Oxo-amphiboles in mantle xenoliths: evidence for H2O-rich melt interacting with the lithospheric mantle of Harrow Peaks (Northern Victoria Land, Antarctica)

Amphiboles are the most widespread hydrous metasomatic phases in spinel-bearing mantle peridotites from Harrow Peaks (HP), Northern Victoria Land (Antarctica). They occur both in veinlets and disseminated in the peridotite matrix (preferentially associated with clinopyroxene and spinel grains). Four amphibole crystals were investigated by single-crystal X-ray diffraction (SC-XRD), electron microprobe analysis (EMPA), secondary ion mass spectrometry (SIMS) and micro-Mossbauer spectroscopy; these crystal-chemical data allow to constrain upper mantle conditions during growth of these amphiboles and the role of volatile circulation during metasomatic processes in the Antarctic region. The HP amphiboles have low Mg# values (69.3-84.1), high TiO2 (2.74-5.30 wt%) and FeOtot contents (3.40 to 6.90 wt%). The Fe3+/Fe-tot ratios are significantly high (0.53-0.66). The W-site is mainly occupied by O2- (0.984-1.187 apfu) plus OH (H2O: 0.70-1.01 wt%) and minor F (0.04-0.24 wt%) and Cl (0.03-0.08 wt%). Consequently, HP amphiboles are actually characterized by a significant oxo component. The aH(2)O values were calculated at 1.5 GPa by dehydration equilibrium equations written as H2O-buffering equilibria among end-member components of amphibole and coexisting peridotite phases. Three out of four HP amphibole-bearing peridotites have values of aH(2)O ranging from 0.122 to 0.335; whereas one sample has aH(2)O remarkably higher (0.782) approaching an ideal H2O basalt solubility. The HP fO(2) values, determined by the olivine-spinel-orthopyroxene oxygeobarometer (Delta QFM = -1.77 : +0.01), are remarkably different from those calculated on the basis of the amphibole dehydration equilibrium and the application of the dissociation reaction (Delta QFM = -2.60 : +6.8). The high aH(2)O and the extremely high fO(2) values, determined by the oxy-amphibole equilibrium with respect to the redox conditions recorded by the co-existing anhydrous minerals (close to QFM buffer), revealed that: i) the amphibole-forming reaction is a relatively recent process with the new phases far from having reached a potential equilibrium with the peridotite matrix; ii) amphibole seems to be formed by the precipitation of migrating H2O-rich melts with a negligible contribution of the peridotite system.