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

Abstract

Amphibole is the hydrous metasomatic phase in spinel-bearing mantle xenoliths from Baker Rocks, Northern Victoria Land, Antarctica. 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+/Sigma Fe ratio was determined by X-ray absorption near edge spectroscopy on amphibole powder. The degree of dehydrogenation determined by SIMS is 0.870-0.994 O3(O2-) a.p.f.u., primary and ascribed to the Ti-oxy component of the amphibole, as indicated by atom site populations; postcrystallization H loss is negligible. Estimates of aH(2)O (0.014-0.054) 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-amphibole equilibrium) is between -2.52 and -1.32 log units below the fayalitemagnetite- 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 (fO(2) 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

Metasomatism in the sub-continental lithospheric mantle beneath the south French Massif Central: Constraints from trace elements, Li and H in peridotite minerals

“…The same observations have been reported in previous studies (Yang et al, 2008;Schmädicke et al, 2013;Bonadiman et al, 2014;Denis et al, 2015). Bonadiman et al However, it is noteworthy that these two Amp-bearing samples have higher / ratios than the / ratio range restricted by mantle xenoliths from worldwide localities (Table 5; Fig.…”

Petrogenesis of the Rifted Southern Victoria Land Lithospheric Mantle, Antarctica, Inferred from Petrography, Geochemistry, Thermobarometry and Oxybarometry of Peridotite and Pyroxenite Xenoliths from the Mount Morning Eruptive Centre

“…error ¼ 1r). Bonadiman et al (2014) also reported the oxygen fugacity of amphibole-bearing mantle xenoliths, based on oxy-amphibole equilibrium; these give Dlog fO 2 values between -1Á32 and -2Á52. Figure 12 shows fO 2 data from xenoliths in continental rifts and various tectonic settings from around the world.…”

“…They obtained Dlog fO 2 values between -0Á2 and -1Á5 for spinel peridotite specimens. Bonadiman et al (2014) reported Dlog fO 2 values from anhydrous olivineorthopyroxene-spinel equilibria of between -0Á30 and -1Á98, which suggests a relatively consistent oxygen fugacity for the anhydrous shallow, rifted, spinel peridotite mantle beneath Victoria Land with a median value of -1Á0 6 0Á1 Dlog fO 2 (n ¼ 20; range -1Á98 to -0Á2; McDonough, 1990); DMM, depleted MORB-source mantle (Workman & Hart, 2005). The HIMU pattern is for Mangaia, Austral Islands [sample M-11 of Woodhead (1996)].…”

“…The references used are as follows: xenoliths from (continental) rift settings, this study; Baikal rift zone, Russia, Ionov & Wood (1992); Lambert-Amery rift, Antarctica, Foley et al (2006); Mt. Lessini, Italy, Siena & Coltorti (1993); SE Australia and Tanzania, Canil et al (1994) and Canil & O'Neill (1996); Northern Victoria Land, Perinelli et al (2012) and Bonadiman et al (2014); Southern Victoria Land, this study. The range and median value (-0Á9 Dlog fO 2 ) for all these localities combined is also shown (xenoliths from rift settings) and compared with xenoliths from subduction settings [Frost & McCammon (2008) and references therein].…”

“…Based mainly on the study of the volcanic rocks it has been argued that the Erebus volcanic province mantle is heterogeneous on a subkilometre scale, with a depleted mantle component refertilized by a mix of HIMU-like and enriched components (Cooper et al, 2007;Sims et al, 2008;Martin et al, 2013). Peridotite and pyroxenite xenoliths from several localities in the Melbourne volcanic province of Northern Victoria Land have been studied in detail, revealing a complex history of partial melting and metasomatism, resulting in a heterogeneous mantle (Wö rner et al, 1989;Zipfel & Wö rner, 1992;Wö rner & Zipfel, 1996;Wö rner, 1999;Coltorti et al, 2004;Perinelli et al, 2006Perinelli et al, , 2008Nardini et al, 2009;Armienti & Perinelli, 2010;Melchiorre et al, 2011;Perinelli et al, 2011;Bonadiman et al, 2014). Oxygen fugacity has been calculated for Northern Victoria Land mantle xenoliths using Mö ssbauer spectroscopic measurements on spinel and is estimated to be between -0Á2 and -2Á5 Dlog fO 2 (Perinelli et al, 2012;Bonadiman et al, 2014).…”