Determination of Methane Stability in the Upper Mantle and Lower Crust. Final Report, July 1, 1989-June 30, 1990.

摘要

In Part A of the report the authors examine the variability in mantle oxygen fugacity f(sub O2) from the Earth's suboceanic upper mantle. The samples they have examined are abyssal spinel peridotites. They chose to study abyssal spinel peridotites as probes of the upper mantle because a sufficient number of samples were available that had a broad global distribution. These provide important constraints on fluid speciation in the system C-O-H, the approximate bulk composition of dominant volatile species in the Earth's upper mantle. Part B of the report presents an analysis of available analytical data on volatiles trapped in quenched MORB glasses from three major oceanic rift systems. The results show that on average, the Earth's suboceanic mantle is more reduced than the subcontinental mantle and that both upper mantle regimes are too oxidized to be in equilibrium with graphite. The results also indicate that conditions favorable to CH4 being an important component of the volatile phase in the upper mantle may be satisfied in the vicinity of mantle plumes (i.e. hot spots). Assuming that the physical and chemical characteristics of mantle plumes are similar beneath the continents, the authors propose that an attractive subject for further research would be to investigate continental supracrustal sedimentary rocks, particularly in regions of continental extension where the manifestation of a subcontinental mantle plume are obvious.