2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 209-4
Presentation Time: 9:45 AM


STODDARD, Philippa S., Geology & Geophysics, Yale University, New Haven, CT 06520, BRANDON, Mark T., Geology and Geophysics, Yale University, New Haven, CT 06520-8109 and RYE, Danny M., Geology and Geophysics, Yale University, PO Box 208109, New Haven, CT 06520-8109, philippa.stoddard@yale.edu

The San Juan Islands in NW Washington State preserve a record of very low temperature-high pressure metamorphism, which formed by structural burial and rapid exhumation between 100 and 84 Ma (Brandon et al., 1988). Metamorphic assemblages include lawsonite, prehnite, Al-pumpellyite, and aragonite. Aragonite is widespread, occurring both in veins and by recrystallization of limestones (Vance, 1968). Maximum metamorphic conditions are constrained to be about 500 Mpa and 100 to 200 C. Our work on southern Lopez Island has found an outcrop where aragonite veins have anomalously light concentrations of carbon isotopes, ranging from 0 to -50 per mil. Six of 38 samples have values in the range -30 to -50 per mil. Such light values are commonly attributed to carbon associated with biogenically produced methane. The outcrop is about 50 m long, and exposes a fault that juxtaposes pillow basalts over mudstones. The outcrop lies within a 3 km thick fault zone, called the Lopez Structural Complex. Structural relationships in this region clearly show that large-scale faulting occurred first, followed by high-pressure metamorphism, and then the development of a pressure-solution cleavage. The aragonite veins at Davis Head show variable deformation indicating that they were emplaced during cleavage formation. Aragonite was the primary carbonate phase, but it has been variably retrograded to calcite by a solid-solid transformation. Other work indicates that isotopes in the carbonate anion do not re-equilibrate during a solid-solid transformation. We propose that the aragonite veins were formed by oxidation of methane. The degree of oxidation was variable, as indicated by the wide range of carbon isotope values. If correct, then the lightest carbon values would represent the isotopic composition of the methane. At low surface pressures, bacterial life is know to remain active to temperatures of ~122 C. Biomolecules are stabilized by pressure, so bacterial life should extend to higher temperatures within the Earth's interior. We suggest that the Lopez Island aragonite veins are evidence of this deep life.
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