Experimental Constraints on Adakitic Metasomatism of Mantle Wedge Peridotites Below Patagonia
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O EOL GIC G A D D A E D C E I H C I L E O S F u n 2 d 6 la serena octubre 2015 ada en 19 Experimental constraints on adakitic metasomatism of mantle wedge peridotites below Patagonia Alexandre Corgne * and Manuel Schilling D. Instituto de Ciencias de la Tierra, Universidad Austral de Chile, Campus Isla Teja, Valdivia, Chile *Contact email: [email protected] Abstract. We performed a series of high-pressure (1.5 and Quaternary alkali lavas in the back-arc region (e.g. GPa) and high-temperature (1000-1300 ºC) experiments to Stern and Kilian, 1996; Gorring et al., 1997). The back-arc investigate the geochemical imprints of adakitic lavas are hosts of frequent mantle xenoliths, the study of metasomatism on mantle wedge peridotite. Reaction which has contributed to a better understanding of the couples were prepared using a powdered adakite from petrological and geochemical variability of the sub- Cerro Pampa (Argentina) placed next to a fragment of continental lithospheric mantle (e.g. Stern et al., 1990, spinel lherzolite from Pali Aike (Chile). Preliminary results 1999; Gorring and Kay, 2000; Laurora et al., 2001; show that the main changes in phase relations are Bertotto, 2002; Kilian and Stern, 2002; Bjerg et al., 2005, incongruent dissolution of olivine and associated 2009; Schilling et al., 2005; Rivalenti et al., 2004; Ntaflos precipitacion of secondary orthopyroxene, incongruent dissolution of primary spinel and formation of secondary et al., 2007; Wang et al., 2008; Dantas et al. 2009). spinel, as well as precipitation of secondary clinopyroxene and in some instances zoned plagioclase. In experiments From the study of mantle xenoliths, it has been proposed with high added water, precipitacion of pargasitic amphibole that the lithospheric mantle below Patagonia has also occurred. Also, trace element contents of secondary undergone variable degrees of melt extraction and distinct clinopyroxene are reported as a potential geochemical types of metasomatism to produce the various mafic and marker of adakitic metasomatism. ultramafic lithologies found in the xenolith collection, which includes spinel/garnet harzburgites, spinel/garnet Keywords: Adakite, Metasomatism, Mantle, Peridotite, lherzolites, spinel websterites, and minor amounts of Subduction wehrlites, dunites, and pyroxenites. However, the exact nature of the metasomatic agents is still unclear in many cases. This unknown has direct implications for the origin 1 Introduction and possible evolution of the subcontinental mantle domains. In particular, in the context of formation of slab Reactive percolation of a wide variety of melts and fluids windows, the respective roles of subduction- and has been called upon to explain the mineralogy and major asthenosphere-derived materials in generating sub- and trace element composition of mantle samples brought continental mantle heterogeneities have remained a topic to Earth´s surface. In that regard, peridotite xenoliths of debate. At the sample scale, the origin of chemical carried to the surface by volcanic activity provide patterns and textures in mantle xenoliths has also remained important information regarding the nature and evolution controversial. Reaction textures of interest include partial of the lithospheric mantle (e.g. Pearson et al., 2003). These mineral replacement, orthopyroxene and clinopyroxene rocks are commonly associated, both in intraplate and rims, websterite veins, glass-bearing sieve-textured subduction settings, with a variety of mafic lithologies that clinopyroxene and spinel, glassy pockets around spinel and are commonly thought to have formed after circulation of amphibole, phlogopite and carbonate precipitation. melts or fluids within the upper mantle (e.g. Brandon et al., 1999). The purpose and difficulty in studying mantle One way to understand more precisely the relative xenoliths thus lie in decrypting their complex history of contributions of proposed metasomatic agents in making depletion and/or enrichment related to partial melting and mantle heterogeneities is to simulate in the laboratory the metasomatic events. Source heterogeneities accumulated exchange reactions that may take place in the sub- over time during successive tectonic events also add to the continental lithospheric mantle. Direct comparison xenolith complexity (e.g. Arai and Ishimaru, 2008). between results from controlled experiments and observations from natural samples should help us improve Patagonia is an interesting region to study sub-continental our understanding of the evolution of the upper mantle mantle processes because its geological context represents beneath the convergent margin. This is the approach a long-lasting convergent margin. Subduction of active followed in the present study focusing on the chemical ridges and subsequent opening of asthenospheric