Eruptive activity of Planchón-Peteroa volcano for period 2010-2011, Southern Andean Volcanic Zone, Chile

*Felipe Aguilera1, 2, Óscar Benavente3, Francisco Gutiérrez3, Jorge Romero4, Ornella Saltori5, Rodrigo González6, Mariano Agusto7, Alberto Caselli8, Marcela Pizarro5

1 Servicio Nacional de Geología y Minería, Avda. Santa María 0104, Santiago, Chile.

2 Present address: Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avda. Angamos 0610, Antofagasta, Chile.
feaguilera@ucn.cl

3 Departamento de Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile.
obenaven@ing.uchile.cl; francisco.gutierrez.ferrer@gmail.com

4 Centro de Investigación y Difusión de Volcanes de Chile, Proyecto Archivo Nacional de Volcanes, Santiago, Chile.
jorge.romerom@alumnos.uda.cl

5 Programa de Doctorado en Ciencias mención Geología, Universidad de Chile, Plaza Ercilla 803, Santiago, Chile.
o.saltori@gmail.com; marcela.pizarro@ing.uchile.cl

6 Departamento de Ciencias Geológicas, Universidad Católica del Norte, Avda. Angamos 0610, Antofagasta, Chile.
r_gonzalez@ucn.cl

7 Departamento de Ciencias Geológicas, Universidad de Buenos Aires, Ciudad Universitaria, Pabellón 2, 1428EHA, Buenos Aires, Argentina.
magusto@gl.fcen.uba.ar

8 Laboratorio de Estudio y Seguimiento de Volcanes Activos (LESVA), Universidad Nacional de Río Negro, Roca 1242, (8332) Roca, Argentina.
atcaselli@unrn.edu.ar

* Corresponding author: feaguilera@ucn.cl

Fig. 1. Schematic map of the Southern Volcanic Zone (SVZ) and location of Planchón-Peteroa volcano. Limits of North Southern Volcanic Zone (NSVZ) and Transitional Southern Volcanic Zone (TSVZ) are shown. Localization of epicentre of 27th February 2010 earthquake is also shown. Additionally, the distribution of others Andean volcanic zones (Northern Volcanic Zone-NVZ, Central Volcanic Zone-CVZ, Austral Volcanic Zone-AVZ) and localization of epicentre of 27th February 2010 earthquake are also shown.

 

 

Fig. 2. Schematic map of the Planchón-Peteroa volcano, main fumarolic fields, crater lakes and location of the sampling sites are shown.

 

Several glaciers are hosted in the summit caldera, but also are filling the highest parts of valleys, especially in the eastern, southern and western flanks of the complex (Naranjo et al., 1999). Glaciers are highly relevant for volcanic hazards, considering that they are the primary source for lahars. The historical records show that lahars have been frequently associated with Planchón-Peteroa eruptions, having occurred in at least 4 eruptions between 1660 and 1998 (González-Ferrán, 1995; Naranjo et al., 1999; Haller and Risso, 2011). The greatest lahar in historical times occurred in March 1762, when the waters of Teno River, which run near to Teno, Romeral and Curicó towns, were stagnant during ten days as a consequence of the lahar (González-Ferrán, 1995).

This work describes the main characteristics of the recent eruptive activity of Planchón-Peteroa volcano (between January 2010 and July 2011) based on: i) field measurements (gas and water geochemistry from fumarolic emissions and crater lake waters), and ii) analysis of the fall deposit, integrated with: i) satellite data (GOES satellite imagery), ii) surveillance camera data from ICES (International Center of Earth Sciences-Argentina) and iii) local witnesses information. The aim of this work is to determine the type and magnitude of the eruption and to establish the origin and deep process related to this unrest period.

2. Geological and volcanological setting

The Planchón-Peteroa volcano overlies a basement constituted by Cretaceous marine and evaporitic sediments, which is composed mostly of limestone, conglomerates, sandstones and siltstones, basaltic to rhyolitic lava flows and pyroclastic rocks (Tertiary) and Middle Tertiary granodiorite plutons (Haller et al., 1985; Tormey, 1989; Tormey et al., 1995; Naranjo et al., 1999).

Tormey (1989), Tormey et al. (1995), Naranjo et al. (1999), Naranjo and Haller (2002) and Tormey (2010) have described widely the evolution of Planchón-Peteroa volcano, which is summarized as follow:

1. Azufre volcano is the earliest (<550 ka) and largest (~69 km³) edifice which is constituted mainly by basaltic andesite lavas and pyroclastic flows, restricted dacitic lava flows, rare andesitic lava and pyroclastic flows, and lahar deposits.
2. Planchón volcano (6 km to the north of Azufre) was built in different stages: i) Planchón I (~14 to ~11 ka), started its activity during the last stages of Azufre activity and is comprised by ~43 km³ of basaltic and basaltic andesite lava flows and pyroclastic deposits; ii) ~11 ka western flank of Azufre-Planchón complex collapsed forming a debris avalanche deposit which travelled ~75 km to the west along Claro and Teno rivers (MacPhail, 1973; Davidson, 1974); iii) Planchón II (~11 to ~7 ka), built within the summit amphitheatre, is a ~5 km³ edifice constituted by basaltic to basaltic andesite lava flows and pyroclastic deposits, and lahar deposits.
3. Peteroa, the smallest (<1 km³) and youngest edifice (<7 ka), is a volcano formed by several scattered vents between Azufre and Planchón volcanoes. Peteroa volcanic products are characterized by andesitic lava flows and dacitic to rhyodacitic pyroclastic deposits.

Several thermal manifestations occur around Planchón-Peteroa volcano and are located 15 km northeast (Vergara valley), 6 km northeast (Valenzuela valley), 5.5 km east (Gendarmería springs), 8 km southeast (Azufre valley) and 20 km southeast (Colorado valley) from Peteroa caldera (Fig. 2). These fluid discharges correspond to springs, bubbling pools, mud pools, fumaroles and steaming ground (Benavente, 2010; Aguilera et al., 2011; Gutiérrez et al., 2012; Saltori, 2014). Additionally, fumaroles and thermal springs are present in a zone located 5 km southeast from Azufre volcano (Fig. 2) (Benavente, 2010; Saltori, 2014).

3. Historical eruptive chronology

The historical volcanic activity has been related exclusively to Peteroa volcano, and it is characterized by a permanent emission of gas from different fumarolic fields. At least 19 eruptions have been observed since 1660, corresponding mainly to phreatic and phreatomagmatic eruptions with VEI ≤2 (Haller et al., 1994; Haller and Risso, 2011). A complete description of the historical eruptive events was made by González-Ferrán (1995) and Naranjo et al. (1999). The first eruption described corresponds to an explosive event during 1660, which occurrence still is debatable, but a recognized explosive eruption was recorded during November-December 1751. The greatest historical eruption occurred 12^th March 1762 (VEI=4), when the explosive and effusive activity was followed by the partial collapse of the volcanic edifice (González-Ferrán, 1995; Haller y Risso, 2011). This collapse produced several debris avalanches and lahars, which affected the valley of Río Teno by stagnating of the river during ten days. Minor explosive eruptions were recorded in 1835, 1837, 1842, 1860, 1869, 1872, 1878 and 1889-1894. During 1937, an explosive and effusive event formed the scoria cone, from where was emitted a single lava flow. This event was accompanied by generation of lahars. Explosive events also occurred in 1938, 1959, 1960, 1962 and 1967. In February 1991, a phreatomagmatic eruption produced an eruptive column of 1 km above the crater, which was dispersed to the east and southeast by 80 km (Naranjo and Haller, 1997, 2002). During this eruption, two new craters were formed (crater 3 and 4; Fig. 2), lahars were produced in the western flank of volcano, whereas Teno and Claro rivers were contaminated with acid waters from the active craters (BGVN, 1991; Haller et al., 1991). The last explosive eruption was recorded during November 1998 (BGVN, 1999).

4. Methodology

4.1. Field measurements

Six field campaigns were carried out during the eruptive period (February, September and October 2010, March and May 2011, and March 2012), in order to: i) obtain tephra fall deposits samples, ii) measure tephra fall deposit thickness, iii) observe the eruptive plume behaviour, and iv) sample gas and water from the fumarolic emissions and from the lakes hosted in the craters, respectively.

Fall deposits samples were obtained from the Peteroa caldera crater, and from the eastern flank of Planchón-Peteroa volcano during 2010 and 2011 campaigns. After digging a square 1 m by side, the fall deposit samples were stored in plastic bags and sealed to avoid the humidity generation. ~500 grs. of tephra were sieved for a size range between -1 and >3.75Φ. The sieved tephra was analysed with a microscope to characterize and classify the componentry, and to determine the statistical parameters associated with each tephra sample. Separation of the different fragment types was carried out using the procedure suggested by Murcia et al. (2008). From each sifter were extracted 5 grs of tephra and were deposited in glass beakers. The samples were washed with Milli-Q water to clean the particles and then covered with HCl 10% vol. A magnetic bar was introduced in each beaker and shacked during 5 min. Afterwards, each sifter was washed three times with acetone with the objective to neutralize and eliminate the HCl, and washed again with Milli-Q water. Finally, the samples were dry in an oven at 80-100 ºC.

Gas sampling was performed at 8 sites within Crater 2 and fumarolic field located in the platform between craters 1 and 2 (Fig. 2). Sampling procedure was carried out in February 2010, March 2011 and March 2012. Fumarolic gases were sampled using a 1 m long titanium tube (diameter: Ø=2.5 cm) connected via a quartz-glass Dewar tubes to a pre-evacuated 60 mL glass thorion-tapped flasks. The flasks were filled with 20 mL of 4N NaOH and 0.15 M Cd(OH)₂, following the methods previously reported by Giggenbach and Goguel (1989), and Montegrossi et al. (2001). Fumarolic condensates were collected using an ice-cooled glass condensing system connected to a gas sampling glass line. Temperatures from fumarolic emissions were measured in the field after gas sampling. Crater lake water samples were collected from craters 1, 2 and 4 in March 2011 and March 2012 (Fig. 2). Sterile polypropylene bottles of 100 ml were used for the cations and anions, whereas 50 ml glass bottles were utilized for the isotopic water composition samples. Cations samples were filtered with a 0.45 µm pore size membrane. No acid was added to the cations samples given the low pH of the water. Temperature, pH and conductivity were measured in the field. Gas and water samples were analysed to obtain the chemical and isotopic composition following the procedures suggested by Vaselli et al. (2006).

4.2. Satellite images analysis

GOES satellite images were obtained from Hawai’i Institute of Geophysics and Planetology site site (http://goes.higp.hawaii.edu last visit 31-12-2013), with, with the objective to determine the direction and distance of eruptive plume. A total of 107 useful images were obtained for the period between 4^th September 2010 and 31^st May 2011, being only acquired images from 12:00 to 22:00 UTC, considering that using daylight images the eruptive plume can be better detected. Three different types of images were used (Harris et al., 2000): i) Visible, corresponds to band 1 image (visible portion of spectrum, 1 km pixel resolution), which is based in the measure of albedo (reflectance); ii) RGB, a false colour image where red band correspond to band 2 (mid-infrared portion of spectrum, 4 km pixel resolution) normalized by the mean band 2 value for the whole image, green band is the band 1, and blue band is the band 4 inverted (thermal-infrared portion of spectrum, 4 km pixel resolution); iii) HOT, a false colour image similar to the RGB, differenced by green band corresponding to the algorithm defined by (band 2-band 4)/(band 2+band 4). The plume was detected in 46 visible images for 18 days, 53 RGB images for 18 days, and 8 HOT images for 4 days. Images were georeferenced using ERDAS Imagine^© 9.1 software, with the objective to obtain an accurate measurement of the maximum distance travelled by the eruptive plumes.

4.3. Surveillance camera images

16 webcam images from ICES (International Center of Earth Sciences-Argentina), located 7 km E from Planchón-Peteroa volcano, were obtained for April, May and July 2011, which were analysed to determine eruptive plume direction and altitude.

