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F u n 2 d 6 la serena octubre 2015 ada en 19 Experimental constraints on adakitic metasomatism of mantle wedge 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 . 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/ harzburgites, spinel/garnet Keywords: Adakite, Metasomatism, Mantle, Peridotite, lherzolites, spinel websterites, and minor amounts of 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 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 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 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. After run completion, samples were quenched rapidly by turning off 515 ST 3 METAMORFISMO Y MAGMATISMO EN ZONAS DE SUBDUCCIÓN

3 Results

3.1 Changes in phase relations and major element chemistry

Chemical reactions occurred mainly at the original peridotite-adakitic melt interface and within veins penetrating along grain boundaries near the interface due to reactive percolation. As shown in Fig. 1, dissolution of olivine grains in contact with the andesitic to dacitic (56- 65 wt% SiO2) subalkaline “adakitic” melt and formation of a layer of orthopyroxene at the melt-olivine interface was observed in all the experiments, with or without added water. This suggests that an incongruent reaction like the following one took place: Ol1 + Liq1 = Opx2 + Liq2. Major element composition of secondary orthopyroxene is En95Fs4Wo1, while primary orthopyroxene, which appears Figure 1. Back-scattered electron image of run #124 (1.5 GPa, unaffected by the adakitic melt, is En90Fs9Wo1. 1100 ºC) showing formation of secondary orthopyroxene (Opx2) and clinopyroxene (Cpx2) at the lherzolite-adakite interface. The crystallization of secondary Ti-rich, Cr- and Na-poor clinopyroxene was also observed in all runs (Fig. 1). In most cases, secondary clinopyroxene remains diopsidic as the primary clinopyroxene (En49Fs5Wo46), except in the anhydrous runs #120 and #123 where a trend towards the enstatite end-member was observed (up to En72Fs5Wo23). The overall preservation of primary clinopyroxene– adakite interface prescribes the formation of secundary clinopyroxene mainly from primary clinopyroxene. Rather, the presence of secondary clinopyroxene over the entire melt-peridotite interface may mostly result from the crystallization of the molten adakite. In addition to secondary clinopyroxene, some runs (#120, #125, #133, #134) also produced zoned plagioclase crystals with composition ranging between An30 and An50.

In runs with spinel present at or near the lherzolite-adakite interface, we observe rims of secondary sieve-textured Figure 2. BSE image and chemical maps of run #128 (1.5 GPa, spinel with sub- to euhedral spinel and melt inclusions 1000 ºC) showing incongruent dissolution of primary spinel (Fig. 2). Where no infiltrated melt is present, there is no (Sp1) and formation of secondary spinel (Sp2). discernable reaction zone. The dissolution process involves a reaction of the type: Sp1 + Liq 1 = Sp2 + Liq2 where Sp2 is a secondary spinel enriched in Cr and depleted in Al and Fe (Fe2+ and Fe3+) in comparison with the primary spinel. The Cr-number (Cr# = 100 × Cr / (Cr + Al) in mole fractions) of Sp2 ranges from 29 to 45, while the Mg- number (Mg# = 100 × Mg2+ / (Mg2+ + Fe2+) in mole fractions) varies between 77 and 83. This compares to lower values of primary spinel of Cr# of 19 and Mg# of 77. Both chemical variations seen in clinopyroxene and spinel could result from selective removal of the low melting component by interaction with the adakitic melt.

In runs with high contents of added water (≥ 9 wt% H2O) such as #125 and #132, we also detected the formation of pargasite amphiboles. These are up to 70 µm large and developed in areas of melt infiltration along clinopyroxene grain boundary (Fig. 3). Figure 3. Back-scattered electron image and chemical maps of run #132 (1.5 GPa, 1050 ºC) showing the formation of pargasitic

amphibole (Amph) under water-rich conditions. 516 AT 1 GeoloGía ReGional y Geodinámica andina