windows reactions and mineralogical transformations of spinel have been held responsible for the formation of the lherzolite associated with adakitic metasomatism. adakitic volcanic arc of the Austral Volcanic Zone, 200 km from the trench, and for the eruption of abundant Neogene 514 AT 1 GeoloGía ReGional y Geodinámica andina 2 Methods the power to the graphite furnace and subsequently decompressed. Recovered capsules were mounted in 2.1 Experimental methods acrylic resin, sectioned longitudinally to expose the reaction interface, and polish down to a 1 µm finish for Reaction couples were prepared using powdered natural SEM, EPMA and LA-ICP-MS analyses. adakite from Cerro Pampa (Argentina) in contact with a cylindrical fragment of fresh unmetasomatized Table 2. Experimental runs performed in the study. protogranular spinel lherzolite from Pali Aike (Chile). Run Lherzolite Temperature Duration Water Initial adakite:peridotite ratios range from 0.2 to 0.4. #120 PM18-2 1250 ºC 89 h Anhydrous Major, minor and trace element geochemistry of the #123 PM18-2 1300 ºC 48 h Anhydrous peridotite samples were characterized prior to starting the #124 PM18-2 1100 ºC 90 h 6 wt% experimental program (Gervasoni et al., 2013; unpublished #125 PM18-2 1000 ºC 40 h 9 wt% data). Distilled and deionized water was added by #128 PM18-2 1000 ºC 72 h 5 wt% micropipette to some of the reaction couples to study its #132 PM18-2 1050 ºC 116 h 12 wt% contribution to adakitic metasomatism. #133 PM18-35 1050 ºC 120 h 7 wt% #134 PM18-35 1000 ºC 110 h 6 wt% Post-experiment water contents of adakitic melts were estimated Table 1. Major element composition of starting materials. by difference from electron probe measurements. Lherzolite 1 Lherzolite 2 Adakite (PM18-2)* (PM18-35)* 2.2 Analytical methods SiO2 62.1 44.5 44.7 TiO2 0.61 0.05 0.08 Al2O3 16.9 2.15 2.3 Conventional SEM observations were made with a ZEISS Cr2O3 0.01 0.35 0.33 electron microscope at the Institut de Physique du Globe FeO 3.17 7.32 7.97 de Paris (France) to understand the overall reactions taking MnO 0.06 0.13 0.13 place in each sample and study textural evolution and MgO 3.73 44.1 44.0 detect points of interest for subsequent fully quantitative CaO 6.33 1.35 1.68 analyses. Semi-quantitative EDS analyses also provided Na O 4.54 0.27 0.31 2 useful information on how phase relations have evolved K O 1.28 0.02 0.01 2 during the experiment. P2O5 0.18 0.03 0.02 NiO 0.01 0.30 0.30 Total 99.1 100.6 101.0 Major and minor element compositions of the run products Mg# 0.68 0.91 0.91 were determined with a CAMECA SX100 microprobe at *From Gervasoni et al. (2013). the Université de Bretagne Occidentale (France), using wavelength-dispersive spectrometry. The electron High-pressure, high-temperature experiments were microprobe used a standard setup: beam current of 20 nA performed in an end-loaded piston-cylinder apparatus at and an acceleration voltage of 15 kV, 10–30 s of peak the Institut de Physique du Globe de Paris (France) using counting, 10 s of background counting, and natural and both 1/2" and 3/4" talc-pyrex cell assembly. A straight synthetic minerals as calibration standards. walled graphite furnace, talc pressure medium and thermal insulator, a Pyrex-sleeve as a viscous barrier against talc Trace element concentrations of some of the run products infiltration, and magnesia and alumina inner spacers and were determined in-situ by LA-ICP-MS at the Université tubing. The cylindrical platinum capsule containing the de Bretagne Occidentale (France), with a Thermo Element reaction couple is partially lined with graphite to minimize 2 ICP-MS coupled to a CompexPro 102 laser ablation Fe-loss and maintain the oxygen fugacity of the module that uses a 193nm excimer laser. The laser beam experiment near the CCO buffer, which corresponds to diameter was adjusted in the 60-90 µm range depending on conditions encountered in the sub-continental mantle phase dimensions. Background gas blanks was first (Frost and McCammon, 2008). Experiments were carried measured on all masses for 40 s before laser ablation. Total out at 1.5 GPa (equivalent to about 50 km depth) and laser firing time, which also depends on phase thickness, 1000-1300 °C (Table 2). This temperature range lies on the was up to 80 s. Ca determined by electron microprobe was upper end of those estimated at 50 km depth in the mantle used as internal standard. The reference NIST 612 and BIR wedge below Patagonia (e.g. Syracuse et al., 2010). In glasses were used as primary calibration and secondary each experiment, the reaction couple was first cold- check standards, respectively. The precision of the LA- pressurized to the target pressure (1.5 GPa). The ICP-MS analytical technique was better than 15% for all temperature was then raised to the target value at a rate of measured elements. 100 °C/min and held there for a duration varying between 48 and 120 h. Temperature was monitored with W-Re thermocouples inserted axially near the capsule.