Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska

VOLGEO-04883; No of Pages 15 Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

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Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska Cynthia Werner a,⁎, Peter J. Kelly b, Michael Doukas b, Taryn Lopez c, Melissa Pfeffer d, Robert McGimsey a, Christina Neal a a

Alaska Volcano Observatory, Volcano Science Center, U.S. Geological Survey, 4200 University Drive, Anchorage, AK 99508, USA Volcano Science Center, Cascades Volcano Observatory, U.S. Geological Survey, 1300 SE Cardinal Court, Vancouver, WA 98683, USA Alaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Drive, Fairbanks, AK 99775, USA d University of Oslo, Department of Geosciences, Oslo, Norway b c

a r t i c l e

i n f o

Article history: Received 3 June 2011 Accepted 3 April 2012 Available online xxxx Keywords: Redoubt Degassing Volatile Vapor saturation CO2 SO2 H2S

a b s t r a c t The 2009 eruption of Redoubt Volcano, Alaska was particularly well monitored for volcanic gas emissions. We report 35 airborne measurements of CO2, SO2, and H2S emission rates that span from October 2008 to August 2010. The magmatic system degassed primarily as a closed system although minor amounts of open system degassing were observed in the 6 months prior to eruption on March 15, 2009 and over 1 year following cessation of dome extrusion. Only 14% of the total CO2 was emitted prior to eruption even though high emissions rates (between 3630 and 9020 t/d) were observed in the final 6 weeks preceding the eruption. A minor amount of the total SO2 was observed prior to eruption (4%), which was consistent with the low emission rates at that time (up to 180 t/d). The amount of the gas emitted during the explosive and dome growth period (March 15–July 1, 2009) was 59 and 66% of the total CO2 and SO2, respectively. Maximum emission rates were 33,110 t/d CO2, 16,650 t/d SO2, and 1230 t/d H2S. Post-eruptive passive degassing was responsible for 27 and 30% of the total CO2 and SO2, respectively. SO2 made up on average 92% of the total sulfur degassing throughout the eruption. Magmas were vapor saturated with a C- and S-rich volatile phase, and regardless of composition, the magmas appear to be buffered by a volatile composition with a molar CO2/SO2 ratio of ~ 2.4. Primary volatile contents calculated from degassing and erupted magma volumes range from 0.9 to 2.1 wt.% CO2 and 0.27–0.56 wt.% S; whole-rock normalized values are slightly lower (0.8–1.7 wt.% CO2 and 0.22–0.47 wt.% S) and are similar to what was calculated for the 1989–90 eruption of Redoubt. Such contents argue that primary arc magmas are rich in CO2 and S. Similar trends between volumes of estimated degassed magma and observed erupted magma during the eruptive period point to primary volatile contents of 1.25 wt.% CO2 and 0.35 wt.% S. Assuming these values, up to 30% additional unerupted magma degassed in the year following final dome emplacement. Published by Elsevier B.V.

1. Introduction Airborne monitoring of gas emissions at Redoubt Volcano, Alaska, began in October 2008 following reports of H2S odors from pilots at 2750 m altitude in the Cook Inlet, workers from the Drift River Oil Terminal, and Alaska Volcano Observatory geologists in the previous 2 months (Neal et al., 2011). These were some of the first significant signs of reawakening of the volcano after 20 years of quiescence (Schaefer, 2012). We report on the CO2, SO2, and H2S gas emissions from the inception of unrest to over a year following dome extrusion, focusing on the syn- and posteruptive period. Pre-eruptive degassing is discussed in detail in

⁎ Corresponding author. Tel.: + 1 907 786 7471; fax: + 1 907 786 7425. E-mail address: [email protected] (C. Werner).

Werner et al. (2012) along with important stream sampling data. Here, we discuss the magnitude and variability of emissions in 2008–10 as compared to the 1989–90 eruptive period. Because airborne measurements of gas emissions were not possible during the explosive period of the 2009 eruption, we also evaluate the amount of “missed” gas in the airborne record by integrating satellite observations of syneruptive SO2 as reported in Lopez et al. (this issue). We discuss the cumulative amount of gas emitted in context of the eruptive behavior and erupted volumes to assess if the degassing occurred as a dominantly closed- (gas remained primarily with the magma) or open- (gas escaped from the magma from which it originated) system. We discuss if the magma was vapor saturated at depth, the CO2/SO2 ratio of that vapor, and if this was constant. Finally, we calculate and refine primary magma volatile contents by combining degassing measurements with erupted magma volumes and trends with time, and determine the amount of unerupted magma that degassed in the year following the eruption.

0377-0273/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.jvolgeores.2012.04.012

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

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C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

2. Background Redoubt Volcano (3108 m) is a glaciated stratovolcano ~ 170 km southwest of Anchorage (Fig. 1), and is one of the most active volcanoes in the Cook Inlet region. Confirmed historical eruptions occurred in 1902, 1966–68, and 1989–90. Observed unrest leading up to the 1989–90 eruption differed from the 2009 eruption in various aspects. Deep (25–30 km) long-period (DLP) earthquakes were recorded several months prior to the 2009 eruption (Fig. 2) whereas none were detected with the existing network in 1989 (see also, Power et al., this issue, 1994). There were, however, shallow long-period earthquakes detected in records back to September, 1989 (Power et al., 1994), or over 3 months preceding the 1989–90 eruption. The onset of the March 15, 2009 phreatic eruption was preceded by ~ 7 weeks of strong shallow tremor that began abruptly in late January 2009 (Buurman et al., this issue), whereas the 1989 initial eruption was preceded by weak precursory tremor with a short, intense swarm of long-period earthquakes 23 h preceding the first explosion (Chouet et al., 1994; Power et al., 1994). Juvenile material from the 1989–90 eruption consisted of hornblende-bearing calc-alkaline andesite and dacite with wholerock SiO2 ranging from 58.2 to 63.4 wt.% SiO2 (Nye et al., 1994), which is similar to some of the andesites erupted in 2009 (57.5 to 62.5 wt.% SiO2, Coombs et al., this issue). Products of the 1989–90 eruption show evidence of two-component magma mixing (Nye et al., 1994; Swanson et al., 1994), whereas the 2009 andesites show more subtle mixing relationships (Coombs et al., this issue). Swanson et al. (1994) estimated magma temperatures of 840– 950 °C, intermediate storage depths of 6–9 km, and oxygen fugacities of NNO +1.5 to 2 log units for 1989–90 magmas. Coombs et al. (this issue) demonstrates that the 2009 magmas that were erupted as final products (intermediate and high silica andesites, ISA and HSA) were fairly similar to those from the 1989–90 eruption, but that the first erupted magma (a low-silica andesite, LSA) was more mafic. Petrologic data suggests that final staging depths were between 4 and 6 km for all magma types erupted in 2009, and oxygen fugacities around NNO +1.4. Depths of storage were corroborated by modeling of Cl/S ratios determined from filter pack data collected in the plume (Pfeffer et al., this issue). Pre-eruptive temperatures of the HSAs were

Spurr Alaska

Anchorage

61o

725–840 °C (Fe–Ti oxides), whereas the LSA equilibration temperature ranged from 900 to 1000 °C (Coombs et al., this issue). The LSA was the dominant eruptive product in the explosive phase, whereas HSA and ISA erupted primarily during final dome extrusion. The conceptual model preferred by Coombs et al. (this issue) is that the LSA rose from an unknown depth in the 8 months prior to eruption, and remobilized stagnant mushy magmas (the HSA) in the mid-crust. Mixed phenocryst populations in the ISA suggest that it is a hybrid between the LSA and HSA. The 1989–90 eruption was characterized by over 20 separate explosions and 14 episodes of lava dome extrusion between December 1989 and June 1990 (Miller and Chouet, 1994), whereas the 2009 eruption was characterized by 19 explosions (nearly all within the first 7 days following the first magmatic explosion) and only 3–4 episodes of lava dome extrusion (Bull and Buurman, this issue). Despite these differences, total erupted volume was similar; a total of 0.05 to 0.13 km 3 (or 50–130 × 10 6 m 3) dense rock equivalent (DRE) magma was erupted in 1989–90, not including juvenile material in lahar deposits (Gardner et al., 1994), compared to 0.08–0.12 km 3 for 2009 (Bull and Buurman, this issue). Prior to the eruption onset in December 1989, there were no degassing measurements, but there were pilot reports of increased melting and steam plumes emanating from the crater in the month preceding the eruption (Miller and Chouet, 1994). Peak gas emissions occurred early in the eruption sequence, with satellite measurements of synexplosive degassing of 175 kt of SO2 during the initial December 1989 eruptive period (Schnetzler et al., 1994). Airborne measurements in January 1990 reported average emissions as high as 72,000 t/d CO2 and ~12,270 t/d SO2 (Hobbs et al., 1991). Based on comparisons of the degassing record to petrologic estimates of S emissions from melt inclusion data, Gerlach et al. (1994) suggested that the magma was vapor saturated with both CO2 and S at 6–10 km depth prior to eruption, consistent with the pre-eruption storage region identified petrologically by Swanson et al. (1994). Where Swanson and Kearney (2008) reported widespread, but very small, anhydrite crystals in the 1989–90 lavas and xenoliths, Coombs et al. (this issue) observed no anhydrite in initial observations of the 2009 lavas, but noted that small blebs of Cu–Fe–S solid solutions were observed. Emissions declined exponentially with time in 1990 and were at background levels by early-mid 1991 (Casadevall et al., 1994, Fig. 2). Background degassing measured at Redoubt in the inter-eruptive interval between the 1989–1990 and 2009 eruptions was b60 t/d CO2 and b10 t/d SO2 (Doukas and McGee, 2007). This lack of elevated passive degassing between eruptions is typical of Cook Inlet Volcanoes (Werner et al., 2011). 3. Instrumentation and methods 3.1. Emission rates

60o

Ke n Pe ai nin su la

Redoubt Volcano

Iliamna

Homer

Augustine 154o

0

50

100

151o

Fig. 1. Map of Cook Inlet region of Alaska showing the location of Redoubt Volcano approximately 170 km southwest of Anchorage.

