CRYSTAL-CHEMISTRY OF FE3- IMPLICATIONS FOR MANTLE METASOMATISM( AND H+ IN MANTLE KAERSUTITE )

Chemical and crystal chemical analyses have been performed on a suite of subcontinental, mantle-derived hornblende (kaersutite) samples. Mos sbauer techniques have been utilized to investigate Fe valence and sit e occupancies, U extraction techniques have been used to determine bul k H contents, proton-induced gamma-ray emission (PIGE) analysis was em ployed to measure F, and electron microprobe techniques coupled with t he above measurements have been utilized to determine major-element co ntents of hornblende. Similar analyses were performed on a suite of me tamorphic amphibole samples from Cosca et al. (1991). Comparison with their wet chemical results on Fe3+/Fe2+ permitted determination of C = 1.22, the correction for differential recoil-free fraction effects, w hich was used to correct the mantle sample Mossbauer data. The results of the analyses for the kaersutite samples show a nearly 1:1 inverse relationship between the Fe3+ and H+ contents. Although the range of F e3+/H+ in the less oxidized kaersutite samples may be explained by par tial H loss during entrainment and ascent, the nearly total dehydrogen ation of the Fe3+-rich megacrysts would require time scales significan tly longer than what is expected for transport. Thus, it seems likely that these oxykaersutite samples grew in a more oxidized metasomatic f luid, where incorporation of H was not required for charge compensatio n. As megacrysts from the same location show wide variation in Fe3+ an d H+, it appears likely that significant variations in the oxidation s tate of the mantle metasomatic fluid occurred over relatively small te mporal or spatial scales.