5. Eruptive chronology

5.1. Activity previous to 2010-2011 eruptive period

Previous to the beginning of this new eruptive period, the activity at Planchón-Peteroa was restricted to fumarolic emission in the sites previously described, and no gas column was observed (Fig. 3). During a visit in February 2009, crater lakes were present in all craters.

 

Fig. 3. Panoramic view from the east side of Peteroa caldera (February 2009).

 

In the following sections are described the 4 phases defined for 2010-2011 eruptive period. The description of the phases included information of eruptive column height (meters above active crater), direction (azimuth) and distance (in km from active crater) reached up by the eruptive plume, places where fall of tephra was observed and dates when the fall of tephra occur. This information is summarized in the Table 1. Additionally, diverse field-based and remote-based observations are included as a complementary data.

5.2. Phase 1

The first eruptive phase started on 4^th January 2010, with the emission of a white column (no ash) 200 m over the active crater (Fig. 4a) (BGVN, 2010) which was permanent during at least 7 months (at 7^th August 2010). During a visit between 25 and 26 February 2010, small phreatic explosions were observed in the SW border of crater 3 (Figs. 2, 4b and c), which were accompanied by an intense activity in the lake (Fig. 4b and c). These phreatic explosions were partially rhythmical, with a frequency of 1 to 2 explosions every minute. The explosions produced the vaporization of the lake and probably are responsible for gas/vapour columns emission since the 4^th January 2010. During this visit, the lakes hosted in craters 1, 2 and 4 did not experience any anomalous activity.

 

Fig. 4. a. Panoramic view from the east side of Peteroa caldera (February 2010); b. Active acid lake hosted in the crater 3, view from the W (February 2010); c. Phreatic explosion in the SW side of crater 3, view from the W (February 2010); d. Eruptive column during 2nd October 2010, view from Romeral, 58 km NW Planchón-Peteroa volcano (image from surveillance camera, OVDAS-SERNAGEOMIN); e. Active crater 3 without acid lake, November 2010, view from the W (courtesy of D. Tormey); f. Eruptive column during February 2011, view from the El Azufre border office, 7 km E from active crater (courtesy of Gendarmería Argentina); g. Panoramic view from the east side of Peteroa caldera (March 2011); h. Eruptive column during February 2011, view from the El Azufre border office, 7 km E from active crater (image from surveillance camera, ICES-Argentina); i. Panoramic view from the east side of Peteroa caldera (March 2012).

 

During 27^th February 2010 occurred the M_W 8.8 Maule earthquake, with an epicentre located ~195 km SW from Planchón-Peteroa volcano. Despite this, no major changes in the volcano activity were observed. During 8^th August 2010, a weak light grey with scarce ash eruptive column was detected. The eruptive column reached up to 200 m above the active crater (crater 3) and was dispersed to the south. This phase was characterized by its very low activity, being classified with a VEI=0, and a maximum VEI=1 during the more intense period of activity (Table 1; Fig. 5).

 

Fig. 5. Date versus Volcanic Explosive Index (VEI) diagram showing the 4 phases that constitute the eruptive period 2010-2011 of Planchón-Peteroa volcano. Black circles (VEI=0), yellow circles (VEI=1), orange circles (VEI=2). VEI determinations (Table 1) were based exclusively on column height, following Newhall and Self (1982). Crater lakes observations (cyan star), most relevant seismic events (yellow star) and gas/water sampling campaigns (green star) are also shown.

 

Between the 9^th August and 3^rd September 2010 is considered a resting phase, where Planchón-Peteroa volcano activity was characterized by the presence of a very weak, but constant gas/steam white column (no ash), reaching a height of up to 100 m above the active crater.

5.3. Phase 2

On September 4 (2010) started the second eruptive phase with the emission of a grey ~1,200 m eruptive column, which was dispersed by 30 km to NNE, producing fall of tephra in Río Teno Mining Company and San Pedro Custom, located 20-22 km NE from the active crater. Fall of tephra were also detected in different sites located 7.8 and 15 km NE, and 10 km N from the active crater (Table 1). Between 6^th and 10^th September, grey columns, occasionally weak, and no higher than 200 m, were dispersed to ESE at a maximum distance of 60 km from the active crater (7^th September). Fall of tephra were also seen during 6, 7 and 10^th September. During the 11^th September-11^th October 2010 period, the eruptive activity was continuous, with the emission of light to dark grey columns, whose heights varied from 200 to 3,000 m. Plumes were dispersed mainly to SE (Fig. 6a), but also to NE and NW (Fig. 6b), at the distance between 17.4 and 344.8 km from the active crater (Table 1). The values of VEI during these dates varied from 1 to 2, with a peak of activity during 1^st and 2^nd October (Figs. 4d and 5; Table 1). Fall of tephra occurred along SE, NE and NW flanks of Planchón-Peteroa volcano during seven days (20, 24, 25, 28 and 30th September, 2^th and 9^th October). The area covered by the accumulated tephra deposits at 2^nd and 10^th October was 852 and 1,265 km², respectively (Table 1). During 12^th October-1^st November 2010 period the eruptive activity diminished and was characterized by the presence of a white (no ash) to light grey (very scarce ash) column, with a limited variation of height between 300 and 400 m, except during 1^st November, when a white gas/steam plume ascent up to 1,000 m above active crater. During this phase, the VEI varied between 0 and 1, whereas eruptions with VEI=2 where recorded in 4^th and 9^th September, and 1^st, 2^nd, 9^th, 10^th and 11^th October 2010 (Table 1; Fig. 5).

 

Fig. 6. GOES satellite RGB images of: a. 26 September 2010,; b. 2 October 2010; c. 4 May 2011 and d. 6 May 2011, showing the displacement of eruptive plumes. Position of Planchón-Peteroa volcano (red triangle) is also shown.

 

A new resting phase occurred between 2^nd November 2010 and 16^th February 2011, and was characterized by the presence of a permanent white gas/steam column of a height of 200-300 m (2^nd November 2010-3^rd January 2011) and 300-650 m (6^th January-16^th February 2010). During the 4 and 5^th January 2011, the columns reached up to 1,500 m height. No crater lake was observed in the crater 3 during 11^th November 2010 (D. Tormey, personal communication, 2011; Figs. 4e and 5), likely caused by a strong vaporization during eruptive phase 2.

