Rocks Beneath Our Toes". The program brings high school students into the field with BU undergraduates to study rocks and minerals collected in their own community. The RoBOT experience culminates with a visit to Boston University, a tour of our geochemical facilities, and the opportunity to work with BU undergraduates in the analysis and exploration of their rock samples with our petrographic microscopes. In six years, this program has reached over 70 Boston area high schools students, over 50 Boston University undergraduates, and seven high school educators. In addition, we have amassed a collection of over 50 rock samples and related "thin sections" for use in future classroom study. Reviews of the program by participants indicate great success and enjoyment. Five graduate students, eight undergraduates, and one post-doc have all been funded and/or trained by grant related research including field work in Greece, Austria, and New England and conference presentations at national and international conferences. Five of these undergraduates have gone on to successful graduate careers in geoscience.
