Amphibole was found as the most widespread hydrous metasomatic phase in spinel-bearing lherzolites, harzburgite and wehrlite xenoliths from Harrow Peaks (HP), Northern Victoria Land (Antarctica). It occurs both in veinlets and disseminated in the peridotite matrix. In order to better understand the mechanism of amphibole formation at upper mantle conditions and to broaden the knowledge of the actual role of water circulation during metasomatic processes in the Antarctic region, four amphibole crystals were fully characterized, by means of single-crystal X-ray diffraction, electron microprobe analysis, secondary ion mass spectrometry and micro-Mössbauer spectroscopy. Compositionally, they present relatively low Mg# values (69.3-84.1) and high TiO2 contents (2.74-5.30 wt.%). The FeOtot contents range from 3.40 to 6.90 wt.%. The measured Fe3+/Fetot ratios (0.53-0.66), reveal anomalously high Fe2O3 contents (2.34-4.52 wt.%). The amount of O2- at the O(3) site (0.984-1.187 apfu), together with the anomalously high Fe2O3 contents, testify for the presence of the oxo component in the Harrow Peaks amphiboles. The volatile content is mainly represented by H2O (0.70-1.01 wt.%) with F and Cl, in the range of 0.04-0.24 wt.% and 0.03-0.08 wt.%, respectively. The coexistence of amphibole with the primary peridotitic phases allows to investigate the chemicophysical parameters that control the amphibole formation. The aH2O values were calculated at 1.5 GPa by dehydration equilibrium equations written as H2O-buffering equilibria among end-member components of amphibole and coexisting peridotitic phases. Three out of four HP amphibole-bearing peridotites show aH2O ranging from 0.122 to 0.335; the fourth sample has a H2O remarkably higher (0.782) and close to an ideal H2O saturation. These values are not unexpected, due to the predominance of the oxo components in the amphiboles. The HP fO2 values were calculated using both olivine-spinel-orthopyroxene oxygeobarometer and amphibole dehydratation/dissociation reaction. Amphibole-derived fO2 values record extremely variable redox conditions (fO2 between QFM-2.60 and QFM +6.8) remarkably different from the fO2 values calculated on the peridotite assemblage (fO2 between QFM -1.77 and QFM + 0.01). In sample HP124, the oxy-amphibole equilibrium is attained at high aH2O and the extremely high fO2 values (up to QFM +6.8) clearly displaced with respect to the redox conditions recorded by the co-existing anhydrous minerals (close to QFM buffer). A possible scenario interprets the amphibole-forming reaction as a relatively recent process, far from having reached a potential equilibrium with the peridotite matrix, and/or amphibole seems to be formed by the precipitation of migrating hydrous rich fluid/melts with a negligible contribution of the peridotite system.
Oxo-amphiboles in mantle xenoliths: a key to understand the hydrous rich metasomatic melts circulating beneath Harrow Peaks, Victoria Land, Antarctica
Ottolini L
2015
Abstract
Amphibole was found as the most widespread hydrous metasomatic phase in spinel-bearing lherzolites, harzburgite and wehrlite xenoliths from Harrow Peaks (HP), Northern Victoria Land (Antarctica). It occurs both in veinlets and disseminated in the peridotite matrix. In order to better understand the mechanism of amphibole formation at upper mantle conditions and to broaden the knowledge of the actual role of water circulation during metasomatic processes in the Antarctic region, four amphibole crystals were fully characterized, by means of single-crystal X-ray diffraction, electron microprobe analysis, secondary ion mass spectrometry and micro-Mössbauer spectroscopy. Compositionally, they present relatively low Mg# values (69.3-84.1) and high TiO2 contents (2.74-5.30 wt.%). The FeOtot contents range from 3.40 to 6.90 wt.%. The measured Fe3+/Fetot ratios (0.53-0.66), reveal anomalously high Fe2O3 contents (2.34-4.52 wt.%). The amount of O2- at the O(3) site (0.984-1.187 apfu), together with the anomalously high Fe2O3 contents, testify for the presence of the oxo component in the Harrow Peaks amphiboles. The volatile content is mainly represented by H2O (0.70-1.01 wt.%) with F and Cl, in the range of 0.04-0.24 wt.% and 0.03-0.08 wt.%, respectively. The coexistence of amphibole with the primary peridotitic phases allows to investigate the chemicophysical parameters that control the amphibole formation. The aH2O values were calculated at 1.5 GPa by dehydration equilibrium equations written as H2O-buffering equilibria among end-member components of amphibole and coexisting peridotitic phases. Three out of four HP amphibole-bearing peridotites show aH2O ranging from 0.122 to 0.335; the fourth sample has a H2O remarkably higher (0.782) and close to an ideal H2O saturation. These values are not unexpected, due to the predominance of the oxo components in the amphiboles. The HP fO2 values were calculated using both olivine-spinel-orthopyroxene oxygeobarometer and amphibole dehydratation/dissociation reaction. Amphibole-derived fO2 values record extremely variable redox conditions (fO2 between QFM-2.60 and QFM +6.8) remarkably different from the fO2 values calculated on the peridotite assemblage (fO2 between QFM -1.77 and QFM + 0.01). In sample HP124, the oxy-amphibole equilibrium is attained at high aH2O and the extremely high fO2 values (up to QFM +6.8) clearly displaced with respect to the redox conditions recorded by the co-existing anhydrous minerals (close to QFM buffer). A possible scenario interprets the amphibole-forming reaction as a relatively recent process, far from having reached a potential equilibrium with the peridotite matrix, and/or amphibole seems to be formed by the precipitation of migrating hydrous rich fluid/melts with a negligible contribution of the peridotite system.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
