Holocene record of large explosive eruptions from Chaitén and Michinmahuida Volcanoes, Chile - PDF

Andean Geology 40 (2): May, 2013 doi: /andgeoV40n2-a?? /andgeoV40n2-a03 Andean Geology formerly Revista Geológica de Chile Holocene record of large explosive

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Andean Geology 40 (2): May, 2013 doi: /andgeoV40n2-a?? /andgeoV40n2-a03 Andean Geology formerly Revista Geológica de Chile Holocene record of large explosive eruptions from Chaitén and Michinmahuida Volcanoes, Chile Álvaro Amigo 1, Luis E. Lara 1, Victoria C. Smith 2 1 Servicio Nacional de Geología y Minería, Programa de Riesgo Volcánico, Avda. Santa María 0104, Santiago, Chile. 2 Research Laboratory for Archaeology and the History of Art, University of Oxford, U.K. ABSTRACT. Tephra fall deposits and one large ignimbrite close to Chaitén and Michinmahuida Volcanoes were analyzed for chemistry and radiocarbon dated to correlate the eruptive units and establish the timing of eruptions. These data suggest that both volcanoes were the source of large (VEI 5) and small to moderate (VEI 5) explosive eruptions throughout the Holocene. Four deposits are associated with volcanic activity from Chaitén Volcano, with two from Plinian eruptions at and (cal) ka BP that also generated pyroclastic density currents. The last event recognized from Chaitén (prior 2008) occurred a few hundred years ago, producing deposits that are similar to those of the 2008 eruption. All products from Chaitén are high-silica rhyolites; whole-rock compositions are indistinguishable but glass compositions are subtly different for some of the units. Seven deposits are related to eruptions of Michinmahuida Volcano, including a Plinian fall deposit at ka BP and a large ignimbrite deposit at ka BP. The chemical compositions of these products range from andesite to dacite. The last substantial explosive eruption event from Michinmahuida Volcano appears to have been a ka BP sub-plinian eruption, although younger scoria fall deposits likely derived from local pyroclastic cones are also found. Both volcanoes pose a wide variety of potential hazards to the region ranging from those derived from ignimbrite-forming eruptions to pyroclastic-cone formation. Valleys adjacent to the volcanoes were the areas most heavily affected by volcanic activity, because they were inundated by pyroclastic density currents and lahars. However, even regions located tens of kilometers east and north of the volcanoes experienced accumulations of tephra, which could harm both agriculture and infrastructure if similar events occurred today. Keywords: Tephra fall deposits, Tephrochronology, Explosive volcanism, Southern Volcanic Zone, Volcanic glass chemistry. RESUMEN. Registro de erupciones explosivas holocenas de los volcanes Chaitén y Michinmahuida, Chile. En las cercanías de los volcanes Chaitén y Michinmahuida se han estudiado un conjunto de depósitos piroclásticos de caída y un depósito ignimbrítico. Estos se han caracterizado mediante análisis químico (de roca total y microsonda en vidrio volcánico) y datados por medio de isótopos de radiocarbono. De esta forma, se ha establecido una nueva cronología eruptiva para estos centros. Los datos obtenidos sugieren que ambos volcanes presentan conspicua actividad explosiva durante el Holoceno de variada magnitud. En particular, se han identificado depósitos de al menos cuatro erupciones asociadas al volcán Chaitén, incluyendo dos Plinianos ocurridas entre y ka antes del presente, que generaron, además, corrientes piroclásticas en los valles adyacentes. El evento más reciente identificado para este volcán, corresponde a una erupción de similar magnitud a la del 2008 y habría ocurrido unos tres siglos antes del presente. Para este volcán, todos sus productos piroclásticos corresponden a riolitas, cuyas composiciones de roca total resultan prácticamente indistinguibles, aunque análisis del vidrio volcánico muestra diferencias. Por otra parte, siete erupciones han sido identificadas