All airborne data for the 2008–10 period of unrest and eruption were obtained using an unpressurized Piper PA-31 Navajo fixedwing aircraft equipped to monitor gas emissions with a few combinations of instruments. Initially, CO2, SO2, and H2S were measured using instrumentation identical to previous studies (Gerlach et al., 1997, 2008; McGee et al., 2001, 2010; Doukas and McGee, 2007). This instrumentation was comprised of a LI-COR 6252 infrared CO2 analyzer, analog Interscan models 4240 SO2 analyzer (0–2 ppm range) and a 4170 H2S analyzer (0–1 ppm range), and a Barringer correlation spectrometer (COSPEC V) for remote measurement of SO2 column abundances. Chemical measurements, ambient temperature, and Global Positioning System (GPS) coordinates and altitude were recorded simultaneously at 1 Hz. Beginning in January 2009, several measurements were conducted using only the COSPEC V, or with a COSPEC and LI-COR LI-7000 CO2 infrared (IR) analyzer that was acquired in late 2008. By early April 2009, a purpose-built instrument package, the Volcano Emissions Research Package (VERP), had been constructed

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

3

Days since first magmatic explosion

a Emission rate (t/d)

30000 25000

25

75 125 175 225 275 325 375 425 475 525 575

March 23, 2009

-175 -125 -75 -25 35000

CO2 SO2 Earthquake hypocenter depth Magmatic explosion

20000 15000 10000

b

3000 2000 1000 0

O ct N 08 ov D 08 ec Ja -08 nFe 09 bM 09 ar Ap 09 r M -09 ay Ju -09 nJu 09 lAu 09 g Se -09 pO 09 ct N 09 ov D -09 ec Ja -09 nFe 10 bM 10 ar Ap 10 r-1 M 0 ay Ju -10 nJu 10 lAu 10 gSe 10 pO 10 ct -1 0

0 0 -10 -20 -30 -40

Cumulative earthquakes

Earthquake hypocenter depth (km)

5000

Days since first magmatic explosion

c Emission rate (t/d)

30000 25000 20000

25

75 125 175 225 275 325 375 425 475 525 575

December 14, 1989

-175 -125 -75 -25 35000

CO2 SO2 Earthquake hypocenter depth Magmatic explosion

15000 10000 y = 12189e-0.01x R2 = 0.83

d

3000 2000 1000 0

Ju lAu 89 g Se -89 p O -89 ct N -89 ov D -89 ec Ja -89 n Fe -90 b M -90 ar Ap 90 r M -90 ay Ju -90 nJu 90 lAu 90 g Se -90 p O -90 ct N -90 ov D -90 ec Ja -90 n Fe -91 b M -91 ar Ap 91 r M -91 ay Ju -91 ne Ju -91 ly -9 1

0 0 -10 -20 -30 -40

Cumulative earthquakes

Earthquake hypocenter depth (km)

5000

Fig. 2. Emission rates of CO2 and SO2 for the (a) 2009 and (c) 1989–90 eruption sequences, and corresponding time series of earthquake hypocenter depths and cumulative number of earthquakes (b and d). For each eruption, the day of the first magmatic explosion was used as a common timeframe for comparing the time series (top axes). Actual dates are shown on the lower axes. Emission data for the 1989–90 eruption obtained from Hobbs et al. (1991) and Casadevall et al. (1994). Dates of magmatic explosions are indicated by yellow triangles. Lines connecting data points in (a) are drawn to help the reader follow the trends, and are not intended to suggest that shorter-term variations in the emission rate did not occur. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

and the majority of measurements reported herein (24 of 35) were collected with this system. Like previous systems, VERP integrates measurements from in situ CO2 (LI-7000) SO2 and H2S sensors (new digital models Interscan 4240 and 4170), and the COSPEC, as well as observations of temperature, pressure, and GPS coordinates and altitude. The primary differences between VERP and the original system are that VERP utilizes a LI-7000 instead of the LI-6252, new digital Interscans, and all data are collected and displayed in real time using a laptop computer and custom LabVIEW software. Technical details describing VERP are included in Kelly et al. (this issue). Examples of raw data from individual transects show that plume anomalies are tracked well in time with both the IR and chemical sensors (Fig. 3). The methods for analyzing the data were identical to procedures used in previous studies (Gerlach et al., 1997, 2008; McGee et al., 2001, 2010; Doukas and McGee, 2007); specialized

methods described in Kelly et al. (this issue) were not used throughout the entire eruption sequence. CO2 and H2S emission rates were calculated from the in situ data (Fig. 3) by the contouring method (see Fig. 4 for an example of a contoured plume cross-section) (Gerlach et al., 1997; McGee et al., 2001) and SO2 emission rates were calculated by both the contouring method and from COSPEC measurements, with COSPEC measurements being the preferred estimate (discussed below). Measurements were made during flights such that the plume was traversed first from top to bottom, and then COSPEC traverses were made just under the plume (thereby reducing light dilution effects, Kern et al., 2010). Emission rates calculated by the contouring method are considered accurate to within approximately ± 20% (Gerlach et al., 1997; McGee et al., 2001; Doukas and McGee, 2007), although they may be subject to sampling biases, which are discussed below (see also Gerlach

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

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C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

ppmv

ppmv

405 400

a) 2/7/2009

CO2

d) 4/5/2009

CO2

b) 2/7/2009

SO2

e) 4/5/2009

SO2

c) 2/7/2009

H2S

f) 4/5/2009

H2S

395 390 385 3 2 1 0

ppmv

0.2 0.1

no data

0

14:41 14:48 14:56 15:03 15:10 15:17 15:24

Alaska Standard Time

14:50

15:05

15:20

15:35

15:50

Alaska Standard Time

Fig. 3. Plume transect for CO2, SO2, and H2S for a pre-eruptive day (February 7, 2009, a, b, and c, respectively) and a day after the last magmatic explosion (April 5, 2009, d, e, and f, respectively). Note how the February data show CO2 peaks that are strong within the plume with concentrations up to 14 ppmv above background, whereas SO2 is barely above detection, and H2S is below detection. Data on April 5 show strong peaks in all three gasses.

et al., 1997, 1998). Average COSPEC SO2 emission rates are estimated to be accurate within approximately ±20%. This estimate includes uncertainty associated with the reproducibility of calibration cells (approximately ±8%), uncertainty in the concentration of the calibration cells (~±3%, Stoiber et al., 1983), uncertainty from wind speed measurements, as well as real variations in the plume during the observation interval. Wind circles (Doukas, 2002) were used for measuring wind speeds and the average uncertainty is estimated to be ±5%, based on one standard error of the measurements. Temporal variability in measured emission rates is probably one of the largest uncertainties in the data and is a function of volcanic activity and meteorological conditions. For COSPEC emission rates measured between January 2009 and August 2010, multiple passes under the plume typically showed temporal variability of about ±10% (one standard deviation about the mean) and the standard error of the mean was generally about ±5%.

33110 t/d CO2

3000 2500

meters

2000 3500

14280 t/d SO2

3000

6 4

2500

SO2 ppmv

2 0

2000 3500

meters

25 20 2 15 CO ppmv 10 5 0

Molar CO2/SO2 ratios were calculated using two different methods: (1) by taking the ratio of CO2 emission rates determined by the contouring method and SO2 emission rates determined using the COSPEC, as in previous studies (e.g., Gerlach et al., 2008), and (2) by analyzing the correlation of in situ concentrations of collocated CO2 (LI-7000) and SO2 (Interscan 4240) plume anomalies from individual plume traverses (Figs. 3 and 5). The in situ measurements were compared by selecting traverses with strong CO2 and SO2 peaks (e.g. Fig. 3d–e), calculating the areas under the CO2 and SO2 anomalies for each traverse, and then fitting a least-squares, best fit line (Fig. 5) to the results from all the selected traverses. Ratios calculated by both methods are presented in Table 1. The estimated uncertainty is about ±30% for CO2/SO2 ratios determined from emission rates (method 1), and about ±20% for ratios determined by in situ measurements (method 2) collected after April 4, 2009. Taking the ratio of emissions is consistent with many previous publications reporting gas ratios (e.g., Gerlach et al., 2008; McGee et al., 2010). However, comparison of SO2 emission rates determined by COSPEC and by contouring suggests that contouring may both under- and overestimate average emission rates (Fig. 6a). The SO2 emission rates determined by COSPEC and contouring agree within the expected uncertainty for 40% of our measurements (9 out of 22) (Fig. 6a inset). Of the remaining 13 measurements, where the mismatch is greater than the uncertainty, nearly all (11 of 13) resulted in contoured SO2 emission rates that were lower than the COSPEC (the median difference for these was − 52%). Such negative sampling

1230 t/d H2S

3000

1.2 0.9

H2S 0.6 ppmv

2500

0.3 0

2000 -6000

-4000

-2000

0

2000

4000

meters Fig. 4. Example of a vertical plume cross-section from May 4, 2009 at Redoubt Volcano shown with 5 × vertical exaggeration. This measurement recorded some of the highest emission rates for the entire degassing sequence. The scale on the right shows the mixing ratio of volcanic gas plume anomalies in ppmv. The vertical axis shows the altitude of the traverses in meters. Each dot corresponds to the GPS location of a data point (all data acquired at 1 Hz sampling rate). In this case, the spatial resolution was approximately 80–90 m in the horizontal direction (governed by the aircraft speed) and 150 m in the vertical direction.

