HR: 0800h
AN: V41C-0731    [Abstracts]
TI: Metamorphic decarbonation in the Neoproterozoic ultrahigh-temperature metamorphism and its environmental implication
AU: * Omori, S
EM: omori@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, O-okayama, Meguro, Tokyo, 1528551, Japan
AU: Santosh, M
EM: santosh@cc.kochi-u.ac.jp
AF: Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi, 7808520, Japan
AB: Metamorphic decarbonation reactions and volcanic degassing leads to significant influx of CO2, a major greenhouse gas, into the ocean-atmosphere system from the solid Earth. The anhydrous mineral assemblages that characterize the Proterozoic granulite facies rocks, including charnockites and ultrahigh-temperature (UHT) rocks, require that water activity was buffered to low levels during their formation. One of the popular models invokes the influx of CO2-rich fluids from the tectosphere mantle to generate dry mineral assemblages. Here we present quantitative estimates on CO2 derived through degassing during UHT metamorphism in the Neoproterozoic through the mineralogical and geological analyses. In an attempt to investigate the link between CO2 liberation from the carbonated tectosphere, UHT metamorphism and major earth processes, we address some of the important issues such as: 1) how the tectosphere had become carbonated; 2) how and when the tectosphere degassed; and 3) what is the difference between Proterozoic orogens and those of the present day. The fate of the Earth as a habitable planet was dictated by a reversal of the fundamental process of formation of oceans by the selective removal of CO2 into mantle in the Hadean time, carbonation of the Archean mantle wedge, and subsequent decarbonation of the carbonated mantle through divergent metamorphism and water infiltration since the late Proterozoic. Our computations show that an extra flux of CO2 was added to the atmosphere through a Himalayan scale UHT metamorphism to the extent of 6E16 to 3E18 mol/my, for a duration of 10 my. A calculation of the impact of the extra CO2 influx to the global mean temperature in the context of carbon cycle and greenhouse effect of CO2 shows that at the peak influx stage, the steady state temperature would be raised by 4 degC from 15 degC and by 13 degC from 4 degC. Our results have important bearing in evaluating the mechanism of melting and the duration of Snowball Earth. Our estimate of the maximum degassing rate during UHT metamorphism suggests that the duration of Snowball Earth in Marinoan was probably shorter, and the recovery from an ice-covered Earth to ocean-covered Earth faster, than in previous estimates.
DE: 0428 Carbon cycling (4806)
DE: 3656 Ultra-high temperature metamorphism
DE: 4930 Greenhouse gases
DE: 9622 Proterozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting