Eruptive Activity at Turrialba Volcano (Costa Rica): Inferences 10.1002/2016GC006525 from 3He/4He in Fumarole Gases and Chemistry of the Products

PUBLICATIONS

Geochemistry, Geophysics, Geosystems

RESEARCH ARTICLE Eruptive activity at Turrialba volcano (Costa Rica): Inferences 10.1002/2016GC006525 from 3He/4He in fumarole gases and chemistry of the products

Key Points: ejected during 2014 and 2015 Eruptive activity resumed at Turrialba volcano (Costa Rica) in 2010 with Andrea Luca Rizzo1, Andrea Di Piazza2, J. Maarten de Moor3,4, Guillermo E. Alvarado5,6, sporadic explosions that increased in Geoffroy Avard3, Maria Luisa Carapezza2, and Mauricio M. Mora6,7 frequency since October 2014 3 4 New data of He/ He in fumarole gases since September 2014 and 1Istituto Nazionale di Geoﬁsica e Vulcanologia, Sezione di Palermo, Palermo, Italy, 2Istituto Nazionale di Geoﬁsica e chemistry of the products erupted 3 between October 2014 and May Vulcanologia, Sezione di Roma1, Roma, Italy, Observatorio Vulcanologico y Sismologico de Costa Rica (OVSICORI), Apdo. 2015 2368-3000, Universidad Nacional, Heredia, Costa Rica, 4Department of Earth and Planetary Sciences, University of New Eruptive activity was triggered by the Mexico, Albuquerque, New Mexico, USA, 5Instituto Costarricense de Electricidad, Apdo. 10032-1000, San Jose, Costa Rica, supply of the plumbing system by a 6Red Sismologica Nacional, Apdo. 2060, San Jose, Costa Rica, 7Escuela Centroamericana de Geologıa, Universidad de 3He-rich magma which led to an increasing juvenile component Costa Rica, Apdo. 2060, San, Jose, Costa Rica

Correspondence to: Abstract A new period of eruptive activity started at Turrialba volcano, Costa Rica, in 2010 after almost A. L. Rizzo, [email protected] 150 years of quiescence. This activity has been characterized by sporadic explosions whose frequency clearly increased since October 2014. This study aimed to identify the mechanisms that triggered the

Citation: resumption of this eruptive activity and characterize the evolution of the phenomena over the past 2 years. 3 4 Rizzo, A. L., A. Di Piazza, J. M. de Moor, We integrate He/ He data available on fumarole gases collected in the summit area of Turrialba between G. E. Alvarado, G. Avard, 1999 and 2011 with new measurements made on samples collected between September 2014 and M. L. Carapezza, and M. M. Mora (2016), Eruptive activity at Turrialba February 2016. The results of a petrological investigation of the products that erupted between October volcano (Costa Rica): Inferences from 2014 and May 2015 are also presented. We infer that the resumption of eruptive activity in 2010 was 3 4 He/ He in fumarole gases and triggered by a replenishment of the plumbing system of Turrialba by a new batch of magma. This is chemistry of the products ejected 3 4 during 2014 and 2015, Geochem. supported by the increase in He/ He values observed since 2005 at the crater fumaroles and by Geophys. Geosyst., 17, doi:10.1002/ comparable high values in September 2014, just before the onset of the new eruptive phase. The presence 2016GC006525. of a number of fresh and juvenile glassy shards in the erupted products increased between October 2014 and May 2015, suggesting the involvement of new magma with a composition similar to that erupted in Received 7 JUL 2016 1864–1866. We conclude that the increase in 3He/4He at the summit fumaroles since October 2015 Accepted 13 OCT 2016 represents strong evidence of a new phase of magma replenishment, which implies that the level of activity Accepted article online 17 OCT 2016 remains high at the volcano.

1. Introduction Turrialba is the southernmost active volcano of the Central America Volcanic Front and is located about 35 km east of San Jose, the capital of Costa Rica (Figure 1). The last eruption prior to 2010 occurred during 1864–1866 [Reagan et al., 2006]. After almost 150 years of quiescence, the volcano entered an unrest phase in 1996 that was initially characterized by anomalous seismicity [Martini et al., 2010]. Sporadic eruptions have occurred at the southwest crater since January 2010, with the opening of a new vent. Afterward the frequency has increased over time and has involved the eruption of fragments of altered preexisting material comprising a small percentage of a juvenile component [Reagan et al., 2011]. The reawakening of Tur- rialba volcano represents a serious hazard due to its proximity to both the capital city and main international airport of Costa Rica, which is only 39 km from the volcano. Preparedness is as important as adequate monitoring of this volcanic activity to minimizing its impact [van Manen, 2014; van Manen et al., 2015]. Nevertheless, forecasting the resumption of eruption at volcanoes is not obvious, above all during the initial vent-opening phases of eruptive activity at long dormant volcanoes. This is especially the case of Turrialba volcano, where vent-opening eruptions have been ongoing since 2010. Indeed, while large magmatic erup-

VC 2016. American Geophysical Union. tions are often preceded by geophysical and geochemical precursory signals, there is still very little docu- All Rights Reserved. mented evidence worldwide of precursory geophysical (i.e., seismicity, deformation, and gravimetry) or

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 1 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Figure 1. (a) Map of the southern sector of Turrialba volcano displaying the location of Falla Ariete and the gas sampling site. The top-left insert shows an image of the eruption that occurred on 13 March 2015. The area indicated in the top right is shown magniﬁed in Figure 1b. (b) Summit crater area with the most important morphological features indicated: the position of the vents that have opened during the past 4 years and the sampling sites for gases and eruptive products. The white square shows the position of the CVTR seismic station (Red Sismologica Nacional [RSN: UCR-ICE]).

geochemical (gas composition and gas flux) signals prior to phreatic or phreatomagmatic eruptions. Among the geochemical tools used for volcano monitoring, the 3He/4He measured in gases emitted from active volcanic area seems to have enormous potential for predicting nonmagmatic unrest. Sano et al. [2015] reported a decadal-scale increase in the 3He/4He toward more-magmatic values at Ontake volcano prior to the fatal phreatic blast that claimed 57 lives in 2014, which apparently represented the only long-to- medium-term precursory signal recorded by the local monitoring network. Similar increases in the 3He/4He—but on a time scale of months to weeks—have been documented at other active volcanoes worldwide, including Mt. Etna (Italy) [Caracausi et al., 2003; Rizzo et al., 2006; Paonita et al., 2012, 2016], Stromboli (Italy) [Capasso et al., 2005; Rizzo et al., 2009, 2015a], Ontake (Japan) [Sano et al., 2015], and Santo- rini (Greece) [Rizzo et al., 2015b]. These observations demonstrate the enormous potential of this tracer in predicting both magmatic and phreatic to phreatomagmatic volcanic unrest, so it could reveal fundamental also at Turrialba volcano. In addition, de Moor et al. [2016a] demonstrated using data obtained at Poas volcano, Costa Rica, that high-frequency gas monitoring is a powerful tool for identifying short-term precursory variations in gas emissions prior to phreatic eruptions. Most recently, de Moor et al. [2016b] show that plume gas compositions as measured by permanent Multi- GAS station located near the active crater at Turrialba show short-term precursory changes in gas composition prior to eruptive episodes at Turrialba. These variations in gas composition measured in the main gas plume reflect dynamic interactions between the hydrothermal system and magmatic gases derived from ascending magma bodies. However, more frequent eruptive activity is making maintenance of this Multi- GAS station risky and unfeasible. In this study, we focus on geochemical monitoring of fumaroles on the flank of the volcano, where gas samples can be collected safely.

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 2 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

After the onset of eruptive activity, the petrological study of the products that erupt from the volcano is fundamental, because it represents a complementary suite of information that can be used to identify the pre- eruptive conditions and processes. In fact, identifying a juvenile component in the erupted products is crucial to distinguishing magmatic or phreatomagmatic events from purely phreatic ones [e.g., Barberi et al., 1992; Nakada et al., 1995; Cashman and Hoblitt, 2004; Alvarado et al., 2016]. Cashman and Hoblitt [2004] identified a juvenile component in the precursory ash that erupted from Mount St. Helens 3 months before the onset of the eruption on 18 May 1980. However, Pardo et al. [2014] highlighted uncertainties in distinguishing phreatomagmatic from phreatic products due to the extreme difficulty of truly differentiating between the juvenile component from well-preserved material from previous eruptions. Those authors argued that excluding the presence of fresh magma input within an eruption from the study of ash deposits only is not straightforward, and that strong evidence that new magma has not erupted is the lack of particles with a distinct glass composition. The problem of distinguishing phreatomagmatic from phreatic ash is very relevant to the Turrialba eruptive activity, since a crucial question is whether the current activity will progress in a final magmatic stage. This article presents results from new 3He/4He measurements of fumarole gases collected in the summit area of Turrialba starting 1 month before the explosions resumed in October 2014. These data are compared with similar measurements carried out since 1999 with the aim of assessing the events that triggered the recent unrest and characterizing the current state of activity. We also present detailed petrological and geochemical analyses of the products that erupted between October 2014 and May 2015 and compare them with the scoriae and ashes that erupted during 1864–1866. The main aim is to propose a likely evolutionary scenario for the current eruptive activity and to evaluate the related hazard implications.