5.4. Phase 3

The 3^rd eruptive phase covers the period from the 17^th February to 20^th May 2011, and was characterized by the presence of a permanent white to grey (scarce ash) column, with heights varying between 200 and 800 m above the active crater. The maximum distance of eruptive plumes were observed on 26^th March, reaching up to 37 km SE from the active crater, and during 3^rd May, when the plumes were displaced mainly to ESE, reaching a maximum travelling distance of ~56.8 km (Table 1). Peaks of activity were recorded during a relative long time lapse (17^th February, 18^th March and 4^th April 2011), when grey columns, reaching heights up to 1,000-1,100 m, being transported to the SE and N. Fall of tephra occurred along the SE and N flanks of Planchón-Peteroa volcano during the 17^th February (Fig. 4f) and 4^th April, respectively. A new increment of the volcano activity (Table 1; Fig. 5) was reported in a short time lapse (17^th, 18^th and 29^th April, 1^st, 2^nd, 4^th and 6^th May 2011). This activity was characterized by the emission of white to grey columns, with heights varying from 500 to 1,200 m, and plumes transported to NW, N, NE, NE, E and SE (Fig. 6c and d), reaching up to 637.6 km distance from the active crater (6^th May; Table 1). During 2^nd, 4^th and 6^th May fall of tephra were detected on the NW, N, NE, NE and ESE flanks of Planchón-Peteroa volcano and, Río Teno Mining Company and San Pedro Custom. Claro River, located in the W side of Planchón-Peteroa volcano, became turbid during 4^th and 5^th May. The area covered by the accumulated tephra deposit at 7^th October was 693 km². Additionally, fall of tephra occurred during the 3^rd, 5^th and 9^th May 2011, in the NW, ENE and SE flanks and in Carrizales (~8 km NW) and Las Leñas (~46 km ENE). VEI during phase 3 varied between 0 and 1, reaching up to 2 during 17^th February, 18^th March, 4^th, 17^th, 18^th and 29^th April, and 1^st, 2^nd and 4^th May 2011 (Table 1; Fig. 5).

Field observations during the 8^th March indicate the presence of a low altitude white column (~100 m; Fig. 4g), as well as the presence of a lake in the crater 3. However, the volume o the crater lake was lower than the one observed on February 2010 (early phase 1). A crater lake was not present in the crater 2 during the 8^th March (only presence of acid bubbling pools in the SW side), whereas crater 1 and 4 presented no changes respect to the observation carried out during the phase 1 (February 2010).

A resting phase occurred between 21^st May and 16^th June 2011, characterized by the presence of a constant very weak gas/steam white column, with a height that varied from 200 to 400 m.

5.5. Phase 4

The last eruptive phase (17^th June-13^th July 2011) was characterized by permanent white to grey columns reaching up 200-500 m height (VEI=1). The two last explosions occurred during the 27^th June (grey column 2,000 m height displaced to NE by 41 km) and 11^th July (grey column 1,000 m height with fall of tephra in the SE flank of the volcano; Fig. 4h), with VEI of 2 (Table 1; Fig. 5).

5.6. Activity after 2010-2011 eruptive period

A permanent low height column (50-100 m) was present in the active crater after the 2010-2011 eruptive period. Low frequency seismic activity was present during August and September 2011 (186 and 274 earthquakes/month, respectively, SERNAGEOMIN, 2011a1 and b2), whereas during October 2011 the seismic frequency increases up to 675 earthquakes/month (SERNAGEOMIN, 2011c³). From November 2011 and February 2012, seismic activity was variable in a restricted range between 317 and 485 earthquakes/month (SERNAGEOMIN, 2011d⁴; 2012a⁵, b⁶ and c⁷). Despite the increasing of seismic activity, eruptive activity was not detected. The last field observations in Planchón-Peteroa craters during March 2012 indicate that only a weak white column (50 m height) was present (Fig. 4i), and all craters hosted crater lakes, which is similar to the observations carried out in February 2010 (early phase 1).

6. Tephra fall deposit analysis

Tephra fall deposits were observed in the whole surface of Peteroa caldera, and mainly on the NW, N, NE, E and SE flanks of Planchón-Peteroa volcano (Fig. 7a). The maximum thickness of tephra fall deposit was observed in the SSE border of crater 3 (4 m thickness; Fig. 7b), decreasing progressively to 1 cm thickness ~20 km SE and ~7 km NW from the active crater, and 0.5 cm thickness ~30 km SE and ~8 km NW from the active crater. Maximum area covered by tephra fall deposit extended up to 1,265 km²  during 10^th October 2010 (Table 1).

 

Fig. 7. Tephra fall deposit on the a) NE flank of Planchón-Peteroa volcano (note a person in the middle-left side of the photo as scale) and b) SE border of crater 3.

 

Several sites located on the NE, E and SE flanks of volcano were sampled during phases 2 and 3. Tephra fall deposits of phases 2 and 3 are almost identical, and are characterized by a size range between -1 Φ (2 mm) and >3.75 Φ (0.075 mm) (Table 2), corresponding mostly to coarse ash. Tephra fall deposits are constituted by lithics fragments (40-80% vol.), quartz (5-50% vol.) and plagioclase (10-25% vol.) crystals. Lithics fragments (>65% vol.) dominate in sizes <1.25 Φ (0.425 mm), whereas for sizes between 1.25 and 3.75 Φ (0.425 and 0.075 mm), lithics fragments and quartz crystals are present in similar quantities (~45% vol.). The smallest fraction, corresponding to >3.75 Φ (0.075 mm), is dominated by quartz crystals (80% vol.) and scarce lithics fragments and plagioclase crystals (10 and 5% vol., respectively). Lithics fragments are constituted mainly by three different types of dark grey to black fragments, corresponding to porphyry lava, glassy porphyritic lava and vesicular lava fragments. Additionally, some lithic fragments are partially oxidized, appearing as reddish brown fragments. Oxide Fe minerals, and less frequently Cu minerals, were occasionally observed as single fragments. Cu minerals have been commonly observed in crater lake environments (e.g., Copahue volcano-Chile, Keli Mutu volcano-Indonesia; Pasternack and Varekamp, 1994; Varekamp and Ouimette, 2001). Granulometric (Table 2) and statistical analysis indicate that tephra fall deposits are multimodal (3 modes; 1, 2 and 3.75 Φ, respectively), suggesting that Planchón-Peteroa eruptions corresponding to highly explosive events, probably related to phreatic processes (Wohletz, 1983). Walker (1971) used statistical parameters such as the graphical median (Md) and graphical standard deviation (σ_G) to determine the type of deposit. In this case, Md 0.11 Φ and σ_G 0.35 Φ suggest that Planchón-Peteroa tephra deposit is related to pyroclastic fall process (Fig. 8), similar to what is suggested by the field measurements and direct observations.

