Experimental tests of mineral equilibria typical of a fertile peridotite performed at mantle pressures and temperatures show that the mantle underlying Earth's oldest continents is considerably more oxidized than has previously been thought . These new results have important implications for 1) the nature of fluids in the sub-continental mantle from which diamonds formed, and 2) for the mobilization of carbon. Once it is oxidized to CO2, an important greenhouse gas, carbon can be incorporated in melts and rise up to the biosphere.
The oxidation state of the minerals in Earth's mantle assemblages (expressed as the 'oxygen fugacity') is important in determining the chemical speciation of volatile elements inside our planet and their degassing over geological time. Mineral assemblages found in garnet-bearing mantle rocks, brought to the surface from depths as great as 220 kilometers, have been used to constrain the oxygen fugacity of the mantle underlying the oldest continents. Such previous studies led to the conclusion that the deepest of these rocks would have been exposed to fluids rich in methane and hydrogen, rather than carbon dioxide and water.
In a Nature article , Vincenzo Stagno, currently a postdoc at the Geophysical Laboratory, Carnegie Institution of Washington, and colleagues from Bayerisches Geoinstitut (Germany) report the results of experimental tests of mineral equilibria that can be used to infer the oxygen fugacity of mantle samples. They found that the equilibrium used in previous studies underestimates oxygen fugacity at the highest pressures of interest. Using a more accurate equilibrium, the authors found that the deepest recovered mantle rocks, which also contain diamonds, were equilibrated with fluids rich in water or silicate melt, rather than carbonate or methane. The experimentally calibrated oxythermometer from their study also implies that as mantle rocks rise and decompress, their oxygen fugacity will increase, causing the oxidation of graphite or diamond to produce carbonate-rich liquids by redox melting, with implications for the release of carbon dioxide from Earth's interior and the extent of the deep carbon cycle.
Figure: The figure shows the interval depth at which redox melting can occur with implications for the onset of geophysically detectable incipient melting, but also for the transfer of carbon from the deep interior to the seafloor ultimately released in form of dissolved CO2 in mid-ocean ridge basalts and smokers.