Petrological and experimental constraints on the pre-eruption conditions of holocene dacite from volcan San Pedro (36°S, Chilean Andes)and the importance of sulphur in silicic subduction-related magmas. Fidel Costa Rodriguez, Bruno Scaillet, Michel Pichavant To cite this version: Fidel Costa Rodriguez, Bruno Scaillet, Michel Pichavant. Petrological and experimental constraints on the pre-eruption conditions of holocene dacite from volcan San Pedro (36°S, Chilean Andes)and the importance of sulphur in silicic subduction-related magmas.. Journal of Petrology, Oxford University Press (OUP), 2004, 45, pp.4, 855-881. hal-00068281 HAL Id: hal-00068281 https://hal-insu.archives-ouvertes.fr/hal-00068281 Submitted on 4 Feb 2013 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. JOURNAL OF PETROLOGY VOLUME 45 NUMBER 4 PAGES 855–881 2004 DOI: 10.1093/petrology/egg114 Petrological and Experimental Constraints on the Pre-eruption Conditions of Holocene Dacite from Volca´n San Pedro (36S, Chilean Andes) and the Importance of Sulphur in Silicic Subduction-related Magmas Downloaded from FIDEL COSTA*, BRUNO SCAILLET AND MICHEL PICHAVANT INSTITUT DES SCIENCES DE LA TERRE D’ORLEEANS,UMR 6113 CNRS-UO, 1A RUE DE LA FEEROLLERIE, 45071 ORLEEANS,FRANCE http://petrology.oxfordjournals.org/ RECEIVED FEBRUARY 4, 2003; ACCEPTED SEPTEMBER 24, 2003 We present an experimental and petrological study aimed at estimat- INTRODUCTION ing the pre-eruptive conditions of a Holocene dacitic lava from The pre-eruptive pressure, temperature, volatile content, ´ Volcan San Pedro (36 S, Chilean Andes). Phase-equilibrium redox state, and petrology of silicic subduction-related experiments were performed at temperatures ( T) from 800 to magmas is a highly investigated topic because of the 950 C, and mainly at 200 MPa, but also at 55, 150, and --- known climatic (e.g. 1982, El Chichoon,and 1991, Pinatubo 406 MPa. Oxygen fugacity ( fO2) ranged from the Ni NiO buffer eruptions; Robock, 2000) and potential hazardous effects at CNRS - ISTO on February 4, 2013 Á þ Á (NNO) to 3 5 log units above (NNO 3 5), and water contents (e.g. eruptions of Mt. Peleee, 1902, or Mount St. Helens, from 3to 6 wt %. We also report several experiments where 1980). Petrological studies that use glass inclusion com- Á --- we added sulphur (0 1 1 wt % S) to the dacite. The main mineral positions [e.g. reviews by Johnson et al. (1994) and Scaillet þ þ assemblage of the dacite (hornblende orthopyroxene plagioclase) & Pichavant (2003)] and geothermobarometers (e.g. Æ Æ Á --- Á is stable at 200 50 MPa, 850 10 C, with 4 5 5 5wt% Manley & Bacon, 2000) provide a way of partially esti- H2O in the melt, and at fO2 of NNO þ 1Á2 Æ 0Á2, in accord with --- mating such parameters, but an alternative and comple- the crystallinity, mineral proportions, and T fO2 determination mentary approach is to perform phase equilibrium from Fe--- Ti oxides of the lava. However, biotite, which is also --- experiments at various pressures (P ), temperatures (T ), present in the dacite, is stable at these same T fO2 conditions only oxygen fugacities ( f O ), and water fugacities ( f H O) to 4 Á 2 2 in experiments with 0 1 wt % S added. This result is in accord reproduce the phase assemblages and compositions of the with the occurrence of pyrrhotite in the lava, and with the presence of volcanic products. The experimental approach has been S in glass inclusions and biotite ( 300 ppm, and up 170 ppm, applied to a number of silicic eruptive products, notably: respectively). Moreover, the zoning patterns and compositions of El Chichoon(Luhr, 1990), Mount St. Helens (Rutherford plagioclase phenocrysts together with the presence of high-temperature et al., 1985; Gardner et al., 1995), Novarupta (Hammer minerals (e.g. clinopyroxene) in the lava suggest that the petrological et al., 2002), Mt. Peleee (Martel et al., 1990, 1999; history of the dacite is more complex than a single near-equilibrium Pichavant et al., 2002), Pinatubo (Rutherford & Devine, 5 crystallization stage, and could be explained by short-lived ( 100 1996; Scaillet & Evans, 1999), Santorini (Cottrell et al., years) temperature fluctuations ( 50 C) in the magma reservoir. 