 

Fig. 8. Empiric diagram for granulometric classification of pyroclastic deposits (after Walker, 1971). Black dot indicate the graphical median (Md) and graphical standard deviation (σG) values for Planchón-Peteroa pyroclastic fall deposit.

 

7. Fluid geochemistry analysis

7.1. Water geochemistry from crater lakes

Water geochemistry analysis from the crater lakes 1, 2 and 4 indicate that all the crater lakes were developed under similar physical and chemical conditions (Benavente et al., 2013; Benavente, 2015; Tassi et al., in press; Table 3). pH varies between 1.49 and 2.91, while temperature (T) is more variable, being colder in crater 1 (7.4 °C), followed by crater 4 (17.1 °C), and the hottest is the lake hosted in the crater 2, which presented a T of 43.2 °C on March 2011 and 30.6 °C on March 2012. The total of dissolved solids (TDS) varies in a limited range from 3,247 to 4,159 mg/l (Table 3). Sulphate is the principal anion in all crater lake waters (Fig. 9a), and its origin seems to be related to the condensation of steam emitted from the fumarolic vents located in the border and bottom of crater lakes and oxidation of SO₂ and H₂S in the near-surface water. Chlorine is the second anion in order of abundance, especially in the crater 1 where its contents reach up to 662 mg/l (Table 3), which seems to be related to the absorption of HCl in the near-surface water. Bicarbonate is absent in all the crater lakes, which is consistent with the low pH values of the lakes (<3). The cations with the highest concentrations correspond to Ca²⁺ (182 to 523 mg/l) and Mg²⁺ (95 to 377 mg/l) (Table 3). As is shown in figure 9b, crater lake samples plot close to the Global Meteoric Water Line (GMWL), indicating that rain and melting of glaciers hosted in Peteroa caldera are the principal water sources for the crater lakes. The subtle δ¹⁸O-shifting to more positive values respect to GMWL depicted by the crater lake samples in figure 9b may be explained as the result of a limited mixing with condensated water vapour from the fumarolic vents surrounding the crater. Even though waters from the lake hosted in crater 2 where sampled in two different periods, no major changes in the physicochemical parameters were observed.

 

Fig. 9. a. HCO3--Cl--SO42- diagram for water samples from crater lakes; b. δ18O-dD diagram for water samples from crater lakes (filled squares) and gas condensates from fumarolic emisions (filled circles). The “andesitic water” field (Taran et al., 1989; Giggenbach, 1992), Global Meteoric Water Line (GMWL), Standard Mean Ocean Water (SMOW) field and local precipitation isotope composition are also shown. Orange symbols for 2011 and red symbols for 2012 samples.

 

7.2. Gas geochemistry from fumaroles

The gas composition from fumaroles sampled during February 2010 (early phase 1) indicates that fumarolic fluids are enriched in H₂S and CH₄ (Table 4; Fig. 10a, b and c), which are typical gas species related to hydrothermal environments (e.g., Giggenbach, 1987, 1996). According to figure 10a, b and c, a limited contribution from atmospheric (i.e., Ar addition) and magmatic sources (SO₂, HCl and HF; Giggenbach, 1996) are also observed in these gas samples. Fumaroles sampled on March 2011 (eruptive phase 3) evidence a limited increasing in the contents of acid species (SO₂, HCl and HF) and a decrease of atmospheric species like Ar and O₂ (Table 4). This seems to be related to an increase (but limited) of the fluids released from a magmatic source as these samples were taken during the 3^rd eruptive phase (Fig. 10a, b and c). This magmatic input can also be observed in figure 9b where the δD- and δ¹⁸O-SMOW values of the fumarolic samples suggest a mixing between a meteoric (mostly corresponding to rain water at 4,000 m a.s.l.) and an “andesitic” source (e.g., Tarán et al., 1989; Giggenbach, 1992). Even though fumarolic fluids at March 2011 present different evidences indicating a magmatic input, the chemical composition of the fluids are still dominated by components originated in a typical hydrothermal environment (e.g., H₂S, CH₄; Table 4).

 

Fig. 10. a. (SO2)-100*(Ar)-(H2S)/10; b. (CH4)-SO2-10*Ar; c. (N2)-50*(CH4)-50Ar ternary diagrams for fumaroles of Planchón-Peteroa volcano. Fumaroles collected in 2010 (yellow filled circles), 2011 (orange filled circles) and 2012 (red filled circles).

 

The most dramatic physico-chemical change occurred after the 2010-2011 eruptive period (March 2012), where a strong increase of the magmatic acid species content occurred. SO₂ abundance increased by three folds with respect to the samples taken on March 2011, whereas HCl and HF contents increased by two order of magnitude (Table 4). Despite of divergent changes in the concentration of the hydrothermal species with respect to the samples taken in 2010 and 2011 (i.e., decreases of CH₄ and increases of H₂S), the relative proportions of SO₂, as well as the contents of the other acid species on March 2012 suggest that fumaroles were strongly dominated by magmatic source fluids on March 2012 (Fig. 10 a and b). Contrary to what intuition would indicate, samples collected during the “magmatic” stage (March 2012), as well as the water samples of the crater lakes and the bubbling pools plot along the GMWL in figure 9b. We argue that the δD and δ¹⁸O temporal variations at the crater fumaroles between March 2011 and 2012 (Fig. 9b) are related to an episode of minimum and maximum availability of rain water and/or melted glacial water in the shallow aquifer hosted at Planchón-Peteroa volcano. Field observations of the crater from where the gas condensates were taken in March 2011, which is also near to the sampling points in March 2012 (crater 2), indicate the presence of an acid lake only in March 2012, supporting this hypothesis.