350

CO2 peak area (ppmv)

meters

3500

3.2. Gas ratios

y = 3.31x R² = 0.98

300 250 200 150 100 50 0

0

20

40

60

80

100

SO2 peak area (ppmv) Fig. 5. The correlation of the CO2 and SO2 peak areas determined from in situ data for plume transects on April 5, 2009 (shown in Fig. 4), resulting in a CO2/SO2 ratio of 3.3 with an R2 of 0.98. All in situ ratios calculated in Table 1 used this method.

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

Date

13-Oct-08 02-Nov-08 26-Jan-09 31-Jan-09 02-Feb-09 07-Feb-09 21-Feb-09 27-Feb-09 15-Mar-09 20-Mar-09 26-Mar-09 04-Apr-09 05-Apr-09 16-Apr-09 20-Apr-09 28-Apr-09 01-May-09 04-May-09 08-May-09 14-May-09 26-May-09 03-Jun-09 03-Jun-09 07-Jun-09 11-Jun-09 01-Jul-09 03-Aug-09 20-Aug-09 23-Sep-09 02-Nov-09 31-Dec-09 08-Apr-10 09-Apr-10 21-Jun-10 20-Aug-10

COSPEC

Contour

SO2

SO2 in situ

CO2 in situ

H2S in situ

Wind speed

Wind stdev

Plume dir.

Traverse distance downwind

Plume width

Plume top

T

P

CO2/SO2 emission rate

CO2/SO2 in situ

In situ

t/d

n

t/d

t/d

t/d

t/d

m/s

m/s

degree

km

km

m

°C

kpa

molar

molar

r2

b 10 b 10 177 180 157 88 46 34 3850 938 3585 16,646 11,353 1952 12,733 13,276 8360 14,277 6559 8917 4307 4223 3824 5598 4224 2647 2066 2546 1481 623 664 494 631 406 268

3 3 2 1 4 3 1 4 2 4 3 5 7 4 6 5 4 8 3 7 4 3 4 5 2 4 4 3 4 4 4 3 4 5 5

— — 48 19 36 8 — 4 139 141 629 1336 880 138 1316 2594 484 1720 249 362 834 113 548 446 427 224 153 231 175 120 148 5 39 8 62

28 31 — 20 25 85 — — — — — 13,201 5494 4299 8509 10,663 9412 16,559 7455 12,237 3271 — — 3059 3761 1982 — — — 298 1446 308 266 160 93

1368 1220 — 7326 3628 9018 5980 — 6588 3858 10,776 20,716 12,820 8827 12,887 17,220 18,413 33,108 16,489 20,250 5935 — — 5987 4728 2836 4529 3378 2284 699 1949 514 821 587 295

11 4 — — 1 — — — — — — 586 198 202 312 536 567 1230 589 1041 172 — — 264 268 102 2 — — 10 41 8 10 5 1

6.8 3.0 5.9 3.9 7.2 9.0 7.0 3.7 7.4 5.5 5.4 11.0 6.2 2.6 5.3 10.7 14.5 7.0 7.6 7.6 6.3 4.0 2.4 4.9 3.8 5.4 13.5 3.2 4.7 4.0 6.9 3.4 5.3 4.1 5.1

0.3 0.0 0.1 0.2 1.1 0.3 0.1 0.2 1.1 0.1 0.2 0.8 0.6 0.5 0.2 1.1 1.0 0.2 0.9 0.2 0.9 0.2 — 0.5 0.2 0.1 1.0 0.3 0.2 0.3 0.3 0.0 0.6 0.4 0.3

244 307 308 36 149 1 183 60 172 349 73 145 84 240 43 170 303 163 178 181 26 207 152 148 132 92 137 150 46 93 360 165 136 230 145

4 5 5 3 5 4 4 6 4 5 16 12 25 20 24 13 9 6 10 15 18 14 24 20 23 19 15 4 4 8 6 5 5 7 7

6.9 7.0 9.0 5.2 8.4 4.5 2.0 7.0 3.2 3.3 8.0 6.0 9.8 6.0 7.9 4.0 4.3 4.9 4.1 7.4 5.7 6.0 10.0 4.6 12.7 4.8 4.2 18.0 5.9 7.5 3.0 3.3 2.8 4.6 3.8

2600 2700 — 3000 3000 3000 2600 — 2900 2700 4000 3800 4300 4100 5000 3400 3500 3500 3700 3400 3500 — 3400 3800 3800 3500 3200 2900 2900 3400 3000 3200 3078 3100 3100

− 10 −8 — − 22 − 18 − 17 −8 — − 23 − 15 − 23 −2 − 16 − 13 − 19 1 −2 −2 −8 −5 0 — 1 1 0 −1 3 4 −9 − 12 −7 − 16 -13 0 −2

75 75 — 73 72 71 79 — 72 77 60 65 67 67 65 74 74 73 70 71 72 — 73 67 69 73 73 75 74 71 74 73 74 72 73

71 57 — 59 34 149 189 — 2.5 6.0 4.4 1.8 1.6 6.6 1.5 1.9 3.2 3.4 3.7 3.3 2.0 — — 1.6 1.6 1.6 3.2 1.9 2.2 1.6 4.3 1.5 1.9 2.1 1.6

69 53 — 511 186 179 — — — — — 2.6 3.3 3.0 2.0 2.3 2.7 2.6 2.7 2.4 2.3 — 1.7 2.8 1.7 1.8 — — — 2.0 1.9 1.9 2.9 2.6 2.0

0.37 0.49 — 0.87 0.73 0.81 — — — — — 0.99 0.98 0.99 0.98 0.99 0.97 0.96 0.99 0.97 0.86 — 0.94 0.91 0.60 0.82 — — — 0.91 0.94 0.83 0.90 0.89 0.61

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

Table 1 Volcanic SO2, CO2, and H2S emission ratesa and plume dimensions measured at Redoubt Volcano during 2009–2010.

Gas emissions rounded in text. Uncertainties for emission rates are estimated at +/− 20%, see text for details, — = not measured. a Significant digits on emission rates are retained to be consistent with Werner et al. (2012) and to reduce rounding errors in calculation of ratios and cumulative emissions (Table 2).

5

6

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

a

16000

20000

COSPEC Contoured

SO2 Contoured (t/d)

18000

14000

SO2 (t/d)

12000 10000 8000

15000

10000

5000

6000

0 0

4000

5000

10000

15000

20000

COSPEC (t/d)

2000 0

b

120

Contoured

4. Results and observations

H2S (t/d)

100 800

4.1. Emission rates and CO2/SO2 ratios

600

The measured CO2, SO2, and H2S emission rates span the period from October 13, 2008 to August 20, 2010 (Figs. 2 and 6, Table 1). Emission rates of CO2 were already high (>1220 t/d) in October and November, 2008, whereas SO2 was near detection limit at that time (~30 t/d). H2S emissions were elevated during October–November, 2008 reaching nearly 11 t/d, and nearly 40% of the total molar S emitted (Fig. 6c). Following an abrupt increase in shallow seismicity at the volcano in late January 2009, CO2 emission rates increased to 3630–9020 t/d until the eruption began in March 2009 (Table 1, Fig. 2). SO2, meanwhile, remained low (≤180 t/d) until the first phreatic explosion on March 15, 2009 (Figs. 2 and 6). As of February 2, 2009, H2S emissions were barely above detection (Table 1, no detection on February 7th, Fig. 3c) and contributed b4% to total molar sulfur (Fig. 6c). This level of pre-eruptive degassing was accompanied by very large in situ CO2 peaks, sometimes in the absence of visual manifestation of such significant degassing. In Fig. 3a, we show that the CO2 peaks measured ~ 4 km downwind of the volcano (Table 1) were on the order of 14 ppmv above background concentrations in February 2009, yet SO2 peaks were very small (b0.05 ppmv), and H2S was barely above detection limits (Figs. 3b and 4c). Similarly, CO2 peaks on January 31, 2009 reached 40 ppmv above background. During the 5 months of unrest, the C/Stot (derived from CO2/(SO2 + H2S)) ratios of the emissions were high, beginning in the range of 40–50 (Werner et al., 2012, CO2/SO2 was 50–70, Table 1, Fig. 6d.), and then increasing to values of 190 once significant melting of glacial ice in the crater began in early February (Table 1, Fig. 6d). The high rates of CO2 degassing starting in January 2009, coupled with evidence of surface heating and increased melting of glaciers, small slurry deposits on the Drift glacier, and periods of heightened seismicity were strong indications that the system was experiencing significant unrest (Bull and Buurman, this issue). The lack of sulfur degassing in the precursory period stands in contrast to measurements during the main magmatic phase of the eruption when SO2 and H2S were easily detected at the ppmv level (Fig. 3e and f). CO2 and SO2 emissions were significantly elevated with respect to the pre-eruptive period starting with the phreatic eruption on March 15, 2009 (Figs. 2 and 6). CO2 exceeded 10,000 t/d in 80% of the measurements collected between March and May, 2009, and SO2 exceeded 10,000 t/d in half of the measurements during this period based on COSPEC observations. Exceptions to this behavior were conspicuous declines in both CO2 and SO2 emissions on March 20, or 2 days prior to the first magmatic explosion on March 22, 2009 (Alaska Standard Time), and on April 16th, when SO2 emissions declined by a factor of 6, and CO2 by roughly 30% in comparison to measurements 10 days before and 4 days after (Table 1). Gas ratios, however, dropped to a low value (average 2.4 for March–December, 2009, or 3.0 for March 2009–August 2010, Fig. 3d, Table 1) beginning

