Crystal-chemistry of amphiboles: implications for oxygen fugacity and water activity in lithospheric mantle beneath Victoria Land, Antarctica.

Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antartica. It occurs in veins or in disseminated form in spinel lherzolites. Both types derive from reaction between metasomatic melts and the pristine paragenesis of the continental lithospheric mantle beneath Northern Victoria Land. To determine the effective role of water circulation during the metasomatic process and amphibole formation, six amphibole samples were fully characterized. Accurate determination of the site population and the state of dehydrogenation in each of these amphiboles was carried out using single-crystal X-ray diffraction, electron microprobe and secondary ion mass spectroscopy on the same single crystal. The Fe3+/ΣFe ratio was determined by X-ray Absorption Near Edge Spectroscopy (XANES) on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870-0.996 O3(O2-) a.p.f.u. , is primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; post-crystallization H loss is negligible. Estimates of aH2O (0.013- 0.054) were determined from the dehydration equilibrium among end-member components assuming that amphiboles are in equilibrium with the anhydrous peridotitic phases. A difference up to 58% in determination of aH2O can be introduced if the chemical formula of the amphiboles is calculated based on 23 O a.p.f.u. without knowing the effective amount of dehydrogenation. The oxygen fugacity of the Baker Rocks amphibole-bearing mantle xenoliths calculated based upon the dissociation constant of water (by oxy-amphybole equilibrium) is between -2.52 and -1.32 log units below the fayalite-magnetite-quartz (FMQ) buffer. These results are systematically lower and in a narrow range of values relative to those obtained from anhydrous olivine-orthopyroxene-spinel equilibria (fO2 between -1.98 and -0.30 log units). A comparative evaluation of the two methods suggests that when amphibole is present in mantle peridotites the application of oxy-amphibole equilibrium is preferred, because ol-opx-sp oxy - calibrations are not "sensitive" enough in recording the effects (if any) of amphibole in the peridotite matrix. Amphibole acts as the main H acceptor among the peridotite minerals and may prevent fluid circulation and buffer oxygen fugacity. The important conclusion of this study is that amphibole within the lithospheric mantle does not always means high water activity and oxidizing conditions.