The 2006–2009 activity of the Ubinas volcano (Peru): Petrology of the 2006 eruptive products and...

Journal of Volcanology and Geothermal Research 270 (2014) xxx–xxx

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The 2006–2009 activity of the Ubinas volcano (Peru): Petrology of the2006 eruptive products and insights into genesis of andesite magmas,magma recharge and plumbing system

Marco Rivera a,⁎, Jean-Claude Thouret b,c,d, Pablo Samaniego b,c,d, Jean-Luc Le Pennec b,c,d

a Observatorio Vulcanológico del INGEMMET (Dirección de Geología Ambiental y Riesgo Geológico), Urb. Magisterial B-16, Umacollo, Arequipa, Perub Clermont Université, Université Blaise Pascal, Laboratoire Magmas et Volcans, BP 10448, F-63000 Clermont-Ferrand, Francec CNRS, UMR 6524, Laboratoire Magmas et Volcans, 5 rue Kessler, 63038 Clermont-Ferrand cedex, Franced IRD, R 163, Laboratoire Magmas et Volcans, 5 rue Kessler, 63038 Clermont-Ferrand cedex, France

⁎ Corresponding author. Tel./fax: +51 54432272.E-mail address: [email protected] (M. Rivera

0377-0273/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.jvolgeores.2013.11.010

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Article history:Received 1 July 2013Accepted 18 November 2013Available online 27 November 2013

Keywords:Explosive activityMagma rechargeMagma mixingThermobarometryUbinasPeru

Following a fumarolic episode that started sixmonths earlier, themost recent eruptive activity of the Ubinas vol-cano (south Peru) began on 27 March 2006, intensified between April and October 2006 and slowly declineduntil December 2009. The chronology of the explosive episode and the extent and composition of the eruptedmaterial are documentedwith anemphasis on ballistic ejecta. A petrological study of the juvenile products allowsus to infer themagmatic processes related to the 2006–2009 eruptions of the andesitic Ubinas volcano. The juve-nile magma erupted during the 2006 activity shows a homogeneous bulk-rock andesitic composition (56.7–57.6 wt.% SiO2), which belongs to amedium- to high-K calc-alkaline series. Themineral assemblage of the ballis-tic blocks and tephra consists of plagioclase N two-pyroxenes N Fe–Ti oxide and rare olivine and amphibole set ina groundmass of the same minerals with a dacitic composition (66–67 wt.% SiO2). Thermo-barometric data,based on two-pyroxene and amphibole stability, records a magma temperature of 998 ± 14 °C and a pressureof 476 ± 36 MPa. Widespread mineralogical and textural features point to a disequilibrium process in theerupted andesite magma. These features include inversely zoned “sieve textures” in plagioclase, inverselyzoned clinopyroxene, and olivine crystals with reaction and thin overgrowth rims. They indicate that the pre-eruptive magmatic processes were dominated by recharge of a hotter mafic magma into a shallow reservoir,where magma mingling occurred and triggered the eruption. Prior to 2006, a probable recharge of a maficmagma produced strong convection and partial homogenization in the reservoir, as well as a pressure increaseand higher magma ascent rate after four years of fumarolic activity. Mafic magmas do not prevail in the Ubinaspre-historical lavas and tephras. However, mafic andesites have been erupted during historical times (e.g. AD1667 and 2006–2009 vulcanian eruptions). Hence, the most recent episode indicates that a resupply of maficmagmas has probably occurred at depth under Ubinas.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

Knowledge of the triggering mechanisms of explosive eruptions isrelevant for forecasting of future eruptions. The injection of hotmagma into a magma reservoir is one of the major mechanisms forproducing magma mixing that has been long recognized as an impor-tant process for triggering explosive eruptions (Sparks et al., 1977).Compositional diversity in erupted products and disequilibrium tex-tures in phenocrysts have been attributed to magma mixing processesat numerous arc-related volcanoes (e.g., Eichelberger, 1975; Huppertet al., 1982; Clynne, 1999; Tepley et al., 2000; Martel et al., 2006). Thetriggering of explosive eruptions by magma recharge and subsequentmixing has also been proposed at several volcanic centers worldwide

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The 2006–2009 activity of tolcanol. Geotherm. Res. (201

(e.g., Askja, Sparks et al., 1977; Pinatubo, Pallister et al., 1992; Karymiskivolcano, Eichelberger & Izbekov, 2000; Soufrière Hills, Murphy et al.,2000; Tungurahua, Samaniego et al., 2011) as well as in Peru, in thecase of the Nevado Sabancaya 1990–1998 eruptions (Gerbe andThouret, 2004). However, forecasting such re-injections remains an ar-duous monitoring challenge for understanding associated volcanic haz-ards. This is particularly true for volcanoes exhibiting long-lasting, low-magnitude eruptions, which may provide little warning of replen-ishment, apart from deep volcano-tectonic or long period events(cf. White, 1996). In this respect, the systematic study of the eruptiveproducts using petrological methods can identify a posteriori rechargemagmatic events, quantify the pre-eruptive magmatic processes lead-ing to the eruption, and recognize potential changes in eruptive behav-ior during long-lasting episodes.

On 27 March 2006, following about six and half months of intensefumarolic activity, Ubinas began to produce gas and gray ash to

he Ubinas volcano (Peru): Petrology of the 2006 eruptive products3), http://dx.doi.org/10.1016/j.jvolgeores.2013.11.010

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elevations b1000 m above the summit. Increased activity followed be-tween April and October 2006, as shown by 3 to 4 km-high ash andgas columns. This intermittent activity steadily but slowly declinedafter September 2007 until December 2009. In this paper, we analyzethe petrological characteristics of the juvenile products of the 2006eruption with emphasis on ballistic ejecta. The major and trace ele-ments and the mineral content of juvenile blocks allow us to place the2006 magma into the framework of the historical eruptions of Ubinas.Historical reports point to long-lasting fumarolic activity alternatingwith a few month- to a few year-long, small to moderate explosiveeruptions (mostly VEI 2) (Rivera et al., 1998; Thouret et al., 2005). In

Fig. 1.Aster image of the Ubinas volcano in the Arequipa–Moquegua region, south Peru. Nine viwithin 16 km from the summit. Inset: The active, frontal volcanic arc of theCAVZ (Central Andeaand north and on the steep slope towards the Río Para to the east and south. The asymmetric esummit caldera and regional faults N25°, 40° and 160°. The N130° and N40° faults may have p

Please cite this article as: Rivera, M., et al., The 2006–2009 activity of tand insights into genesis of andesite..., J. Volcanol. Geotherm. Res. (201

this context, we complement the petrological database with severalsamples from theAD1667 eruption of Ubinas. The rationale for compar-ing the 2006 blocks with the AD 1667 scoriae is the following: (1) theVEI 3 (Siebert et al., 2011) and volume (0.05 km3) indicate that theAD 1667 eruption is the largest event over the past 500 years; (2) theerupted magma has a basic andesite composition; and (3) togetherwith the 2006–2009 events, the AD 1667 eruption represents the bestexamples for the most probable eruptive scenarios at Ubinas. The tex-tural and mineralogical characteristics and the mineral-melt equilibri-um allow us to constrain the pre-eruption, physical P–T conditions ofmagma by using the two-pyroxenes and Al-hornblende thermo-

llages and one town ‘Ubinas’with a combined population of about 4760 people are locatednVolcanic Zone) in south Peru. Ubinas has beenbuilt on the high-relief plateau to thewestdifice has been built on a complex tectonic setting that includes a N130° fault crossing thelayed a role in the past and recent flank failures of the Ubinas edifice towards the south.


image of Fig.�1


3M. Rivera et al. / Journal of Volcanology and Geothermal Research 270 (2014) xxx–xxx

barometers. Our ultimate goal is to better understand the triggeringmechanisms associated with the renewed 2006 eruptive activity andto quantify the processes involved in magma differentiation.

2. Geology and eruptive history of the Ubinas volcano

The Ubinas volcano (16° 22′ S, 70° 54′ W; 5672 m above sealevel — asl) is located ~75 km east of Arequipa in the Western Cor-dillera of the Central Andean Volcanic Zone (CAVZ) (Figs. 1 & 2).The volcano is built upon a 65 km-thick, mature continental crustthat consists of Mesozoic and Cenozoic sedimentary and volcanicformations unconformably overlying a Precambrian basement(Kink et al., 1986; Mégard, 1987). Ubinas is a composite cone witha roughly circular base, slightly elongated in the SE–NW direction,with an estimated volume of 36 km3 (Thouret et al., 2005). The Ubinasasymmetric edifice straddles the edge of a high plateau made up of LateMiocene–Early Pleistocene ignimbrites and lavas (Marocco and delPino, 1972) and the valleys of Río Ubinas to the south and Río Para tothe east (Fig. 1). In the summit area, steep lava flows (5672 m asl) aretruncated by an elliptical summit caldera (Fig. 4A), ~1.4 km in theN–S di-rection with a floor at about 5380 m asl (Fig. 4A). This summit calderamay have resulted from a series of large Plinian eruptions that tookplace before Holocene times (between 20 and 14 ka) and as recently asc.980 year BP (Thouret et al., 2005).

Ubinas is composed of two successive edifices (Thouret et al., 2005;Rivera, 2010): (1) The mostly effusive ‘Ubinas I’ edifice is characterizedby the emplacement of andesitic and dacitic lava flows between 440and 370 ka. The south flank of the volcano suffered a sector collapse,producing a debris-avalanche whose deposits have an estimated vol-ume of ~2.8 km3 in the valley of the Río Ubinas, as far as 12 km SE ofthe summit; and (2) the overlying ‘Ubinas II’ composite edifice is an800 m-high cone composed by andesitic and dacitic lavas and pyroclas-tic deposits dated between 370 ka and the Holocene. This edifice wasbuilt during five eruptive stages (Thouret et al., 2005; Rivera, 2010).During the last stage, between 20 and 1 ka, the eruptive behavior ofthe Ubinas volcano has been dominantly explosive, with Plinian erup-tions alternating with mild eruptive episodes. The last Plinian eruption

Fig. 2. Photograph depicting the eruptive activity of Ubinas (5670 m asl). Continuous degasinhabitants), located 6 km SE from the volcano's summit.


(VEIs 4–5), dated at 980 ± 60 year BP, produced ~1 km3 of andesiticpumice (Thouret et al., 2005).

Over the past 500 years, Ubinas produced mostly tephra with atrend towards more mafic magma compositions in recent times(Thouret et al., 2005; Rivera, 2010). Albeit frequent, the historicalUbinas explosive activity was small to moderate in magnitude (VEIs2–3). Twenty-five unrest episodes have been reported at Ubinas since1550 AD (Hantke and Parodi, 1966; Rivera et al., 1998; Thouret et al.,2005; Rivera et al., 2010; Siebert et al., 2011; Table 1) that rank Ubinasas themost active volcano in Peru,with a recurrence of four to seven un-rest episodes per century. Most event episodes, however, consist of in-creasing fumarolic activity associated with phreatic events (e.g. 1–7September 2013), but a limited number of VEIs 2–3 eruptions havealso been described (Siebert et al., 2011). The VEI 3 vulcanian AD 1667eruption produced about 0.05 km3 of scoria-flow and fall depositswith a mafic andesitic composition. The VEIs 2–3 vulcanian andphreatomagmatic eruptions produced minor tephra fallout, small-volume lithic or scoria-rich pyroclastic flows and lahars (Rivera et al.,1998, Thouret et al., 2005). Ash from these events frequently damagedcultivated land, due to tephra fall and rain-induced lahars that traveledthrough Río Ubinas valley: this was again the case during the last 2006–2009 eruptive episode. This unrest affected approximately 4760 people(INEI, 2007) living in seven villages within 12 km from the volcano.

