HR: 08:30h
AN: V41D-03 INVITED     [Abstracts]
TI: How the Upper Mantle Became Oxidized
AU: * Kasting, J F
EM: kasting@essc.psu.edu
AF: Penn State University, 443 Deike, University Park, PA 16802 United States
AB: Today, Earth's upper mantle has an average oxygen fugacity near the quartz-fayalite-magnetite (QFM) redox buffer (1), although significant departures from this redox state occur in different localities and at different depths (2). However, early in Earth history, following the Moon-forming impact, the upper mantle was almost certainly uniformly more reduced. The impactor that formed the Moon was probably Mars-sized or larger (3) and had already differentiated an iron core. Successful models of lunar formation must account for the fact that the Moon has only 25 percent of Earth's iron abundance (4). This can be accomplished if the iron core of the impactor was accreted by the Earth, while the Moon was formed from the mantles of the impactor and the Earth. Other large impactors would also have brought in metallic iron, and all such large impacts would have melted large portions of Earth's mantle. It is therefore inevitable that the Earth's upper mantle began its existence with an oxygen fugacity at or below iron-wstite (IW). How the upper mantle became oxidized from IW up to QFM is an interesting question. Much of the oxidation could have taken place during brief steam atmosphere stages following impacts (5,6) when hydrogen escape to space was extremely rapid (7). Continued oxidation could have been caused by cycling of volatiles through the mantle, accompanied by outgassing of reduced gases (8) and by subduction of ferric iron that had been oxidized at the surface (9). Oxidation of the uppermost 700 km of the mantle from QFM to IW would have required the equivalent of about half an ocean of water, assuming that the hydrogen was lost to space. This could have been accomplished in less than 2 b.y. if the average H$_2$ outgassing rate was a few times the present value, 5x10$^{12}$ mol/yr (10). The timing of mantle oxidation has important consequences for the composition of Earth's atmosphere at the time when life originated because it controls the oxidation state of volcanic gases. If redox indicators (Cr and V) from ancient rocks have been correctly interpreted (11,12), the process of mantle oxidation was essentially complete by 3.5 Ga. However, mantle oxidation would have hung up somewhat below QFM by conversion of graphite (or diamond) to CO$_2$ or carbonate, before rising to QFM. This process may therefore help explain why atmospheric O$_2$ did not rise until ~2.3 Ga (13,14), nearly half a billion years after the invention of oxygenic photosynthesis (15). References: 1. Holland, H.D. The Chemical Evolution of the Atmosphere and Oceans. Princeton Univ. Press, Princeton (1984). 2. Woodland, A.B. and Koch, M. Earth Planet. Sci. Lett. 214, 295 (2003). 3. Cameron, A. G. W. In Origin of the Earth and Moon, R. M Canup and K. Righter (eds.), p. 133, Univ. of Arizona Press, Tucson (2000). 4. Wood, J.A. In Hartmann, W.K., et al. (eds.) Origin of the Moon, p. 17, Lunar and Planetary Inst., Houston, TX (1986). 5. Matsui, T. and Abe, Y. Nature 319, 303 (1986). 6. Matsui, T. and Abe, Y. Nature 322, 526 (1986). 7. Pepin, R.O. Icarus 92, 2 (1991). 8. Kasting, J.F., et al., J. Geol. 101, 245 (1993). 9. Lecuyer, C. and Ricard, Y. Earth Planet. Sci. Lett. 165, 197 (1999). 10. Holland, H.D. Geochim. Cosmochim. Acta 66, 3811 (2002). 11. Delano, J.W. Origins of Life Evol. Biosph. 31, 311 (2001). 12. Canil, D. Earth Planet. Sci. Lett. 195, 75 (2002). 13. Holland, H. D. In Early Life on Earth, S. Bengtsson, ed., p. 237. New York, Columbia Univ. Press (1994). 14. Farquhar, J., et al., Science 289, 756 (2000). 15. Brocks, J.J., et al., Science 285, 1033 (1999).
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1645 Solid Earth
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting