Determining Anatectic Melt Composition of Nanogranite Inclusions in High-Grade Metapelites from the Ivrea-Verbano Zone (Italy)

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Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012

DETERMINING ANATECTIC MELT COMPOSITION OF NANOGRANITE INCLUSIONS IN HIGH-GRADE METAPELITES FROM THE IVREA-VERBANO ZONE (ITALY).

Ph.D. candidate: ALICE TURINA, I course Tutor: Prof. BERNARDO CESARE Co-Tutor: Prof. STEFANO POLI Cycle: XXVII

Abstract Nanogranites are tiny polycrystalline inclusions representing the anatectic melt that was trapped during incongruent melting reactions in minerals of high-grade metamorphic rocks. Their characterisation therefore allows to determine the chemistry of primary anatectic melts, which is otherwise one of the biggest unknowns in crustal petrology. I am analysing the composition and the microstructures of nanogranites in amphibolite- to granulite-facies metapelites from the Ivrea-Verbano Zone. These results, together with those from classical petrology and thermodynamic modelling, will shed light on the metamorphic history of the rocks. To analyse the composition of the melt nanogranites were re-melted using a piston-cylinder apparatus. Experiments were performed at 800, 850 and 900 °C and 8-12 Kbar. The inclusions did not totally homogenise at 800 °C but show evidences of over-heating at 900 °C, while at 850 °C the results are still unclear. Further experiments will constrain the temperature of melting.

Introduction Melt inclusions are drops of silicic liquid trapped in “host” minerals during growth (Sorby, 1858). They have been extensively investigated in igneous petrology because they can preserve some chemical and physical features of the original magmatic system and thus provide information that otherwise would be lost (Lowestern, 1995; Frezzotti, 2001; Bodnar and Student, 2006). Recently, anatectic melt inclusions, called nanogranites, have been discovered in high-grade metamorphic rocks that experienced partial melting (Cesare et al., 2009). These inclusions can be trapped in peritectic minerals formed during incongruent melting reactions and thus, if they remain a closed system, their composition is representative of that of primary anatectic melts (Cesare et al., 2011; Ferrero et al., 2012). In crustal petrology the composition of the very first melt that forms during anatexis of various source rocks can be either inferred from: i) the composition of leucosomes in migmatites, that can be modified as a consequence of fractional crystallization or segregation (Sawyer, 1987; Solar and Brown, 2001) or ii) experimental works, that cannot totally reproduce natural conditions, especially at low melting degrees (Vielzeuf and Schmidt, 2001). Both methods suffer from limitations that could be overcome through the in situ analysis of natural anatectic melt inclusions; therefore it is important to develop new techniques to determine the composition of the melt trapped as nanogranites (Bartoli et al., in press). The aim of my Ph.D. project is to characterise nanogranites trapped in peritectic garnet of metapelites from the Ivrea-Verbano Zone. This area is of great interest because it is a widely studied example of an exhumed crustal section that underwent regional metamorphism and partial melting of Varisican age (Handy and Zingg, 1991; Schnetger, 1994; Handy et al., 1999). The sequence evolves from amphibolite-facies rocks of shallower crustal levels (locally called Kinzigites) to deeper granulites (Stronalites) through well-recognized mineral isogrades and P-T conditions (Schmid and Wood, 1976; Zingg, 1980; Henk et al., 1997). During this project I am analysing the composition of melt inclusions both in amphibolitic and granulitic metapelites to gain more information on the composition and the volatile contents of primary anatectic melts and possibly to understand the formation and evolution processes of the melt. In order to achieve a complete petrologic characterisation I will accomplish a petrographic and microstructural study of the inclusions and mineral assemblages, to shed light on the P-T conditions of formation and trapping. Moreover, nanogranites from the Kaligandaki valley in Central Nepal (Himalaya) will also be characterised to compare melt compositions in different geological and geodynamic settings.

Methods • Petrographic and microstructural observations are carried out through optical microscopy (both in reflected and transmitted light) and scanning electron microscopy EDS spectroscopy, BSE imaging

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012

and mapping. The minerals that trap nanogranites are polished to uncover the tiny cryptocrystalline aggregates they contain, then analysed with a Field Emission Scanning Electron Microscope (FE- SEM) using EDS maps to determine the constituent mineral phases. Quantitative analyses are not possible due to the small dimensions of crystals that range from some nanometres up to 5 micrometres. The polishing process is a major limit in the work because it often results in mechanical removal of the inclusion; we are now developing new methods to solve this problem. • To obtain the composition of the primary melts, nanogranites need to be fully re-homogenised at the temperature of trapping. Therefore re-melting experiments are performed using a single-stage piston cylinder at the Department of Earth Sciences, University of Milan. Fragments of garnet that contain nanogranites are extracted from the rock and sealed in a golden capsule with silica powder, then loaded in the apparatus. At the end of the experiments the capsules are mounted in epoxy and uncovered through polishing. Successfully re-melted inclusions are then analysed using EMP for major elements, with correction for Na and K loss, following the method explained in Morgan and London (2005). • X-ray micro-tomography has been used to perform a non-destructive microstructural investigation for the analysis of inclusions distribution in garnets.