400 200 0

c

SO2 / Stot

0.9 0.8 0.7 0.6 0.5

10 Ju

b-

n-

10

9 -0 ct

Fe

M

Ju

ar

40

n-

-0

9

80

emission ratio in situ ratio

O

120

10 8 6 4 2 0 09

CO2/SO2

d

160

CO2 / SO2

April 16, 2009 and Dec. 31, 2009 the contoured SO2 emission rate was ~ 2.2 times higher than the COSPEC emission rate, resulting in high CO2/SO2 ratios (up to 6) calculated by method (1) but not for method (2) for these dates (Table 1). For these reasons, calculating a ratio by combining emission rates determined by contouring and remote sensing observations (method 1) may not be as accurate as an in situ ratio (method 2). Generally, the calculated CO2/SO2 ratios using both methods are similar throughout the eruption (Table 1), but the in situ ratios are considered more accurate and less prone to technique-based discrepancies. It is important to remember that remote sensing instruments are also subject to biases (Kern et al., 2010; McGee et al., 2010), and we can be most confident in emissions and ratio estimates where the various methods converge.

g-

10

0 -1 ne Ju

Au

0

10

r-1 Ap

bFe

ec D

O

ct

-0

-0

9

9

09 gAu

-0

9

9

09

r-0

ne Ju

Ap

-0

bFe

ec D

O

ct

-0

8

8

0

Date Fig. 6. Emission rates and ratios during study period. (a) SO2 emission rates with time as calculated with the COSPEC and by contouring in situ SO2 data. The inset shows the correlation of the COSPEC with the in situ SO2 data; often contouring of the in situ data underestimates the emission rate compared to COSPEC as discussed in Section 3.2. (b) H2S emission rates with time; (c) the amount of SO2 as a proportion of total sulfur with time, which increases in the pre-eruptive period, reaching greater than 0.9 by January 2009 (calculated from in situ data); (d) the proportion of CO2 to SO2 with time. Open symbols in the inset of (d) show the CO2/SO2 ratio as calculated from emission rates, whereas filled symbols show the in situ ratio (see text for description). Lines connecting data points are drawn to help the reader follow the trends, and are not intended to suggest that shorter-term variations in the emission rate did not occur. Dashed lines indicate periods with the greatest uncertainty.

biases are likely caused due to under-sampling the plume where the highest plume concentrations are not sampled with the chosen vertical traverse interval (as also suggested by McGee et al., 2010). For this reason, COSPEC emission rates are considered more reliable. However, such under-sampling would also affect CO2 data and thus act to underestimate the CO2/SO2 of a typical emissions-based ratio estimate determined by method (1). Though not as common, contouring can produce emission rates that exceed COSPEC results. This could be due to several factors, including an actual decrease in emissions during COSPEC observations, radiative transfer effects (i.e. light dilution, multiple scattering), or perhaps plume migration. For example, on

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

7

Date

last magmatic eruption 4/5/09

1.5

100

OMI data

10%

80

60 12%

12/15/89 Eruption

Cumulative Gas (10 6 t)

2.0

Dome Growth

1.0

0.5

40 Airborne and Satellite CO2 ‘08-09 Airborne only CO2 ‘08-09 Airborne and Satellite SO2 ‘08-09 Airborne only SO2 ‘08-09 Airborne and Satellite SO2 ‘89-90

Gas Emitted (%)

phreatic eruption 03/15/09

2.5

20

0 0

100

200

300

400

500

600

700

Days Since Beginning of Unrest Fig. 7. Cumulative gas emissions with time for the 2009 and 1989 eruptions showing the difference between trends when using only the airborne data versus using both airborne and satellite estimates. Estimated cumulative CO2 extrapolated from the combined airborne and satellite SO2 emission rates and assuming a CO2/SO2 ratio based on airborne data (described in text). Satellite data for SO2 published for 2009 eruption in Lopez et al. (this issue), and 1989 data in Schnetzler et al. (1994). Significant periods in each eruption are shown with dashed vertical lines; the shaded area indicates the period of dome growth in 2009. Note highest cumulative gas output during explosive eruptions, and emissions remained relatively high through the dome-growth period. Both eruptive periods indexed to a common timeframe of the beginning of reported unrest (Oct. 13, 2008 and Oct. 1, 1989). Cumulative estimates for different periods of the eruption in 2009 tabulated in Table 2. Overall cumulative airborne measurements underestimated total emissions by ~ 10–12% relative to estimates including satellite data. Missing data within the first 100 days of the 1989–90 eruption likely results in underestimated total SO2 emission.

March 15, 2009, and remained low regardless of eruptive activity and level of degassing. The highest emission rates measured using airborne techniques were observed on April 4, 2009 (~ 9 h following explosive activity), and on May 4, 2009 (Fig. 4) during a period of intense seismic activity and rapid dome extrusion (Diefenbach et al., this issue). Measurements indicated 20,720 and 16,650 t/d on April 4 and 33,110 and 14,280 t/d on May 4 (Fig. 4), for CO2 and SO2 respectively. H2S increased proportionately with other gas emissions during the eruption (Fig. 6b). The largest H2S emission rate (1230 t/d) was measured on May 4, coincident with the largest SO2 emission (Fig. 6), and second-largest CO2 emission for the Redoubt eruption (Fig. 2a). The percentage of H2S contributed to total molar S, however, remained minor throughout the eruption (Fig. 6c), increasing from b7% during the May 2009 seismic swarm to maximum of 14% in June 2009. The average contribution of H2S to total molar S was 8% from February 2009 to August 2010. Airborne emission rates could not be measured regularly during a 10-day explosive period between March 22 and April 4 (Alaska Standard Time) due to ash in the plume (only one measurement was

made during this period on March 26, 2009, Table 1). As discussed in detail in Lopez et al. (this issue), and integrated into our discussion below, emission rates derived from Ozone Monitoring Instrument (OMI) total mass data during this period ranged from below detection limit to ~ 84,100 t/d SO2 (averaging ~25,700 t/d). Emissions of both CO2 and SO2 declined exponentially subsequent to the May 2009 peak in emissions (Fig. 2). Approximately 70–80% of the degassing was complete by the end of dome growth in July 2009 (Fig. 7), yet emissions stayed elevated above the inter-eruptive background levels for Redoubt and other Cook Inlet volcanoes (Werner et al., 2011) for approximately 1 year following the cessation of dome growth. The last measurements reported herein (August, 2010) were on the order of a few hundred t/d of CO2 and SO2. 4.2. Cumulative emissions Cumulative emissions were calculated using airborne measurements only, and including the contribution from OMI measurements (Lopez et al., this issue). Cumulative emissions were calculated as the integrated amount of gas emission over a period of time assuming

Table 2 Cumulative emissionsa over different time periods and percent of total degassing, including OMI estimates. Time period

Oct 13, 2008–March 15, 2009 (precursory degassing) March 15–April 4, 2009 (explosive period only) April 4–July 1, 2009 (final dome growth period) July 1, 2009–August 20, 2010 (post eruptive degassing) Total degassing (Oct 13, 2008–August 20, 2010) a

ECO2

ESO2

% Total

% Total

Cum %

Cum %

t

t

ESO2

ESO2

ESO2

ESO2

313,448

47,116

14

4

14

4

730,842

422,617

32

33

46

37

611,917

419,672

27

33

73

70

616,188

382,401

27

30

100

100

2,272,395

1,271,805

100

100

Significant digits on emission rates were retained in calculation of cumulative emissions to mitigate rounding errors. Emissions rounded in text.

