HR: 09:30h
AN: V21F-07    [Abstracts]
TI: Hydrogen and Carbon Isotope Ratios in the Archean Pilbara Greenstone Belt: Contraints on the Mechanisms of Oxygen Buildup in the Atmosphere
AU: * Gregory, R T
EM: bgregory@smu.edu
AF: Stable Isotope Laboratory, SMU Department of Geological Sciences PO Box 750395, Dallas, TX 75275 United States
AU: Richards, I J
EM: irichard@smu.edu
AF: Stable Isotope Laboratory, SMU Department of Geological Sciences PO Box 750395, Dallas, TX 75275 United States
AU: Ferguson, K M
EM: kferg@smu.edu
AF: Stable Isotope Laboratory, SMU Department of Geological Sciences PO Box 750395, Dallas, TX 75275 United States
AB: Hydrogen and carbon isotope ratios of secondary phases formed during seafloor hydrothermal alteration provide constraints on important reservoirs affected by oxygen build-up in the atmosphere. We determined hydrogen and carbon isotope ratios from the well-preserved, ~3.5 Gyr Pilbara greenstone belt to constrain:1) whether hydrogen loss is a viable mechanism for changing the oxidation state of the Archean atmosphere and the lithosphere and 2) the relative proportions of the inorganic carbon (carbonates) and the reduced carbon reservoir. The hydrogen isotope values on greenstones (from the core of the North Pole dome to the base of the Fortescue Group) and granites (from the North Pole, Shaw, Mt Edgar and Coruna Downs batholiths) exhibit the following ranges: -84 < δD < -22 (mean -64; 33 analyses) and -36 < δD < -87 (mean -66; 17 analyses), respectively. A single Fortescue (Late Archean) basalt gave a δD value of -34. The secondary calcite in the greenstones exhibits the following range: -3.7 < δ¹³C < 1.7 (mean -0.4; 79 determinations). The hydrogen isotope ratio of the ocean is extremely sensitive to hydrogen loss because of the large (several hundred per mil) negative fractionation between hydrogen gas and water vapor. These results indicate that if hydrogen loss affected the oxidation state of the early atmosphere, it should have happened before 3.5 Gyr and not be delayed until the Archean to Proterozoic transition; i.e. the Archean δD values are indistinguishable from Phanerozoic values. Because of its retrograde solubility and the relative insensitivity of the δ¹³C value of precipitated secondary carbonate to temperature, the δ¹³C values of hydrothermal calcite in pillow lavas are proxies for the carbon isotope ratio of the inorganic carbonate reservoir sampled by the ancient ocean. The secondary calcite δ¹³C values are remarkably similar to values measured on similar calcites from Tertiary and Mesozoic oceans indicating that a complementary ¹³C-depleted reservoir comparable in size to the present one existed by 3.5 Gyr.
DE: 1000 GEOCHEMISTRY
DE: 1041 Stable isotope geochemistry (0454, 4870)
DE: 3610 Geochemical modeling (1009, 8410)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8125 Evolution of the Earth (0325)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005