2. Chronology of Turrialba Volcano Reactivation Turrialba volcano has experienced at least six magmatic eruptive periods during the past 3400 years. The most-recent eruption prior to 2010 occurred during 1864–1866 and was characterized by a sequence of phreatic explosions that transitioned to phreatomagmatic activity and climaxed with Strombolian eruptions [Reagan et al., 2006]. After that eruptive phase, the volcano entered a quiescence state that lasted until the late 1990s. The recent reactivation of Turrialba was characterized by increasing seismicity, ground deformation, and fumarole activity during the mid-1990s to early 2000s, and during 2005–2008 by the expansion of fumarole fields between and within the central and southwest craters, and along the western and south- western outer flanks of the edifice [Martini et al., 2010; Vaselli et al., 2010]. These structural changes have

been accompanied by increases in the temperature and concentration of magmatic ﬂuids (SO2, HCl, and

HF) at the crater fumaroles since 2005 [Vaselli et al., 2010], and by a noticeable increase in the SO2 flux by up to 2 orders of magnitude since late 2007 [Martini et al., 2010; Conde et al., 2013], which strongly indicate an enhanced degassing of magmatic fluids. The seismic activity and gas flux peaked during 2009 and 2010 [Conde et al., 2013], with the first explosive event occurring on 5 January 2010 [Reagan et al., 2011; Campion et al., 2012], which opened a vent inside the southwest crater. Ash emissions occurred in January 2012, opening new fractures with high-temperature fumaroles from which ash emissions took place again in May 2013. The activity at Turrialba escalated on 30 October 2014 with an energetic explosion at 11:35 P.M. (local time) that caused the collapse of the eastern side of the southwest-crater wall and the ejection of blocks up to 2 m in diameter near the eruptive vent. Ash emissions continued for 2 weeks, and a more-energetic explosion occurred on 9 December. This period was followed by 3 months of strong degassing and seismicity. Another eruptive phase started on 8 March 2015, with >32 intermittent ash emissions events occurring on at least 20 days up to 18 May. The most-energetic explosions during this period occurred on 12 March (Fig- ure 1a), 7 April, 24 April, and on 6 May. Despite further weak ash emission events occurring in June and August, the volcano remained quiet until explosive activity resumed for 2 weeks during mid-October 2015 with ash columns (500 m high) and small pyroclastic surges affecting the summit area (OVSICORI-UNA, 2015). The activity then increased significantly from January to July 2016 (when this manuscript was submit- ted for publication), with the occurrence of several ash eruptions.

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 3 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Figure 2. (a) The products that erupted during 2014 and 2015 are characterized by a light gray, heterogeneous basal component containing abundant hydrothermal minerals, while the top ﬁne ash layer is darker, denser, and more homogeneous. The middle orange altered layer corresponds to the hiatus between the 2014 and 2015 eruptive episodes. The alteration occurred because of a long exposure to the gas plume. (b) The products of the 1864–1866 eruption are well preserved in the crater region and show orange basal phreatic units cov- ered by thinly bedded gray phreatomagmatic deposits and a massive scoriaceous unit. The scale bar reported at the right of the strati- graphic sketch is indicative and does not correspond with the height on the left column foe which the scale can be evaluated considering the hammer size.

3. Methods 3.1. Sampling of Gases and Ashes The high-temperature fumarole on the western rim of the southwest crater has been sampled twice (in Sep- tember 2014 and February 2015, when its temperature was 2108C and 1648C, respectively), while a low- temperature site (boiling temperature) along the active fault called Falla Ariete has been sampled 6 times (from January 2015 to February 2016) (Figure 1). We determined the 3He/4He in the gas samples at the noble gases isotope laboratory of Istituto Nazionale di Geoﬁsica e Vulcanologia (INGV) in Palermo, Italy. The geochemical characterization of these fumaroles and their magmatic-derived signature have been widely investigated in previous studies [e.g., Tassi et al., 2004; Hilton et al., 2010; Vaselli et al., 2010; Di Piazza, 2014; Di Piazza et al., 2015]. We collected six samples of ashes and lapilli from the explosions that occurred during October–December 2014 and on 12 March, 7 April, and 6 May 2015. These samples were collected in the summit area of the volcano within a few hundred meters of the eruptive vent by digging pits in fresh deposits, or by sampling ash from the soil immediately after the explosive events (Figure 2a).

3.2. Analytical Procedures 3.2.1. 3He/4He Measurements in Gases The element and isotope compositions of both He and Ne in fumaroles were measured by admitting the gases into an ultra-high-vacuum (1029 to 10210 mbar) puriﬁcation line, in which all of the species in the gas mixture except noble gases were removed. Before isotope analysis, He (3He and 4He) and Ne (20Ne) isotopes were separated from Ar by adsorbing the latter in a charcoal trap cooled by liquid nitrogen (77 K). He and Ne were then adsorbed in a cryogenic trap connected to a cold head cooled with an He compressor to <10 K. He was desorbed at 42 K and admitted into a GVI-Helix SFT mass spectrometer. After restoring the ultra high vacuum in the cryogenic trap, Ne was released at 82 K and then admitted into a Thermo-Helix MC Plus mass spectrometer. The same procedure was adopted for the He-isotope and Ne-isotope measurements of the air standard.

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 4 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Table 1. 3He/4He (in Units of R/Ra, and in Units of Rc/Ra After Correction for Air Contamination), the 4He/20Ne, and the Temperature of Fumarole Gases Collected at the Turrialba Summit Area From 2005 to 2016a Date Temperature Err 6 Sampling Site (mm/dd/yy) (8C) R/Ra 4He/20Ne Rc/Ra (Rc/Ra) This Study Southwest Crater 09/05/14 210.0 7.94 902.5 7.94 0.06 Southwest Crater 02/04/15 164.0 7.48 890.2 7.49 0.06 Falla Ariete 01/27/15 7.46 18.8 7.57 0.06 Falla Ariete 05/27/15 89.2 7.08 3.7 7.65 0.08 Falla Ariete 09/18/15 5.24 0.9 7.61 0.07 Falla Ariete 10/28/15 6.82 2.3 7.75 0.08 Falla Ariete 12/18/15 6.75 1.8 8.00 0.07 Falla Ariete 02/10/16 6.01 1.1 7.93 0.06 Di Piazza et al. [2015] West Crater (HT) 03/16/11 456.0 7.88 44.8 7.93 0.08 West Craterb 03/20/11 90.0 7.92 60.6 7.96 0.08 Central Crater 03/20/11 83.0 7.52 7.4 7.82 0.09 Central Crater 03/20/11 70.3 7.77 170.7 7.78 0.10 Central Crater 03/20/11 85.0 7.81 30.8 7.88 0.08 Vaselli et al. [2010] West Craterb Feb-99 7.33 144.0 7.34 West Craterb Apr-05 90.0 7.32 1496.0 7.32 West Craterb Mar-08 92.0 7.66 680.0 7.66 West Crater (HT) Mar-08 278.0 7.58 5571.0 7.58 Central Crater Sep-01 89.0 7.48 1496.0 7.48 Ariete Fault Mar-08 94.0 7.71 18.0 7.84 Hilton et al. [2010] West Craterc Jan-01 90.0 7.73 7.74 0.07 West Craterc Mar-01 90.0 8.10 191.4 8.01 0.10 West Craterc Jul-01 84.0 7.69 179.7 7.70 0.07 West Craterd Jul-09 92.0 7.68 558.7 7.68 0.20 Central Crater Jun-05 88.0 6.53 5.7 6.85 0.20 Central Crater Jun-07 88.0 7.44 132.6 7.48 0.30 Central Crater Jan-09 89.0 7.09 872.0 7.09 0.20

aData for 2005–2011 are from Hilton et al. [2010], Vaselli et al. [2010], and Di Piazza et al. [2015]. HT indicates high-temperature fumarole collected in the inner rim of southwest crater and corresponds to our sampling site. bFumarole collected at the top of the southwest-crater rim. cFumarole collected at the top of the inner walls of the southwest-crater rim. dFumarole collected at the top of the outer walls of the southwest-crater rim.

Values of the 3He/4He are expressed in units of R/Ra (where Ra is the 3He/4He of air, which is equal to 1.39 3 1026) and are corrected for atmospheric contamination based on the 4He/20Ne ratio [e.g., Sano and Wakita, 1985]. Hereafter we report the 3He/4He ratio corrected for atmospheric contamination in units of Rc/Ra. In the gas samples collected in this study, the 4He/20Ne ratio ranges from 0.9 to 902 (air 4He/20Ne 5 0.318), with samples collected from September 2015 onward showing the lowest ratios (4He/20Ne 2.3; Table 1). These samples display the largest difference between raw and corrected 3He/4He values (up to 2.3 Ra), but the relative variations observed between September 2015 and February 2016 (see section 4.1) can be considered accurate because they were recorded at comparable 4He/20Ne values (Table 1). The errors were generally within 60.06 Ra. Typical blanks for He and Ne were <10214 and <10216 mol, respectively. Further details about the sampling and analytical procedures are available in Di Piazza et al. [2015] and Rizzo et al. [2015b]. 3.2.2. Petrological and Geochemical Analyses of Ashes Sample preparation, observation of polished sections, and chemical analyses were carried out at the INGV laboratories in Rome, Italy. Ashes and lapilli were dried at room temperature (258C), weighed, and sieved at half-/ intervals. Sieved samples were cleaned using several bath cycles in distilled water and thereafter in acetone. Under a binocular microscope, 50 particles of the freshest-looking component with a grain size of 2 < / < 1 were selected from each sample. Glassy particles were embedded in resin, ground down to expose the particles, polished (until a 0.1 lm-grain polycrystalline diamond slurry), and coated with carbon. Backscattered-electron (BSE) images of polished sections were obtained using scanning electron microscopy. Chemical analyses of glasses (from the 2014 and 2015 explosions; Table 2) and minerals (from the 2015 explosions only; Table 3) were performed using an electron microprobe (JXA8200, JEOL) combined with energy-dispersive and wavelength-dispersive spectrometer detectors (composed of 5 spectrometers

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 5 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Table 2. Results From Electron Microprobe Analyses of Residual Glass of Turrialba Ashes That Erupted During 2014 and 2015a