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

8

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

linear interpolation between measurements. The total CO2 and SO2 output calculated from airborne measurements were 2.05 × 10 6 and 1.11 × 10 6 tonnes, respectively (Fig. 7) between Oct. 13, 2008 and Aug. 20, 2010. If we include the contribution of SO2 emission on all days when OMI analyses were completed (Lopez et al., this issue) and a gas flight had not been completed (i.e., on days with both, the airborne measurement was used), the cumulative total SO2 increases to 1.27 × 10 6 tonnes (Table 2), or roughly 12% higher over the same time period (Fig. 7). It should be noted, however, that OMI analyses were only conducted through June 12, 2009, therefore measurements before March 23, 2009 and after June 12, 2009 were calculated using airborne methods alone. We estimated how the lack of airborne measurements during the peak of the eruption might have affected total CO2 by pairing the airborne CO2/SO2 ratio with OMI measurements in the days following the airborne measurement. CO2/SO2 ratios were multiplied by the OMI emission rates subsequent to that airborne measurement until the next airborne measurement, thus assuming the CO2/SO2 ratio stayed constant between airborne measurements, which was generally true within 0.5 of a unit. Given this assumption, cumulative CO2 emissions over the period mentioned above would increase to 2.27 × 10 6 t (Table 2), or about 10% higher (Fig. 7). 5. Discussion 5.1. Pre-eruptive conditions and conceptual model of eruption The geochemical data set collected prior to the eruption was one of the most complete sequences from an arc volcano. Measurements included airborne emission rates of CO2, SO2, and H2S, stream geochemical sampling and flow rate estimation, and glacial ice–melt rates (Bleick et al., this issue) starting 6 months prior to the 2009 Redoubt eruption, as discussed in Werner et al. (2012). A summary of that work is presented below. Early in the unrest (October–November, 2008), CO2 emissions were elevated while SO2 was low, resulting in high CO2/SO2 ratios. Analysis of water samples from streams draining from beneath Drift glacier down valley from the vent area indicated that scrubbing of sulfur by the glacial meltwaters played a relatively minor role in obtaining a high CO2/SO2 ratio in gas emissions (Werner et al., 2012). With pronounced increases in glacial ice melt starting in late January throughout February, 2009 (Bleick et al., this issue), SO2 emissions, which had climbed to ~180 t/d (Table 1), started to decline, reaching b50 t/d again by the end of February (resulting in a CO2/SO2 of 190). Meanwhile CO2 emissions were extraordinary for a non-erupting volcano. Concentrations of sulfate (SO42 −) in the stream draining the summit region increased during this period, suggesting that shallow scrubbing of SO2 by ice melt was taking on a more significant role than earlier (Werner et al., 2012). However, based on the observed SO42 − concentrations, hydrogen and oxygen isotopic composition of the meltwaters, and estimated flow rates, Werner et al. (2012) suggest that the streamflow represented a mixture of condensed magmatic steam and glacial melt water that had a residence time in the summit area just long enough to allow pH neutralization (to ~ 5) through reaction with local rocks, sediment, or both (Werner et al., 2012). The data suggest that meltwaters scrubbed a few hundred t/d of SO2, but not the >2100 t/d SO2 expected from degassing of a magma in the mid-to upper-crust with a CO2/SO2 ratio of 2–2.4 at the observed CO2 emission rates (Werner et al., 2012). The primary CO2/SO2 ratio of the magmatic degassing was assumed from measured compositions during eruptive and unrest events at Redoubt and other Cook Inlet volcanoes in the past (i.e. CO2/SO2 ~ 2, Werner et al., 2011), and was observed following the onset of the 2009 eruption (Table 1). Thus, Werner et al. (2012) argue that high CO2 emission rates and high CO2/SO2 starting 5 months prior to the onset of the magmatic eruption in March

2009 were likely due to degassing of the low silica andesite in the mid- to lower crust (prior to mixing with the cooler high silica andesite), but that deep hydrothermal processes (i.e. scrubbing within the edifice, e.g. Symonds et al., 2001) could not be ruled out. Evidence for deep magma migration prior to the 2009 eruption is provided by other studies. For instance, GPS monitoring detected farfield horizontal displacements as early as May 2008. Strong near-field deformation associated with the explosive phase was consistent with a volume reduction from a mid- to deep-crustal elongate chamber down to 11 km depth, though the deeper end of the reservoir is poorly constrained (up to 18 km depth, Grapenthin et al., this issue). Starting in December 2008, seismic monitoring detected long-period earthquakes in the lower crust (between 28 and 32 km depth), suggesting that magma movement near the base of the crust was occurring (Power et al., this issue). Finally, while the petrologic data from the eruption can only definitively show that the final staging of all erupted products was in the middle to upper crust (4 to 6 km below the edifice), Coombs et al. (this issue) suggest that the low silica andesite that erupted first rose from an unknown depth in the months prior to the eruption. Textual evidence suggests that this magma mixed with the other magma types to form hybrid andesites and initiated eruption of these magmas. There was no evidence of mixing with a mafic magma, even though repeated intrusion of mafic magma is thought to drive explosive eruptions at Redoubt and other volcanoes (Nye et al., 1994; Swanson et al., 1994; Wolf and Eichelberger, 1997; Scaillet and Pichavant, 2003; Moune et al., 2009). Coombs et al. (this issue) leave open the possibility that the high CO2 degassing was involved in the rejuvenation of the shallow magma bodies in the mid-crust through a mechanism called ‘gas sparging’ (Bachmann and Bergantz, 2006; Coombs et al., this issue), but this is not their favored model. While gas percolation around the magma bodies would be consistent with the degassing observations, percolation through the magma bodies is not supported by the dominant closed-system degassing behavior and explosivity of the eruption that was observed, as is demonstrated below. An alternate scenario discussed in Werner et al. (2012) that could have resulted in high pre-eruptive CO2/SO2 ratios is that the magmatic gas was transported through a saturated magmatic–hydrothermal system, removing nearly all SO2 in that process. While no hot springs are known to exist on the flanks of the volcano (Motyka et al., 1993), it is plausible that such a system could exist at Redoubt (Burnham, 1997; Giggenbach, 1997; Motyka et al., 1993; Simon and Ripley, 2011). Furthermore, seismic tremor in the month preceding the eruption is thought to have had a boiling or ‘hydrothermal’ signature consistent with the growth and collapse of bubbles (Buurman et al., this issue). Whether the edifice would have been capable of scrubbing thousands of t/d of SO2 given the very high gas flow rates observed (Table 1), remains debatable (Shinohara, 2008; Symonds et al., 2001). Regardless of the specific origin of the early magmatic degassing or the role of deep hydrothermal scrubbing potential, the pre-eruptive data highlight that CO2 emission rates and CO2/SO2 ratios are key, but often unmeasured, precursors to eruption at arc volcanoes. 5.2. Eruptive degassing The magnitude of eruptive degassing at Redoubt was consistent with other Cook Inlet volcanoes, all of which do not exhibit significant open-system degassing between eruptions (Werner et al., 2011, and references therein). Average emission rates during the eruptive period (March 15–July 1, 2009) were 12,590 and 7070 t/d of CO2 and SO2, respectively, with maximum values slightly more than a factor of two higher than the averages (Table 1). Eruptive emissions of CO2 at Redoubt were comparable to peak emissions during the 1980–81 eruption of Mount St. Helens, whereas SO2 emissions were higher (peak SO2 emissions were between 2000 and 3500 t/d at Mount St. Helens) (McGee and Casadevall, 1994). As one might expect, SO2 emissions

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

during the (short-lived) eruptive phase at Redoubt were substantially higher than peak emissions at other andesitic volcanoes such as Soufriere Hills that experience significant open-system degassing between periods of explosive activity (Edmonds et al., 2003). Average CO2 and SO2 emissions during the eruptive period are equivalent to 5.4 and 13%, respectively, of the global daily average flux from arc volcanoes as calculated by Fischer (2008), whereas extending the observation window to include all emissions over 2009 reduces the contribution to 2 and 6% of global annual fluxes. Such high percentages, as cautioned by Hilton et al. (2007), highlights that eruptive periods display elevated levels of degassing with respect to quiescent periods. Thus, both regular measurements between eruptions as well as high-frequency measurements during eruptive periods are needed to accurately estimate time-averaged emission rates of volatiles from arc volcanoes. The CO2/SO2 ratio of the gasses following the onset of the eruption (stable molar composition of 2.4 ± 0.1, where the uncertainty is one standard error of mean) is similar to the CO2/SO2 ratio of volcanic gasses from other silicic to andesitic volcanoes worldwide (Scaillet and Pichavant, 2003; Wallace and Edmonds, 2011). Unfortunately, data coverage during the explosive phase of the eruption is sparse. As such, it is difficult to specifically assess if the low silica andesite that erupted mainly in the explosive phase had a vapor phase CO2/SO2 that differed from the high and intermediate silica andesites, which comprised the majority of the dome growth. The one airborne measurement made during the explosive phase gave a slightly elevated CO2/SO2 emissions ratio (4.4 on March 26, 2009, Table 1) compared to the measurements made during the dome growth period. However the difference is minor and the lack of in situ data for this measurement leaves the significance of this ratio in question. Note that elevated CO2/SO2 emissions ratios were also observed on April 16 and December 31, 2009, that were not confirmed with the in situ data. The very high emission rates of CO2 and SO2 coupled with a fairly constant CO2/SO2 beginning with the March 15 phreatic eruption points to the release of an exsolved vapor phase, which is discussed in detail below. While SO2 was clearly the dominant sulfur species emitted to the atmosphere during the eruption, H2S emission rates were some of the highest ever reported, certainly for Alaskan volcanoes (Doukas and McGee, 2007; McGee et al., 2010). In addition, H2S displayed increasing trends with CO2 suggesting that SO2 was less dominant at depth. Specifically, during the height of the eruptive activity emission rates of b2000 t/d CO2 resulted in SO2/Stot ratios > 0.94, whereas CO2 emission rates greater than this resulted in SO2/Stot between 0.86 and 0.94. Such trends are consistent with thermodynamic modeling of other oxidized and silicic magmatic systems that demonstrate that H2S is the dominant sulfur species at depth (e.g. at Mount St. Helens, Gerlach et al., 2008), or at least has a more dominant role relative to SO2 (Clemente et al., 2004). Chlorine degassing as measured through filter pack analysis of S/Cl ratios (Pfeffer et al., this issue) was not detected until the end of the explosive period (April 4), even though evidence existed for shallow degassing of magma prior to that time (Pfeffer et al., this issue). Magmas that erupted explosively from depth retained significant Cl in the melt (0.15–0.2 wt.% Cl), whereas slow extrusion allowed for further exsolution (down to 0.03 wt.% Cl in glass, Coombs et al., this issue). Cl concentrations in the pumice fragments from explosive eruptions may reflect the pre-eruptive Cl content in the melt at the depth of storage (Coombs et al., this issue). These data support a pressure dependence of the amount of Cl retained in the melt, a dependence on the rate at which the magma reached the surface (Balcone-Boissard et al., 2010), or post-emplacement loss of additional Cl as dome crystallization of microlites would drive up SiO2, thereby driving down the solubility of Cl (e.g. Harford et al., 2003). HCl emissions calculated from pairing S/Cl ratios from filter pack measurements with the SO2 emission rates measured in this study

