Crystal chemistry of Fe3+ and H+ in mantle kaersutite: Implications for mantle metasomatism

Abstract

Chemical and crystal chemical analyses have been performed on a suite of subcontinental, mantle-derived hornblende (kaersutite) samples. Mdssbauer techniques have been utilized to investigate Fe valence and site occupancies, U extraction techniques have been used to determine bulk H contents, proton-induced y-ray emission (PIGE) analysis was employed to measure F, and electron microprobe techniques coupled with the above measurements have been utilized to determine major-element contents of hornblende. Similar analyses were performed on a suite of metamorphic 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, which was used to correct the mantle sample M0ssbauer data. The results of the analyses for the kaersutite samples show a nearly l:l inverse relationship between the Fe3+ and H+ contents. Although the range of Fe3+/H+ in the less oxidized kaersutite samples may be explained by partial H loss during entrainment and ascent, the nearly total dehydrogenation of the Fe3*-rich megacrysts would require time scales significantly longer than what is expected for transport. Thus, it seems likely that these oxykaersutite samples grew in a more oxidized metasomatic fluid, where incorporation of H was not required for charge compensation. As megacrysts from the same location show wide variation in Fe3* and H+, it appears likely that significant variations in the oxidation state of the mantle metasomatic fluid occurred over relatively small temporal or spatial scales.