Sample N SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2OP2O5 SO3 Total 6-May-15 cl1 5 55.4 2.7 14.3 9.1 0.17 4.8 8.5 2.5 1.5 0.83 0.020 99.8 cl4 5 55.9 2.8 14.1 9.5 0.13 4.7 8.1 2.6 1.3 0.87 0.028 99.9 cl2 5 59.7 2.1 14.3 7.6 0.21 3.0 5.3 3.1 3.6 0.99 0.002 99.9 cl6 6 60.0 2.0 13.8 8.1 0.17 2.7 5.2 3.4 3.4 1.00 0.013 99.8 cl3 5 57.0 3.2 13.3 10.1 0.11 4.3 5.7 3.2 2.0 0.93 99.9 cl5 4 58.5 2.1 13.2 9.5 0.21 3.3 5.9 2.6 3.6 0.87 0.018 99.9 cl7 5 58.0 2.0 13.9 9.3 0.18 3.4 6.5 2.9 2.9 0.86 0.011 99.9 cl9 4 58.3 2.3 13.7 8.4 0.14 3.4 6.1 3.4 3.3 0.94 0.024 99.9 cl8 3 59.1 1.9 14.0 9.0 0.18 3.0 5.5 3.1 2.8 0.93 0.026 99.8

7-Apr-15 cl20 5 58.9 2.0 13.6 8.7 0.18 3.2 5.8 3.4 3.2 0.90 0.023 99.9 cl22 4 60.2 2.0 14.1 8.9 0.19 3.2 3.7 3.3 3.4 0.85 0.035 99.9 cl23 4 59.1 2.0 13.6 8.7 0.18 3.1 5.8 3.6 3.0 0.81 0.015 99.9 cl24 4 59.7 2.0 13.5 8.4 0.18 2.8 5.4 3.9 2.9 1.04 0.011 99.9 cl21 4 57.3 2.0 13.6 9.2 0.16 3.4 6.8 2.8 3.8 0.78 0.014 99.9 cl19 5 57.5 1.8 14.5 8.5 0.18 3.8 6.5 3.2 3.0 0.79 0.017 99.8

3-Mar-15 cl10 3 65.5 1.5 14.2 5.8 0.13 1.7 3.6 3.1 3.7 0.66 0.015 99.8 cl11 4 58.7 1.6 14.1 8.5 0.23 3.6 6.5 3.3 2.7 0.60 0.013 99.9 cl14 4 76.7 0.7 11.7 1.8 0.00 0.2 0.6 2.7 5.3 0.23 0.017 99.9

October 2014 cl1 2 56.7 1.9 14.8 9.1 0.21 3.3 5.7 4.0 3.2 0.82 99.8 cl3 2 54.1 3.1 14.1 9.8 0.17 4.2 6.5 4.3 2.7 0.93 0.038 99.9 cl4 2 58.1 1.8 14.6 8.4 0.17 2.9 5.3 4.3 3.3 0.83 0.060 99.8 cl5 2 58.4 2.3 14.2 9.2 0.17 3.2 5.6 2.8 2.7 0.93 0.037 99.7 cl6 2 57.1 1.8 14.6 8.3 0.15 2.9 5.9 3.6 4.6 0.89 0.072 99.9 cl7 2 56.6 2.1 14.2 9.5 0.11 3.2 6.2 3.9 3.0 0.82 0.195 99.8 cl8 2 57.1 1.8 14.7 8.9 0.17 2.9 5.9 3.8 3.6 0.93 0.073 99.9 cl9 2 55.9 1.7 14.5 10.4 0.19 3.6 6.4 3.2 2.8 1.08 0.244 99.8 cl10 2 57.8 2.0 14.2 9.3 0.15 3.4 6.4 2.8 2.6 0.92 0.098 99.8 cl11 2 56.9 1.9 14.6 9.0 0.20 3.1 6.0 4.0 3.5 0.76 0.001 99.8 cl12 2 57.6 2.0 14.7 8.6 0.07 3.1 6.0 3.6 3.2 0.87 0.112 99.8 cl13 2 61.9 2.2 13.7 9.1 0.18 2.3 4.9 1.5 2.9 1.01 0.186 99.8 cl14 2 59.6 2.2 15.1 8.9 0.16 2.3 4.9 2.1 3.4 1.05 0.150 99.9 cl15 3 57.4 2.1 14.9 8.6 0.09 3.1 6.1 3.6 3.1 0.83 0.064 99.9 cl16 2 59.0 2.2 14.1 9.1 0.21 3.1 6.9 2.0 2.6 0.77 0.061 99.9

November 2014 cl1 2 56.6 1.9 14.2 9.7 0.25 3.3 6.5 4.0 2.7 0.69 0.099 100.0 cl2 2 62.8 1.5 14.5 6.3 0.19 2.0 3.6 4.4 3.7 0.80 99.9 cl3 2 53.8 2.5 14.6 9.9 0.13 4.6 8.2 3.7 1.4 0.75 0.061 99.8 cl4 2 57.4 2.1 14.3 8.7 0.16 2.9 5.4 4.0 3.8 0.96 0.086 99.7 cl5 3 55.6 2.8 14.6 9.2 0.26 4.0 7.3 4.2 1.0 0.95 100.0 cl6 3 63.9 1.3 14.5 6.1 0.03 1.7 3.4 4.1 4.1 0.73 0.086 99.9 cl7 2 58.6 2.1 14.5 9.5 0.19 3.1 6.8 2.4 1.7 0.92 0.098 99.9 cl8 2 55.7 1.9 14.6 10.1 0.24 3.3 6.1 3.8 3.3 0.80 0.036 99.9 cl9 2 58.3 2.0 14.9 9.5 0.10 3.2 5.9 2.3 2.8 0.97 0.086 99.9 cl10 2 59.7 2.0 15.2 9.5 0.11 3.2 6.4 0.9 1.6 0.89 0.099 99.7 cl11 3 63.0 1.5 15.0 6.1 0.12 1.7 3.9 4.3 3.5 0.57 0.124 99.8 cl12 2 56.1 2.7 15.2 9.5 0.22 4.1 5.9 3.0 2.2 0.86 99.9 cl13 2 53.7 2.9 14.3 9.4 0.16 4.9 8.7 3.8 1.4 0.81 0.001 100.0 cl14 2 56.6 3.5 14.1 11.3 0.33 4.3 5.1 1.2 2.3 1.05 0.112 100.0 cl15 2 54.8 3.0 14.3 10.4 0.19 4.8 7.1 2.8 1.7 0.80 0.125 100.0 cl16 2 54.0 3.1 14.0 10.6 0.25 4.8 8.4 2.5 1.3 0.99 0.086 100.0 cl17 2 55.2 1.7 16.0 8.7 0.07 3.8 7.0 3.7 2.7 0.66 0.187 99.7 cl18 2 58.7 2.2 14.7 9.7 0.15 3.2 6.8 2.1 1.7 0.72 0.102 100.0 cl19 2 56.3 3.6 14.0 11.5 0.20 4.2 6.2 1.0 1.8 0.92 0.113 99.9 cl20 2 55.8 2.9 14.6 10.1 0.09 4.5 6.5 2.6 2.0 0.87 99.9

December 2014 cl1 2 56.3 2.0 13.9 9.6 0.18 3.3 6.2 3.5 3.9 0.76 0.013 99.7 cl2 2 58.0 2.0 14.2 9.0 0.15 2.8 6.0 3.8 3.0 0.91 99.8 cl4 2 57.8 2.1 14.0 8.8 0.15 3.1 6.0 3.8 3.1 0.90 0.038 99.9 cl5 2 57.0 2.0 14.3 9.3 0.24 3.1 6.0 3.7 3.2 0.93 0.088 99.7 cl6 2 56.1 1.8 15.4 9.0 0.30 3.7 6.6 3.3 2.8 0.78 0.000 99.9

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 6 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Table 2. (continued)

Sample N SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2OK2OP2O5 SO3 Total cl7 2 55.3 1.8 15.7 9.1 0.11 3.9 7.1 3.6 2.6 0.61 0.173 99.9 cl8 2 57.0 1.5 15.7 8.5 0.09 3.5 6.5 3.3 2.9 0.79 0.063 99.9 cl9 2 54.4 3.3 13.9 10.5 0.23 4.7 8.0 2.2 1.6 0.86 99.7 cl10 2 57.2 2.3 14.4 9.4 0.22 3.1 5.7 3.5 2.9 0.93 0.101 99.8 cl11 2 55.2 1.8 15.8 8.8 0.17 4.0 7.1 3.4 2.7 0.78 0.063 99.9 cl12 2 57.6 2.2 14.4 9.3 0.17 3.2 5.4 3.4 3.3 0.90 0.127 99.9 cl13 2 55.7 1.8 15.1 9.8 0.20 3.6 6.7 3.6 2.8 0.85 100.0 cl14 2 54.8 3.5 13.9 9.9 0.20 4.2 7.7 3.0 1.8 0.88 0.063 100.0 cl15 2 56.1 1.6 15.8 9.0 0.16 4.0 6.8 3.2 2.8 0.66 100.0

aMajor oxides are reported as wt %. N 5 number of analysis of which we reported the average values.

with 12 crystals). We used a slightly defocused beam with a size of 5 lm, with a counting time of 5 s for the background and 15 s for the peak. 3.2.3. Daily Dominant Frequency of Seismic Signals Continuous seismic record from CVTR station of the Red Sismologica Nacional (RSN: UCR-ICE) was passed through a fourth-order Butterworth band-pass ﬁlter from 1 to 45 Hz. The signal was then divided in 10 min- long samples, to which the fast Fourier transform was applied and the dominant frequency was then calculated between 1 and 25 Hz. Each sample was tapered using a Hann function. The mean, standard deviation, and mode were calculated daily.