9

resulted in HCl emissions on the order of 1200–3600 t/d between April 4 and May 4, 2009, a period of active dome extrusion (Pfeffer et al., this issue). This range of emission rates is comparable to the average emission rates for the 1989–90 dome growth period at Redoubt (~1760–2100 t/d HCl, Gerlach et al., 1994), and also similar to emission rates published for other active andesitic systems (e.g. up to 13,600 t/d at Soufriere Hills, Edmonds et al., 2002). 5.3. Comparison of 2009 with 1989 emission rates Emission rate trends with time in 2009 were very similar to that observed in 1989–90 following the first magmatic eruption (Fig. 2), despite differences in the details of the eruptive style and duration. Emissions in excess of 10,000 t/d CO2 and SO2 were observed during the height of both eruptions. The 1989–90 eruption was characterized by explosive activity associated with repeated dome growth and failure for over 4 months (125 days) after the first magmatic eruption, whereas the 2009 eruption experienced explosive activity for only 2 weeks, followed by months of dome growth. The magnitude of degassing 125 days after the first magmatic eruption (Fig. 2) in both eruption sequences was, however, consistent (on the order of 2500–5000 t/d SO2). In 2009, some of the highest emission rates were measured coincident with a significant seismic swarm (Buurman et al., this issue) approximately 1 month following the last magmatic explosion (May 2009) during a period of elevated lava extrusion rate (Diefenbach et al., this issue), whereas the 1989 dataset was sparse during this time (~50 days since the first magmatic eruption, Fig. 2). Following that period, the 2009 emission rate began an exponential decline in emissions with nearly the same time constant as that observed in 1990–1991 (Fig. 2). Emissions dropped to non-eruptive, but still elevated levels for Cook Inlet volcanoes (i.e., b1500 t/d, Werner et al., 2011) around 275–325 days following the first magmatic eruption in both the 1989–90 and 2009 degassing sequences (Fig. 2). The last measurements reported herein (August, 2010, Table 1) were a factor of 2–3 higher than those measured during the same period (roughly 525–550 days, Fig. 2) following the 1989–90 eruption. 5.4. Cumulative gas emissions in 2009 and 1989 Nearly 14% of the cumulative CO2 liberated for the 2008–09 sequence occurred prior to the first phreatic eruption on March 15, 2009 (Fig. 7, Table 2), while the SO2 output relative to the total eruptive degassing was near zero (b4%). The first 9–10 days of the magmatic eruption recorded some of the highest emission rates and overall made up approximately 28 and 29% of the total growth for cumulative CO2 and SO2 through August, 2010, respectively (Fig. 7). In the period directly following the explosive eruptions, slopes of the cumulative trends for CO2 and SO2 are higher than that of the trends that include satellite data (Fig. 7), suggesting that the OMI estimates of the gas emissions are lower on average than as measured by airborne techniques, as discussed in Lopez et al. (this issue). There exists a distinctive break in slope in the cumulative CO2 trend at the end of the May period of increased extrusion and the start of the exponential decline associated with passive degassing. By the end of dome growth (July 1, 2009, Diefenbach et al., this issue), ~70% of the cumulative CO2 and SO2 had been emitted. The fact that the majority of gas emitted during periods of explosive activity (~30%) or dome growth (30%, Table 2) suggests that the majority of degassing occurred as a closedsystem, meaning that most of the exsolved gas stayed with the magma until eruption or very near the surface. The minor increase in cumulative degassing by including the satellite-based data (10–12%, Fig. 7) suggests only a minor deficiency in the airborne record. Because airborne measurements could not be collected during explosive eruptions, validation of the calculated OMI SO2 emission rates for explosive eruptions is not possible, yet

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

10

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

comparisons between OMI satellite and airborne SO2 emission rates during non-explosive periods suggest that the OMI methods underestimate SO2 in most cases by ~−28% (total range of +23% to − 79) (Lopez et al., this issue). Thus, applying an average +28% correction to the OMI-derived emission rates during the explosive period when airborne measurements could not be made (March 27–April 3, 2009) had a follow-on effect of increasing the total cumulative degassing by less than 3%. It follows that airborne measurements appear to do a reasonable job of estimating total gas emissions if measurements are frequent, but pairing airborne measurements with OMI-derived emission rates likely results in more accurate estimates of total degassing. Significant gaps exist in the time series of the emission rates measured during the first 100 days of the 1989–90 eruptive activity (Figs. 2 and 7), though the emission rates measured after day 100 are much better constrained (Casadevall et al., 1994) than for the 2009 data presented here. As the highest emission rates were measured in the first 100 days of the eruptive period in 2009 (Fig. 2), we infer the total degassing for the 1989–90 eruption (0.88 × 10 6 t, Fig. 7) is possibly underestimated. The total erupted material in the 2009 eruption (0.08–0.12 km 3, Bull and Buurman, this issue) is nearly identical to that of the 1989–90 eruption (Gardner et al., 1994) and thus it is plausible that emission rates should be comparable. 5.5. Calculating primary volatile contents from cumulative emissions and eruptive volumes Primary volatile contents of magmas at depth are difficult to constrain, especially for gasses like CO2 that exsolve from melts at extreme pressures in the lower crust and even upper mantle (Mysen et al., 1975; Holloway and Blank, 1994). Thus, melt inclusions, which record pre-eruptive conditions at depth, will often not accurately reflect a primary CO2 content due to previous separation into the gas phase (Blundy et al., 2010). The same is also true for sulfur in felsic melts such as those in this study, where considerable decreases in the solubility of S occurs with increases in SiO2 (Wallace and Edmonds, 2011). If, however, a gas emission record is dense enough in time, and good estimates of eruptive volumes also exist, then the original primary volatile content can be estimated from

these data (Gerlach et al., 1994, 2008). The calculations assume that the degassed vapor originated from the erupted magma, which while speculative, is arguably most appropriate for volcanoes like Redoubt that do not experience significant open-system degassing between major eruptions (Doukas and McGee, 2007). We consider three different periods when the gas emissions and volume of erupted magma were well constrained (Table 3). To estimate the volatile content we first make the simplifying assumption that the (OMIadjusted) emission of gas measured during a given time period was initially dissolved in the melt and lost (exsolved to the gas phase and released to the atmosphere) during eruption. The CO2 content in the melt is calculated using the relationship (Gerlach et al., 1994):   9 ΔCO2m ¼ 10 ECO2 =ρm ϕm V

ð1Þ

where ΔCO2m is the amount of CO2 that has been lost from the melt in ppm (later converted to wt.% by multiplying times 10 − 4), ECO2 is the mass of CO2 emitted in tonnes (Table 3), ρm is the melt density in kg/km 3 (2.2 × 10 12; Coombs et al., this issue), ϕm is the melt volume fraction (0.52 ± 0.07 based on eruptive products; Coombs et al., this issue, and a value of 1 is used for an all melt endmember), and V is the DRE volume of the erupted magma (0.08–0.12 km 3; Bull and Buurman, this issue). Similarly, we also calculate the primary S content of the melt:   9 ΔSm ¼ 10 ESO2 =2ρm ϕm V

ð2Þ

where ΔSm is the amount of sulfur that has been lost from the melt in ppm and the constant 2 takes into the account the difference in mass formula weights of SO2 and S. For the main eruptive and effusive phase of March 15 to July 1, 2009 when 1.3 Mt of CO2 and 0.84 Mt of SO2 (0.42 Mt S) were emitted, we calculate ΔCO2m of 0.9–1.7 wt.% and ΔSm of 0.3–0.5 wt.% (Table 3). If we assume that all the CO2 and SO2 emitted during the eruption and pre-eruptive period (from Oct. 13, 2008 to July 1, 2009; 1.6 Mt CO2, 0.88 Mt SO2 or 0.44 Mt S) was originally sourced from the erupted magma volume, the ΔCO2m value increases to 1.1–2.1 wt.% and ΔSm increases to 0.3–0.6 wt.% (Table 3). If we