Activities of the first year During the first year of Ph.D. new samples were collected in two different periods of fieldwork; the first one (28-30 May 2012, in collaboration with Prof. Siegesmund) was aimed at the investigation of Valle Strona (northern Ivrea Zone), where the metamorphic evolution is better constrained and continuously mapped. 28 samples of amphibolites and granulites were collected for thin section analyses. In the second one (10-12 October 2012, in collaboration with Prof. Boriani) I went to Valle d’Ossola and Valle Sesia (southern Ivrea Zone) to obtain samples (38) from different areas. Some thin sections from Valle Fiorina (northern Ivrea Zone, provided from Prof. Mazzucchelli) were also observed to detect the presence of nanogranites. Up to now I have performed three experiments, one of which was accomplished during my master’s degree and the other two during this Ph.D. first year, on samples from the Ivrea-Verbano Zone (provided by Dr. Tanya Ewing) and from the Kaligandaki valley (provided by Prof. Dario Visonà). These experiments were conducted at 900, 850 and 800 °C and 8-12 Kbar confining pressure. Some of the activities of the first year regarded the improvement in sample preparation. For the first re- melting experiments we obtained garnet fragments through milling of rocks and handpicking or through separation of “chips” from thick sections. Now I am testing the possibility of drilling cores of garnet containing inclusions using a hollow-core drill. This will permit a major improvement in the efficiency of the experiments. To better understand the 3D distribution of inclusions and the trapping mechanism, three X-ray computed micro-tomography datasets were collected and inclusions were studied through image-analysis and 3D calculations to obtain the volume and position with respect to the garnet core. For this preliminary analysis we decided to investigate inclusions in garnets from El Hoyazo (Spain) because, due to their formation process (Cesare et al., 2007; Acosta-Vigil et al., 2007), these inclusions are not polycrystalline, a feature that could be a limitation in the processing of data.

Results The preliminary microscopic observations have permitted the identification of nanogranite inclusions in the majority of amphibolite-facies samples from Valle Strona while in granulite-facies metapelites these inclusions are much less widespread. Nanogranites are also present in most of the garnets from migmatitic paragneisses of the Kaligandaki valley. Usually, inclusions from the Ivrea Zone are smaller (10-15 µm) than those from Himalaya (10-20 µm). Petrographic observations confirm that the transition from amphibolites to granulites in Valle Strona is marked by the decrease of biotite and increase of garnet, as defined by Schmid and Wood (1976). Not all the samples collected represent metapelitic rocks sensu stricto, which contain the mineral assemblage: Qtz+Sil+Bt+Grt+Kfs+Pl±Gr±Ms (abbreviations after Kretz, 1983), since more arenaceous protoliths can produce assemblages that also contain Opx. 2

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012

The spatial distribution of inclusions in thin sections suggests a preferentially ring-shaped arrangement localized between the core and the rim of garnets. Since this characteristic feature is reproduced in different samples we decided to investigate the 3D distribution through micro-tomography. The results show that in El Hoyazo garnets the distribution of melt inclusions is bell-shaped, with a maximum concentration at about 1/3 distance between the core and the rim. This suggests a mechanism of primary trapping of the inclusions on garnet growth surfaces. Conversely, mineral phases (i.e. zircons, apatites, monazites) trapped as single homogeneous inclusions are evenly distributed in the whole garnet. Inclusions in garnets from both the Ivrea-Verbano Zone and the Kaligandaki valley have been re-melted in three different experiments. The first was performed at T=900° C, P=8 Kbar and time=24h. The inclusions from both samples show clear evidences of over-heating and reaction with the host garnet such as Opx formation and decrepitation. The second one was run at T=800° C, P=11 Kbar and time=24h. The inclusions started to melt but the process was not completed (Fig. 1a and 1c). For the last experiment we set T=850° C, P=12 Kbar and time=24h. In this last one the results are quite unclear: all the Himalayan inclusions show evidence of over-heating (Fig. 1d) while those from the Ivrea-Verbano Zone show different features even within the same sample due to the fact that some inclusions have clearly over- heated but some are not even totally melted (Fig. 1b).

a b c d

Fig. 1: Experimentally re-melted inclusions from the Ivrea-Verbano Zone at 800° C (a) and 850° C (b) and from the Kaligandaki valley at 800° C (c) and 850° C (d).