4. Results and Discussion 4.1. 3He/4He in Fumarole Gases Figure 3a shows the 1999–2016 time series of the 3He/4He at Turrialba fumaroles located at the central and southwest craters as well as those located at Falla Ariete (Figure 1). Analytical data and fumarole tempera- tures are shown in Table 1. Data from 1999 to 2011 are from Hilton et al. [2010], Vaselli et al. [2010], and Di Piazza et al. [2015], and those from September 2014 to February 2016 were obtained in the present study. The 3He/4He at the fumaroles ranges from 7.32 to 8.01 Ra at the southwest crater (1999–2015), from 6.85 to 7.88 Ra at the central crater (2001–2011), and from 7.57 to 8.00 Ra at Falla Ariete (2008–2016) (Figure 3). In detail, the 3He/4He at the fumaroles from the central crater decreased from 7.48 in 2001 to 6.85 Ra in 2005 and increased (with some variability) from 6.85 Ra in 2005 to 7.88 in 2011. At the southwest crater, the 3He/4He increased from 7.34 Ra in 1999 to 8.01 Ra in 2001, followed by a decrease to 7.32 Ra in 2005. There- after it increased again to 7.96 Ra in 2011 and 7.94 Ra in 2014, and then decreased to 7.49 Ra in the single sample collected in 2015 (Figure 3a). Increasingly dangerous conditions in the crater region since 2015 forced us to stop sampling the southwest-crater fumaroles. Thus, since February 2015 the sample collection focused on the Falla Ariete boiling-temperature fumaroles, which previously displayed 3He/4He of 7.84 Ra in 2008 and varied from 7.57 to 8.00 Ra during 2015 and 2016, with a clearly increasing trend from October 2015 onward (Figure 3b). The maximum 3He/4He were recorded in March 2011 (7.93–7.96 Ra) [Di Piazza et al., 2015] and September 2014 (7.94 Ra) at the Southwest Crater, and in December 2015 (8.00 Ra) at Falla Ariete. The morphology of the summit area has been progressively modified since the beginning of the eruptive activity in January 2010, leading to the disappearance of the central-crater fumarole field in October 2014. The lower and more-variable 3He/4He measured in these fumaroles are consistent with a larger contribution of crustal 4He at the area, possibly reflecting a contamination of magmatic gases by hydrothermal fluids or by interaction with crustal rocks [Hilton et al., 2010; Vaselli et al., 2010], or it could be related to the low flux that characterized this fumarole field. However, the general trend of the 3He/4He variation is similar to that at the southwest fumaroles (Figure 3a). Data collected over many years demonstrate that the magmatic 3He/4He signatures in fumarole gases at Turrialba volcano are strongest at the southwest crater, irrespective of whether the gases have a high or low temperature [Hilton et al., 2010; Vaselli et al., 2010; Di Piazza et al., 2015]. The fumaroles at Falla Ariete exhibit comparable or even higher 3He/4He values than the crater fumaroles (Figure 3), which allowed us to sample gases representative of the magmatic source in safer conditions (Table 1).

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 7 IZ TA.EUTV CIIYA URAB OCN 8 VOLCANO TURRIALBA AT ACTIVITY ERUPTIVE AL. ET RIZZO Table 3. Results of Electron Microprobe Analyses of Olivine, Pyroxene, and Plagioclase Crystals of the 2015 Samples Onlya Date 03/12/15 03/12/15 03/12/15 03/12/15 03/12/15 03/12/15 03/12/15 03/12/15 03/12/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 Spot name cl21plg1 cl23plg1 cl23plg2 cl24plg1 cl24plg2 cl24plg3 cl25plg1 cl25plg2 cl25plg3 cl10plg1 cl10plg2 cl10plg3 cl11plg1 cl11plgmf cl12plg1 cl12plg2 cl12plg3

SiO2 54.44 54.11 55.45 54.47 52.77 55.76 53.50 52.06 55.59 56.33 55.04 54.91 53.78 53.71 51.31 53.90 52.76 Al2O3 25.77 26.69 25.59 23.31 26.36 24.75 26.81 27.03 25.27 25.67 26.18 25.97 26.68 26.19 28.23 26.49 26.65 Fe2O3 1.13 1.01 1.14 2.90 1.05 1.09 0.87 0.90 0.84 0.99 1.11 1.10 0.98 1.14 1.02 1.26 1.33 CaO 11.09 11.43 10.27 10.33 11.88 9.80 11.28 12.32 10.20 10.44 10.86 10.75 11.83 11.56 13.43 12.01 12.43 Na2O 4.43 3.88 4.71 4.24 4.21 4.94 4.35 3.80 4.96 4.75 4.65 4.77 4.03 4.29 3.37 3.82 3.95 K2O 0.39 0.37 0.50 0.89 0.38 0.62 0.32 0.24 0.39 0.32 0.34 0.29 0.33 0.31 0.26 0.33 0.35 Tot. 97.24 97.49 97.65 96.14 96.64 96.95 97.13 96.35 97.25 98.50 98.17 97.79 97.62 97.20 97.60 97.82 97.47 Formula Based On 8 Oxygen eceity epyis Geosystems Geophysics, Geochemistry, Si 2.530 2.505 2.559 2.576 2.477 2.591 2.490 2.450 2.574 2.573 2.531 2.535 2.492 2.502 2.393 2.495 2.461 Al 1.412 1.457 1.393 1.300 1.459 1.355 1.471 1.500 1.379 1.382 1.419 1.413 1.458 1.438 1.552 1.445 1.466 Fe 0.039 0.035 0.039 0.103 0.037 0.038 0.030 0.032 0.029 0.034 0.039 0.038 0.034 0.040 0.036 0.044 0.047 Ca 0.552 0.567 0.508 0.523 0.598 0.488 0.562 0.621 0.506 0.511 0.535 0.532 0.587 0.577 0.671 0.595 0.621 Na 0.399 0.349 0.421 0.389 0.384 0.445 0.393 0.347 0.445 0.420 0.414 0.427 0.362 0.388 0.304 0.343 0.358 K 0.023 0.022 0.029 0.054 0.023 0.037 0.019 0.014 0.023 0.019 0.020 0.017 0.019 0.019 0.015 0.020 0.021 An 56.68 60.48 52.99 54.19 59.54 50.29 57.73 63.25 51.95 53.77 55.21 54.49 60.63 58.68 67.72 62.14 62.15 Ab 40.94 37.20 43.96 40.26 38.22 45.90 40.34 35.31 45.67 44.25 42.76 43.79 37.38 39.42 30.73 35.81 35.77 Or 2.39 2.33 3.05 5.55 2.24 3.80 1.94 1.44 2.38 1.98 2.03 1.72 1.99 1.90 1.55 2.05 2.08 Ca 1 Na 1 K 0.974 0.937 0.959 0.966 1.004 0.970 0.974 0.983 0.974 0.950 0.969 0.975 0.969 0.983 0.991 0.958 1.000 Si 1 Al 3.942 3.962 3.952 3.875 3.937 3.946 3.961 3.950 3.953 3.955 3.950 3.948 3.950 3.940 3.946 3.940 3.926

Date 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 04/07/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 Spot name cl12plg4 cl13plg1 cl13plg2 cl13plg3 cl13plg4 cl15plg1 cl15plg2 cl16plg1 cl16plg2 cl16plg3 cl16plg4 cl2plg1 cl2plg2 cl2plg3 cl2plg4 cl2plg5 cl2plg6

SiO2 52.93 54.53 52.38 54.24 54.15 52.33 54.33 52.81 54.57 53.50 53.35 51.60 53.56 53.89 50.73 55.42 52.87 Al2O3 26.43 25.58 26.95 25.72 25.74 27.72 28.17 27.05 26.25 27.10 26.76 28.30 26.97 26.36 28.98 25.71 27.30 Fe2O3 1.20 1.09 1.13 1.13 0.96 0.97 1.15 1.03 1.26 1.24 1.28 0.81 1.12 0.81 0.67 0.86 0.95 CaO 12.03 11.02 12.92 11.49 11.40 13.68 13.21 12.49 11.53 11.98 11.82 13.37 11.50 10.94 14.12 10.46 11.87 Na2O 3.91 4.53 3.91 4.44 4.40 3.46 3.70 3.77 4.20 3.71 4.10 3.05 3.91 4.33 2.81 4.68 3.89 K2O 0.32 0.37 0.28 0.40 0.38 0.23 0.31 0.31 0.50 0.36 0.46 0.24 0.32 0.35 0.13 0.46 0.36 Tot. 96.82 97.13 97.57 97.41 97.01 98.38 100.86 97.46 98.31 97.88 97.78 97.37 97.38 96.68 97.45 97.58 97.25 Formula Based On 8 Oxygen Si 2.478 2.537 2.443 2.521 2.524 2.421 2.446 2.458 2.513 2.474 2.475 2.406 2.486 2.514 2.368 2.558 2.462 Al 1.459 1.403 1.482 1.409 1.414 1.512 1.495 1.484 1.425 1.478 1.464 1.555 1.476 1.450 1.595 1.399 1.498 Fe 0.042 0.038 0.040 0.040 0.034 0.034 0.039 0.036 0.044 0.043 0.045 0.028 0.039 0.028 0.024 0.030 0.033 Ca 0.604 0.549 0.646 0.572 0.569 0.678 0.637 0.623 0.569 0.594 0.588 0.668 0.572 0.547 0.706 0.517 0.592 Na 0.355 0.409 0.354 0.400 0.397 0.310 0.323 0.340 0.375 0.333 0.369 0.275 0.352 0.392 0.254 0.419 0.351 K 0.019 0.022 0.017 0.024 0.023 0.014 0.018 0.018 0.029 0.021 0.027 0.014 0.019 0.021 0.008 0.027 0.022 An 61.75 56.06 63.56 57.46 57.55 67.69 65.16 63.50 58.45 62.65 59.75 69.77 60.68 56.99 72.93 53.70 61.37 Ab 36.31 41.73 34.80 40.18 40.17 30.97 33.00 34.63 38.54 35.12 37.50 28.74 37.31 40.85 26.24 43.48 36.39 Or 1.94 2.22 1.64 2.36 2.28 1.35 1.84 1.87 3.01 2.23 2.74 1.49 2.00 2.16 0.82 2.82 2.24 Ca 1 Na 1 K 0.978 0.980 1.016 0.996 0.989 1.002 0.978 0.982 0.973 0.948 0.984 0.958 0.942 0.960 0.968 0.963 0.965 Si 1 Al 3.938 3.940 3.925 3.931 3.939 3.933 3.941 3.943 3.938 3.952 3.940 3.961 3.961 3.965 3.962 3.957 3.960 Date 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 05/06/15 Spot name cl4plg1 cl5plg2 cl5plg3 clplg2 clplg3 clplg4 clplg5 cl7plg1 cl7plg2 cl8plg1 cl8plg2 cl9plg1 cl9plg2 cl9plg3 cl9plg4 cl9plg5 cl9plg6 cl9plg7

