The oxidation ratio of iron in coexisting biotite and hornblende from granitic and metamorphic rocks: The role of P, T and f(O-2)

Previously published and new data on the composition of coexisting biotite and hornblende from granitic and metamorphic rocks show that the degree of iron oxidation, R [= Fe3+/(Fe2+ + Fe3+)], is different in these two mineral s; the R of hornblende is greater. Granulite-facies minerals have the great est difference in R, whereas in granitic rocks, those minerals show the lea st difference. The oxidation of biotite and hornblende under high-level con ditions is accompanied by the crystallization of magnetite, and newly forme d oxidized mafic minerals have a lower Fe/(Fe + Mg) and R than the original ones. Under mesozonal and catazonal conditions, the increase in pressure p revents the formation of magnetite, and oxidation is accompanied by a signi ficant increase in R; these changes in the chemical composition of hornblen de are supplemented by an increase in Al. Since the Al content of hornblend e is known to be an indicator of pressure, such a correlation of R and Fe3 content with aluminum content points to an increase of these parameters wi th a rise of pressure.