para el volcán Michinmahuida, incluyendo un depósito ignimbrítico emplazado en el Holoceno temprano entre ka antes del presente y una erupción Pliniana ocurrida entre de ka antes del presente. Las composiciones de los piroclastos de este volcán varían desde andesitas a dacitas, y la últma erupción habría correspondido a una del tipo subpliniana ocurrida hace unos 500 años; no obstante, se han identificado depósitos más recientes probablemente derivados de la actividad de conos piroclásticos. En suma, ambos volcanes representan potencial peligro en la región, cuyas zonas más afectadas corresponderían a los valles adyacentes a los volcanes, ya sea por corrientes piroclásticas o lahares. Sin embargo, regiones distantes decenas de kilómetros, particularmente hacia el norte y este de los volcanes, han experimentado acumulaciones importantes de tefra, cuyos efectos serían devastadores si eventos de similar magnitud ocurrieran en el futuro. Palabras clave: Depósitos piroclásticos de caída, Tefrocronología, Volcanismo explosivo, Zona Volcánica Sur, Química de vidrio volcánico. 228 Holocene record of large explosive eruptions from Chaitén and Michinmahuida Volcanoes, Chile. 1. Introduction Chaitén Volcano (42.83 S, W, 1,120 m a.s.l.) and Michinmahuida Volcano (42.79 S, W, 2,405 m a.s.l.) are located in the southern segment of the Andean Southern Volcanic Zone (SVZ), which extends between Calbuco (41.3 S) and Hudson (45.9 S) Volcanoes (Stern, 2004). Unlike most of the volcanic centers located along the SVZ, these two volcanoes are separated by only 20 km in an east-west alignment, which is an unusual spatial configuration (Fig. 1). Michinmahuida is a large stratovolcano located atop the main trace of the Liquiñe-Ofqui Fault Zone (LOFZ), with an elongate NE-SW summit ridge and an ice-filled caldera in the upper part of the edifice. It is one the largest volcanoes located in the southernmost SVZ (ca. 165 km3). The composition reported for its volcanic products range from basaltic-andesite to dacite (e.g., Naranjo and Stern 2004; Kilian and López-Escobar, 1992; López-Escobar et al., 1993). In contrast, Chaitén Volcano is a small complex of rhyolitic domes of ca. 1 km3 after the 2008 eruption (Pallister et al., 2013, this volume), built inside of a caldera located west of the LOFZ. Detailed surveys of the stratigraphy and evolution of both volcanoes are not available, although reconnaissance investigations show that they differ significantly in terms of chemical signatures (Kilian and López-Escobar, 1992; Watt et al., 2013, this volume). Chaitén Volcano erupted explosively in May 2008, raising the attention of the scientific community on this remote region of northern Patagonia (e.g., Folch et al., 2008; Carn et al., 2009; Watt et al., 2009). FIG. 1. Location map for Chaitén and Michinmahuida Volcanoes. Location of the Mallines section (a) and the upper southern flank of Michinmahuida Volcano (b) are indicated as well as the main trace of the Liquiñe-Ofqui Fault Zone (LOFZ) as a dashed line. Contour lines every 500 m a.s.l. are shown. The inset shows the location of the studied area in the southernmost SVZ context, main volcanoes and the ODP 1233 deep-sea core site. Amigo et al./ Andean Geology 40 (2): , After two years of both explosive and effusive activity, about 1 km 3 of dense rhyolitic magma was erupted (Alfano et al., 2011; Major and Lara, 2013, this volume; Pallister et al., 2013, this volume). That event is the only certain eruption during historical times, even though historical reports point to Chaitén Volcano as the source of an explosive eruption in the region during the 17 th century (Lara et al., 2013, this volume). Previous studies suggest that after a large Plinian eruption in the early Holocene, Chaitén Volcano was dormant until 2008 (Naranjo and Stern, 2004). However, Watt et al. (2013, this volume) demonstrate that Chaitén erupted explosively again after the early Holocene paroxysmal event. In fact, based on a proximal pyroclastic sequence, they provide