3. Chronology of the 2006–2009 activity

Based on the phenomenology of Ubinas activity (Fig. 3), we dividethe 2006–2009 eruptive episode in five phases (Table 2). Table 2displays the characteristics of events, ash and ballistics (Figs. 4 & 5),the recorded ash and gas plumes and SO2 flux, and sources of data(e.g., Appendix Fig. A).

Phase 1: The onset of the eruptive activity was characterized bystrong fumarolic activity between mid-August 2005 and 26 March2006 (Fig. 3). This activity included vapor-rich emissions bearinglow amounts of magmatic gas (Appendix Fig. A). A shallow magmabody was probably degassing below Ubinas vent.

sing of Ubinas in March 2006 (phase 1) as seen from the village of Ubinas (about 1800


image of Fig.�2


Fig. 3.Diagram showing the five phases of the 2006–2009 eruptive episode based on observations from the INGEMMET campsite in a campsite located at 4600 m asl, 4 km SWof volcano.Maximum heights of eruption columns between April 2006 and December 2009 above the summit of the volcano (5670 m asl, i.e. 400 m above the vent). Stars * indicate the explosiveevents that expelled most of the ballistic blocks analyzed in this work.

Table 1Eruptions of Ubinas reported from AD 1550 (based on El Pueblo newspaper, 1936, 1937, 1951 and 1969; Hantke and Parodi, 1966; Valdivia, 1995; Rivera et al., 1998; Siebert et al., 2011).

Year Start End Type of activity VEI Comments

1550 Explosive eruption 3?1599 07 February 22 February Moderately explosive eruption 2 Gray fine ash fell nearby the city of Arequipa 75 km W of Ubinas.1600 Explosive eruption Probably refers to the AD 1600 Plinian eruption of Huaynaputina volcano.1662 Explosive eruption 2? Ash dispersed as far as Sama and Locumba located between 90 and 120 km SW of Ubinas.1667 Large explosive eruption 3 Small volume of dark gray ashfall- and scoria-flowdeposits onNWandN flanks of Ubinas.1778 Explosive eruption High fumarolic activity and ash emission.1784 Explosive eruption 2 High fumarolic activity and ash emission.1826 Explosive eruption 21830 Explosive eruption 21862 Explosive eruption 21865 Explosive eruption 2 Abundant ash emission.1867 24 May 28 May Explosive eruption 2 Abundant ash emission.1869 October Explosive eruption 21906 October Explosive eruption 21907 October Explosive eruption 21912–1913? Explosive eruption 2 Ash fell in Chojata andYalahua located 18 km to the E andNE of Ubinas respectively and in

Ubinas town. Ash damaged crops and cattle died due to intoxication from water and/orgrass mixed with ash.

1923–1925? Explosive eruption Gray fine ash fell nearby the city of Arequipa.1936 03 January July Explosive (phreatic) eruption

and strong fumarolic activity2 Ash fell in the Ubinas valley affecting people and crops.

1937 May July Explosive eruption 2 Ash damaged crops while at least 4 people and numerous cattle died due to unknownepidemics, e.g. intoxication from consumption of water, vegetables and/or fruits mixedwith ash.

1951 May 21 October Explosive eruption 2 Ash fell in the Ubinas valley causing damage to crops and affecting people health.1956 June Explosive eruption and strong

fumarolic activity2 Ash emissions caused damage to crops and six villages in the Ubinas valley.

1969 May December? Explosive eruption 2 In the Ubinas valley, ash destroyed crops and affected population health. At least fivepeople and numerous cattle died due to intoxication from consumption of water,vegetables, and/or fruits mixed with ash.

1995–1996 December April? Strong fumarolic activity 1 High fumarolic activity startled the population living in five villages in the Ubinas valley.2006–2009 March December Small phreatic, phreatomagmatic

and vulcanian eruptions, andstrong fumarolic activity

2 Constant ash emissions affected peoples' health. About 1500 people were evacuated fromfive villages of the Ubinas valley in April and June 2006, firstly to Anascapa, secondly toChacchagen shelters.

2013 September Explosive (phreatic) activity andstrong fumarolic activity

1 Ash fell 30 kmnorth,west and northwest of Ubinas, affecting people and cattle health. Ashcolumns 2 to 3 km high occurred between 1st and 7th of September.


Please cite this article as: Rivera, M., et al., The 2006–2009 activity of the Ubinas volcano (Peru): Petrology of the 2006 eruptive productsand insights into genesis of andesite..., J. Volcanol. Geotherm. Res. (2013), http://dx.doi.org/10.1016/j.jvolgeores.2013.11.010

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Table 2Event characteristics and measured parameters that enable us to distinguish five phases during the 2006–2009 eruptive episode.

PhasesCharacteristics

Phase 1High fumarolic activity, weakSO2 degassing

Phase 2Phreatic and phreatomag-maticactivity

Phase 3Vulcanian explosions

Phase 4Vulcanian explosions and strongdegassing

Phase 5Weak degassing and interspersedexplosions

Duration 6.5 month-long onset ofactivity, mid-August 2005-26March 2006

About 3 weeks: 27 March –

~19 April 20066 months: 20 April 2006 toend of October 2006

1.6 year: November 2006 to April 2008 1.8 year:May2008 -December 2009

Eruptive events⁎ Increasing fumarolic activity 27 March: strong, phreaticemissions of fine grey ash.14 - 18 April 2006: two largeexplosions with ballistics.19 April 2006: first appear-anceinside vent of a plug 60 m across,andesitic lava.

Between 14, 20 and 26 April, increase innumber and size of explosions, but moresporadic afterwards.End of April - August 2006, intermittentactivity.Eruptive activity declined in Sept. 2006,but increased slightly in Oct. 2006.

Nov. 2006 -Jan. 2007, mild activity.Febr.- May 2007: slight increase,several explosions (Fig. 5C).17 May - 19 Nov. 2007, mild activity.20 Nov. 2007 - 14 Jan. 2008, small butmore frequent explosions.Mid-Jan. - April 2008, slight increase.

2008: degassing interspersed withat least 12 small explosive events.Nov. 2008 - Dec. 2009, fumarolicactivity.

Degassing1andmeasured2SO2

Mid-August 2005 and 26March2006 (Fig. 3 & Appendix A): 1-2kt SO2/day. Sulphur-richplumes averaging 993 ppm⁎m3

maximum 1550 ppm⁎m3

27 March 2006: sharp increase inemissions of magmatic gas (5.5-6.5kt SO2/day).

October: gas and ash emission.SO2 measured on 11 May: 1800 ppm⁎m3.4-7.5 kt SO2/day.

Jan. - April 2006: mild degassing events.b6 kt SO2/day Nov. 2006-Jan. 20076-11.5 kt SO2/day Febr.-Sept. 2007.

Permanent 200 to 800 m-highvapor-rich columns.Oct. 2008 - Dec. 2009, degassing.Oct. 2008-2009 ≤4.5 kt SO2/day.

Seismic activity3 Very weak activity Little tremor, LP events, explosions 24 June-16 July: less tremor, no LPs16 July-Oct. more tremors, LPs, explosions,rare VTs

November 2006 scattered activitymuch less LPs

Weak activity interspersed withexplosions

Height of ash-ladencolumns above thecaldera rim (5400 masl.)4

n.a. 27 March 2006: 1000 m. Continuous columns with increasingelevation up to 3000 m.26-27 April, up to 3000 m.End April - August 2006: up to 3000 and4000 m.September (esp. 18): up to 2000 m.October: between 400 and 2000 m.

Nov. 2006-Jan. 2007: up to 1200 m(Fig. 5B)Feb.-May 2007: columns N 2000 m(Fig. 5C).5 & 14 Jan. 2008: 1500 and 3000 m.Mid Jan. - April 2008: permanent,between 500 and 2500 m.

Mid May 2008 - Oct 2008, 1000 to1500 m.Nov. 2008 - Sept. 2009, 200 to 1500m (Fig. 5D).Oct. to Dec. 2009, between 400 and1500 m.

Ash dispersaldistance and direction

n.a. 7, 22 & 29May; 2, 18 & 23 June; 10, 19 & 22July; and 12 August: fine ash 40 to 80 kmfrom the vent.Sept.: fine ash N20 km towards E, SE, S andN. Oct :N20 km towards SE, E, N.

Feb-May 2007: ash dispersed N20 kmaround the volcano.Jan. 2008: tephra dispersed 60 kmtowards E, S and SW.

Fine ash 40 km from the vent.Oct. to Dec. 2009: fine ash a fewtens of km towards S.

Ash layer on summitcaldera floor and ash fallaround the summit

n.a. Four to 8 cm-thick ash layer (Fig. 4):glass, crystals, accidental blocks, andash-sized accessory fragments.

Grey ash layer II (Fig. 4) withdisseminated lithic lapilli.

Jan. - April 2008: occasional, thin fineash-fall layer around the summit.Layer III: coarse ash with disseminatedjuvenile blocks (Fig. 4).

Occasional, thin ash-fall layer aroundthe summit.

Ballistics: size,distance, velocity(calculated with “Eject!1.1”: Mastin, 2001)

14-18 April 2006: juvenile andaccessory lava blocks 70 cm across,500 m from the vent on thecaldera floor; exit velocity100 - 120 m/s.

14-26 April & 7 May: juvenile andesiticlava ballistics up to 40 cm within 1 km ofthe vent. Impact craters up to 2 m acrosson the caldera floor (Fig. 4);exit velocity of 140 - 160 m/s.

Dec. 2006, and Jan., March, April, May,and Sept. 2007: juvenile ballistics up to50 cm across within 1 km of the venton the caldera floor; exit velocity140 - 160 m/s.

Inferred processes at &below vent

Probable presence of shallowbody magma at low depth.

Magma intrusion heralded byphreatomagmatic activity.

Vulcanian eruptive style alternating withstrong degassing.

Vulcanian explosions and degassing. Degassing magma body. Decliningactivity.

1 Degassing andmeasured SO2 indicate abundantmagmatic gasmixedwith ash columns. This contrasts with fumarolic activity (weak emission of vaporwith a very little amount ofmagmatic gases≤1 kt/day), which characterizes common Ubinasactivity between eruptive events.

2 SO2 fluxes derived from OMI satellite data, courtesy of S. CARN (Michigan State University): see Appendix Fig. A. Note of caution: some “spikes” may be due to explosions. Other limited data stem from Cruz & Clegg (2006) who used a Flyspecspectrometer in January 2006 and May 2007.

