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

Abstract

The Fe 3+ /Fetot 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+ /Fetot, whereas garnet, cordierite and ilmenite are virtually Fe 3+ -free (0-1% Fe 3+ /Fetot) 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 metamor- phism; therefore, its disappearance by melting represents an important event of iron reduction during granulite formation, because haplogranitic melts usually incorpo- rate small amounts of ferric iron. Iron reduction is accompanied by the oxidation of carbon and the production of CO2, according to the redox reaction: 2Fe2O ðBtÞ 3 þ C ðGrÞ ) 4FeO ðCrd;Grt;Ilm;OpxÞ þ CO ðfluid;melt;CrdÞ 2 : Depending on the nature of the peritectic Fe-Mg mineral produced (garnet, cordierite, orthopyroxene), the CO2 can either be present as a free fluid component, or be completely stored within melt and cordierite. The oxi- dation of graphite by iron reduction can account for the in situ generation of CO2, implying a consequential ra- ther than causal role of CO2 in some granulites and migmatites. This genetic model is relevant to graphitic rocks more generally and may explain why CO2 is present in some granulites although it is not required for their formation.