7.3 Geothermometry from gaseous phase

Even though no major changes in outlet tempera-tures at the fumarolic vents were observed during and after  the 2010-2011 eruptive period (Table 4), the chemical changes detected in the previous revision (section 7.2) can be an indicator of variations in the physicochemical conditions of fluids at depth (e.g., temperature, pressure, redox), as has been observed in other active volcanoes (e.g., Fischer et al., 1997; Vaselli et al., 2010; Tassi et al., 2013). Accordingly, a thermodynamic review is done with the objective to detect possible variations in the physicochemical conditions of fluids at depth.

According to Giggenbach (1993), theoretical temperatures of magmatic-hydrothermal fluids in the CH₄-CO₂-H₂ system can be calculated by the following reaction:

CH₄+2H₂O ↔ 4H₂+CO₂     (1)

The temperature dependence of the equilibrium constant of reaction (1) in the equilibrated vapour and liquid is given by:

log(X_CH4/X_CO2)_v = 4R_H+5181/T(K)     (2)

log(X_CH4/X_CO2)_l=4R_H+5181/T+log(B_CO2)-log(B_CH4)     (3)

where R_H = log(X_H2/X_H2O), log(X_H2O)≈log(f_H2O) =4.9-(1820/T), T is in K, and B_CO2 and B_CH4 are the vapour/liquid distribution coefficients of CO₂ and CH₄, respectively (Giggenbach, 1987, 1991). The B_CO2 and B_CH4 values at different temperatures are calculated on the polynomial equations reported by Sepúlveda et al. (2007). Assuming that Ar in magmatic-hydrothermal fluids have an atmospheric origin and is controlled by the equilibrium between the atmosphere and air-saturated water (log(r_Ar) = -6.52, where r_Ar is the Ar mol-ratio), Giggenbach (1993) suggested that at equilibrium, H₂ concentrations in the equilibrated vapour and liquid were related to R_H by the following equations:

log(X_H2/X_Ar)_v=R_H-log(r_Ar)     (4)

log(X_H2/X_Ar)_l=R_H-log(r_Ar)-log(B_H2)     (5)

where the vapour/liquid distribution coefficients B_H2 was calculated from the same data sources used for those of CO₂ and CH₄. In figure 11a the log(X_H2/X_Ar*) versus log(X_CH4/X_CO2) grid is constructed on the basis of equations 2, 3, 4 and 5 at different RH and temperatures. The Ar* values, used in figure 11a instead of the Ar molar concentrations, were calculated as follows:

Ar*=Ar-(O₂/22)     (6)

 

Fig. 11. a. log(H2/Ar*) versus log(CH4/CO2) diagram for the thermal discharges sampled at Planchón-Peteroa volcano. Ar*=Ar-O2/22; RH=log(H2/H2O) (Giggenbach, 1987). b. 3log(CO/CO2)+log(CO/CH4) versus log(H2O/H2)+log(CO/CO2) binary diagram for the Planchón-Peteroa volcano fumaroles after Chiodini and Marini (1998). Theoretical compositions were calculated considering gas solubility and thermodynamic data for pure water. Symbols as figure 10.

 

The O₂/22 values correspond to the amounts of Ar added by atmospheric contamination, because O₂ is completely absent in pristine hydrothermal fluids and only occurs as the result of shallow level atmospheric contamination (Giggenbach, 1993). The gases collected at the crater during the complete eruptive period plot along the liquid equilibrium curve at R_H values near -3.1, with temperatures increasing from ~290 ºC for 2010 samples (eruptive phase 1) to ~330 ºC for 2011 samples (eruptive phase 3) (Fig. 11a). Such redox conditions are lower than those dictated by the FeO/FeO_1.5 “rock” buffer (R_H=-2.8) and they can be ascribed to inputs of oxidizing deep magmatic-related fluids into the shallow hydrothermal environment, which agree with the presence of SO₂ in the crater gases during the 2010-2011 unrest period (Table 4; Giggenbach, 1987). The distribution of the gases sampled after eruptive period (2012) shows higher oxidizing conditions (R_H≈-3.25) with respect to the prior period, although there is no significant increment of the equilibrium temperature respect to 2011 samples (Fig. 11a). The evolution of equilibrium conditions indicated by the CO₂-CH₄-H₂-Ar systems is consistent with the changes from a magmatic-hydrothermal (in 2010-2011) to a “magmatic” stage (in 2012).

Chemical equilibria in the H₂-CO₂-CO-CH₄-H₂O system can also be considered for geothermometric evaluation of the Planchón-Peteroa fumarolic fluids (Chiodini and Marini, 1998). The equilibrium conditions can be adequately described by the following two redox-independent reactions (Chiodini and Marini, 1998):

CO₂+H₂↔H₂O+CO                                                  (7)

CH₄+3CO₂↔4CO+2H₂O                                        (8)

The analytical values of the thermodynamic constants of equations (7) and (8), i.e., log(H₂O/H₂)+log(CO/CO₂) and 3log(CO/CO₂)+log(CO/CH₄), are controlled by the temperature of an original saturate liquid phase (T_o) and either adiabatic single-step steam separation temperature (T_s; Fig. 11b). As shown in figure 11b, despite some spread, fumarolic samples of Planchón-Peteroa volcano plot within the vapour+liquid field. The evolution of the equilibrium conditions in figure 11b indicates the occurrence of an increasing “energetic” boiling of the magmatic-hydrothermal reservoir during the unrest episode (2010-2011). In February 2010 vapor phase was separated from a liquid zone (T_o~275 °C) at a temperature T_s>200 °C and a pressure >16 bar; whereas in March 2011 fumarolic gases plot near the liquid line at T_o~350 °C and a T_s<150 °C. The relatively low H₂ and CO contents of sample Pet-5 in March 2011 (Table 4), which plots far from the gases collected in 2011, are likely related to the extremely low flux of this fumarole (T~43 °C; Table 4), which favors oxidation of these highly reactive gas compounds at shallow depth. Fumarolic gases collected during the “magmatic” stage (2012), on the other hand, plot close to a T_o and T_s up to 325 °C and 275 °C, respectively (Fig. 11b), evidencing a steam separation at higher pressures (~60 bar).