Conclusions and Future The experiments suggest that the trapping temperature of melt inclusions should be between 800 and 850 °C for the Ivrea Zone and slightly lower, around 800 °C, for the Kaligandaki valley. The fact that inclusions start to re-melt at 800° C but do not reach a complete homogenisation could either be due to the fact that i) this temperature is close to, but not equal to, the right trapping temperature or that melting is not perfectly eutectic but can occur in a temperature interval or ii) the temperature is the correct one but the kinetics of the melting reaction need more time to attain homogenisation. New experiments are planned in the near future on amphibolite- and granulite-facies samples from Ivrea and on garnets from Kaligandaki, varying time and temperature, to better constrain the temperature of melting in these two areas. When this is achieved, the inclusions will be analysed to obtain information on the chemical composition and volatile content, and then these analyses will be compared. Petrographic observation will be performed on the new samples from Valle d’Ossola and Valle Strona and, if possible, experiments will be carried out to extend our observations and interpretations to the whole area. In the meantime, the microstructural investigation of inclusions relationship with other solid-phase inclusions and distribution will continue through the improvement of non-destructive investigation techniques (micro-tomography). The future petrographic work will focus on the mineral chemistry of assemblages in the different rocks while the equilibrium-phase modelling will be accomplished later on, probably at the end of second year of Ph.D., during a planned period as visiting student ad Virginia Tech University (USA).

References ACOSTA-VIGIL, A., CESARE, B., LONDON, D. and MORGAN VI, G.B. 2007. Microstructures and composition of melt inclusions in a crustal anatectic environment, represented by metapelitic enclaves within El Hoyazo dacites, SE Spain. Chemical Geology, 237, 450-465. 3

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012

BARTOLI, O., CESARE, B., POLI, S., BODNAR, R.J., ACOSTA-VIGIL, A., FREZZOTTI, M.L. and MELI, S. 2012. Recovering the composition of melt and the fluid regime at the onset of crustal anatexis and S-type granite formation. Geology, in press. BODNAR, R.J. and STUDENT, J.J. 2006. Melt inclusions in plutonic rocks: petrography and microthermometry. In: Melt Inclusions in Plutonic Rocks. Mineralogical Association of Canada Short Course, 36, 1-26. CESARE, B., ACOSTA-VIGIL, A., FERRERO, S. and BARTOLI, O. 2011. Melt inclusions in migmatites and granulites. Journal of the Virtual Explorer, 40, 2. CESARE, B., FERRERO, S., SALVIOLI-MARIANI, E., PEDRON, D. and CAVALLO, A. 2009. Nanogranite and glassy inclusions: the anatectic melt in migmatites and granulites. Geology, 37, 627- 630. CESARE, B., MAINERI, C., BARON TOALDO, A., PEDRON, D. and ACOSTA-VIGIL, A. 2007. Immiscibility between carbonic fluids and granitic melts during crustal anatexis: a fluid and melt inclusion study in the enclaves of the Neogene Volcanic Province of SE Spain. Chemical Geology, 237, 433-449. FERRERO, S., BARTOLI, O., CESARE, B., SALVIOLI-MARIANI, E., ACOSTA-VIGIL, A., CAVALLO, A., GROPPO, C. and BATTISTON, S. 2012. Microstructures of melt inclusions in anatectic metasedimentary rocks. Journal of Metamorphic Geology, 30, 303-322. FREZZOTTI, M.L. 2001. Silicate-melt inclusions in magmatic rocks: application to petrology. Lithos, 55, 273-299. HANDY, M.R., FRAZN, L., HELLER, F., JANOTT, B. and ZURBRIGGEN, R. 1999. Multistage accretion and exhumation of the continental crust (Ivrea crustal section, Italy and Switzerland). Tectonics, 18(6), 1154-1177. HANDY, M.R. and ZINGG, A. 1991. The tectonic and rheological evolution of an attenuated cross section of the continental crust: Ivrea crustal section, souther Alps, northwestern Italy and southern Switzerland. Geological Society of America Bulletin, 103(2), 236-253. HENK, A. FRANZ, L., TEUFEL, S. and ONCKEN, O. 1997. Magmatic underplating, extension and crustal re-equilibration: insights from a cross-section through the Ivrea Zone and Strona-Ceneri Zone, northern Italy. Journal of Geology, 105, 367-377. KRETZ, R. 1983. Symbols for rock-forming minerals. American Mineralogist, 68, 277-279. LOWESTERN, J.B. 1995. Applications of silicate-melt inclusions to the study of magmatic volatiles. In: Magmas, Fluids and Ore Deposits. Mineralogical Association of Canada Short Course, 23, 71-99. MORGAN VI, G.B. and LONDON, D. 2005. Effect of current density on the electron microprobe analysis of alkali aluminosilicate glasses. American Mineralogist, 90, 1131-1138. SAWYER, E.W. 1987. The role of partial melting and fractional crystallization in determining discordant migmatite leucosome compositions. Journal of Petrology, 28, 445-473. SCHMID, R. and WOOD, B.J. 1976. Phase relationships in granulitic metapelites from the Ivrea- Verbano Zone (Northern Italy). Contributions to Mineralogy and Petrology, 54, 255-279. SCHNETGER, B. 1994. Partial melting during the evolution of the amphibolite- to granulite-facies gneisses of the Ivrea Zone, Northern Italy. Chemical Geology, 113, 71-101. SOLAR, G.S. and BROWN, M. 2001. Petrogenesis of migmatites in Maine, USA: possible source of peraluminous leucogranite in plutons?. Journal of Petrology, 42, 789-823. SORBY, H.C. 1858. On the microscopical structure of crystals, indicating the origin of minerals and rocks. Quarterly Journal of the Geological Society of London, 14, 453-500. VIELZEUF, D. and SCHMIDT, M.W. 2001. Melting relations in hydrous systems revisited: application to metapelites, metagreywakes and metabasalts. Contributions to Mineralogy and Petrology, 141, 251- 26. ZINGG, A. 1980. Regional metamorphism in the Ivrea Zone (Southern Alps, N-Italy): field and microscopic investigations. Schweizerische Mineralogische und Petrographische Mitteilungen, 60, 153-179.