3 Description of seismic activity based on Macedo et al.’s report, 2011.4 Column height and other parameterswere estimated by INGEMMET personnel from a camp at 4500masl located 4 km SW from the summit. Estimates were correlatedwithmeteorological data acquired by SENAHMI andwith thermal anomalies

detected by GOES (e.g. 08/31/2006; 06/19/2007) and MODIS satellite images (e.g. 11/24/2006).⁎ Non-eruptive event: 17 January 2007, a rainfall- and snowmelt-triggered lahar travelled N30 km from the Ubinas’ S flank to the Río Tambo valley. Estimated volume: 260,000 m3.

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Fig. 4. A. Photograph looking SE in the summit caldera of the Ubinas volcano at 5420 m asl. B. Stratigraphic section showing the 39 cm-thick tephra-fall deposit 20 m way fromthe crater rim on the caldera floor (I) Reddish, coarse ash and lapilli layer rich in hydrothermally altered lithics; (II) Gray ash layer with disseminated lapilli-sized lithicfragments; (III) Layered horizon of coarse ash with disseminated blocks of juvenile material. Picture looking SW taken on 2 December 2010. The AD 1667 tephra-fall deposithas not been indicated in the picture although it probably underlies the 2006 tephra deposit. C. An impact crater of 7.2 m in diameter was formed by a 2-m-across bomb onthe caldera floor ~300 m from the crater on 7 May 2006 (picture looking NW taken by V. Aguilar).


Pa

Phase 2 suddenly began on 27March 2006 and consisted of phreaticand phreatomagmatic activity (27 March to ~19 April 2006). Strongphreatic explosions produced fine gray ash emissions up to 1000 mabove the summit. Between 14 and 18 April 2006, two large explo-sions ejected lava blocks as large as 70 cm in diameter 500 maway from the crater on the summit caldera floor. Ashfall formed a4 to 8 cm-thick layer inside the caldera (Fig. 4B). On 19 April 2006,incandescent lava, 60 m in diameter, was for the first time observedinside the vent (Fig. 5A).Phase 3was characterized by vulcanian explosions between 20 April2006 and late October 2006. In 20 and 26 April, explosive activityincreased in number and size based on estimated ash and gas col-umn heights, seismic activity and the OMI-derived record of SO2

data (S. Carn, pers. com.) (Table 2). The ash-laden eruption columnsrose up to 3000 m above the caldera rim. SO2 emissions (OMI data)were four to sixfold that of phase 1. Between 14 and 26 April, andsporadically later, increased explosive activity hurled ballistic, juve-nile andesitic lava blocks up to 40 cm across within 1 km of thecrater, with 2 m-wide impact craters (7 May 2006, Fig. 4C). Fromthe end of April to August 2006 the activity remained intermittent,with ash columns rising up to 3000 and 4000 m above the summit.Fine-grained ash was dispersed around the cone as far as 40 to80 km from the vent. The eruptive activity declined after September2006 (Fig. 3, Table 2). In October 2006, a slight increase in the activ-ity was expressed by gas and ash columns between 400 and 2000 mabove the summit, while fine ash was dispersed more than 20 kmtowards SE, E, and N.

lease cite this article as: Rivera, M., et al., The 2006–2009 activity of thend insights into genesis of andesite..., J. Volcanol. Geotherm. Res. (2013),

Phase 4 consisted of vulcanian explosions alternating with strongdegassing events between November 2006 and April 2008(Fig. 5B–D). From November 2006 to January 2007 the activitywas generally mild, with ash emissions rising up to 1200 mabove the summit. Between February and May 2007, a slightincrease in the eruptive activity was observed (Fig. 5C). Duringthese months several explosions were accompanied by eruptioncolumns exceeding 2000 m in height and ash was dispersedmore than 20 km away around the volcano. From 17 May to 19November 2007 the activity remained mild. From 20 November2007 to 14 January 2008, small explosions were more frequentand plumes rose up to 1500 and 3000 m above the summit, andtephra was carried as far as 60 km from the volcano towards E,S and SW. Between mid-January and April 2008 a slight increasein the activity was reflected by mild degassing events producinga permanent 500 to 2500 m-high plume and occasional ashemissions.Phase 5 shows a declining activity. Degassing events alternatedwith at least twelve explosive events that produced 1000 to1500 m-high eruption columns between mid May 2008 andOctober 2008, dispersing ash as far as 40 km from the vent.SO2 fluxes decreased by a factor of 1–2 with respect to Phases2–4, but remained twofold higher than that of phase 1. FromNovember 2008 to December 2009 the activity remained mild,with gas and ash columns rising between 400 and 1500 m-height, and fine ash drifting a few tens of km towards thesouth.

Ubinas volcano (Peru): Petrology of the 2006 eruptive productshttp://dx.doi.org/10.1016/j.jvolgeores.2013.11.010

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Fig. 5. A. Photographs depicting theUbinas 2006–2009 eruptive activity. The incandescent lava plug appeared at the bottom of the vent on 19 April 2006 (phase 2). B. The 25 January 2007explosion propelled an ash column up to 1.6 km above the caldera rim and dispersed ash towards the east (phase 4, picture taken from the village of Sacuaya 4 km SE of the summit).C. The 16 April 2007 explosion propelled an ash column up to 3 km above the caldera rim (phase 4). D The 15 March 2009 explosion propelled an ash column up to 1.5 km above thecaldera rim (phase 5). Picture looking NE taken by R. Amache.


4. Characteristics of the 2006–2009 tephra

4.1. Field collection, measurements and analytical methods

Fieldwork was carried out soon after each of the 2006 eruptions,with the aim of sampling the tephra, including the juvenile ballisticblocks. From the onset of the eruptive episode until April 2008, 31 ashsamples were collected on the floor of the summit caldera (Fig. 4) andon the flanks of the cone. At the end of the eruptive episode inNovember 2009, the tephra-fall deposit inside the caldera wasabout 40 cm-thick (Fig. 4B), whereas it reached 4 cm in-thickness4 km away the vent, and only 1 cm thick 6 km away in Ubinas vil-lage. Fig. 6 portrays the isopachs (Fig. 6A) and the thickness decayrate of ashfall layer (Fig. 6B). The diameter of ash particles decreasedfrom amaximum of 1 to 0.3 mmover a distance of 3 to 5 km from thevent (Fig. 6). Further away, at 12 km from the vent, the tephra-falldeposit contained 40 to 60 vol.% of 0.05–0.1 mm-sized particles.

Short-lived, strong phreatomagmatic and vulcanian explosionsejected ballistic blocks outside the 300 m-deep vent onto the calderafloor and/or the outward upper flanks (see stars associated to columnheights in Fig. 3) between April 2006 and September 2007. During theApril 2006 explosions (phase 2) hydrothermally altered blocks up to70 cm in diameter and some juvenile blockswere hurled to amaximumdistance of 500 m of the crater with an exit velocity of 100–120 m/s(using “Eject! 1.1” software, Mastin, 2001: Table 2). The explosions oflate April 2006 (phase 3) ejected dense juvenile andesitic lava blocksand poorly vesicular blocks up to 40 cm to within 1 km from the craterwith a calculated exit velocity of 140–160 m/s. Impacts of the ballisticsformed pits 2 m in diameter upon landing on the caldera floor. Moreviolent explosions on 27 April 2006 and 7, 22 and 29 May 2006(phase 3; Fig. 4C) and later explosions during the fourth phase hurledjuvenile ballistic blocks, including breadcrust bombs with quenched


rims, up to 50 cm in diameter within 0.8–1 km m from the craterwith a calculated exit velocity of 130–160 m/s.

Eleven samples of ballistic blocks from the third eruptive phase in2006were collected and analyzed. The juvenile, dense and poorly vesic-ular blocks are the products of four explosions between April andOctober 2006 (Table 4). All samples were collected on the uppermostwestern flank and on the caldera floor between 1 and 2 km distancefrom the vent. We have complemented the sampling with several AD1667 scoriae and lava clasts. Petrographic observations of thin sectionsof ballistic blocks were helpful to document the textural relationshipsand to identify mineral phases. Appendix B provides a detailed accountof the analyticalmethods. Table 4 provides allmajor, trace elements andisotopic analytical results. Tables 5 to 9 include a selection of micro-probe analyses of major and trace elements in minerals and glass.

4.2. Componentry of tephra

The ash was examinedwith optical and polarizingmicroscopes for aqualitative investigation of texture and a componentry analysis(Table 3). Ash particles collected during the phase 2 are angular andslightly rounded and include a wide range of components that areranked in order of abundance in Table 3: grains of zeolites, hydrother-mal minerals, grains of gypsum and pyrite, Al-sulfate, Fe–Ti oxide,quartz and few crystals of plagioclase, pyroxene and black shards frag-ments. This assemblage of lithic ash and aminor amount of glass shardsand/or juvenile crystals suggests that phreatic or phreatomagmaticactivity produced the volcanic ash from phase 2. In contrast, ash parti-cles fromphase 3 included a large amount of fresh, juvenile componentsand a small amount of weathered and hydrothermally altered particles.The ash particles of phase 4 contained mainly crystal fragments ofplagioclase, pyroxene, amphibole, Fe–Ti oxide, glass shards and a fewhydrothermally-altered clasts. The increase in free crystals and crystal


image of Fig.�5


Fig. 6.A) Isopachmap of the ashfall layer emplaced fromMarch 2006 to November 2009. Thismap indicates the location of villages (black squares) shown for reference. B) Plot of the ashlayer thinning rate, comparing thickness versus square root of isopach area (A0.5). The dashed line is the best exponential fit to the data, which allows calculation of tephra volume (seetext). The total volume of the ashfall layer is ~7 × 106 m3.


fragments with respect to weathered and hydrothermally altered frag-ments in the phase 4 ash assemblage suggests increasing fragmentationof a juvenile crystal-richmagma and concurrent decreasing interactionswith the hydrothermal system. The volcanic conduit was progressivelycleaned.

4.3. Tephra volume and volcanic explosivity index 2

To evaluate the size of the 2006–2009 eruptive episode we consid-ered a set of tephra thickness (T) measurements obtained from phases2 to 4 at 26 locations. The isopachmap in Fig. 6A provides themaximumthicknesses of the tephra-fall deposit measured during the last twoyears of the 2006–09 eruptive episode before wind and runoff erosion.Although these measurements were obtained over different phasesand integrated in one data set, the isopach contour lines show a simpleconcentric pattern centered on the active vent (Fig. 6B). Assuming ellip-tical contour shape for each T value in Fig. 6A, the calculated isopacharea (A) yields 0.61 km2 at T = 40 cm, 6.94 km2 (10 cm), 34.61 km2

(4 cm), 80.22 km2 (2 cm), 127.40 km2 (1 cm) and 202.47 km2

(0.5 cm). A plot of T vs. A0.5 (Fig. 6B) reveals a quite regular thinningrate, which fits a single exponential segment (dashed line in Fig. 6Bwith R2 = 0.964). Using the expression of Pyle (1989) based on theexponential decay assumption, the bulk tephra volume is 6.8 × 106 m3


(with extrapolated thickness T0 31.35 cm at the vent and thicknesshalf-distance bt 1.29 km). Similarly, assuming two segments withpower law fits (Rose et al., 1973) and an inflexion point at a thicknessof 2 cm, the calculated tephra volume is 7.3 × 106 m3. Estimates inferredfrom the trapezoidal rule approach of Fierstein and Nathenson (1992)yield a bulk volume of 6.5 × 106 m3, which does not account for thetephra volume beyond the 0.5 cm isopach. As a result, the range of bulkvolumes between 6.50 and 7.3 × 106 m3 points to a small-sized eruptionranked as VEI 2. An unknown but small volume of lava intruded at thevent and ballistic blocks should be considered in order to obtain thevolume of magma for the entire 2006–2009 episode.