Table 3 Cumulative emissions over different time periods and calculated volatile contents. Volume magma

ΔCO2m

ΔSm

ΔCO2m normalizedb

ΔSm normalizedb

ΔCO2m melt frac = 1

ΔSm melt frac = 1

km3

wt.%

wt.%

wt.%

wt.%

wt.%

wt.%

0.06 0.04

0.9 1.8

0.27 0.53

0.8 1.5

0.22 0.44

0.6 0.8

0.16 0.24

(2) March 15–July 1, 2009 (eruptive period—explosive and dome growth only) 1,342,759 421,144 2.2 0.59 0.12 1,342,759 421,144 2.2 0.45 0.08

0.9 1.7

0.27 0.53

0.7 1.4

0.22 0.44

0.5 0.8

0.16 0.24

(3) May 4–16, 2009 (period of heightened dome extrusion) 235,598c 50,663 2.2 0.59 235,598c 50,663 2.2 0.45

0.0124 0.0124

1.5 1.9

0.31 0.41

1.2 1.6

0.26 0.34

0.9 0.9

0.19 0.19

(4) Oct 13, 2008–July 1, 2009 (eruption including precursory degassing) 1,656,208 444,702 2.2 0.59 1,656,208 444,702 2.2 0.45

0.12 0.08

1.1 2.1

0.29 0.56

0.9 1.7

0.24 0.47

0.6 0.9

0.17 0.25

0.34 0.67 0.36 0.71

0.9 1.3

0.24 0.36 0.26 0.38

Time period

ECO2

ES

t

t

Melt density 1012 kg/km3

(1) March 15–April 4, 2009 (explosive period only) 730,842 211,309 2.2 730,842 211,309 2.2

(5) Oct 13 2008–August 2010 (all degassing 2,272,395 635,903 2,272,395 635,903 Including H2S 675,558 675,558 a b c

Melt fraction (0.52 +− 0.07)a

0.59 0.45

and eruptive volume, assumes no passive degassing of unerupted magma) 2.2 0.59 0.12 1.5 0.41 1.2 2.2 0.45 0.08 2.9 0.80 2.4 2.2 0.59 0.12 0.43 2.2 0.45 0.08 0.85

Coombs et al. (this issue). Normalized by a whole-rock density of 2653 kg/m3, Coombs et al. (this issue). The cumulative emission for May 16th was linearly extrapolated.

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

consider a period of heightened effusive activity from May 4–16th, 2009 when 0.24 Mt of CO2 and 0.01 Mt of SO2 were emitted, we calculate ΔCO2m of 1.5–1.9 wt.% and ΔSm of 0.3–0.4 wt.%. Finally, the only increase we see over this range of estimates is if we assume that all the degassing from Oct. 13, 2008 to August 2010 was sourced from the erupted magma, which results in ΔCO2m of 1.5–2.9 wt.% and ΔSm of 0.4–0.8 wt.% (Table 3), and the upper limits are obtained only when the lowest erupted volume is paired with the lowest melt fraction. These upper limits are likely overestimated because it is reasonable to expect that degassing of unerupted magma occurred in the year following the end of lava extrusion and that some amount of degassing from the dome lavas contributed to the observed gas emissions following emplacement. These estimates of primary CO2 and S span a large range mainly due to the uncertainty in the volume of erupted material (Bull and Buurman, this issue) and melt content (Coombs et al., this issue). In general, however, they are only slightly higher than the maximum proposed estimate for the 1989–90 eruption using the same methods (ΔCO2m = 1.1 wt.%), which was considered a minimum due to uncertainty both in the total gas emission and the CO2/SO2 ratio (Gerlach et al., 1994). Gerlach et al. (1994) suggested a whole-rock normalized value (calculated by normalizing to the whole-rock density) of 0.6 wt.% CO2, whereas our values span from 0.7 to 1.7 wt.% CO2 (for all periods October 2008–July 2009). The lowest estimates (ΔCO2m = 0.5–0.9 wt.%) are calculated if we assume a melt fraction of 1 (Table 3). Overall, it is plausible that the highest primary CO2 melt concentration of 1.7 wt.% is valid as rare melt inclusions do record such concentrations at arc volcanoes (0.4–1.7 wt.% CO2 at Mount St. Helens, Blundy et al., 2010), yet Blundy et al. (2010) recognize that perhaps these are not primitive inclusions due to the low H2O contents. We suggest that more moderate contents (ΔCO2m ~ 1.25 wt.%) are perhaps more reasonable in the case for Redoubt as is discussed below. The estimates of primary sulfur content (ΔSm = 0.27–56 wt.%, or 2700–5600 ppm) were fairly consistent regardless of which time period was chosen. Whole-rock normalized values drop to 0.22–0.47 wt.% S, and assuming the magma had experienced no crystallization (i.e., setting the melt fraction to 1), results in 0.16 to 0.24 wt.% S (Table 3). These estimates in S content are similar to of the range of 2200–3900 ppm S for the 1989–90 magmas (Gerlach et al., 1994) and typical of primitive magmas from arc settings that typically contain 900–2500 ppm (Wallace, 2005) but sometimes extend up to 7000 ppm S (Métrich et al., 1999; Wallace and Edmonds, 2011). While H2S emissions were not included in all calculations to be comparable with other studies, the estimation of primary sulfur content was found to be relatively insensitive to this addition (Table 3), which is expected given that on average H2S was b 10% of the total S throughout the eruption (Table 1, Fig. 6c). 5.6. Vapor saturation of the magma Multiple lines of evidence support the hypothesis that the magma was saturated with a carbon and sulfur-rich volatile phase. From a petrologic perspective, the composition of plagioclase crystals from low silica andesites (LSA) erupted in the beginning of the eruption are also consistent with growth under H2O-saturated conditions with about 4.0–4.9 wt.% H2O dissolved in the melt (determined by plagioclase– melt hygrometry), and temperatures between ~890–960 °C based on oxide pairs (Coombs et al., this issue). Without melt inclusion analysis, however, it is impossible to assess how much water would have been in the vapor phase. Given the range of water expected to be dissolved in the melt based on petrologic analysis, we estimate how much CO2 would have been dissolved at various pressures (Newman and Lowenstern, 2002), as shown in Table 4. The results show that the dissolved CO2 concentrations for a rhyolitic melt at 925 °C with 4.0–4.9 wt.% H2O

11

Table 4 Saturation pressures for rhyolitic melt at 925 °C with 4.0–4.9 wt.% H2O and various dissolved CO2 concentrationsa. H2O (wt.%)

CO2 (ppm)

P (MPa)

Depth (km)

4.0 4.0 4.0 4.0 4.4 4.4 4.4 4.4 4.9 4.9 4.9 4.9

0 100 1000 2000 0 100 1000 2000 0 100 1000 2000

115 133 285 439 135 152 303 455 160 177 326 475

4.3 5 10.8 16.6 5.1 5.7 11.4 17.2 6 6.7 12.3 17.9

Dissolved H2O content and reservoir temperature are based on estimates by Coombs et al. (this issue). a Saturation pressures calculated with Volatile Calc (Newman and Lowenstern, 2002).

at various pressures and depths are trivial with respect to the primary volatile contents calculated from the emissions in Table 3. For instance, at 150 MPa (~6 km depth, assuming a pressure depth conversion based on the granitic bedrock, 2700 kg/m 3) and 925 °C, a rhyolitic melt with 4.4 wt.% H2O could only dissolve ~ 100 ppm CO2 (Table 4, Newman and Lowenstern, 2002), which suggests that minimally 98% of the CO2 was present as a vapor phase in the shallow storage region. Even if we assume the magma was originally much deeper in the crust (10–18 km), only 1000–2000 ppm CO2 could have dissolved in the melt (Table 4), again suggesting that the majority of the CO2 was in the vapor phase. Likewise, models of sulfur solubility also suggest that minimal sulfur (200–1000 ppm) remains dissolved in hydrous and oxidized rhyolitic melts at 150 MPa, though the estimates around NNO + 1.5 are quite varied (Clemente et al., 2004; Liu et al., 2007; Moretti and Baker, 2008; Baker and Moretti, 2011). If sulfur contents were originally higher in the primary magma, as would be expected for a hotter and more mafic magma that drove this eruption (Coombs et al., this issue) and typically drive eruptions in the region (Nye et al., 1994; Swanson et al., 1994; Wolf and Eichelberger, 1997; Scaillet and Pichavant, 2005; Moune et al., 2009), then likely a substantial percentage of the sulfur was also in the vapor phase. While no glass or melt inclusion volatile data are available for the 2009 eruption, the highest recorded concentrations were b60 and 360 ppm S in the rhyolitic and dacitic melts, respectively, for the Redoubt 1989–90 eruption (Gerlach et al., 1994). Assuming 2009 melts had similar dissolved S melt contents, this would suggest that a minimum of 78% of the S would have been in the vapor phase at depth based on the primary contents in Table 3. The existence of a S-rich vapor phase at intermediate depths is consistent with high partition coefficients determined from experimental data for felsic melts (Webster and Botcharnikov, 2011; Webster et al., 2011) and is commonly observed for silicic systems worldwide, as reviewed by Wallace and Edmonds (2011). Thus, overall the results lead us to conclude that a H2O–CO2–S-rich accumulated vapor phase was present at depth and was likely the source for the majority of volatiles emitted over the course of the eruption. 5.7. Perturbation of the system in May 2009 A perturbation of the ongoing dome-building eruption occurred in May 2009, starting approximately 1 month following the last explosive event. The perturbation was characterized by an intense pulse of higher gas emissions (Figs. 2 and 8) and seismic activity (Buurman et al., this issue), which in turn (or simultaneously, it is not clear from the data as shown in Fig. 8) led to an increase in the