Scuola di Dottorato in Scienze della Terra, Dipartimento di Geoscienze, Università degli Studi di Padova – A.A. 2011-2012

SUMMARY OF ACTIVITY IN THIS YEAR

Courses and workshops:

R. ANGEL: “Scientific communication”. Department of Geosciences, Padua University, 29 October – 28 November 2012. S. ANDÒ, D. BERSANI, A. CASAGLI, M.L. FREZZOTTI, G. GODARD, M. PLACIDI, D. SMITH, F. TECCE, M. TRIBAUDINO: “Raman Spectroscopy in Earth Sciences”, II annual school, University of Milan-Bicocca, 24-25 October 2012. B. DEVIVO, R.J. BODNAR, L.V. DANYUSHEVSKY,J.D. WEBSTER: “Fluids in the Earth”. 7th annual shortcourse, University of Naples, 15-19 October 2012. BRUKER ITALIA: “Theoretical and practical aspects of scanning electron microscopi and microanalysis”. Workshop organized by Bruker Italia, Milan, 9 October 2012. S. CHACKRABORTY, R. DOHMEN, T. MUELLER, T. FROCKENBERG, M. TIRONE, K. FAAK, S.BORINSKY, : “Application of diffusion studies to the determination of timescales in geochemistry and petrology”. DMG-MSA workshop, Ruhr-Universitat Bochum (Germany), 1-5 October 2012. R. ABART, E. PETRISHCHEVA, S. JAHN, R. MILKE, G. DRESEN: “Diffusion in geological materials”. DMG short course. University of Vienna (Austria), 10-14 September 2012. R.J. BAKKER: “Fluid inclusions short course”. University of Leoben (Austria), 7-9 September 2012. EUROPEAN MICROBEAN ANALYSES SOCIETY (EMAS): “Electron Probe microanalysis of materials today – practical aspects”. 10th EMAS regional workshop, Department of Geosciences, Padua University, 17-20 June 2012 E. CALANDRUCCIO: “Corso avanzato di inglese parlato”. Department of Geosciences, Padua University, 17-24 May 2012. L. GULIK: “Corso avanzato di inglese scientifico”. Department of Geosciences, Padua University, 16-20 April 2012. L. SALMASO, L. CORAIN, S. BONNINI, R. ARBORETTI: “Statistica applicata alla sperimentazione scientifica (edizione 2012)”. Department of Geosciences, Padua University, February 2012.

Communications:

CESARE, B.; FERRERO, S.; BARTOLI, O.; ACOSTA-VIGIL, A.; TURINA, A.; POLI, S.; EWING, T. 2012. Nanogranites in anatectic metapelites: reading the message in the bottle. 34th International Geological Congress, Brisbane, Australia.

CESARE, B.; FERRERO, S.; BARTOLI, O.; ACOSTA-VIGIL, A.; TURINA, A.; POLI, S.; EWING, T.; BODNAR, R.J. 2012. Nanogranites in anatectic metapelites: building up the database. 22nd Goldschmidt Conference, Montréal, Canada.

Other:

Training in the use of EMPA JEOL JXA 8200 SuperProbe and microprobe analyses, Department of Earth Sciences, University of Milan.

Web-editing of the research group website: www.eurispet.eu/acme/home.html