SiO2 52.19 54.95 55.58 52.41 52.92 53.45 54.69 54.96 54.93 52.36 54.02 53.45 54.00 52.84 54.21 52.39 53.30 54.18 Al2O3 27.84 26.07 24.71 26.98 26.91 26.39 25.66 25.60 26.45 27.38 26.57 26.66 27.72 27.88 26.49 28.00 26.75 27.04 Fe2O3 0.98 1.14 1.41 0.96 0.95 1.00 0.79 1.08 0.88 0.78 0.98 0.79 0.94 0.90 0.99 0.83 1.03 1.10 CaO 12.92 10.47 10.00 12.40 11.92 11.12 10.97 10.70 10.82 12.42 11.11 11.75 12.31 12.89 11.27 13.43 11.94 11.94 Na2O 3.49 4.73 4.81 3.83 3.86 4.34 4.42 4.62 4.34 3.74 4.43 4.10 3.90 3.61 4.15 3.46 3.94 4.13 K2O 0.16 0.54 0.63 0.36 0.41 0.44 0.45 0.53 0.48 0.34 0.47 0.36 0.30 0.27 0.47 0.24 0.36 0.38

Tot. 97.58 97.90 97.14 96.93 96.98 96.73 96.98 97.49 97.89 97.00 97.57 97.10 99.16 98.37 97.58 98.34 97.31 98.77 10.1002/2016GC006525 Formula Based On 8 Oxygen Si 2.427 2.535 2.582 2.454 2.472 2.499 2.544 2.546 2.529 2.447 2.503 2.490 2.465 2.437 2.510 2.421 2.480 2.484 Al 1.526 1.418 1.353 1.489 1.482 1.455 1.407 1.398 1.436 1.509 1.452 1.464 1.492 1.516 1.446 1.525 1.468 1.462 Fe 0.034 0.039 0.049 0.034 0.033 0.035 0.028 0.038 0.030 0.027 0.034 0.028 0.032 0.031 0.035 0.029 0.036 0.038 Ca 0.644 0.517 0.498 0.622 0.597 0.557 0.547 0.531 0.534 0.622 0.552 0.587 0.602 0.637 0.559 0.665 0.595 0.587 Na 0.315 0.423 0.433 0.348 0.350 0.394 0.398 0.415 0.387 0.339 0.398 0.371 0.345 0.322 0.373 0.310 0.356 0.367 K 0.009 0.032 0.037 0.022 0.024 0.026 0.027 0.032 0.028 0.020 0.028 0.021 0.017 0.016 0.027 0.014 0.021 0.022 An 66.50 53.21 51.44 62.76 61.48 57.04 56.26 54.32 56.23 63.41 56.46 59.94 62.39 65.33 58.26 67.25 61.23 60.09 Ab 32.54 43.52 44.73 35.06 36.03 40.31 40.97 42.46 40.81 34.55 40.71 37.88 35.80 33.08 38.87 31.33 36.57 37.65 Or 0.96 3.27 3.83 2.18 2.49 2.66 2.77 3.22 2.95 2.04 2.83 2.17 1.81 1.60 2.86 1.41 2.19 2.26 Ca 1 Na 1 K 0.968 0.972 0.968 0.991 0.971 0.976 0.972 0.978 0.949 0.981 0.977 0.979 0.965 0.975 0.959 0.989 0.972 0.976 Si 1 Al 3.953 3.952 3.935 3.944 3.954 3.954 3.951 3.944 3.966 3.956 3.955 3.954 3.957 3.953 3.956 3.946 3.948 3.946

aMajor oxides are reported as wt %. Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Figure 3. Time series of the 3He/4He values (reported in units of Rc/Ra) in fumaroles at the central and southwest craters and at Falla Ariete during (a) 1999–2016 and (b) 2014–2016. The green line indicates the maximum Rc/Ra values measured in ﬂuid inclusions (FIs) of basaltic- andesitic magmas that erupted during 1864–1866 [after Di Piazza et al., 2015]. Gray circles indicate the explosive events that have occurred at Turrialba volcano since 2010. The black points show the daily dominant frequency mode of seismic records at the CVTR seismic station from January 2009 to April 2016.

The study of noble gases in fluid inclusions hosted in rocks that erupted at Turrialba during the last 7 ka revealed that the maximum 3He/4He would be expected to be 8.1 Ra [Di Piazza et al., 2015]. This would be mainly determined by the signature of the most-mafic magma types found in the Turrialba volcanic complex. Thus, 3He/4He values in fumarole gases approaching 8.1 Ra should reflect the direct degassing of a mafic and uncontaminated magma. In addition, because the solubility of He in silicate melts is low, and 3 4 comparable to that of CO2 [Paonita, 2005, and references therein], the increase in the He/ He should reflect preferential degassing from a mafic magma deep-seated in the plumbing system, as observed in other magmatic systems worldwide [e.g., Caracausi et al., 2003; Rizzo et al., 2015a]. Therefore, the increase in the 3He/4He observed at the southwest-crater fumaroles from 1999 to 2001 indicates a phase of degassing at depth of a mafic and 3He-rich batch of magma and the arrival of more- magmatic fluids at the surface. This is confirmed by the enhanced degassing rate observed in 2000 from the fumaroles at the central and southwest craters, with the formation of a large fracture (named Quemada) discharging fluids at 908C between the craters [Tassi et al., 2004; Vaselli et al., 2010]. Seismic swarms (>9000 events/year in 2001), radial inflation, and changes in the fumarole-gas composition were also detected during 2001 and 2002 [Barboza et al., 2003a, 2003b; Tassi et al., 2004; Martini et al., 2010]. The 3He/4He at the southwest-crater and central-crater fumaroles decreased after 2001 (i.e., from 8.01 to 7.32 Ra at the

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 9 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Figure 4. BSE imaging of ash particles emitted by the explosions during (a, b) October–December 2014, (c, d) March 2015, and (e, f) May 2015. (a, b) Porphyritic block and a particle with a molten surface. (c) A particle with strong crystallization. (d) A glassy shard. (e, f) Glassy and spongy shards found in the products of the most-recent explosions, both of which may represent fresh magma.

southwest crater and from 7.48 to 6.85 Ra at the central crater) (Figure 3a), suggesting a reduced supply of gas from the primitive magmatic source and an enhanced release of hydrothermal fluids. Accordingly, the seismicity was significantly lower than the maximum exhibited in 2001 [Hilton et al., 2010, who reported seismic data from the Global Volcanology Program, www.volcano.si.edu]. There was a new increase in the 3He/4He after 2005 at the central-crater and southwest-crater fumaroles, indicating the recurrence of degassing at depth of a mafic and 3He-rich batch of magma and the arrival of more-magmatic fluids at the surface. This may have persisted up to September 2014, when the same high 3He/4He ratio was found at southwest-crater fumaroles (Figure 3a), though there may have been unrecord-

ed variations since no samples were obtained during 2011–2013. Considering that the SO2 flux from the craters remained high between the first ash emission in 2010 and the beginning of the eruptive cycle in October 2014, with average values above 500 t/d [Conde et al., 2013; OVSICORI-UNA, 2015], it can be inferred that the strong degassing of a primitive magma source continued during the period with no measurements, as suggested by the high 3He/4He at the fumaroles (Figure 3a). The 3He/4He at the southwest-crater fumaroles decreased in February 2015 (from 7.94 to 7.49 Ra) toward values falling between those measured during 2005–2008 (Figure 3). This suggests a reduced supply of gas from the primitive magmatic source and an enhanced release of hydrothermal fluids. Samples collected at Falla Ariete in February, May, and September 2015 yielded similar 3He/4He values (Figure 3b), which indicates the same source of volatiles and that active magma degassing was continuing at depth. Since October 2015, the 3He/4He at Falla Ariete increased toward the maximum value (8.00 Ra, Figure 3b), indicating that fresh magma was again replenishing the plumbing system beneath Turrialba. Moreover, the eruptions at Turrialba escalated again during the first half of 2016, peaking in May (Figure 3b), and the level of activity of the volcano could increase further in the following months.

