Fe3+ reduction during biotite melting in graphitic metapelites: another origin of CO2 in granulites

The Fe(3+)/ Fe(tot) of all Fe-bearing minerals has been analysed by Mossbauer spectroscopy in a suite of biotite-rich to biotite-free graphitic metapelite xenoliths, proxies of an amphibolite-granulite transition through progressive biotite melting. Biotite contains 9 to 16% Fe(3+)/Fe(tot), whereas garnet, cordierite and ilmenite are virtually Fe(3+) - free (0 - 1% Fe(3+)/Fe(tot)) in all samples, regardless of biotite presence. Under relatively reducing conditions (graphite-bearing assemblages), biotite is the only carrier of Fe(3+) during high-temperature metamorphism; therefore, its disappearance by melting represents an important event of iron reduction during granulite formation, because haplogranitic melts usually incorporate small amounts of ferric iron. Iron reduction is accompanied by the oxidation of carbon and the production of CO(2), according to the redox reaction: 2Fe(2)O(3)((Bt)) 3+ C((Gr)) double right arrow 4FeO((Crd, Grt, Ilm, Opx)) + CO(2)((fluid, melt, Crd)). Depending on the nature of the peritectic Fe-Mg mineral produced ( garnet, cordierite, orthopyroxene), the CO(2) can either be present as a free fluid component, or be completely stored within melt and cordierite. The oxidation of graphite by iron reduction can account for the in situ generation of CO(2), implying a consequential rather than causal role of CO(2) in some granulites and migmatites. This genetic model is relevant to graphitic rocks more generally and may explain why CO(2) is present in some granulites although it is not required for their formation.