evidence for three mid- to late-holocene eruptions. We complement the study by Watt et al. (2013, this volume) by focusing our attention on large events from Chaitén and Michinmahuida Volcanoes. Our objective is to establish more complete chronologies of their large eruptive events. To get a complete view of these large events, special attention is paid to both very proximal sites, where complete exposures were found, and to distal outcrops where major eruptions still can be recognized. Here, we present stratigraphic correlations, chemistry (whole-rock and microprobe) and 14 C radiometric dates, which show that both volcanoes have been sites of large eruptions throughout the Holocene. Therefore, these volcanoes pose a significant threat to communities across the southern regions of Chile and Argentina. 2. Methods The area around Chaitén and Michinmahuida Volcanoes is largely uninhabited, densely vegetated and has very few roads. Access was variably by 4-wheel-drive truck, horseback, walking and, in a couple of remote places, by helicopter. Samples were collected from ca. 50 sections around both volcanoes, mainly around the southern, northern and southeastern sides of the volcanoes; the northeastern part of the area remains unvisited Whole-rock chemistry Several authors have used the distinctive compositions of pyroclasts derived from Chaitén and Michinmahuida in order to identify the sources of tephra deposits. These identifications are based mostly on the rhyolitic composition of Chaitén, because it is the only Holocene source of rhyolite in the southern part of the SVZ. We conducted major- and trace-element analyses for 12 whole-rock samples from Michinmahuida and 3 whole-rock samples from Chaitén (Table 1). Pyroclastic samples were first crushed to less than mm and then compositional analyses were done at the Servicio Nacional de Geología y Minería (SERNAGEOMIN) laboratory by X-Ray fluorescence (XRF) for major elements, and by inductively coupled plasma mass spectrometry (ICP-MS) for trace and rare-earth elements. Relative errors in measurements are usually less than 0.5% for major elements and less than 3% for trace elements. Semi-quantitative analysis of mineral content was obtained through X-ray diffraction (XRD) analyses at the SERNAGEOMIN s laboratory on a Panalytical XPert PRO diffractometer, for a q scan at 0.02 every 5 seconds. Bulk samples were previously powdered to mm using an agate mortar Glass chemistry Glass shards from 6 samples from Chaitén, 3 samples from Michinmahuida and 1 ash sample from a deep sea core (ODP 1233 site) were analyzed (Table 2). Pumice lapilli were lightly crushed, wet sieved, and dried in an oven at 60 C. Glass shards were selected and then mounted in epoxy resin blocks and polished for analysis. More than 300 individual glass shards were analyzed using a wavelength-dispersive electron microprobe at the Research Laboratory for Archaeology and the History of Art, University of Oxford. An accelerating voltage of 15 kv, beam current of 6 na, and defocussed (~10 micron diameter) beam were used. Peak counting times were 30 s for Si, Al, Fe, Ca, K, Ti; 40 s for Cl and Mn; 60 s for P, and 10 s for Na. The electron microprobe was calibrated using a suite of mineral standards, and the PAP absorption correction method was used for quantification. Accuracy of the electron microprobe analyses was assessed using a suite of secondary glass standards (MPI-DING glasses; Jochum et al., 2006). During all runs the secondary standards were within 2 standard deviations of the preferred values (see Jochum et al., 2006). Analytical errors associated with the concentrations are: 0.3% relative standard deviation for Si, 1% for Al, 3% Ca, K, and Na, 7% 230 Holocene record of large explosive eruptions from Chaitén and Michinmahuida Volcanoes, Chile. TABLE 1. WHOLE-ROCK MAJOR AND TRACE ELEMENT COMPOSITIONS DETERMINED BY XRF AND ICP-MS. Sample Source Location SiO 2 Al 2 O 3 TiO 2 Fe 2 O 3 (t) CaO MgO MnO Na 2 O K 2 O P 2 O 5 LOI Total V Cr Co Ni Zn