4.2. Petrographic and Chemical Analyses of Products That Erupted During 2014 and 2015 Six samples of ash that erupted during the sequence of explosive events that occurred between October 2014 and May 2015 were collected in the Turrialba summit area. BSE imaging of these samples (Figure 4) revealed that the shape of the particles vary from irregular to angular. Fresh glassy particles are not

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 10 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Figure 5. Classiﬁcation diagrams of the main mineralogical phases of Turrialba rocks contained in ashes erupted in 2015, compared with phenocrysts of Turrialba products erupted in 1864–1866: a pyroxene ternary diagram; the olivine forsteritic molar percentage; and a plagioclase ternary diagram. It was not possible to analyze mineral phases in the 2014 samples.

abundant and are found mainly in the most-recent deposits (e.g., the sample collected on 6 May 2015). Most of the erupted particles are microlite rich, many are characterized by the presence of euhedral-to- subhedral phenocrysts and microlites of plagioclase and clinopyroxene (commonly zoned), and a few are olivine and magnetite. Samples from the explosions during October–December 2014 are strongly dominated by highly altered particles, with sulﬁdes, native S, and signs of hydrothermal alteration (Figures 4a and 4b); some particles from the 2014 explosions display fresh-looking glasses, but particles with signs of plagioclase alteration and molten surfaces have also been observed. De Moor et al. [2016b] recently showed that many of the fresh-looking particles in an ash sample from the eruption on 29 October 2014 show geochemical evidence for cryptic alteration. Ashes from the explosions during March and April 2015 show euhedral and commonly zoned plagioclase and clinopyroxene (Figures 4c and 4d), and an increased proportion of fresh-looking particles. The ashes from 6 May are the least altered of the entire suite (Figures 4e and 4f), not appearing to be modiﬁed by recycling or alteration processes such as those from the previous eruptions. Glassy morphologically unaltered particles present spongy outlines, with very well-rounded and clean vesicles, and are completely aphyric or display very small crystals of plagioclases (Figures 4e and 4f). The chemical characterization of glasses and minerals (Tables 2 and 3) was performed on only the freshest particles from each explosive event (similar to particles shown in Figures 4e and 4f). Figure 5 shows the mineral chemistry of the freshest ash particles that erupted at Turrialba volcano in 2015. These samples greatly differ in the degree of crystallinity, ranging from completely glassy to 70% of crystals content. Plagioclase

is ubiquitous as a euhedral phenocryst phase ranging in composition between An50 and An67; clinopyrox-

enes are euhedral-to-subhedral with an augitic-to-diopsidic composition (Wo4–47,En43–72, and Fs8–23) often

zoned; olivine crystals (Fo72–75) and oxides (Mt56–69) occur as phenocrysts and microphenocrysts or microlites. The composition of mineral phases found in the fresh ashes of explosions during 2015 matches that of the 1864–1866 eruptive products [Di Piazza, 2014; Di Piazza et al., 2015], though the crystals from 2015 typi- cally show a narrower variability (Figure 5).

Based on the SiO2 and alkali contents (Table 2), microanalysis of glasses revealed that most of the samples are basaltic-andesite to trachy-andesite in composition (Figure 6), with a few anomalous exceptions displaying a more-evolved composition (dacite and rhyolite). The silica content varies from 55 to 65 wt % while

total alkalies from 2.5 to 8.0 wt %, and only one sample displaying a rhyolitic composition (SiO2 77 wt % and alkali 8 wt %; Figure 6). The compositional range of major elements in ashes is broader for October and November 2014 than from December 2014 to May 2015 (Table 2). Particularly, some of the clasts

erupted in October and November 2014 have anomalously low Na2O(<2%), which are not observed in the following eruptions (Table 2; Figure 6). In particular, the ashes that erupted in April and May explosions

show SiO2 contents ranging between 55 and 60 wt %, alkali compositions between 4 and 7wt%, and MgO contents between 3 and 5 wt %. We attribute the lower alkali content of the October and

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 11 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Figure 6. Diagram of the total alkali versus silica content [after Le Bas et al., 1986] comparing the compositions of the samples collected in this study with those of bulk rocks and glasses of the 1864–1866 eruption from Di Piazza et al. [2015].

November 2014 particles (Figure 6) to leaching by hydrothermal fluids or interaction with volcanic gases in the plume [Alvarado et al., 2016], which is a process commonly observed in ash particles that erupt from other volcanoes [Dellino et al., 2001; Delmelle et al., 2007; Moune et al., 2007; Islam and Akhtar, 2013, and references therein]. Similarly, de Moor et al. [2016b] found strong evidence for cryptic alteration in the fresh- looking particles that erupted on 29 October 2014. It is notable that this loss of alkali did not occur in the particles from the samples that erupted later. This evidence is in accordance with the petrographic observations described above, in which we recognized that samples from the explosions occurred during October– December 2014 are strongly dominated by highly altered particles, with sulfides, native S, and signs of hydrothermal alteration (Figures 4a and 4b). The absence of geochemical evidence for alteration in the 2015 magmatic particles supports the evolution of eruptive behavior to a more-open conduit system connecting the shallow magma body (or bodies) to the surface. We suggest that the 2014 fresh-looking particles were emplaced shortly (weeks to months) pri- or to the eruption onset in the shallow subsurface as breccia dikes deposits. They were then exposed to hydrothermal fluids and magmatic gases streaming through the system prior to reaching the surface as entrained material during the eruption on 29 October 2014, whose gas compositions were more hydrothermal in character than the 2015 eruptions [de Moor et al., 2016b]. In contrast, the eruptions on April and May 2015 were purely magmatic (based on their gas chemistry), suggesting that these eruptions formed conduits that directly connected the magmatic system to the surface. This would mean that the fresh-looking particles in the later eruptions did not interact with hydrothermal fluids prior to or during the eruption, thereby preserving their pristine magmatic compositions. The pristine particles in the 2015 ash are thus convincingly juvenile in character from both geochemical and petrographic standpoints. The samples from October 2014 provide ambiguous evidence, in that the fresh-looking particles appear pristine both morphologically and petrographically, whereas the geochemical data presented here (alkali leaching) and the F enrichment reported by de Moor et al. [2016b] show that these samples were actually altered by hydrothermal interactions.

Figure 7 shows variation diagrams of selected major elements versus the SiO2 content. The evolutionary trend observed in the truly pristine juvenile 2015 glasses resembles that of the glasses from the 1864–1866

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 12 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Figure 7. Variation diagrams of major elements versus SiO2 for Turrialba rocks. The black dashed lines indicate the chemical modiﬁcation due to fractional crystallization (same legend as in Figure 6). Oxides are expressed in wt %. Green areas are from electron microprobe analyses of glasses of the 1864–1866 eruption [Di Piazza, 2014].

eruptive products. The different degrees of evolution are also related to the various extents of crystallization observed in the ash samples.

4.3. Eruption Trigger and Composition of Involved Magma There was a transient increase in 3He/4He at the southwest-crater fumaroles between 1999 and 2001, which indicated an enhanced output of magmatic volatiles from a mafic magma deep-seated in the plumbing system of the volcano (Figure 3a). Periodic measurements of 3He/4He in fumarole gases since 2005 have identified a 6 year-long increasing trend related to the degassing of a mafic and 3He-rich magma, which was replenishing the Turrialba plumbing system. Figure 3 also shows the dominant frequencies of the continuous seismic record at Turrialba during 2009–2016. The trend of this spectral parameter displays periods of high (>5 Hz) and low frequencies (<3 Hz); the former are interpreted to be caused by shear fractures due to shallow pressurization or by an enhanced hydrothermal activity, while the latter indicate resonance in a fluid-filled cavity due to pressure changes induced by magma or gas transport from depth [e.g., Chouet, 1986], or a simple mechanical failure [Eyre et al., 2015]. The dominant frequency was generally moderate to low during 2009 and 2010 (Figure 3a), which appears to be consistent with the magma recharge from depth inferred by the 3He/4He values. We argue that the replenishments during 1999–2001 and 2005–2009 (up to 2011) were causally related to the resumption of the eruptive activity in 2010. Indeed, shallower lev- els of the plumbing system have been progressively involved, as testified by the reactivation of the summit area in 2001, with the opening of new degassing vents and the occurrence of significant morphological changes during 2004–2008 [Martini et al., 2010; Vaselli et al., 2010]. The increases in the temperature and

concentration of magmatic ﬂuids (SO2, HCl, and HF) at crater fumaroles [Vaselli et al., 2010] and the

enhanced degassing of SO2 from the craters [Conde et al., 2013] clearly indicated the arrival of magmatic fluids at the surface. The gradual replenishment of magma in the shallow portion of the plumbing system pro- vided heat and gas to the overlying, sealed, and vapor-dominated hydrothermal system. This would have caused the pressurization and mobilization of fluids, partial displacement of the hydrothermal system beneath the craters, opening of the system following breakage of the sealing carapace by phreatic explosions, and the consequent release of fluids, ash, lithics, and rocks. From 2011 onward, there was a generally

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 13 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

high frequency of seismicity (Figure 3a), indicating a shallow pressurization of the conduits and the involvement of the hydrothermal system. The presence of a diverse mixture of hydrothermally altered and reworked material in the ash deposits derived from the explosions that resumed in 2010, indicates that these were not rich in juvenile component. Indeed, Reagan et al. [2011] found that only 1% of basaltic-andesitic glassy shards could be attributed to a juvenile component among the 2010 eruptive products. More recently, Alvarado et al. [2016] inferred that a juvenile component is present since 2010, but in a very small quantity. The most important observation from our samples is that the amount of allegedly juvenile component appears to increase with time, as indicated by the colors in Figure 2a progressing from heterogeneous light gray to more-homogeneous darker gray, and by the transition from a hydrothermally leached composition to a homogeneous composition of the fresh glasses evident in Figures 6 and 7. This is supported by the optical observations and BSE imaging of the December 2014 to May 2015 eruptive products (Figure 4), which have ash particles with more fresh glasses and aphyric-to-porphyritic textures. These ashes display a narrow and more-maﬁc chemical compo-

sition (basaltic-andesite to trachy-andesite, SiO2 55–60 wt %, Figure 6) increasing with time, and thus we consider these to be representative of a fresh magma. On the other hand, the products that erupted in October and November 2014 displayed a wider variability in chemistry (Figures 6 and 7), which partly results from the destruction and eruption of an important volume of the preexisting rim located between the central and southwest craters, and it is due to a ﬁrst phase of vent opening. These observations are in agreement with the progressive decrease of the frequency of seismicity recorded since 2015 (Figure 3b), indicating a major involvement of magmatic ﬂuids rather than the hydrothermal system. In general, the mineral and major elements chemistry of fresh particles (Figures 5–7) resemble those of the 1864–1866 eruptive products [Di Piazza, 2014; Di Piazza et al., 2015], suggesting that the composition of the magma involved in the current unrest of Turrialba volcano is similar to the magma that erupted during 1864–1866. The variability observed in the chemical evolution of the glasses could be the result of the involvement of different layers of a magma batch having different degrees of crystallinity (from 5% crystals observed in basaltic-andesitic shards to 35–39% crystals in the trachy-andesitic ones). The evolution trend observed in the glass matrix (Figure 7) is compatible with a progressive differentiation by crystallization of a magma having a pristine composition similar or identical to the magma that erupted during 1864–1866. Two hypotheses for the provenance of the juvenile component in the recent ashes are as follows: 1. The fresh particles are representative of the magma body that is directly responsible for the degassing and unrest, is newly intruded (i.e., recently arrived from the lower crust or the mantle), and has essentially the same composition as the 1864–1866 magma body due to similar underlying mantle melting and crustal level differentiation processes. 2. The pristine particles represent remobilization of the upper part of an extensive magmatic system that also drove the 1864–1866 eruption, with reactivation of this previously emplaced magma (still containing melt and therefore still mobile) caused by the injection of new magma into the base of the plumbing system.