1999), Soufrie`re Hills (Barclay et al., 1998), and Unzen (Sato et al., 1999). In most of these studies, the natural phase assemblages were reproduced using a volatile com- KEY WORDS: sulphur; dacite; experiments; biotite; Andes position consisting mainly of H2O. The petrological and *Corresponding author. Present address: Institut fuu€r Geologie, Mineralogie, und Geophysik, Ruhr-Universitaa€t, Bochum, Bochum 44780, Germany. Telephone: þþ49 234 322 4393. Fax: þþ49 234 Journal of Petrology 45(4) # Oxford University Press 2004; all rights 321 4433. E-mail: [email protected] reserved JOURNAL OF PETROLOGY VOLUME 45 NUMBER 4 APRIL 2004 experimental work carried out on the El Chichoonand (Costa et al., 2002) and quenched mafic inclusions (QMI); Pinatubo eruptions, and the abundance of S in glass (3) 0Á5km3 of two-pyroxene dacite with abundant inclusions in phenocrysts from silicic subduction-related QMI; (4) 0Á1km3 of two-pyroxene andesite with rare magmas (e.g. Scaillet & Pichavant, 2003), however, show QMI. The last volcanic activity that rebuilt the summit that S can be also an important volatile to take into cone involved basaltic andesites and mafic andesites account for understanding the phase assemblages and (0Á2km3). Costa & Singer (2002) proposed that the composition. zoned magma reservoir is the result of incomplete In this paper we combine experimental and petrolo- magma mixing between a mafic end-member, repres- gical data for a Holocene dacitic lava flow from Volca´n ented by the last basaltic andesites, and the first erupted San Pedro (36S, Chilean Andes) and we show that its and most silica-rich dacite. We have performed experi- main mineral assemblage (hornblende, plagioclase, and ments on this silica-rich dacite, which lacks any quenched orthopyroxene) and proportions can be experimentally inclusions, and which we infer to have resided in the reproduced under a relatively limited set of P, T, f O2, and upper parts of the San Pedro zoned magma reservoir. f H2O conditions. However, to explain the full mineral Although its bulk-rock composition (66 wt % SiO2, assemblage and textures of the dacite it was also neces- 2Á7wt%K2O; Table 1) is similar to older dacites erupted sary to: (1) perform S-bearing experiments to account for at the TSPC (e.g. Ferguson et al., 1992; Singer et al., 1995; Downloaded from the presence of biotite in the lava at the same pre-eruptive Feeley & Dungan, 1996; Costa & Singer, 2002) or neigh- conditions, and (2) to integrate the experimental results bouring volcanoes (e.g. Quizapu; Hildreth & Drake, with detailed geochemical and petrographical data for 1992), its mineral assemblage includes biotite, which the dacite to account for the presence of clinopyroxene appears to be rare in other Holocene dacites erupted in and the zoning patterns of plagioclase phenocrysts. the southern volcanic zone of the Andes (Hildreth & http://petrology.oxfordjournals.org/ The outline of the paper is as follows: after describing Moorbath, 1988). the geology and petrography of the San Pedro Holocene zoned eruption we give constraints on the pre-eruptive Pre-eruptive conditions estimated from the conditions from the phase assemblage of the dacite dacite lava phase compositions alone. Next, we present the experimental results, which The dacite contains 29 wt % of crystals, mainly include S-free and S-bearing experiments. Finally, we plagioclase ( 17%) and amphibole ( 11%), but also integrate the experimental results with the geochemical euhedral biotite, clinopyroxene, orthopyroxene, titano- and petrographical data for the Holocene magma reser- magnetite, ilmenite, apatite, zircon, pyrrhotite, and chal- voir, and we discuss whether the pre-eruptive conditions copyrite, set in a matrix of rhyolitic glass (Table 1). Two and amount of S inferred for the San Pedro dacite can be or three crystals per thin section of orthopyroxene- at CNRS - ISTO on February 4, 2013 applied to other subduction-related dacites. mantled olivine (Fo82--- 69) are also present, and they have been attributed as being derived from basaltic QMI as found in other lavas of the same eruption GEOLOGICAL SETTING AND (Costa & Singer, 2002). Plagioclase phenocrysts are oscil- latory zoned and consist of repetitive normal zoning - PRE ERUPTIVE CONDITIONS patterns (typically from An65 to An50), but most rim ESTIMATED FROM PHASE compositions are An40--- 35 (Singer et al., 1995). There COMPOSITIONS IN THE is also a small population of plagioclase xenocrysts (An84--- 82), which are probably derived from the same DACITIC LAVA mafic inclusions as the olivine xenocrysts (Singer et al., Volca´n San Pedro ( 2km3) is the Holocene volcanic 1995; Costa & Singer, 2002). The proportion of plagio- edifice of the Quaternary Tatara--- San Pedro volcanic clase xenocrysts is about 10% of the total plagioclase complex (TSPC, 36S, Chilean Andes; Singer et al., population (Singer et al., 1995), which transferred to 1997; Dungan et al., 2001; Costa & Singer, 2002). Pre- wt % of the phases in the lava is about 1Á6 wt % and so vious work distinguished between an older phase com- has little compositional leverage on the bulk-rock compo- prising a main cone-building stage made of basaltic sition. Amphibole is magnesiohornblende or tscherma- andesitic and dacitic lavas, and a younger phase that kite [classification after Leake et al.