Rb Sr Zr Ba Pb Nb Cs Hf Ta T U Y La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Michinmahuida Volcano AA Amarillo ignimbrite at Turbio Chico River valley (bomb) 42º54.77 S/ 72º23.71 W LL Amarillo ignimbrite at Turbio Chico River valley (scoria) Pyroclastic cone at Amarillo River valley (bomb) 42º54.77 S/ 72º23.71 W AA º57.19 S/ 72º28.04 W AA C Pyroclastic cone in Amarillo River valley (scoria) 42º57.64 S/ 72º28.08 W AA Reddish scoria at Turbio Chico River valley (Mic1) 42º53.69 S/ 72º23.30 W AA A Grey pumice at Turbio Chico River valley (youngest Michinmahuida eruption) 42º55.31 S/ 72º23.79 W AA C Mallines section (1 st scoria) 42º53.30 S/ 72º20.36 W AA D Mallines section (2 nd scoria) 42º53.30 S/ 72º20.36 W AA F Mallines section (3 th scoria) 42º53.30 S/ 72º20.36 W AA I Mallines section (4 th scoria) 42º53.30 S/ 72º20.36 W AA O Mallines section (5 th scoria) 42º53.30 S/ 72º20.36 W AA R Mallines section (6 th scoria) 42º53.30 S/ 72º20.36 W Chaitén Volcano Mallines section AA G (1 st rhyolite) 42º53.30 S/ 72º20.36 W 10 10 5 AA L Mallines section (2 nd rhyolite) 42º53.30 S/ 72º20.36 W 10 5 AA P Mallines section (3 th rhyolite) 42º53.30 S/ 72º20.36 W 5 Amigo et al./ Andean Geology 40 (2): , for Fe, 20% for the lower abundance elements (Mg, Ti and Cl) and ~50% for Mn and P which are just above detection limit. Due to variable secondary hydration, all the glass analyses presented were normalized to 100% for comparative purposes Dating Organic material in soils and sediment above and below the tephra layers helps to constrain ages of the explosive events. Samples were carefully obtained in the field, avoiding contamination such as modern roots mainly through clearing and digging back exposures, and then packaged in aluminum foil and dried at low temperature. Accelerator Mass Spectrometry (AMS) analyses were done and most samples were calibrated using the OxCal4.1 software (Bronk Ramsey, 2009) and the Southern Hemisphere calibration curve SHCal04 (McCormac et al., 2004), and were corrected for fractionation (Table 3). Ages are given as calibrated ranges before present (BP, where present is 1950) at the 95.4% confidence level Sieving and grain-size Bulk pyroclastic samples were collected in most places, with reworked deposits avoided. All samples were dried in a box oven at no more than 50 o C for less than 48 hours. Granulometric analyses of the coarse f r a c t i o n w e r e d o n e o n s a m p l e s s i e v e d a t o n e - p h i ( ϕ ) intervals from -4ϕ (16 mm) to +4ϕ (1/16 mm). Each fraction was then weighed to 0.01 g on a balance, and weight percentages calculated. Once raw grain size data were collected, size statistics such as median diameter (Md ϕ ), sorting (σ ϕ ) and skewness (α ϕ ) were computed (Table 4). Sorting values (σ ϕ ) range from , corresponding to moderately to poorly sorted deposits. Componentry analysis was also performed on selected samples in order to investigate the variety of particle compositions. Grain-size distributions of the fine fraction of samples ( 4ϕ) were quantified using laser diffraction spectrometry (LDS), employing a Malvern Mastersizer Hydro 2000G at the Department of Geology of the University of Chile. The refractive indices for size-distribution calculation were 1.48 and 1.50; based on the high SiO 2 content of the ash, the absorption index was set to 0.10, as discussed by Horwell (2007) Volume estimates Volume estimates of tephra deposits were computed using a new empirical method proposed by Bonadonna and Costa (2012), based on the integration of the Weibull function. This method combines the advantages of exponential and power-law fits of the thickness-versus-distance data. The Weibull function is less sensitive than other functions to missing data, because it does not rely on the selection of segments needed for the application of other functions (for a review see Bonadonna and Costa, 2012). This insensitivity to missing data is relevant for deposits identified in this study where only a few isopach contours are available. Despite the exposure and distribution dependence of volume estimations, we expect uncertainties much less than an order of magnitude. Dense rock equivalent (DRE) volumes are not presented here, because systematic distributions of both pyroclasts and deposit density are not available. 