5. Petrological characteristics of ballistic juvenile blocks

The 2006 juvenile blocks are dark to gray, dense to poorly vesicular,porphyritic andesites bearing 20–25 vol.% of phenocrysts (300 μm–

1.8 mm), 30–40 vol.% of microphenocrysts (100–300 μm), and35–50 vol.% of matrix glass. Glomerophenocrysts of plagioclase,clinopyroxene, orthopyroxene, and Fe–Ti oxide are abundant. Allthe samples show a similar mineral assemblage including plagio-clase, clinopyroxene, orthopyroxene and magnetite with scarceamphibole and olivine. Olivine phenocrysts occur in the April–May2006 ballistic blocks whereas no olivine exists in the October 2006


image of Fig.�6


Table 3Description of content and components for tephra erupted during phases 2 to 4 in 2006.

Eruptivephase

Componentry (juvenile, accessory, accidental) Proportion(in vol.%)

Comments

2 Light-toned aggregates of micron-sized ash grains with a few dark clasts. The light-tonedcrystalline grains belong to zeolites and other hydrothermal minerals.

30–40 Ash textures suggest that phase 2 particles wereproduced by phreatic to phreato-magmatic activity.

Tiny grains (5 mμ across) of gypsum, pyrite, Al-sulfate, magnetite and quartz. 20–30Light-toned free crystals including fragments of plagioclase. 25–20Rounded to sub-angular, yellowish to pinkish ash particles of weathered plagioclase. 10–15Dark-toned pyroxene and amphibole crystals, as blocky or tabular particles, or flakes.Some amphibole flakes are fresh-looking and many other crystals show no weathering.

5–10

Dark shard fragments with small vesicles. 5–10Gray agglomerates of tiny (black and white) crystals with weathered grayish plagioclase. 5Massive silica grains with no internal structure and silica clots of micron to sub-micron-sizedparticles (up to 300 μm). One inherited quartz crystal was identified.

3 Dark shard fragments with small vesicles. 30–40 Ash particles include a large amount of fresh, juvenilecomponents and a little amount of weathered orhydrothermally altered particles.

Slightly elongated to angular white plagioclase crystals. 15–20Greenish sub-angular pyroxene. 10–15Weathered or hydrothermally altered crystal agglomerates. 10–15Pinkish, small sized sub-angular grains of plagioclase. 2–6Dark fragments of amphibole crystals. 2–4Fe–Ti oxide particles. 2–4Black glass fragments and glass shards. 20–30

4 Fragments of light-toned free plagioclase crystals. 15–20 Ash particles include a large amount of fresh, juvenilecomponents.Agglomerates of light-toned crystals of plagioclase. 15–20

Accessory fragments composed of light-toned, fresh and altered lava particles. 4–10Sub-angular free crystals of pyroxene. 5–8Pinkish angular crystals of plagioclase. 4–8Fe–Ti oxide particles. 2–4Free amphibole crystals. 1–2


samples and are scarce in AD 1667 samples. Scattered, subhedralphenocrysts of amphibole 200–400 μm in size (b1 vol.%) areobserved in some ballistic blocks ejected in April and May 2006, butare absent in the AD 1667 samples. The glassy to microcrystallinegroundmass is characterized by a relatively low content (15–20 vol.%)of microlites (100–60 μm) of plagioclase, orthopyroxene, clinopyroxene,and Fe–Ti oxide. The dark gray, glassy matrix of the juvenile blocks andbombs of the 2006 and AD 1667 erupted products shows a high-K daciticcomposition (66.2–68.7 wt.% SiO2, 5.0–5.8 wt.% K2O).

5.1. Whole-rock geochemistry of juvenile blocks

The compositions of the 2006 juvenile blocks are shown in Fig. 7together with those of samples that erupted during the historical activ-ity of Ubinas, including AD 1667 scoriae (Rivera et al., 1998; Thouret

Fig. 7.Alkali-silica diagram (Le Bas et al., 1986) showing the compositions of 2006 eruptedlava blocks and scoriae at Ubinas. Note the wide range of compositions of the historicalmagmas prior to 2006 compared to the homogeneous compositions of the products thaterupted in 2006.


et al., 2005). All samples from the 2006 eruptive episode display homo-geneous andesitic compositions with 56.7–57.6 wt.% SiO2 (normalizedto an anhydrous basis, Table 4, Fig. 7). No chemical differences havebeen observed between the juvenile blocks that erupted betweenApril and October 2006. The AD 1667 samples also show homogenouscompositions, with slightly lower silica (55.7–56.1 wt.% SiO2) andhigher magnesium contents (4.2–4.7 wt.% MgO, Fig. 7). Other historicalproducts show heterogeneous compositions, with a silica content rang-ing between 55 and 67 wt.% SiO2, and define a high-K calc-alkaline(2.0–2.3 wt.% K2O)magmatic trend. However, we emphasize that sam-ple compositions from both 2006 and AD 1667 eruptions plot at themafic end of the Ubinas magmatic series, corresponding to the mostprimitive magmas that erupted during at least the past five centuriesat this volcano.

Fairly good correlations exist between most major and traceelements and silica content (Fig. 8). Most major oxides (except Na2O,not shown in Fig. 8) and some trace elements (e.g., Sr, Ni, and V) arenegatively correlated to silica content, although some scattering isobserved for Ni. Large-ion lithophile elements (LILE; e.g., K, Rb, Ba, Th,not shown in Fig. 8) are positively correlated to the silica content,whereas light and middle rare earth elements (LREE and MREE; e.g., Laand Sm) are negatively correlated to the silica content. High fieldstrength elements (HFSE; e.g., Nb) and heavy rare earth elements(HREE; e.g., Yb) show no clear variation with increasing silica content(Fig. 8). The trace elements patterns (Fig. 9) clearly point to variableenrichment in LILE (e.g. Rb, Ba, Th, K) and LREE (e.g. La, Ce), a depletionin HREE (e.g. Yb) and strong negative anomalies in HFSE (e.g. Nb). All arearchetypal characteristics of calc-alkaline rocks of continental margins.

Sr and Nd isotopic ratios in the studied samples show homoge-neous values with high 87Sr/86Sr (0.70674–0.70678), and low valuesof εNd (−6.01 to −6.48, Table 3). These values fall into the field ofmagma compositions emitted by Ubinas during the last five centu-ries (Thouret et al., 2005; Rivera, 2010). These values, which are con-sistent with those of other CAVZ volcanic rocks, are given as evidencefor crustal contamination or assimilation (Davidson et al., 1991;Delacour et al., 2007; Mamani et al., 2010).

Previous studies of the Ubinas magmatic products (Thouret et al.,2005; Rivera, 2010) showed that the composition of parental magmashas been relatively homogeneous through time and essentially results


image of Fig.�7


Table 4Whole-rocksmajor (wt.%) and trace elements (ppm) analyses for the 2006Ubinas juvenile products. The analytical precision (2σ) is b1% formajor elements (except for Fe, Na: 2% and LOI:~10%) and around 5% for trace elements. The trace elementswere analyzedby ICP-MS. The analytical errors estimated according to rock standards JB3 and JA2 are about 15–20% forNb andTa and b10% for other trace elements.

Sample Ubi06-04 Ubi06-05-1 Ubi06-05-2 Ubi06-05-3 Ubi-10-18A Ubi-10-18B Ubi-0613 Ub-0614

Eruption date April 2006 April 2006 April 2006 April 2006 April 2006 May 2006 7 May 2006 24 May 2006

Rock type Gray dark block Gray dark block Gray dark block Gray dark block Gray dark block Gray dark block Gray dark block Gray dark block

SiO2 56.8 57.1 57.3 56.7 56.8 56.5 56.50 55.88Al2O3 17.2 17.2 16.5 17.1 17.3 17.6 17.52 17.14Fe2O3 7.68 7.73 8.21 7.68 7.51 7.92 7.73 9.28MgO 3.22 3.28 3.82 3.27 3.30 3.16 3.26 3.26CaO 6.49 6.5 6.69 6.48 6.50 6.68 6.58 6.57Na2O 4.03 4.03 3.87 3.97 4.00 4.04 3.99 3.95K2O 2.31 2.27 2.14 2.27 2.49 2.28 2.34 2.21TiO2 1.12 1.12 1.15 1.12 1.16 1.13 1.12 1.10P2O5 0.48 0.49 0.48 0.49 0.411 0.48 0.50 0.49MnO 0.12 0.12 0.13 0.12 0.11 0.12 0.12 0.11LOI −0.03 −0.15 0.0H2O PF(1000°) 0.00H2O (110°) −0.31TOTAL 99.45 99.84 100.30 99.20 99.49 99.76 99.66 99.99Rb 62 56 51 57 65 62 55 60Sr 912 901 871 889 885 922 907 910Y 20 22 20 19 17.6 18.8 16.6 17.7Zr 226 223 213 218 205 214 197 207Nb 12 12 10 11 11.2 11.2 10.2 9Ba 947 925 893 936 984 960 878 946La 39.4 40.7 40.1 40.6 39 37 33.2 42.1Ce 76 84.6 84.2 85.4 80 80 73.3 87.7Nd 40.4 42 41.8 42 36 38 34.6 39Sm 7.1 7.5 7.4 7.3 6.7 7.1 7.1 7.9Eu 1.8 1.9 1.9 1.88 1.71 1.82 1.7 2.1Gd 5.7 6.0 6.0 5.92 4.7 5.6 5.2 5.9Dy 4.0 4.1 4.1 4.05 3.4 3.6 3.5 3.9Er 1.7 1.7 1.7 1.73 1.5 1.7 1.6 2.0Yb 1.5 1.5 1.5 1.48 1.36 1.40 1.35 1.5Sc 9 7 15 16 12.3 10.4 10 10V 144 149 163 143 165 153 137 160Cr 10 10 10 7 21 10 15 14Co 23 24 23 23 19 19 17 18Ni 5 7 8 9 15.7 9.5 17 20Th 7.8 8.0 7.5 7.91 8.7 6.5 7.2 7.6Pr 10.1 10.6 10.4 10.5 9.5 9.6Tb 0.7 0.7 0.7 0.7 0.6 0.9Ho 0.7 0.7 0.7 0.7 0.6 0.7Tm 0.2 0.2 0.2 0.2 0.2 0.2Lu 0.2 0.2 0.2 0.2 0.2 0.2Hf 5.9 5.9 5.6 5.9 5.8 6.0Ta 0.7 0.7 0.5 0.6 0.6 0.7Pb 11.3 13.5 9.9 13.4 14.0 2087Sr/86Sr 0.706761 0.706764±2σ error 0.000006 0.000006143Nd/144Nd 0.512329 0.51233±2σ error 0.000006 0.000006


from partial melting of a mantle source metasomatized by hydrousfluids. In addition, Ubinas samples show an important depletion inHREE and Y and large variations in Sr/Y (28.2–65.9) and LREE/HREE(e.g. (La/Yb)N = 16.2–33.9) ratios (Thouret et al., 2005). Becauseamphibole and/or garnet is the only phase able to incorporate largeamounts of HREE and Y, depletion of these elements in arc magmas isusually interpreted as an indicator for a deep-crustal fractionation pro-cess near the base of the N60-km-thick continental crust (Mamani et al.,2010). In sum, we propose that the petrogenetic process and the mag-matic sources have not evolved significantly over the past five centuriesbased on the remarkable geochemical homogeneity of the 2006 and AD1667 mafic products.