Hypothesis 1 is based primarily on the observation that the magma that erupted during the 1963 eruption of Irazu volcano (only 12 km west of Turrialba) ascended from the mantle very rapidly, on time scales of months to years [Ruprecht and Plank, 2013]. Moreover, Hypothesis 2 cannot be discounted without identifying a unique geochemical or petrological fingerprint that conclusively distinguishes the new juvenile component from the 1864–1866 eruptive products, or without direct constraints on the timing of magma ascent intrinsically based on these samples. The reactivation of shallow magma in established and long- lived magma reservoirs by injections of magma into the base of extensive and complex plumbing systems has been inferred for numerous volcanic eruptions worldwide [e.g., Claiborne et al., 2010; Larsen et al., 2010]. However, deciding between these mechanisms is a somewhat secondary aspect from a hazards perspec- tive. The presence of a true juvenile component in our studied products—which was convincingly identified in those erupted in 2015—clearly indicates that a magmatic eruption is possible at Turrialba within the near future. The change in the magmatic component of the ash in early 2015 from cryptically altered to pristine is a significant development in the eruptive behavior of Turrialba volcano. Variations in the 3He/4He and in the frequency of seismicity (Figure 3b) similarly show a more-magmatic signature in early 2015, and they

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 14 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

are indicative of changes in the plumbing system, thus providing insight into the composition of the magma. The evolutionary stages that led to the magmatic eruption of 1864–1866 comprised phreatic explosions that transitioned to phreatomagmatic and Strombolian activity (Figure 2b) [Reagan et al., 2006]. Meanwhile, the recent occurrence of explosions with a juvenile component could indicate that the eruptive activity at Turrialba is currently at an intermediate stage and hence would indicate that future eruptions will include a more-magmatic component.

4.4. Hazard Implications and Evolutionary Scenario Turrialba volcano represents a serious threat to the population of Costa Rica. Indeed, ash fall has been reported in the capital San Jose on 21 days since 29 October 2014 (Figure 1). Ash from the eruption on 12 March 2015 (column heights up to 2 km) caused the closure of 12 schools and the evacuation of farms within 2 km of the crater (affecting 50 people and 300 cattle). That eruption also caused 111 flights to be cancelled at SJO International Airport (affecting 7000 passengers). Air traffic was disrupted again on 21 March, 23 March, 4 May, and 18 May, with numerous flight cancellations. Aviation safety is a primary con- cern since ash can reach the airport within an hour of an eruption. Prolonged eruptive activity at Turrialba could have a severe impact on the economy of Costa Rica since: (1) tourism is one of the primary sources of foreign income, (2) San Jose is pivotal to commercial activity in the country, and (3) the impacted area includes some of the most important farmland in the country. The attention of volcanologists and the local authorities is now mostly focused on the possible evolution of the present activity toward a magmatic eruption. We can use the data obtained in our 3He/4He monitoring and from the chemistry of the erupted ashes to speculate on the evolutionary scenario that may be expected over the coming months. As reported in section 4.1, the 3He/4He displayed high values until Sep- tember 2014 (Figure 3), suggesting that the resumption of eruptive activity in 2014 was related to a new degassing phase at depth. This was followed by a decrease in the 3He/4He that occurred during a period of high-frequency seismicity (Figure 3b). In our opinion, this indicates a reduced supply of the primitive magma at depth and/or an enhanced release of shallow hydrothermal fluids that mix with magmatic fluids, suggesting a decrease in the activity level; indeed, the rate of explosive events reduced markedly between December 2014 and March 2015. Nevertheless, a new gradual increase in the 3He/4He has been recorded since then, reaching again the maximum values that we measured in December 2015 (8.0 Ra, Figure 3b). During this period, the seismicity was characterized by a progressive decrease in the dominant frequency toward very low values (<2 Hz), which is strongly suggestive of the new arrival of deep magmatic fluids. This is also consistent with the eruptions increasing in frequency and energy during the first half of 2016, and the explosive activity was continuing on the date of manuscript submission (early July 2016). The investigation of the fresh particles suggested that the amount of juvenile component increased from 2014 to 2015, as indicated in a progression from hydrothermally leached to homogeneous fresh glasses (Figures 6 and 7). We conclude that it is reasonable to expect that other eruptions will occur over the coming months, and evolution toward a magmatic eruption is possible.

5. Concluding Remarks After almost 150 years of quiescence, a series of sporadic explosions started at Turrialba volcano (Costa Rica) in January 2010, increased in frequency from October 2014, and is still ongoing, ejecting blocks and ash dominated by altered preexisting material. On a few occasions, ash fallout has reached the capital San Jose and caused the closure of the Juan Santamaria international airport of Costa Rica, as well as evacua- tions and damage locally. The 3He/4He measurements, coupled with characterization of the eruptive products between October 2014 and May 2015, have led us to draw the following conclusions: 1. The resumption of explosive activity in 2010 has been triggered by a gradual replenishment of a maﬁc and 3He-rich batch of magma deep-seated in the plumbing system whose last phases of recharge began in 2005. This magmatic recharge has pressurized the shallow plumbing system and partially displaced the shallow hydrothermal system located beneath the craters. 2. Altered and reworked materials with variable compositions dominated the eruptive products in the ﬁrst eruptive stages during October and November 2014. A minor content of fresh glassy shards with a

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 15 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

more-homogeneous chemistry was recognized from December 2014 to May 2015, whose proportion seems to increase with time. Their composition matches that of the products that erupted during 1864– 1866, indicating that the magma feeding the ongoing eruptive phase has similar features. 3. When crater fumaroles could not be anymore sampled for safety reasons, we focused the monitoring on Falla Ariete fumaroles that are located on the flank of the volcano. At these fumaroles, the 3He/4He has increased since October 2015 with a peak of 8.0 Ra being recorded in December 2015, which is the high- est for the past 10 years. Such values are consistent with those measured in the fluid inclusions of the basaltic-andesitic magmas emitted during 1864–1866 (8.1 Ra), indicating that magmatic volatiles degassed from a new batch of magma at depth are arriving at the surface. The first half of 2016 was characterized by a new increase in the frequency and energy of eruptions. This implies that the level of activity of the volcano is still high and that other explosions are very likely to occur over the coming months.

Acknowledgments References We thank Mariano Tantillo for Alvarado, G. E., et al. (2016), Are the ashes from the latest eruptions (2010–2016) at Turrialba volcano (Costa Rica) related to phreatic or performing the laboratory analyses of phreatomagmatic events?, J. Volcanol. Geotherm. Res., in press. noble gases isotopes, and Andrea Barberi, F., A. Bertagnini, P. Landi, and C. Principe (1992), A review on phreatic eruptions and their precursors, J. Volcanol. Geotherm. Res., Cavallo and Manuela Nazzari for 128, 273–285. assistance during electron microprobe Barboza, V., et al. (2003a), Changes in the activity of Turrialba volcano: Seismicity, geochemistry and deformation, Seismol. Res. Lett., 74, analyses. We are also grateful to 215. OVSICORI-UNA and INGV in Palermo Barboza, V., et al. (2003b), Changes in the activity of Turrialba volcano: Seismicity, geochemistry and deformation, paper presented at 8th and Rome for supporting all of the field and laboratory activities. We IAVCEI Gas Workshop, March–April, Nicaragua and Costa Rica. finally thank English Science Editing Campion, R., M. Martinez-Cruz, T. Lecocq, C. Caudron, J. Pacheco, G. Pinardi, C. Hermans, S. Carn, and A. Bernard (2012), Space- and for language revisions before ground-based measurements of sulphur dioxide emissions from Turrialba volcano (Costa Rica), Bull. Volcanol., 74, 1757–1770. submission, Evelyn Furi€ and Chad Capasso, G., M. L. Carapezza, C. Federico, S. Inguaggiato, and A. Rizzo (2005), Geochemical monitoring of the 2002–2003 eruption at Strom- Deering for the helpful suggestions boli volcano (Italy): Precursory changes in the carbon and helium isotopic composition of fumarole gases and thermal waters, Bull. Vol- during revisions, which improved the canol., 68(2), 118–134. manuscript. The data analyzed for this Caracausi, A., F. Italiano, A. Paonita, A. Rizzo, and P. M. Nuccio (2003), Evidence of deep magma degassing and ascent by geochemistry of paper are reported in the Tables (1–3) peripheral gas emissions at Mount Etna (Italy): Assessment of the magmatic reservoir pressure, J. Geophys. Res., 108(B10), 2463, doi: and are available by contacting the 10.1029/2002JB002095. corresponding author. Cashman, K. V., and R. P. Hoblitt (2004), Magmatic precursors to the 18 May 1980 eruption of Mount St. Helens, USA, Geology, 32(2), 141–144. Chouet, B. (1986), Dynamics of a fluid-driven crack in three dimensions by the finite difference method, J. Geophys. Res., 91(B14), 13,967– 13,992. Claiborne, L. L., C. F. Miller, D. M. Flanagan, M. A. Clynne, and J. L. Wooden (2010), Zircon reveals protracted magma storage and recycling beneath Mount St. Helens, Geology, 38, 1011–1014.

