Intellectual Merit. This project addresses the role of H-O-C fluids in heat transfer associated with formation of a Paleozoic belt of metamorphic 'hot spots' exposed in a north-south transect across New Hampshire (NH). At each of these localities, steep gradients in metamorphic temperature and rock geochemistry are centered on syn-metamorphic quartz±graphite vein networks. Chamberlain and Rumble (1988) proposed the hypothesis that these features represent zones where large quantities of hot fluids ascended through fracture networks during Acadian regional metamorphism. On the other hand, numerical modeling studies have shown that typical regional metamorphic devolatilization is unlikely to transport sufficient heat to strongly perturb regional geotherms. It would appear that hot spots can only be produced if the fluid fluxes are enormous and the timescales of flow are extremely short, but it is unknown if such fluxes and timescales are realistic. A multidisciplinary approach is proposed to test this hypothesis. Field work will focus on well-exposed hot spot localities near Bristol and Nelson, NH. The hypothesis must pass five crucial tests. (1) Fluid fluxes must have been large. Time-integrated fluid fluxes through veins will be estimated by quantifying chemical and isotopic (O, H) mass transfer in the vein networks. (2) Fluid flow must have been in a direction of decreasing temperature. The nature and extent of chemical and isotopic metasomatism will determine the direction of fluid flow, and elucidate fluid sources and pathways. (3) Timescales of flow must have been short (<10^6 yrs). The absolute timing of hot spot formation will be determined using Sm/Nd dating of garnet in metamorphic rocks, and U/Pb dating of monazite and zircons in metamorphic rocks and any magmatic dikes in the field areas. Furthermore, chemical diffusion profiles in calcite, apatite, and garnet will be used to constrain timescales of peak heating. (4) The timing of peak thermal conditions must have been nearly synchronous across isograds (test via Sm/Nd and U/Pb dating). (5) Metamorphic pressures must have been roughly constant across isograds at the present level of exposure. Thermobarometry will be done to determine peak conditions, regional T/P gradients, and P-T-t paths. Knowledge of the time-integrated fluid fluxes and timescales of flow will allow estimation of the actual fluid fluxes through the hot spots as well as gradients in the fluxes across the field areas. This information, together with the P-T-t history, age(s) of fluid flow, and field relations, will provide the initial and boundary conditions needed for modeling of fluid flow (2-dimensional) and its effect on regional thermal structure. If the hypothesis fails then other alternatives will be investigated, including magmatism and upwelling of lower crustal gneiss domes. The Ague lab will take primary responsibility for thermobarometry, mass transfer analysis, and diffusion and flow modeling; the Baxter lab for Sm/Nd garnet dating; and the Chamberlain lab for stable isotope and U/Pb work.

Broader Impacts. Large fluid fluxes poentially transport mass as well as heat. Therefore, if correct, the hypothesis of fluid-driven heating would bear on problems of societal relevance including ore metal transport and the transfer of greenhouse gases out of metamorphic belts. Human resources will be developed because Ph.D. graduate student and undergraduate involvement is critical. PIs and students will take part in field work and, in Years 2 and 3, coordinate "group meetings" at international conferences. All of the PIs have advised women students and are committed to diversity, including the involvement of underrepresented minority groups in science. Ague spearheaded development of the new Hall of Minerals, Earth, and Space (HoMES) at the Yale Peabody Museum, allowing integration of new research results into Earth science displays viewed by over 150,000 visitors annually. A large fraction of these visitors are schoolchildren, many of whom live in the urban centers of New Haven, Bridgeport, and other Connecticut cities. Moreover, Ague has separate NSF funding to develop educational programs based on the new Hall for area schoolteachers. Baxter will incorporate the field area into his annual "RoBOT: Rocks Beneath Our Toes" outreach program as a part of his Fall mineralogy class. The program engages Boston area high school students and BU undergraduates.

Project Report

The purpose of this research was to document the role fluids play in shaping the mid-levels of the continetal crust during mountain building. The area we studied lies in the northern Appalachians that now exposes rocks that underwent high temperatures and pressures some 400 million years ago. About 30 years ago one fo the PIs on this grant (Chamberlain - Stanford) identified what he called "hot spots" in which fluids rising through the deforming crust during mountain building created hot spots as high-temperature fluids were concentrated into narrow zones. Interestingly, these hot spots are the sites of old graphite mines in which Henry David Thoreau worked to collect graphite for the famous Thoreau pencils. As part the Stanford-side of this project, Chamberlain collected rocks and graphite from within and around the mine site and analyzed these for the stable isotopes of oxygen, hydrogen and carbon. The isotopes of these elements allow us to discern the conditions underwhich the fluids deposited the graphite. What we found was that the source of the carbon for the graphite came from the adjacent shales and carbonate rocks and were formed as fluids from these two rock types mixed deep within the Earth's crust. Although of interest to how these graphite deposits formed this finding has far wider applicability - it tellls us something about the long-term carbon cycle. The long-term carbon cycle represents the resevoirs and fluxes of carbon in the Earth's surface that control our global thermostat. Volcanic activity and metamorphism releases carbon dioxide to the atmosphere and chemical weathering and carbonate formation in the oceans removes it. These competing processes serve to regulate carbon dioxide in the atmosphere and have kept our planet from freezing or boiling for nearly 4 billion years. For one of the key players in this cycle, metamorphism, however it is not clear how it contributes to the carbon cycle. Some have argued that it is a net source of carbon dioxide. In contrast, others argue that it is a sink for carbon dioxide. Our study shows that the carbon that was liberated from the shales was once organic matter and the carbon from the carbonate rocks was inorganic carbon. Both of these rock types would be a net source of carbon dioxide if the carbon dioxide-bearing fluids released from these rocks during mountain building and metamorphism reached the Earth's surface. However, the fluids did not make it to the surface and instead mixed in the crust and the carbon precipitated as graphite. Since graphite is ubiquitous throught the Appalachians we argue that, at least in this case, metamorphism is a net sink for carbon dioxide rather than a source. This finding now needs to be integrated with our long-term carbon models to see how the formation of graphite regulates the long-term carbon cycle. We suspect that it will be a major player. Thoreau would have liked this finding.

National Science Foundation (NSF)
Division of Earth Sciences (EAR)
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Jennifer Wade
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Stanford University
Palo Alto
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