8. Discussion

The renewing of Planchón-Peteroa volcano activity (phase 1) was characterized by an almost permanent emission of a ~200 m gas/steam column during 7 months as a consequence of small phreatic explosions in the SW border of crater 3. This activity seems to be related to heat and fluid transfer from a shallow magmatic-hydrothermal reservoir with a minimal release of deep magmatic gases. This hydrothermal signature can be assumed as a “normal” gas composition during quiescent periods of Planchón-Peteroa volcano.

The phase 2 corresponds to the more intense eruptive phase along all eruptive period, with columns reaching up to 3,000 m of height, plumes dispersed at maximum distance of 344.8 km from the active crater and frequent tephra fall. Tephra fall deposits reached its greatest area at 10^th October 2010 (1,265 km²), and were exclusively constituted by no juvenile fragments. Some of these fragments were partially altered, being characterized by the presence of Fe-oxides, and less frequently Cu-minerals. Considering the absence of juvenile products, there is no undoubtable evidences of new magmatic batch from depth that can be regarded.

The eruptive phases 3 and 4 were characterized by a permanent emission of gas/steam columns varying from 200 to 800 m of height that were occasionally associated with emission of ash columns of 1,000-1,200 m height. Plumes were dispersed at variable distances (from 37 to 637.6 km), with occasional fall of tephra. The componentry of the fall deposits was identical that the one during phase 2, with sole presence of no juvenile fragments. The heat transfer probably began to progressively stabilise from phase 3 to 4 (which had increased previously from phase 1 to 3, as observed by the thermodynamical review, with a temperature increasing from ~290 to ~330 ºC ), producing less intense and less frequent explosive events. Fluids composition was still dominated by reduced hydrothermal fluids, but with a subtle increase of deep oxidized magmatic fluids, producing a limited increase in the concentration of typical magmatic species (e.g., SO₂; Table 4). The re-establishment of the acid lake in the crater 3 is probably a consequence of repose period between phases 2 and 3, when rain and melting of glaciers during summer, in addition of water supply from from condensation of hydrothemarmal fluids, allowed the replenishment of this lake.

After the 2010-2011 eruptive period, the volcanic activity was characterized by  a permanent low height steam column (50-100 m), but no explosive activity was observed. Fluid composition during March 2012 shows a dramatic increase of the  magmatic component contents (e.g., SO₂, HCl and HF) which is accompanied by higher oxidizing conditions (R_H=-3.25) and separation of steam at higher pressure (~60 bar; Fig. 12a and b). In consequence, once the eruptive period finished, release of magmatic fluids from a deep reservoir to the surface become effective, but without eruptive activity associated.

 

Fig. 12. Schematic models of the transition from a hydrothermal-magmatic episode (c) to magmatic episode (d) at Planchón-Peteroa volcano caused by a temporary breaching of a normally self-sealed zone (modified from Fournier, 2007). See text for discussion.

 

A key observation for understanding the mechanisms responsible for the eruption pattern and the recorded geochemical evolution of the fumarolic fluids at Planchón-Peteroa volcano could be related to the N-S orientation of the subsidence that affected the neighboring volcanic systems (e.g., Calabozos caldera) within months of the 27^th February 2010 MW 8.8 Maule earthquake (Pritchard et al., 2013). Planchón-Peteroa volcano is emplaced along a NNE-SSW high-angle (>60°) thrust fault which is favorably orientated for dextral shear with respect to the prevailing stress field affecting the Andean region (i.e., subhorizontal maximum principal compressive stress-σ_Hmax trending N 60° E; Farías, 2007; Cembrano and Lara, 2009). Correspondingly, the N-S orientation of the subsidence reported by Pritchard et al. (2013) may reflect the long-term feature, i.e., the shape of both the hydrothermal-magmatic reservoir and the underlying magmatic system. As Pritchard et al. (2013) suggested, the existing NNE-striking magmatic and hydrothermal systems (e.g., Calabozos caldera) would have been disturbed by the static and dynamic stress change of the Maule earthquake through the formation and/or reactivation of numerous cracks suitably oriented with respect to the NNE-SSW co-seismic stress field, allowing fluids and heat to escape from the magmatic-hydrothermal reservoirs. According to this hypothesis, the enhanced fumarolic activity observed during the 1^st eruptive phase (February 2010) should be a consequence of this intense escape of fluids from a reservoir where vapour and liquid were in equilibrium at a T~275 °C, a pressure >60 bar and redox conditions slightly more oxidized (R_H≈-3.1) that those dictated by the FeO/FeO_1.5 buffer (Fig. 12a). Such fluid releases would have drastically reduced the fluid pressure within the magmatic-hydrothermal reservoir causing the phreatic eruptions observed during the 1^st eruptive phase, but without an evident surface deformation.

In March 2011, acid gases content at crater fumaroles, as well as the CO₂/CH₄ and N₂/Ar ratios slightly increased compared to those observed in February 2010 (Table 4), whereas R_H values remained relatively constant. These temporal changes suggest that after an almost uninterrupted release of fluids from the magmatic-hydrothermal reservoir during the 2010 unrest episode (Fig. 12a), a partial consumption of the liquid-dominated system overlying the magmatic zone would have occurred (e.g., Chiodini et al., 2004). Such process is clearly observed in figure 11b where fumarolic gases sampled in March 2011 plot close to the pure liquid equilibrium line at T_o~350 °C, being this composition typical of vapors separated from a liquid reservoir when complete vaporization of the liquid zone occurs (e.g., Guagua Pichincha-Ecuador, Montserrat-West Indies, Aguashuca-Salvador, Tambora-Indonesia; Chiodini and Marini, 1998).