Conde, V., S. Bredemeyer, E. Duarte, J. F. Pacheco, S. Miranda, B. Galle, and T. H. Hansteen (2013), SO2 degassing from Turrialba volcano linked to seismic signatures during the period 2008–2012, Int. J. Earth Sci., 103(7), doi:10.1007/s00531-013-0958-5. Dellino, P., R. Isaia, L. La Volpe, and G. Orsi (2001), Statistical analysis of textural data from complex pyroclastic sequences: Implications for fragmentation processes of the Agnano-Monte Spina Tephra (4.1 ka), Phlegraean Fields, southern Italy, Bull. Volcanol., 63, 443–461. Delmelle, P., M. Lambert, Y. Dufrene, P. Gerin, and N. Oskarsson (2007), Gas/aerosol–ash interaction in volcanic plumes: New insights from surface analyses of ﬁne ash particles, Earth Planet. Sci. Lett., 259, 159–170. De Moor, J. M., A. Aiuppa, J. Pacheco, G. Avard, C. Kern, M. Liuzzo, M. Martinez, G. Giudice, and T. P. Fischer (2016a), Short-period volcanic gas precursors to phreatic eruptions: Insights from Poas Volcano, Costa Rica, Earth Planet. Sci. Lett., 442, 218–227. De Moor, J. M., et al. (2016b), Turmoil at Turrialba volcano (Costa Rica): Degassing and eruptive behavior inferred from high-frequency gas monitoring, J. Geophys. Res. Solid Earth, 121, 5761–5775, doi:10.1002/2016JB013150. Di Piazza, A. (2014), The evolution of Turrialba volcano plumbing system, PhD thesis, 220 pp., Univ. degli Studi Roma Tre, Roma. Di Piazza, A., A. L. Rizzo, F. Barberi, M. L. Carapezza, G. De Astis, and C. Romano (2015), Geochemistry of the mantle source and magma feeding system beneath Turrialba volcano, Costa Rica, Lithos, 232, 319–335. Eyre, T. S., C. J. Bean, L. De Barros, F. Martini, I. Lokmer, M. M. Mora, J. F. Pacheco, and G. J. Soto (2015), A brittle failure model for long- period seismic events recorded at Turrialba volcano, Costa Rica, J. Geophys. Res. Solid Earth, 120, 1452–1472, doi:10.1002/2014JB011108. Hilton, D. R., C. J. Ramirez, R. Mora-Amador, T. P. Fischer, E. Furi, P. H. Barry, and A. M. Shaw (2010), Monitoring of temporal and spatial variations in fumarolic helium and carbon dioxide characteristics at Poas and Turrialba volcanoes, Costa Rica (2001–2009), Geochem. J., 44, 431–440, doi:10.2343/geochemj.1.0085. Islam, M. S., and S. Akhtar (2013), A critical assessment to the performance of alkali-silica reaction (ASR) in concrete, Can. Chem. Trans., 1(4), 253–266. Larsen, J. F., C. J. Nye, M. L. Coombs, M. Tilman, P. Izbekov, and C. Cameron (2010), Petrology and geochemistry of the 2006 eruption of Augustine volcano, in The 2006 Eruption of Augustine Volcano, edited by J. A. Power et al., U.S. Geol. Surv. Prof. Pap., 335–382. Le Bas, M. J., R. W. Le Maitre, A. Streckeisen, and B. Zanettin (1986), A chemical classiﬁcation of volcanic rocks based on the total alkali-silica diagram, J. Petrol., 27(3), 745–750. Martini, F., F. Tassi, O. Vaselli, R. Del Potro, M. Martinez, R. Van de Laat, and E. Fernandez (2010), Geophysical, geochemical and geodetical signals of reawakening at Turrialba volcano (Costa Rica) after almost 150 years of quiescence, J. Volcanol. Geotherm. Res., 198, 416–432. doi:10.1016/j.jvolgeores.2010.09.021. Moune, S., F. Faure, P. G. Gauthier, and K. W. W. Sims (2007) Pele’s hairs and tears: Natural probe of volcanic plume, J. Volcanol. Geotherm. Res., 164, 244–253. Nakada, S., Y. Miyake, H. Sato, O. Oshima, and A. Fujinawa (1995), Endogenous growth of dacite dome at Unzen volcano (Japan), 1993– 1994, Geology, 23(2), 157–160.

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 16 Geochemistry, Geophysics, Geosystems 10.1002/2016GC006525

Paonita, A. (2005), Noble gas solubility in silicate melts: A review of experimentation and theory, and implications regarding magma degassing processes, Ann. Geophys., 48(4/5), 647–669. Paonita, A., A. Caracausi, G. Iacono-Marziano, M. Martelli, and A. Rizzo (2012), Geochemical evidence for mixing between fluids exsolved at different depths in the magmatic system of Mt Etna (Italy), Geochim. Cosmochim. Acta, 84, 380–394, doi:10.1016/j.gca.2012.01.028. Paonita, A., A. Caracausi, M. Martelli, and A. L. Rizzo (2016), Temporal variations of helium isotopes in volcanic gases quantify pre-eruptive refill and pressurization in magma reservoirs: The Mount Etna case, Geology, 44(7), 499, doi:10.1130/G37807.1. Pardo, N., et al. (2014), Perils in distinguishing phreatic from phreatomagmatic ash; insights into the eruption mechanisms of the 6 August 2012 Mt. Tongariro eruption, New Zealand, J. Volcanol. Geotherm. Res., 286, 397–414. Reagan, M. K., E. Duarte, G. J. Soto, and E. Fernandez (2006), The eruptive history of Turrialba volcano, Costa Rica, and potential hazards from future eruptions, Spec. Pap. Geol. Soc. Am., 412, 235–258. doi:10.1130/2006.2412(13). Reagan, M., M. Rowe, E. Duarte, and E. Fernandez (2011), Juvenile glass fragments in phreatic explosion debris from Turrialba volcano, Cos- ta Rica. Goldschmidt Conf. Abstracts, Min. Mag., 75(3), 1700. Rizzo, A., A. Caracausi, R. Favara, M. Martelli, P. M. Nuccio, A. Paonita, A. Rosciglione, and M. Paternoster (2006), New insights into magma dynamics during last two eruptions of Mount Etna as inferred by geochemical monitoring from 2002 to 2005, Geochem. Geophys. Geo- syst., 7, Q06008, doi:10.1029/2005GC001175. Rizzo, A., et al. (2009), Geochemical evaluation of observed changes in volcanic activity during the 2007 eruption at Stromboli (Italy), J. Vol- canol. Geotherm. Res., 182, 246–254. Rizzo, A. L., C. Federico, S. Inguaggiato, A. Sollami, M. Tantillo, F. Vita, S. Bellomo, M. Longo, F. Grassa, and M. Liuzzo (2015a), The 2014 effu- 3 4 sive eruption at Stromboli volcano (Italy): Inferences from soil CO2 flux and He/ He ratio in thermal waters, Geophys. Res. Lett., 42, 2235–2243, doi:10.1002/2014GL062955. Rizzo, A. L., F. Barberi, M. L. Carapezza, A. Di Piazza, L. Francalanci, F. Sortino, and W. D’Alessandro (2015b), New mafic magma refilling a quiescent volcano: Evidence from He-Ne-Ar isotopes during the 2011–2012 unrest at Santorini, Greece, Geochem. Geophys. Geosyst., 16, 798–814, doi:10.1002/2014GC005653. Ruprecht, P., and T. Plank (2013), Feeding andesitic eruptions with a high-speed connection from the mantle, Nature, 500(7460), 68–72. Sano, Y., and H. Wakita (1985), Geographical distribution of 3He/4He ratios in Japan: Implications for arc tectonics and incipient magma- tism, J. Geophys. Res., 90(B10), 8729–8741, doi:10.1029/JB090iB10p08729. Sano, Y., T. Kagoshima, N. Takahata, Y. Nishio, E. Roulleau, D. L. Pinti, and T. P. Fischer (2015), Ten-year helium anomaly prior to the 2014 Mt Ontake eruption, Sci. Rep., 5, 13069. Tassi F., O. Vaselli, V. Barboza, E. Fernandez, and E. Duarte (2004), Fluid geochemistry in the period 1998-2002 at Turrialba Volcano (Costa Rica), Ann. Geophys., 47, 1501–1511. Van Manen, S. M. (2014), Hazard and risk perception at Turrialba volcano (Costa Rica); implications for disaster risk management, Appl. Geogr., 50, 63–73. Van Manen, S. M., G. Avard, and M. Martınez (2015), Co-ideation of disaster preparedness strategies through a participatory design approach: Challenges and opportunities experienced at Turrialba volcano, Costa Rica, Des. Stud., 40, 218–245, doi:10.1016/ j.destud.2015.06.002. Vaselli, O., F. Tassi, E. Duarte, E. Fernandez, R. J. Poreda, and A. D. Huertas (2010), Evolution of fluid geochemistry at the Turrialba volcano (Costa Rica) from 1998 to 2008, Bull. Volcanol., 72(4), 397–410, doi:10.1007/s00445-009-0332-4.

RIZZO ET AL. ERUPTIVE ACTIVITY AT TURRIALBA VOLCANO 17