5.2. Mineral characteristics and composition, compared with AD1667 products

Plagioclase, which dominates the mineral assemblage in the Ubinassamples (15–25 vol.%), occurs as euhedral and subhedral phenocrysts


(b1–2 mm),microphenocrysts, andmicrolites in thematrix. Plagioclasephenocrysts in the 2006 samples show a wide compositional rangefrom An39 to An78 (Table 5) whereas microlites display more restrictedcompositions (An44–62). In contrast, plagioclase phenocrysts in theAD 1667 samples reveal a smaller compositional range from An51 toAn67. On the basis of textural characteristics, three populations ofphenocrysts have been observed in samples of both 2006 and AD1667 eruptive products: (1) clear, euhedral crystals, with normal(e.g. An59 → 52), oscillatory (e.g. An53 → 49 → 52, An51 → 65 → 52) and re-verse (e.g. An52 → 62) zoning patterns; (2) sieve-cored phenocrysts,where the cores are completely riddled with glass and have overgrownwith clear rims (Fig. 10); and (3) sieve-ringed phenocrysts, where aclear core is mantled by a dissolution–resorption rim zone. Some ofthese phenocrysts often show corroded rims (Fig. 10). The plagioclasephenocrysts of the second and third populations display strong reversezoning patterns. Fig. 10 illustrates zoning patterns across the profiles ofa sieve-ringed plagioclase phenocryst, which displays a homogeneouscore (An36–45) surrounded by an overgrowth rim with a calcic-rich



Table 4Whole-rocksmajor (wt.%) and trace elements (ppm) analyses for the 2006Ubinas juvenile products. The analytical precision (2σ) is b1% formajor elements (except for Fe, Na: 2% and LOI:~10%) and around 5% for trace elements. The trace elementswere analyzedby ICP-MS. The analytical errors estimated according to rock standards JB3 and JA2 are about 15–20% forNb andTa and b10% for other trace elements.

Ubi-10-18C Ubi-10-20 Ubi-0618 Ubi-77 UBI-10-19 Ubi99-10 Ash-1 crater Ash-2 south wall Ash-3 Querapi Ash Ubinas

October 2006 October 2006 28 October 2006 1667 1667 1667 April 2006 April 2006 April 2006 April 2006

Gray dark block “Breadcrust” bomb Black block Black scoria Black scoria Black scoria Ash Ash Ash Ash

55.4 56.2 56.75 56 55.5 55.15 59.10 61.21 60.08 59.1617.9 17.5 17.44 16.35 16.7 16.35 13.32 13.10 13.25 13.637.55 7.98 7.74 7.75 7.74 7.92 4.21 4.04 4.00 4.053.52 3.34 3.26 4.23 4.38 4.68 1.03 1.08 0.97 0.847.30 6.65 6.53 7.13 7.00 7.25 5.16 4.62 4.32 5.324.10 4.04 3.97 4.04 4.00 4.10 1.98 2.00 2.01 1.852.24 2.28 2.34 2.18 2.13 2.23 2.04 2.08 2.09 1.941.23 1.18 1.11 1.37 1.28 1.44 1.17 1.18 1.13 1.140.44 0.47 0.51 0.56 0.45 0.54 0.38 0.38 0.39 0.370.11 0.12 0.11 0.10 0.11 0.11 0.04 0.04 0.04 0.07

−0.25 −0.15 −0.1 0.3 −0.091.10 0.74 0.98 1.17

10.29 9.12 10.67 9.5799.58 99.59 99.66 100.01 99.21 99.75 99.81 99.60 99.92 99.1058 54 58.6 44 51 39

941 899 923 1140 1021 1135 803 735 722 80818.6 18.9 18.0 17.3 16.0 18 10 9 10 9

207 210 204 245 190 223 205 206 207 20110.8 11.4 11.1 11.1 11.3 9

946 1152 934 1150 1044 1156 987 923 927 101437 37 35.9 48.5 35 35.378 79 78.9 97 75 78.537 38 37.9 48 36 347.3 7.7 6.9 8.3 7.1 6.11.92 1.90 1.8 2.1 1.74 1.995.2 5.1 5.2 5.8 4.9 4.843.6 3.6 3.4 3.6 3.2 2.81.5 1.6 1.6 1.5 1.4 1.31.37 1.4 1.4 1.2 1.3 1.0

13.3 11.4 10 14.6 15.6 16 10 9 9 10169 161 141 182 185 180 114 113 109 11628 14 n.a. 98 110 106 64 59 59 6220 21 18 60 24 25 17 14 15 1517.1 12.6 5.1 41 46 42 149 43 56 666.8 6.6 7.8 5.6 4.3 4.6

9.7 8.70.6 0.60.7 0.50.3 0.20.2 0.26.4 6.00.7 0.5n.a. 12.30.7067750.0000080.5123220.000006


composition (An50–65). Suchpatterns suggest that the phenocrystswerein disequilibrium with the surrounding melt during the magmaticevolution.

Clinopyroxene phenocrysts (4–6 vol.%) appear as either phenocrysts(up to 800 μm) or microphenocrysts (100–250 μm). Following thestructural formulae calculation of Lindsley (1983), clinopyroxene is amagnesium-rich augite (En40–47 Wo38–46 Fs11–19, according to the clas-sification scheme of Morimoto et al., 1988; Table 6). The low Wo, highEn and magnesium number (Mg#) of euhedral phenocrysts are relatedto the most primitive and higher temperature compositions (Lindsley,1983). In the 2006 samples, clinopyroxene phenocrysts display Mg#varying from 72 to 82, whereas slightly magnesium-rich compositionsare documented in the AD 1667 samples (Mg# 77 to 87). In both 2006and AD 1667 samples, clinopyroxenes show slight normal zoningpatterns (e.g., Mg# 82 → 81) but most of them display reverse zoning(e.g., Mg# 65 → 80).

Orthopyroxene appears as either euhedral or subeuhedral pheno-crysts (up to 1000 μm) and microphenocrysts (100–300 μm) in all


rock samples (2–4 vol.%). They show a ferroan-enstatite composi-tion (Wo3–8 En65–74 Fs23–33; Morimoto et al., 1988). Phenocrystshave Mg# from 67 to 77 (Table 7). Most orthopyroxene phenocrystsshow normal zoning (Mg# 76 → 72) but some of them display a slightreverse zoning (Mg#78–80, Fig. 11). In addition, orthopyroxene over-growth rims of olivine phenocrysts display fairly high Mg# (66–78).In contrast, orthopyroxene has not been observed in the AD 1667scoriae.

Amphibole, which is an accessory phase (b1 vol.%) in the 2006samples, has not been found in the AD 1667 products. Amphibole hasrounded (anhedral) habits and common reaction rims (20 to 150 μmin width) of the so-called “black-type” weathering (or “opacite”),which is formed by a cryptocrystalline aggregate of Fe–Ti oxides, plagio-clase and pyroxene. Some phenocrysts are totally weathered (Fig. 11).Compositionally, amphibole shows 5.9–6.1 atoms per formula units(apfu) of Si and (Na + K)A N 0.5 apfu. They are therefore classified asmagnesio-hastingsite according to Leake et al. (1997) (Table 8). Thecompositional range is narrow (Mg# 65–73; 11.7 to 13.3 wt.% Al2O3)



Fig. 8. Plots showing major and trace elements versus silica content for Ubinas rocks.



image of Fig.�8


Fig. 9. Spiderdiagram showing the trace elements patterns for 2006 samples comparedwith the eruptive products of Ubinas (gray field) over the past 1000 years. Data normal-ized to primitive mantle composition (after Sun and McDonough, 1989). Note that the2006 samples are more depleted in LILE and slightly enriched in REE than other Ubinassamples.

Table5

Selected

plag

ioclasean

alyses

forthe20

06Ubina

ssamples.

Sample

Ub0

6-04

Ub0

6-04

Ub0

6-04

Ub0

6-04

Ub0

6-04

Ub-13

Ub-13

Ub-13

Ub-13

Ub-13

Ub-13

Ub-13

Ub-13

Ub-13

Ub-13

Ub-14

Ub-14

Ub-14

Ub-14

Ub-18

Ub-18

Ub-18

Ub-18

Ub-18

Ana

lyses

45

68

911

1221

2628

4142

1711

310

825

2641

4345

4647

4850

cr

mi

cr

cr

cr

cc

rmi

cr

cr

cr

cc

rc

r

SiO2(w

t.%)

52.40

53.72

53.98

53.40

55.72

51.05

56.03

54.82

52.40

57.00

51.40

53.32

55.15

53.03

49.72

57.79

55.61

53.36

55.24

51.54

59.94

51.67

55.91

58.39

TiO2

0.08

0.09

0.11

0.08

0.06

0.12

0.12

0.00

0.07

0.03

0.04

0.09

0.04

0.03

0.03

0.11

0.10

0.08

0.08

0.05

0.15

0.07

0.06

0.00

Al 2O3

28.96

27.75

27.49

28.05

26.98

29.39

26.71

28.16

28.93

26.25

30.06

28.73

27.46

29.02

31.02

25.19

26.96

28.30

27.19

30.09

24.13

30.23

27.48

26.21

FeOa

0.71

0.87

0.70

0.67

0.70

0.52

0.74

0.63

0.74

0.51

0.57

0.76

0.85

0.67

0.70

0.65

0.71

0.66

0.86

0.70

0.58

0.68

0.35

0.32

MnO

0.01

0.04

0.00

0.03

0.00

0.00

0.02

0.00

0.01

0.03

0.00

0.02

0.04

0.04

0.00

0.00

0.02

0.01

0.03

0.03

0.00

0.03

0.00

0.00

MgO

0.06

0.07

0.05

0.10

0.09

0.05

0.08

0.08

0.10

0.04

0.05

0.07

0.09

0.04

0.08

0.06

0.07

0.07

0.12

0.03

0.04

0.05

0.03

0.01

CaO

12.55

11.28

11.23

11.75

10.04

13.03

9.65

11.23

12.12

8.96

13.35

11.73

10.17

12.27

14.61

8.30

10.12

11.83

10.36

13.28

6.55

13.35

9.49

8.17

Na 2O

4.30

4.84

4.79

4.66

6.07

3.93

5.45

4.86

4.28

5.81

3.71

4.45

5.13

4.31

2.79

6.39

5.29

4.56

5.26

3.63

6.83

3.39

5.62

6.18

K2O

0.40

0.48

0.45

0.32

0.55

0.41

0.60

0.46

0.39

0.71

0.28

0.37

0.63

0.45

0.23

0.69

0.54

0.35

0.56

0.29

1.16

0.29

0.60

0.81

Cr2O3

0.09

0.00

0.00

0.04

0.00

0.00

0.00

0.01

0.00

0.01

0.00

0.01

0.03

0.01

0.00

0.00

0.01

0.00

0.02

0.00

0.00

0.00

0.00

0.01

Total

99.56

99.13

98.81

99.09

100.21

98.50

99.40

100.27

99.04

99.34

99.45

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Olivine is also an accessory mineral (1–2 vol.%), occurring assubhedral and anhedral phenocrysts and microphenocrysts (b500 μm)in the April and May 2006 juvenile blocks. Olivine displays narrowcompositional ranges (Fo58–78, Table 8) and common normal zonations(e.g., Fo78 → 72) in the 2006 samples and narrower compositions(Fo74–79) in the AD 1667 products. Most olivine phenocrysts showresorbed zones and reaction rims made up of small crystals ofplagioclase, pyroxene, and Fe–Ti oxide, while others are found withovergrowth rims of orthopyroxene (Fig. 11).