3. Results Major explosive eruptions have been reported for both Michinmahuida and Chaitén Volcanoes in previous studies. A thick andesitic scoria fall layer has been recognized several kilometers east of the volcanoes and is associated with Michinmahuida Volcano. This deposit is called Mic1 and dated at 7.3 ka BP (Naranjo and Stern, 2004). Naranjo and Stern (2004) also identified a rhyolitic pumice fall, called Cha1, and associated it with an eruption of Chaitén Volcano. Watt et al. (2011a) recognized the same deposit 160 km north in the Hualaihué region. They estimated its bulk tephra volume at 3.5 km 3, and dated it at 9.75 ka BP. Naranjo and Stern (2004) also identified a second rhyolitic pumice layer, the Mic2 unit, east of the volcanoes. Although it is rhyolite, it has a dispersal pattern similar to that of Mic1, and they assigned it to an eruption of Michinmahuida. In contrast, Watt et al. (2009; 2013, this volume) correlated the unit to a proximal pyroclastic sequence north of Chaitén, and argued that it represents an eruption of Chaitén, not Michinmahuida. They named this event Cha2 and estimated its bulk tephra volume at 4.7 km 3. That event has a radiocarbon date of about 4.95 ka BP. Watt et al. (2013, this volume) also estimated a bulk tephra volume of 1.5 km 3 for the Mic1 deposit. On 232 Holocene record of large explosive eruptions from Chaitén and Michinmahuida Volcanoes, Chile. TABLE 2. AVERAGE (AND STANDARD DEVIATION) GLASS MAJOR ELEMENT COMPOSITIONS AS DETERMINED BY ELECTRON MICROPROBE. NORMALISED TO 100% ANHYDROUS BASE. Chaitén Volcano Sample SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2 O AA M (n=25) Rhyolite between Cha1 and Cha2 (Mallines section) AA B (n=27) pre-2008 lahar, Chaitén River valley AA P (n=33) Cha1 (Mallines section) AA (n=31) Pumice flow, Chaitén River valley AA G (n=31) Cha2 (Mallines section) AA B (n=32) Few centuries old ash (El Amarillo) Michinmahuida Volcano LL (n=4) Amarillo ignimbrite (Turbio Chico River valley) AA A (n=25) Grey pumice (Turbio Chico River valley) AA A (n=13) Glassy lithics in grey pumice (Turbio Chico River valley) ODP site 1233 silicic clasts (n=7) mafic clasts (n=3) Amigo et al./ Andean Geology 40 (2): , TABLE 3. RADIOCARBON AGES REPORTED IN THIS STUDY. Sample name Material Location Coordinates Event (volcano) Conventional age±error BP Calibrated age range BP d 13 C Observation AA B Wood El Amarillo 43º2.50 S/72º28.74 W pre-2008 ash (Chaitén) 130± present Wood 1 cm above ash fall deposit AA A Charred wood Mallines section 42º53.30 S/72º20.36 W AA B Wood Turbio Chico River 42º55.31 S/72º23.79 W scoria fall with several pulses (Michinmahuida or scoria cone) dacitic grey pumice (Michinmahuida) 160± present Paleosol 1-2 cm above fall deposit 390± Wood 1-2 cm underneath fall deposit AA D Wood (radiometric) Chaitén River 42º54.29 S/72º41.62 W Non volcanic debris flow 870± Wood within deposit AA B Wood (radiometric) Mallines River 42º S/72º W Non volcanic debris flow 1,130±30 1, Wood within upper part of deposit AA E Charred wood Mallines section 42º53.30 S/72º20.36 W AA A AA B Organic sediment Organic sediment Turbio Chico River Turbio Chico River scoria fall younger than Mic1 (Michinmahuida) 1,310±30 1,280-1, Paleosol 1-2 cm underneath fall deposit 42º53.30 S/72º20.36 W Cha2? (Chaitén) 4,130±40 4,812-4, Paleosol 1-2 cm underneath fall deposit 42º53.69 S/72º23.30 W Cha2? (Chaitén) 4,190±40 4,824-4, Paleosol 1-2 cm underneath fall deposit AA B Wood (radiometric) Gigios River 42º48.66 S/72º39.72 W Cha2 (Chaitén) 4,470±30 5,273-4, Wood near the base of the deposit AA A Wood (radiometric) Gigios River 42º48.66 S/72º39.72 W Cha2 (Chaitén) 4,570±30 5,306-5,
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