Continuation of the degassing process from the magmatic-hydrothermal reservoir would have led to a total consumption of the liquid-dominated systems, lowering abruptly the load experienced by the underlying magmatic chamber at Planchón-Peteroa (e.g., Fournier, 2007). Fournier (2007) argued that such process may induce rapid vesiculation within the water-saturated magma, resulting in the rupture of the self-sealed zone surrounding the magmatic body (i.e., sealing carapace; Fisher et al., 1996), and consequently a rapid loss of the exsolved fluids from the crystallizing magma and that flashed from any brine that had previously resided in plastic rock beneath the self-sealed zone would have occurred (Fig. 12b). The increasing seismic activity observed from October 2011 to February 2012 at Planchón-Peteroa volcano and its surroundings (SERNAGEOMIN, 2011c³ and d⁴; 2012a⁵, b⁶ and c⁷) along with the clear magmatic signature observed in March 2012 at the crater fumaroles is consistent with this hypothesis (e.g., Fournier, 2007). Discharge of magmatic fluid across a narrow ring directly into brittle rock where hydrostatic pressure conditions prevails (Fig. 12a) strongly favors the evolution of SO₂-rich and H₂S-poor fluids, explaining the relatively high SO₂/H₂S ratios measured (>2; Table 4) at the crater fumaroles during March 2012 (e.g., Fournier, 2007; Liotta et al., 2010). The relatively low HCl contents (<14.1 mmol/mol) measured during the “magmatic” stage at the crater fumaroles may be a consequence of the relatively low temperature at which the vapor phase was separated (T_S<275 °C), as HCl₍₀₎ is preferentially partitioned into the liquid phase at T<400 °C (Fournier, 2007). A similar scenario would have also occurred during the 2^nd eruptive phase, where a sudden rupture of the self-sealed zone would have caused the observed phreatic eruptions (Fig. 12b).

9. Conclusions

We conclude that the eruptive period 2010-2011 of Planchón-Peteroa volcano is related to the increasing of heat and mass transfer from shallow hydrothermal-deep magmatic sources, perhaps favoured by theformation and/or reactivation of cracks produced by the Maule earthquake (27^th February 2010). The chemistry of the crater fumaroles suggests two distinct phases of volcanic activity at Planchón-Peteroa: 1) During the 2010-2011 unrest episode, the crater fumaroles had a composition typically of volcanic emissions in magmatic-hydrothermal stages, dominated by CO₂ (H₂S) and with comparable amounts of hydrothermal- and magmatic- related components such as CH₄, and SO₂, HCl and HF, respectively. The CO₂/CH₄ ratios slightly increased from 3,300-4,100, in February 2010, to 7,400-10,700, in March 2011, likely resulting from the evolution of the hydrothermal-magmatic reservoir to higher temperatures and/or oxidizing conditions. 2) In March 2012, the SO₂, HCl and HF abundances of the crater fumaroles increased by three folds, indicating that magmatic fluids overwhelmed the previous “hydrothermal-magmatic signature”. The change from a magmatic-hydrothermal stage to a subsequent phase dominated by magmatic components, seems to be a consequence of a degassing episode from a relatively “old” (i.e., highly degassed) magma body. The explosive activity dominated by phreatic eruptions, which ejected exclusively no juvenile material. Tephra fall deposit minimum volume was calculated in 0.0088 km³, and following Legros (2000) methodology, considering the 0.5 cm isopach and a density of 2,61 g/cm³, the erupted mass calculated was 2.3×10¹⁰ kg. According to the classification proposed by Pyle (2000), the eruptive activity of Planchón-Peteroa volcano for 2010-2011 period, had a magnitude (M) 3.36, corresponding to a VEI 1-2 based in the classification of Newhall and Self (1982). The composition of tephra ash deposit and its physical parameters (e.g., area, volume, grain size distribution) are consistent with the low magnitude of the eruption, low VEI value and eruption type (phreatic). The low magnitude of these phreatic explosions prevented the glacier melting, avoiding the lahars formation, which could affect the Teno and Claro rivers, like occurred during previous events (González-Ferrán, 1995; Naranjo et al., 1999).

Acknowledgements
This work was funded by “Proyecto de Iniciación FONDECYT No. 11100372: Role of climate, continental crust and subducted sediments in the degassing style and fluid composition of Andean volcanoes” (F. Aguilera). V. Marfull (Centro de Investigación y Difusión de Volcanes de Chile, Proyecto Archivo Nacional de Volcanes), R. Tormey (Entrix Inc.), Gendarmería Argentina (Paso Vergara) and local witnesses gave us important field-based information that improve our database related with this eruptive process. Finally we would like to acknowledge to C. González, M. Inostroza and S. Layana for their help during satellite images processing and tephra samples analysis. Our acknowledgements to Andean Geology editor (Dr. L. Lara) by its valuable comments of the manuscript.

 

¹ SERNAGEOMIN. 2011a. Reporte de Actividad Volcánica (RAV), Región del Maule, agosto 2011. Servicio Nacional de Geología y Minería, Informe (Unpublished), No. 25: 2 p.

² SERNAGEOMIN. 2011b. Reporte de Actividad Volcánica (RAV), Región del Maule, septiembre 2011. Servicio Nacional de Geología y Minería, Informe (Unpublished), No. 26: 2 p.

³ SERNAGEOMIN. 2011c. Reporte de Actividad Volcánica (RAV), Región del Maule, octubre 2011. Servicio Nacional de Geología y Minería, Informe (Unpublished), No. 27: 2 p.

⁴ SERNAGEOMIN. 2011d. Reporte de Actividad Volcánica (RAV), Región del Maule, noviembre 2011. Servicio Nacional de Geología y Minería, Informe (Unpublished), No. 28: 2 p.

⁵ SERNAGEOMIN. 2012a. Reporte de Actividad Volcánica (RAV), Región del Maule, diciembre 2011. Servicio Nacional de Geología y Minería, Informe (Unpublished), No. 29: 2 p.

⁶ SERNAGEOMIN. 2012b. Reporte de Actividad Volcánica (RAV), Región del Maule, enero 2012. Servicio Nacional de Geología y Minería, Informe (Unpublished), No. 30: 2 p.

⁷ SERNAGEOMIN. 2012c. Reporte de Actividad Volcánica (RAV), Región del Maule, febrero 2012. Servicio Nacional de Geología y Minería. Informe (Unpublished), No. 31: 2 p.

 

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