Fe–Ti oxide is present as euhedral microphenocrysts (b200 μm) dis-persed in the groundmass (2–4 vol.%) and inclusions in clinopyroxene,orthopyroxene, olivine and plagioclase phenocrysts of both 2006 andAD 1667 products. They correspond to titanomagnetites (11–17 wt.%TiO2; structural formulae calculated according to Stormer, 1983) anddo not show any exsolved lamellae of ilmenite (Table 9). Some pheno-crysts of titanomagnetite show reverse zoning with TiO2-rich rims(11.5–13.5 wt.%).

6. Discussion

6.1. Physical and chemical conditions of magma storage and crystallization

The pre-eruptive P–T–XH2O conditions of Ubinas magmas may beconstrained by comparing the observed mineral assemblage with thatobtained during experimental studies on hydrous mafic andesiticmagmas. On the basis of the major element composition, the primitiveandesites of Mt. Pelée (Martel et al., 1999), the Mexican Volcanic Belt(Moore and Carmichael, 1998; Blatter and Carmichael, 2001) and Mt.Shasta (Grove et al., 2002; Krawczynski et al., 2012) may be consideredcompositionally similar to those of the Ubinas volcano. The experimen-tal works onmafic andesite magmas have shown that amineral assem-blage comprising plagioclase, orthopyroxene, clinopyroxene, and Fe–Tioxides (with minor amounts of amphibole and olivine) reflects a com-plex crystallization history through a wide range of P–T–XH2O condi-tions (N300 MPa, 900–1050 °C, N4 wt.% H2O). On the basis of thesefirst-order experimental parameters, and in order to better constrainthe P–T conditions of Ubinas magmas, we have used two thermo-barometers from the literature.

Over the past four decades, different calibration schemes haveaimed to constrain temperature from enstatite-diopside partitioning(Davis and Boyd, 1966; Wood and Banno, 1973; Wells, 1977; Lindsleyand Andersen, 1983). We use the recently calibrated thermometer of


image of Fig.�9


Fig. 10. Chemical zonation in plagioclase phenocrysts. Left-hand panels correspond to back-scattered electron images (A and C) showing the location of two almost orthogonal profiles.Right-hand panels (B and D) are the An content vs. distance from the rim. We emphasize the occurrence of thin (50–100 μm), An-rich overgrowth rims developed outside the resorptionzones. Note the extreme reverse zonation for plagioclase crystals, suggesting the occurrence of mingling process in the 2006 lavas. R = rim, C = core.


Putirka (2008), which has been tested for intermediate to silicicmagmas under a quite large temperature range. We selected onlynear-rim compositions of euhedral orthopyroxene–clinopyroxenepairs and tested the equilibrium between themby comparing the calcu-lated Fe–Mg exchange coefficient with its accepted value (1.09 ± 0.14;Putirka, 2008). The estimated temperature ranges between 949 and1021 °C (n = 10) for a fixed pressure of 450 MPa (see below). Westress that pressure has little effect on temperature estimates. For in-stance an increase in 100 MPa induces a rise of ~5 °C, i.e. a variationof less than one standard deviation of the estimated temperatures.

The experimental calibration of the Al-in-hornblende barometer(Jonhson and Rutherford, 1989) has been widely used to estimate thepre-eruption pressure of amphibole-bearing calc-alkaline magmas.However, the lack of quartz in the mineral assemblage makes this ba-rometer inapplicable to Ubinas andesites. Recently, Ridolfi et al.(2010) reviewed the amphibole stability for a large compositionalrange of calc-alkaline magmas and proposed that most calc-alkalinemagmas crystallize near the stability curve of amphibole. The authorsproposed several empirical P–T–ƒO2–XH2O formulations based on singlecompositional components such as the silica index (Si*) and the alumin-ium content (AlT) of amphibole. The formulations may be applied tocalcic amphiboles with Al# ≤ 0.21 and Mg / (Mg + Fe2+) N 0.5,which is the case of the 2006 Ubinas magmas. The formulations suggestthat amphibole crystallized at temperatures of 998 ± 14 °C (n = 11), apressure of 476 ± 36 MPa and a fO2 of −9.5 ± 0.1 (NNO + 1). Wenote that temperature estimates obtained using Ridolfi et al.'smethod fall in the temperature range inferred from the two-pyroxenethermometer. The temperature estimates are in the upper limit foramphibole stability in arc magmas, suggesting that they representmaximum values. Our estimated pressure dataset is consistent withamphibole crystallization in other well-studied magmatic systems


such as those of Mt. St. Helens, Soufrière Hills, Redoubt, and Reventador(Ridolfi et al., 2010). The application of this formulation for the entireUbinas magmatic series, which also includes silica-rich andesites andrhyolites, yields two distinct populations of amphibole phenocrysts(Rivera, 2010) (Fig. 12). The first group includes amphiboles crystalliz-ing at low pressures (100–200 MPa) found in silica-rich samples,while the second group of high-pressure amphiboles (250–550 MPa)corresponds to amphibole phenocrysts hosted in low-silica andesitesand mafic andesites. We emphasize the fact that the amphibolesdescribed here correspond to the high-pressure population.

6.2. Magma recharge: insight from mineral textures and compositions

On the basis of the mineralogical and geochemical characteristicsof the entire Ubinas magmatic series, Thouret et al. (2005) proposedthat the main petrogenetic process able to explain its chemical di-versity is fractional crystallization of an andesitic magma leavinga cumulate composed of plagioclase, ortho- and clinopyroxeneand amphibole. As a result, the dacitic and rhyolitic magmas char-acterizing the Early Holocene eruptions of Ubinas were generated.More recently, Rivera (2010) suggested that fractional crystalliza-tion alone does not explain the variable enrichment in LILE aswell as the Sr–Nd isotopic signature of Ubinas magmas, invokingvariable amounts of crustal assimilation. In this paper, we focusedon the petrological characterization of the mafic end-member ofthe Ubinas magmatic series, represented by the 2006 and AD1667 eruptive products. In these samples, several petrographicfeatures and chemical zoning patterns of the phenocrysts suggestthat disequilibrium processes are ubiquitous in Ubinas magmas(Figs. 10 and 11):


image of Fig.�10


Table 7Selected orthopyroxene analyses for the 2006 Ubinas samples. End-member components of “quadriteral” pyroxenes (mol.%) normalized to atomic Ca + Mg + ΣFe = 100with ΣFe = Fe2+ + Fe3+ + Mn (Morimoto et al., 1988); #Mg = 100 Mg/(Mg + Fe).

Sample Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-18 Ub-18 Ub-18 Ub-18

Analyses 1 2 3 12 13 6 8 9 18 24 13 14 8 28 35 39 8 9 57 58 60 61

r c c c c c c c m c c r Overgrewin olv

Overgrewin olv

c c r r c r c r

SiO2 (wt.%) 52.35 53.51 53.62 53.92 52.63 53.02 53.97 53.65 54.00 53.59 54.45 53.06 49.37 53.19 54.81 53.59 49.37 54.00 53.55 54.01 53.74 52.05TiO2 0.49 0.25 0.22 0.31 0.37 0.23 0.31 0.18 0.20 0.28 0.28 0.52 0.31 0.36 0.29 0.16 0.31 0.40 0.48 0.48 0.33 0.46Al2O3 1.23 0.66 0.82 0.95 1.67 0.85 1.24 1.04 0.78 0.65 0.57 1.64 0.41 0.64 0.82 0.83 0.41 0.69 1.21 0.99 0.81 1.59Cr2O3 0.04 0.04 0.01 0.03 0.20 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.03 0.02 0.00 0.00 0.03 0.00 0.00 0.00 0.00 0.00FeOta 16.77 16.62 16.65 16.79 15.96 20.86 16.68 18.19 17.34 17.24 16.23 16.60 23.72 19.39 15.66 18.26 23.72 19.83 17.27 16.89 17.28 17.79MnO 0.49 0.56 0.60 0.56 0.52 0.61 0.56 0.59 0.47 0.54 0.66 0.56 0.71 0.70 0.43 0.76 0.71 0.65 0.54 0.52 0.60 0.56MgO 24.68 25.69 25.66 25.41 26.11 23.01 26.15 25.49 25.58 25.00 25.33 24.66 23.89 21.21 26.72 23.75 23.89 21.38 24.09 25.65 24.67 23.37CaO 2.19 1.48 1.60 1.63 1.48 1.27 1.61 1.11 1.65 1.74 1.80 2.06 2.73 4.11 1.86 1.57 2.73 3.48 3.84 1.74 1.60 2.56Na2O 0.04 0.01 0.03 0.04 0.06 0.02 0.04 0.02 0.04 0.05 0.10 0.02 0.15 0.08 0.04 0.00 0.15 0.16 0.06 0.06 0.04 0.05K2O 0.01 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.05 0.05 0.00 0.01 0.02 0.01 0.00 0.06 0.00 0.02 0.01 0.01Total 98.29 98.85 99.22 99.65 98.82 99.88 100.57 100.28 100.08 99.10 99.49 99.18 101.31 99.69 100.65 98.92 101.31 100.66 101.03 100.34 99.09 98.43Mg# 73.9 74.3 74.4 73.1 76.9 66.9 75.2 73.0 73.6 72.7 73.6 72.7 77.8 66.1 75.9 69.9 77.8 65.8 73.1 73.8 71.8 71.2Wo 4.47 2.94 3.20 3.25 3.01 2.54 3.18 2.20 3.26 3.50 3.56 4.14 5.9 8.3 3.61 3.16 5.92 7.10 7.66 3.42 3.20 5.24En 70.02 71.40 71.26 70.13 73.92 64.60 72.14 70.76 70.73 69.50 70.14 69.07 72.2 59.9 72.68 66.80 72.20 60.42 66.92 70.71 68.82 66.79Fs 25.51 25.66 25.53 26.62 23.06 32.86 24.68 27.04 26.01 27.00 26.30 26.79 21.9 31.8 23.72 30.04 21.87 32.48 25.41 25.87 27.98 27.97

c: phenocryst core, r: phenocryst rim, m: microlite.a All iron as FeO.

Table 6Selected clinopyroxene analyses for the 2006 Ubinas samples. End-member components of “quadrilateral” pyroxenes (mol.%) normalized to atomic Ca + Mg + ΣFe = 100withΣFe = Fe2+ + Fe3+ + Mn (Morimoto et al., 1988); #Mg = 100 Mg/(Mg + Fe).

