A field trip for geologists interested in the Tectonic Flux initiative of the Deep Carbon Observatory was held in New Hampshire, 29-30 July 2013. Hydrothermal graphite deposits give direct evidence of the mobilization, transport, and precipitation of carbon from supercritical fluids throughout mid to lower depths in Earth’s continental crust. Profs. Jay Ague (Yale University) and Ethan Baxter (Boston University) led participants on a spirited exploration of the Bristol, NH, deposit where graphite nodules are found along contacts between pegmatite dikes and quartzitic-pelitic granulite facies metamorphic rocks. Research by Jay Ague, Xu Chu, and Meng Tian demonstrates graphite deposition at the base of Earth’s crust, under 8 kbars pressure and 800 degrees C. Measurements by Nora Sullivan and Ethan Baxter of Sm-Nd garnet geochronology in the Bristol area suggests regional metamorphism spanned ~50 million years from 400-350 Ma. Continuing study of zoned garnet geochronology will reveal the sequence and duration of multiple thermal excursions and fluid flow events, including graphite mineralization, over this prolonged interval of geologic time.
Staff Scientist Douglas Rumble (Geophysical Laboratory) guided field trippers to the Mt. Kearsarge, Franklin Pierce, Walpole, and Osgood graphite deposits. One of the most important lessons to be learned from the New Hampshire graphite occurrences is that the precipitation of graphite from supercritical crustal fluids is likely to be far more common than currently recognized. Systematic mapping of graphite mineralization in metamorphic belts would make it possible to improve estimates of the amounts of carbon transported during metamorphism. Supporting collaborative measurements of radiogenic isotopes would provide age constraints to estimate rates of mobilization, transport, and precipitation of graphite. The combined results of these studies would produce quantitative data on “Tectonic Fluxes” of carbon during metamorphism accompanying continent-continent plate collisions and continent-oceanic plate subduction.
The historic graphite deposits of New Hampshire, USA, offer a wide range of graphite mineralization for research from mid- to high-grade regional metamorphic environments. The deposits are localized along igneous-wall rock intrusive contacts or fill fractures associated with faults or ductile shear zones. Minerals co-precipitated with graphite include tourmaline, fuchsite, arsenopyrite, muscovite, chlorite, and ilmenite. Wall-rock alteration is common where graphite replaces metamorphic silicate minerals forming halos of graphitization around veins.
Meeting report contributed by D. Rumble, staff scientist at the Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA. Rumble is co-PI with Ed Young of the Department of Earth and Space Sciences at UCLA, CA, USA for development of a high-resolution mass spectrometer for analysis of methane gas samples.