3.2 Trace elements in clinopyroxene The upper mantle beneath Patagonia, Argentina, documented by xenoliths from alkali . Journal of South American Earth Sciences 18, 125–145. Run #132 produced a layer of secondary clinopyroxene Bjerg E.A., Ntaflos T., Thöni M., Aliani P., Labudia C.H., 2009. sufficiently large to perform trace element measurements Heterogeneous lithospheric mantle beneath Northern Patagonia: by LA-ICP-MS. Corresponding rare earth element patterns Evidence from Prahuaniyeu garnet- and spinel-peridotites. are shown in Figure 4 and compared to that of primary Journal of Petrology 50, 1267-1298. clinopyroxene and adakitic melt. The former displays a Brandon A.L., Becker H., Carlson R.W., Shirey S.B., 1999. Isotopic rather flat pattern, while the latter displays an enriched constraints on time scales and mechanisms of slab material transport in the mantle wedge: evidence from the Simcoe mantle pattern inherited from the original natural adakite from xenoliths, Washington, USA. Chemical Geology 160, 387-407. Cerro Pampa used in the experiment. Adakitic Dantas C., Grégoire M., Koester E., Conceição R.V., Rieck Jr. N., metasomatism produces secondary pyroxene with MREE- 2009. The lherzolite–websterite xenolith suite from Northern enrichment relative to primary clinopyroxene, but with Patagonia (Argentina): Evidence of mantle–melt reaction similar La and lower Lu content. Lattice strain effects processes. Lithos 107, 107-120. probably explain the limited incorporation of LREE in Gervasoni, F., Conceição, R.V., Jalowitzki, T.L.R., Schilling, M.E., Orihashi, Y., Nakai, S., Sylvester, P. (2013). Heterogeneidades secondary clinopyroxene (Blundy et al., 1998). do manto litosférico subcontinental no extremo sul da Placa Sul- americana: influência da subducção atual e interações litosfera- astenosfera sob o Campo Vulcânico de Pali Aike. Pesquisas em Geociências, 39 (3): 269-285. Gorring M.L., Kay S.M., 2000. Carbonatite metasomatized peridotite xenoliths from southern Patagonia: implications for lithospheric processes and Neogene plateau magmatism. Contributions to Mineralogy and Petrology 140, 55-72. Gorring M.L., Kay S.M., Zeitler P.K., Ramos V.A., Rubiolo D., Fernandez M.I., Panza J.L., 1997. Neogene Patagonian plateau lavas: continental associated with ridge collision at the Chile Triple Junction. Tectonics 16, 1–17. Kilian R., Stern C.R., 2002. Constraints on the interaction between slab melts and the mantle wedge from adakitic glass in peridotite xenoliths. European Journal of Mineralogy 14, 25-36. Laurora A., Mazzucchelli M., Rivalenti G., Vannucci R., Zanetti A., Barbieri M.A., Cingolani C.A., 2001. Metasomatism and melting in carbonated peridotite xenoliths from the mantle wedge: the Gobernador Gregores case (Southern Patagonia). Journal of Petrology 42, 69-87. Figure 4. REE spidergram showing a relatively flat pattern for Ntaflos T., Bjerg E.A., Labudia C.H., Kurat G., 2007. Depleted primary clinopyroxene (Cpx1) and MREE-enriched pattern for lithosphere from the mantle wedge beneath Tres Lagos, southern secondary clinopyroxene (Cpx2) and adakitic melt in run #132. Patagonia, Argentina. Lithos 94, 46-65. Pearson D.G., Canil D., Shirey S.B., 2003. Mantle samples included in volcanic rocks: Xenoliths and diamonds. In: (ed. Carlson Acknowledgements R.W.) The Mantle and Core, Treatise on Geochemistry, vol. 2 (eds. H. Holland and K.K. Turekian). Elsevier–Pergamon, Oxford. pp.171-275. The authors acknowledge financial support from the Rivalenti G., Zanetti A., Mazzucchelli M., Vannucci R., Cingolani, Fondecyt Regular Projects nº1130635 (AC) and C.A. (2004). Equivocal carbonatite markers in the mantle n°1100724 (MS) and administrative support from the xenoliths of the Patagonia backarc: the Gobernador Gregores Universidad Austral de Chile and the French National case (Santa Cruz Province, Argentina). Contributions to Center for Scientific Research (CNRS). We also Mineralogy and Petrology 147, 647-670. Schilling M., Conceição R.V., Mallmann G., Koester E., Kawashita acknowledge fieldwork support from Yuji Orihashi (U. K., Herve F., Morata D., Motoki A., 2005. Spinel-facies mantle Tokyo), Ryo Anma (U. Tsukuba) and Rommulo xenoliths from Cerro Redondo, Argentine Patagonia: Conceição (UFRGS), and technical assistance from James petrographic, geochemical, and isotopic evidence of interaction Badro and Julien Siebert (IPGP), and from Jessica between xenoliths and host . Lithos 82, 485-502. Langlade and Claire Bassoullet (IUEM). Stern C.R., Kilian R., 1996. Role of the subducted slab, mantle wedge and in the generation of adakites from the Andean Austral Volcanic Zone. Contributions to Mineralogy and Petrology 123, 263-281. References Stern C.R., Kilian R., Oiker B., Hauri E.H., Kyser T.K., 1999. Evidence from mantle xenoliths for relatively thin (<100 km) Arai S., Ishimaru, S., 2008. Insights into petrological characteristics continental lithosphere below the Phanerozoic crust of of the lithosphere of mantle wedge beneath arcs through southernmost South America. Lithos 48, 217-235. peridotite xenoliths: a review. Journal of Petrology 49, 665-695. Syracuse E.M., van Keken P.E., Abers G.A., 2010. The global range Bertotto G.W., 2002. Xenolitos ultramaficos en el Cerro De la of subduction zone thermal models. Physics of the Earth and Laguna, volcanismo basaltico de retroarco en el sureste de la Planetary Interiors 183, 73–90. provincia de Mendoza, Argentina. Revista de la Asociación Wang J., Hattori K.H., Li J., Stern C.R., 2008. Oxidation state of Geológica Argentina 57, 445–450. Paleozoic subcontinental lithospheric mantle below the Pali Aike Bjerg E.A., Ntaflos Th., Kurat G., Dobosi G., Labudia C.H., 2005. volcanic field in southernmost Patagonia. Lithos 105, 98-110. 517