Sample Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-14 Ub-14 Ub-04 Ub-18 Ub-18 Ub-18

Analyses 11 1 7 10 11 12 25 1 4 5 6 9 37 38 43 44 23 24 71 51 52 53

c c c c c c c c r c r c c r c r c r c c r c

SiO2 (wt.%) 52.47 52.51 49.54 51.00 51.94 51.63 50.71 51.39 51.55 51.87 51.22 50.20 51.22 50.63 51.10 52.25 50.84 50.76 48.46 51.73 52.05 52.37TiO2 0.45 0.34 0.89 0.98 0.78 0.59 0.75 0.57 0.60 0.78 0.88 0.99 0.60 0.71 0.69 0.58 0.91 0.85 0.93 0.68 0.58 0.45Al2O3 1.26 1.50 2.99 3.13 2.56 1.87 1.92 1.51 1.77 2.02 2.39 3.88 1.91 2.54 3.25 1.68 2.45 2.32 3.52 2.00 1.79 1.43Cr2O3 0.01 0.00 0.00 0.00 0.02 0.01 0.00 0.00 0.00 0.01 0.00 0.05 0.01 0.07 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.04MgO 15.16 14.66 14.61 15.06 15.01 16.11 13.92 13.74 15.71 14.97 14.85 14.61 15.82 15.02 13.83 15.91 15.05 15.12 15.02 14.25 14.73 14.68FeOta 9.70 10.43 9.84 8.98 8.73 8.81 11.17 11.26 9.87 9.07 9.21 9.10 8.79 9.08 9.60 9.44 9.54 9.11 9.16 10.57 9.72 9.34MnO 0.38 0.36 0.29 0.33 0.31 0.29 0.41 0.51 0.40 0.35 0.37 0.26 0.27 0.29 0.29 0.43 0.31 0.35 0.29 0.38 0.28 0.33CaO 19.87 20.35 20.64 19.89 20.60 19.70 19.67 19.60 18.32 20.15 20.31 20.39 19.65 20.14 20.50 19.17 19.92 20.45 17.37 20.01 20.30 20.34Na2O 0.40 0.53 0.45 0.35 0.41 0.37 0.44 0.52 0.31 0.41 0.41 0.38 0.36 0.42 0.56 0.28 0.44 0.33 0.39 0.40 0.44 0.38K2O 0.01 0.00 0.00 0.00 0.03 0.00 0.00 0.03 0.03 0.01 0.00 0.00 0.02 0.03 0.00 0.05 0.00 0.03 0.04 0.00 0.02 0.00Total 99.73 100.67 99.30 99.72 100.40 99.37 98.98 99.15 98.56 99.65 99.65 99.85 98.65 98.94 99.85 99.78 99.47 99.33 95.18 100.03 99.91 99.35Mg# 76.3 76.4 83.3 77.9 78.4 82.0 73.7 72.4 76.2 77.0 78.5 79.4 81.5 81.0 75.7 77.6 79.5 81.0 78.1 73.5 76.4 75.0En% 43.2 41.5 41.6 43.5 43.0 45.5 40.3 39.9 45.4 43.1 50.4 42.3 45.1 43.2 40.5 45.1 43.0 43.1 45.8 41.0 42.2 42.2

c: phenocryst core, r: phenocryst rim, m: microlite.a All iron as FeO.

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Fig. 11. Textural and mineralogical features pointing to magma mixing in the 2006 lavas. A. Normally-zoned olivine phenocryst with orthopyroxene overgrowth. B. Olivine skeletal phe-nocryst with orthopyroxene overgrowth. C. Reverse-zoned clinopyroxene phenocryst. D. Amphibole phenocryst with a reaction rim (200 μmwide) composed of pyroxene, Fe–Ti oxidesand plagioclase. Each of the microphotographs shown the sample number in the upper left corner.


(1) Reversely zoned plagioclase phenocrysts present common“wavy” dissolution surfaces between the core and the rim, andCa-rich overgrowth rims (Fig. 10). Additionally, anhedral pheno-crysts show rounded corners, embayments zones and ‘sieve’textures. The phenocryst textures suggest that the crystalsunderwent resorption episodes in response to physical and/orchemical changes in themagma reservoir. Such changes are usu-ally associated with magma mixing (e.g., Tepley et al., 2000;Izbekov et al., 2002; Martel et al., 2006). Experimental studieshave reproduced complex reverse zoning within plagioclasephenocrysts: they demonstrate that temperature and chemicalcompositions are key parameters to promote the developmentof the disequilibrium textures (Nakamura and Shimakita,1998). However, the associated major compositional variations(N10% An) in plagioclases indicate that the dissolution eventsare related to mafic magma recharge and subsequent magmamixing (Singer et al., 1995; Ginibre et al., 2002).

(2) Some clinopyroxene and orthopyroxene phenocrysts showreverse zoning in particular in the outermost rims (Fig. 11),suggesting that the injection of a high-T mafic magma occurredjust before the eruption. According to Nakagawa et al. (1999), re-verse zonation in clinopyroxene phenocrysts is a strong evidencefor magma mixing.

(3) Overgrowth rims of orthopyroxenes in microphenocrysts ofolivine suggest that the olivine crystals are in disequilibriumin the hosting melt (Fig. 11). This may result from changes inphysical and chemical conditions during crystal growth.The disequilibrium features result from the recharge of anolivine-bearing mafic magma into a compositionally zonedreservoir.

(4) Some phenocrysts of titanomagnetite show an increase of theTiO2 content from the core to rim. This feature has been ex-plained by a change in temperature and/or oxygen fugacity


of the magma induced by the recharge of an ascending hotmafic magma into a more differentiated and less hot magmareservoir (Devine et al., 2003).

(5) All amphibole crystals observed in the 2006 juvenile blocksare subhedral and anhedral phenocrysts characterized bybreakdown and resorption rims (20–150 μm wide, Fig. 11).Such features in amphibole are commonly described in sever-al calc-alkaline products (Rutherford and Hill, 1993; Browneand Gardner, 2006; Ridolfi et al., 2010). Mechanisms foramphibole breakdown include: (1) a temperature increaseassociated with the so-called isobaric heating (Ridolfi et al.,2010); (2) the slow magma ascent (or isothermal decompres-sion) related to decreasing water content and oxygen fugacityof the melt (e.g., Rutherford and Hill, 1993; Pichavant et al.,2002; Browne and Gardner, 2006); or, (3) likely amphibolexenocrysts from the magma chamber that have been incorpo-rated in the most recent magma (Gardner et al., 2002; Arceet al., 2005). Based on geobarometry data, the amphibolesfrom the Ubinas mafic andesites crystallized between 16and 19 km in depth (see below), and thus the reaction rimsmay result from the magma ascent towards the surface, asproposed by Rutherford and Hill (1993).

In sum, the afore-mentioned mineralogical features suggest ubiqui-tous evidence of mineral disequilibrium in the andesitic magma bodyproduced by replenishment of the reservoir with more primitive andhigh-Tmagmas. Although the 2006Ubinas samples donot showmacro-scopic evidence of magma mingling and mixing such as mafic enclaves(e.g. Bacon, 1986) or banded textures (e.g. Clynne, 1999), we proposethat magma recharge and subsequent mixing and homogenization arethe dominant petrogenetical process at the origin of the 2006 and AD1667 magmas. This view of Ubinas mafic andesite genesis is in agree-ment with recent studies suggesting that andesites are mixtures of


image of Fig.�11


Table 8Selected amphibole and olivine analyses for the 2006 Ubinas samples.

Sample Ub06-04S Ub-13 Ub-14 Ub-14 Ub-14 Ub-14 Ub-14 Ub-10-18c

Ub-10-18c

Ub-10-18c

Ub-10-18c

Ub06-04 Ub06-04 Ub-14 Ub-14 Ub-14 Ub-14 Ub-14 Ub-14 Ub-14 Ub-14 Ub-13 Ub-13 Ub-13 Ub-13 Ub-18 Ub-18

Analyse Amp16

Amp33

Amp12

Amp13

Amp14

Amp33

Amp34

Amp 36 Amp 37 Amp 52 Amp 53 Ol 15 Ol 19 Ol 1 Ol 4 Ol 10 Ol 11 Ol 21 Ol 22 Ol 27 Ol 28 Ol 11 Ol 12 Ol 23 Ol 24 Ol 88 Ol 89

c c c c r c r c r c r c c c r c r c r c r c r r c c rSiO2

(wt.%)41.53 41.24 40.21 40.60 41.44 40.93 40.88 41.59 41.29 41.30 41.02 37.67 37.62 38.43 37.11 38.55 37.89 38.08 36.30 38.05 37.95 37.46 36.89 37.17 37.62 36.37 36.11

TiO2 3.27 3.05 3.42 3.21 3.17 3.41 3.23 3.53 3.49 3.67 3.76 0.03 0.01 0.02 0.05 0.00 0.03 0.01 0.03 0.04 0.16 0.03 0.00 0.00 0.04 0.06 0.01Al2O3 13.04 12.59 13.14 13.25 12.73 12.67 12.64 12.50 12.51 12.67 12.82 0.00 0.04 0.00 0.04 0.05 0.03 0.04 0.01 0.00 0.01 0.00 0.09 0.01 0.02 0.01 0.02Cr2O3 0.00 0.11 0.02 0.00 0.16 0.01 0.01 0.04 0.08 0.17 0.08 0.00 0.00 0.07 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.01 0.00FeOt⁎ 11.81 10.72 11.12 11.35 10.69 11.90 11.86 9.83 10.20 9.77 10.61 26.57 22.70 21.31 30.34 22.18 25.30 21.50 31.47 24.77 25.76 28.37 30.89 31.07 30.14 34.39 34.47MnO 0.09 0.09 0.18 0.11 0.07 0.10 0.14 0.08 0.11 0.14 0.09 0.51 0.38 0.32 0.66 0.29 0.46 0.35 0.75 0.35 0.44 0.72 0.78 0.75 0.66 0.75 0.76MgO 13.96 14.24 13.84 13.82 14.59 13.69 13.61 14.68 14.72 14.63 14.16 35.52 38.15 38.98 31.43 38.71 36.15 39.16 30.92 37.03 36.32 33.36 30.38 31.41 31.96 28.56 28.62CaO 11.33 11.54 11.42 11.13 11.48 11.41 11.71 11.49 11.70 11.50 11.57 0.15 0.11 0.16 0.24 0.17 0.16 0.13 0.22 0.15 0.13 0.19 0.23 0.16 0.22 0.17 0.22Na2O 2.43 2.27 2.36 2.45 2.40 2.32 2.42 2.36 2.33 2.59 2.55 0.01 0.00 0.00 0.02 0.04 0.05 0.00 0.00 0.01 0.00 0.00 0.00 0.05 0.02 0.00 0.03K2O 0.81 0.78 0.79 0.75 0.79 0.73 0.77 0.86 0.90 0.83 0.85 0.00 0.02 0.00 0.00 0.02 0.00 0.01 0.01 0.02 0.02 0.03 0.10 0.01 0.00 0.00 0.04Total 98.28 96.64 96.49 96.68 97.52 97.18 97.26 96.96 97.34 97.26 97.51 100.46 99.04 99.30 99.89 100.03 100.09 99.30 99.70 100.42 100.79 100.17 99.36 100.61 100.69 100.31 100.28Mg# 81.94 82.34 82.30 83.54 84.57 80.26 77.20 82.70 82.89 81.93 79.26 70.44 74.96 76.53 64.88 75.7 71.8 76.4 63.7 72.7 71.6 67.7 63.7 64.3 65.4 59.7 59.7

Amp: Amphibole, Ol: Olivine; c: phenocryst core, r: phenocryst rim.⁎ all iron as FeO.

Table 9Selected Fe–Ti oxide and glass analyses for the 2006 Ubinas samples.

Sample Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub06-04 Ub-13 Ub-13 Ub-13 Ub-14 Ub-14 Ub-14 Ub-14 Ub-14 Ub-14 Ub-18 Ub-18 Ub-18 Ub-13 Ub-13 Ub-13 Ub-13 Ub-13 Ub-14 Ub-14 Ub-14

Analyses Ox 10 Ox 16 Ox 20 Ox 22 Ox 23 Ox 8 Ox 36 Ox 40 Ox 5 Ox 29 Ox 37 Ox 38 Ox 39 Ox 40 Ox 65 Ox 66 Ox 70 G 32 G 33 G 34 G 13 G 14 G 6 G 7 G 9

c c c c c c c c c c c r c r c c c

SiO2 (wt.%) 0.09 0.06 0.08 0.05 0.09 0.09 0.07 0.06 0.12 0.03 0.13 0.13 0.10 0.17 0.12 0.11 0.08 66.14 66.59 65.65 67.81 67.88 67.06 67.85 68.12TiO2 13.59 15.36 12.28 11.96 12.35 14.13 12.16 16.39 11.27 11.15 13.59 13.67 13.37 13.49 16.74 16.35 16.74 1.46 1.35 1.35 0.79 0.87 1.05 1.44 1.26Al2O3 2.65 2.21 3.43 3.48 3.27 2.90 3.68 2.08 4.88 3.89 2.74 2.68 3.13 3.21 1.61 1.66 1.69 14.02 14.12 13.90 14.50 15.49 15.28 14.35 13.89Cr2O3 0.02 0.04 0.06 0.00 0.17 0.05 0.03 0.01 0.07 0.07 0.07 0.04 0.00 0.04 0.01 0.02 0.00 0.00 0.02 0.02 0.00 0.00 0.00 0.03 0.00MgO 2.16 2.25 3.10 2.81 2.87 2.65 2.91 2.58 3.77 2.84 2.38 2.50 2.71 2.64 1.68 2.11 2.03 0.70 0.65 1.13 0.60 0.45 0.43 0.51 0.63FeOa 75.05 73.39 75.71 75.2 75.91 74.70 75.04 73.37 72.97 75.18 75.12 75.20 74.30 74.32 72.98 73.28 74.35 4.45 4.14 5.46 3.58 3.21 3.54 3.71 3.87MnO 0.47 0.52 0.40 0.38 0.39 0.45 0.43 0.54 0.37 0.29 0.42 0.47 0.40 0.37 0.63 0.55 0.59 0.14 0.07 0.19 0.08 0.13 0.10 0.05 0.12CaO 0.03 0.00 0.06 0.07 0.02 0.00 0.03 0.11 0.10 0.35 0.01 0.08 0.04 0.09 0.25 0.08 0.00 1.82 1.54 2.17 1.62 2.39 2.05 1.21 1.26Na2O 0.00 0.00 0.00 0.00 0.01 0.04 0.02 0.03 0.01 0.00 0.00 0.00 0.01 0.03 0.00 0.00 0.00 4.08 4.04 3.93 4.67 4.78 4.19 4.33 4.45K2O 0.00 0.00 0.02 0.00 0.00 0.01 0.03 0.01 0.00 0.01 0.03 0.05 0.02 0.02 0.01 0.02 0.01 5.29 5.44 5.37 5.10 5.23 5.03 5.66 5.81Total 94.06 93.83 95.14 93.95 95.08 95.03 94.41 95.18 93.56 93.80 94.51 94.80 94.09 94.39 94.02 94.18 95.49 98.11 97.95 99.17 98.75 100.43 98.73 99.14 99.40

Ox: Fe–Ti oxide, G: Glass; c: microphenocryst core, r: microphenocryst rim.a All iron as FeO.

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Fig. 12. P (and depth)–T diagram for the amphiboles of theUbinas volcano. Points represent data for the scarce amphibole phenocrysts of the 2006 lavas and the grayfield encompasses therange of amphibole phenocrysts for the entire Ubinas edifice (after Rivera, 2010). The amphibole from the 2006 lavas corresponds to the deeper reservoir, at an estimated depth rangebetween 16 and 20 km. Dashed lines indicate the thermal stability curve of amphibole (Ridolfi et al., 2010). Depth values are obtained frompressure estimates, using the density for crustalrocks (see text).

Fig. 13. Schematic cross section illustrating the plumbing system below the Ubinassummit caldera.


mafic and silicicmagmas instead of products of fractional crystallizationprocess (Eichelberger et al., 2006; Reubi and Blundy, 2009). In theUbinas mafic andesitic magmas, the homogeneous whole-rock compo-sitions contrast with the heterogeneous mineral phases and composi-tions. Instead of a mixing process between a primitive mafic andesiteand more differentiated end-members, we therefore consider a mixingbetween different batches of andesitic magmas with different degreesof differentiation. Mixing represent the driving mechanism to eruptthese mixed andesites as proposed by Kent et al. (2010). This has prob-ably been the case of the AD 1667 and 2006–2009 Ubinas eruptions.

6.3. Magmatic plumbing system and eruption triggering mechanism

On the basis of the petrological study of the 2006 lavas, and togetherwith data from Thouret et al. (2005) and Rivera (2010), we propose amodel for the Ubinas magmatic plumbing system (Figs. 12 & 13).Phase equilibrium constraints and amphibole geobarometric estimatesindicate that the crystallization pressure was around 476 ± 36 MPa.Assuming a bulk density of 2700 kg/m3 for upper crustal rocks in theCentral Andes (Kono et al., 1989), themagma storage region is estimat-ed to be located between16 and19 kmbelow the summit (Fig. 12). Thisdepth range is consistentwith previous estimates of Rivera (2010), whoargued that a shallow reservoir between 4 and 7 kmwas likely locatedabove a deep magmatic reservoir between 9 and 20 km below Ubinas'summit. Our model is based on the size and style of the 2006–2009events (Table 2) and the petro-chemical characteristics of the eruptedproducts. We thus infer that a new batch of hot mafic magma ascendedfrom the deep chamber into the conduit or in the shallowmagma cham-ber containing a cooler andesitic magma (Fig. 13), hence remobilizingthe resident magma by the input of heat and/or volatiles. This magmainput triggered convection and homogenization of the shallow storage,as previously proposed for other andesitic composite cones such asSoufrière Hills (Murphy et al., 2000; Zellmer et al., 2003), Shiveluch(Humphreys et al., 2006) and Tungurahua (Samaniego et al., 2011).

Higher in the plumbing system, the magmatic conduits and thehydrothermal system of Ubinas have been recently constrained by theseismic study of Inza et al. (2011). The authors suggest that the sourceof the volcanic explosions in 2006 was located between 400 and1600 m below the vent, whereas a much deeper source of long-periodevents (2300–3700 m below the summit) is likely related to the over-pressurized hydrothermal system as previously proposed by Thouretet al. (2005). Recent geoelectrical (SP), audio-magnetotelluric and soil


image of Fig.�12

image of Fig.�13


New

Resaltado

New

Resaltado


gas surveys have enabled Gonzalez et al. (submitted for publication) todelineate an active 10 km-wide hydrothermal system located 1–3 kmbelow the caldera floor. Before the onset of the eruptive activity inMarch 2006, the ascending magma interplayed with the large pressur-ized hydrothermal system, and triggered the initial phreatomagmaticstyle (phase 1) as observed at Ngauruhoe in 1974–1975 (Naim andSelf, 1978). Evidence for such interactions is the occurrence of violent,‘canon-like’ explosions, the presence of ash containing secondaryminerals from the hydrothermally-altered conduits rocks (Table 3),and bread-crust juvenile bombs ejected at the onset of the eruption.

7. Conclusion

The most recent explosive activity of Ubinas began at the end ofMarch 2006, increased between April and October 2006, and wascharacterized by low tomoderate explosivity (VEI 2). The activity slow-ly declined until December 2009. This eruptive episode was character-ized by vulcanian explosions that expelled blocks and bombs into thesummit caldera and on the upper flanks of the volcano, and producedfrequent ash emissions with a regional impact. All 2006 samples havehomogeneous andesitic compositions (56.7–57.6 wt.% SiO2) and con-tain a mineral assemblage composed of plagioclase, orthopyroxene,clinopyroxene and magnetite, with scarce amphibole and olivinecrystals. Phenocryst textures and mineral chemistry indicate frequentdisequilibrium features such as mineral re-absorptions, overgrowthsrims and reversely zoned phenocrysts. All the features are commonlyascribed to reheating of a slowly cooling resident magma body by newhigh-T magma batch and subsequent magma mixing between them.Thermo-barometric results point to magmatic temperatures around1000 °C and pressures in the range of 450–500 MPa, which correspondto depths of 16–20 km below the summit. Therefore, the 2006–2009eruptive episode was probably triggered by the recharge and subse-quent magma mixing of a hot mafic andesitic magma into a coolerandesitic magma. However, given that the 2006 mafic andesites spana narrow range of chemical composition, an almost complete homoge-nization seems to be achieved. This process led to over-pressurizationof themagma chamber triggering themild 2006–2009 eruptive activity.The repeated ascent of small-volume magma batches from the shallowreservoir into the upper conduit is responsible for heating and subse-quently interacted with the hydrothermal system. These interactionspromoted brief but intense explosive events, which produced ballisticejecta and a small amount of fine-grained tephra. Small erupted tephravolumes of mafic andesite composition characterize the intermittenteruptive activity recorded at Ubinas in 2006–2009 and other historicaleruptions. Comparison with the AD 1667 products provides no furtherevidence for the presence of large magma bodies below Ubinas. Thissituation may characterize Ubinas activity over the past 500 years.

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.jvolgeores.2013.11.010.

Acknowledgments

This work stems from a PhD thesis by the first author, funded by the‘Institut de Recherche pour le Développement’ (IRD) hosted at LMV andthe Instituto Geológico Minero y Metalúrgico of Perú (INGEMMET). Wethank J. Cotten and C. Liorzou (Université of Bretagne Occidentale) andG. Wörner (University of Göttingen) for providing the geochemicalanalyses. We thank our colleagues of the INGEMMET Volcano Observa-tory in Arequipa for fieldwork assistance and the multi-institutionalscientific committee for sharing information during the Ubinas crisisin 2006. S. Carn has provided us with the OMI satellite SO2 data andJ.-L. Devidal has helped us with the microprobe analyses. Constructivecomments from Editor M. Rutherford, B. Scaillet and an anonymous re-viewer have significantly improved the manuscript. This is Laboratoryof Excellence ClerVolc contribution no. 78.


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