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

50 45

1.5

40 1.3 35 1.1

average emission in late April

0.9

25

0.7

20

0.5 3-Apr-09

30

100

S = 0.27 wt. %

dome volume 6 3 (10 m )

15 S = 0.35 wt. %

13

80

11 S = 0.45 wt. %

60

9

July 1

7 40

S = 0.8 wt. %

C/S Molar

Cummulative Gas (10 6 t)

1.7

55

Airborne CO2 May period Airborne and Satellite SO2 accelerated dome growth dome volume 6 3 (10 m )

Estimated Cumulative Magma Volume (10 6 m3 DRE)

1.9

Cumulative Dome Volume (10 6 m3)

12

5 20 ave. = 2.4

1

0 12-Mar-09

3

20-Jun-09

28-Sep-09

6-Jan-10

16-Apr-10

25-Jul-10

15 13-Apr-09 23-Apr-09 3-May-09 13-May-09 23-May-09

2-Jun-09

12-Jun-09

Fig. 8. A detail of cumulative magma erupted (dome growth, Diefenbach et al., this issue) and degassing during the May period of increased dome extrusion. Note that the CO2 emissions in May were on average higher than had been observed in April.

rate of dome growth (Diefenbach et al., this issue). Samples of lava erupted during this period, paradoxically, reflect a compositionallydistinct and cooler high-silica andesite magma nearly identical to a small-volume component of the final 1990 lava dome (Coombs et al., this issue). When we apply Eqs. (1) and (2) to period between May 4 and May 16, 2009, when emissions and magma extrusion were well constrained, we calculate volatile contents that are slightly higher, but fairly consistent with those calculated for the whole eruptive period (Table 3). Thus, the vapor phase composition appears to be independent of magma composition and degree of evolution. The reason for the increase in emission rate and magma extrusion is unclear, but what is remarkable is that the conduit system was able to release this quantity of gas (50 kt/d of CO2 and SO2 alone) and extrude a viscous, vesicular, and cooler magma without explosion. If the increased degassing and lava effusion rates in May 2009 resulted from recharge into the magma storage region or closer to the surface, then it suggests this process was pulsatory rather than continuous in nature and lasted on the order of weeks.

5.8. Passive degassing of unerupted magma and narrowing the range of primary volatile contents As the majority of gas was emitted during the period of active lava effusion or explosive activity (Fig. 7), it follows that the volatiles were mainly trapped as bubbles within the melt. However, passive, open-system, degassing of un-erupted magma occurred for over a year following the cessation of dome growth. Specifically, ~ 25% of the total CO2 (0.61 Mt) and SO2 (0.38 Mt) was emitted from July 2009 to August, 2010, after lava effusion had ceased (Fig. 7, Table 2), which suggests that volatiles could separate and be transported independently of melt. One way to estimate the amount of un-erupted magma, and narrow the range of the calculated pre-eruptive volatile contents, is to convert the individual degassing measurements into estimated magma volumes degassed with time and compare volume trends calculated from degassing to actual trends of magma volume erupted (Figs. 9 and 10). Only certain volatile contents will result in consistency between the two volume trends. The results require that (1) the degassing trend is a function of the magma volume erupted, (2) the erupted magma had a constant original volatile content, and (3) little open system degassing occurred prior to eruption. This analysis was completed for two periods, the dome-growth period (April, 2009– July–August, 2009) when magma volume erupted was best constrained (Fig. 9), and over the course of the whole degassing sequence for which the final erupted magma volume was 0.12 km 3 (Bull and Buurman, this issue) (Fig. 10).

Fig. 9. Cumulative magma degassed from the onset of the final dome extrusion (April 4, 2009) until the end of the eruption based on dome extrusion measurements (orange triangles, Diefenbach et al., this issue) and calculated using the SO2 emission rate data and assuming original melt contents that varied from 0.27 to 0.56 wt.% S (red squares). Best correlation between dome extrusion and degassing is obtained when 0.35 wt.% S is assumed. Average CO2/SO2 ratios (white squares) during this period also are shown to increase slightly for four consecutive measurements during the period of increase dome growth in May 2009. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

In Fig. 9, the magma volume calculations are indexed with a common start date of April 4, when the final dome began growing (Diefenbach et al., this issue). The cumulative degassing measurements converted into magma volumes are shown in Fig. 9 for a variety of assumed primary S contents. A S content of 0.35 wt.% results in a close fit to the slope of the erupted dome volume, as measured by photogrammetry (Diefenbach et al., this issue), and results in complete degassing of the dome lava by July 1, 2009, when the dome stopped extruding (Fig. 9). Using a primary S content of 0.27 wt.% (the minimum from our calculations, Table 3) indicates that the magma would have degassed completely prior to being extruded, which is not likely. Assuming a higher primary S content of 0.56 wt.% indicates that the magma associated with dome extrusion would not be completely degassed until January 2010, which also seems unlikely. While we will never be able to determine the original volatile content, this exercise allows us to refine our original range to values that match the dome extrusion data more closely. The consistency in time of the trends of magma volumes over the dome-growth period as shown by sulfur degassing in Fig. 9, assuming a primary content of 0.35 wt.% S, is mirrored by the CO2 and SO2 trends observed in Fig. 10, where the cumulative volume trends are now indexed by the date of the first measurement (October 13, 2008). Here we only show the solution for 1.25 wt.% CO2 and 0.35 wt.% S. Again, lower volatile contents would have resulted in higher calculated magma volumes and higher contents result in lower magma volumes. However, if we assume these volatile contents are reasonable, then in the year following the end of dome extrusion, Fig. 10 shows that ~15 to 32% additional magma degassed in excess of that that erupted (based on a total erupted volume of 0.12 km 3, Bull and Buurman, this issue). The volume of this magma (0.02–0.04 km 3) is more than would be contained in a typical conduit system (assuming a simplistic model of a cylinder up to 50 m diameter that extends to the magma storage region). Thus, one interpretation is that the long slow decline in emissions from July 2009 to August 2010 was related to continued degassing from the unerupted magma body at depth, and the gradual cooling and sealing of the conduit system. In this scenario, posteruptive seismic events, like one observed in December 2009, may have been related to minor releases of built-up pressure as magma in the conduit cooled and sealed. There is some limited evidence that sealing and release of trapped volatiles may have been occurring in late December 2009 since the SO2 emission rate measured on Dec.

Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

140 120

explosive eruptions

160

dome extrusion

13

140 degassing of unerupted magma

100

130 120 110 100

maximum estimated total erupted volume (0.12 km3)

90 80 70

80

60 50

60

40 40

6

3

Dome volume (10 m ) Airborne and Satellite CO2 Airborne and Satellite SO 2 Airborne only CO2 Airborne only SO

20

30 20

Percent Magma Volume Degassed (106 m3)

Estimated Magma Volume Degassed (106 m3)

C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

10

2

0

Oct-08 Dec-08 Feb-09 Apr-09 Jun-09 Aug-09 Oct-09 Dec-09 Feb-10 Apr-10 Jun-10 Aug-10

Fig. 10. Cumulative magma degassed (October, 2008–August, 2010) based on explosive and dome extrusion measurements (orange triangles, Diefenbach et al., this issue; Bull and Buurman, this issue) and calculated using the CO2 and SO2 emission rate data sets, assuming and original melt contents of 1.25 wt.% CO2 and 0.35 wt.% S. Emissions track the erupted material (orange triangles) well over the duration of the eruption assuming these melt contents. Furthermore, degassing of unerupted magma would have started in the time period between the end of dome extrusion (July, 2009) and November, 2009 (i.e., shown on the graph when the magma volume based on degassing data exceeds the actual erupted magma volume). Given these assumptions, approximately 30% more melt degassed than erupted by August, 2010. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

31, 2009 (1450 t/d) was higher than that measured a month earlier on Nov. 2 (850 t/d). The highlights of the above discussion are that: (1) high estimates of primary melt CO2 and S contents were determined from the degassing record and erupted volumes, (2) prolonged but minor amounts of open-system degassing were observed before and after the period of lava eruption, (3) even though the system was vapor-saturated in the mid-crust, the majority of gas remained with the erupted magma and degassed dominantly (60%) as a closed-system, and (4) even though geochemically distinct magmas were erupted between the explosive and effusive phase, the co-existing vapor phase apparently had a constant composition with respect to C and S. These findings are supported by studies that discuss melt inclusion data and vapor buffering of magmas at depth (Rust et al., 2004; Roberge et al., 2009; Blundy et al., 2010; Wallace and Edmonds, 2011). These studies demonstrate that silicic magmas have CO2 contents in melt inclusions that are elevated with respect to what traditional openor closed-system degassing trends would predict. Thus, the magmas are either accompanied by vapor with elevated CO2, or have undergone ‘fluxing’ with CO2-rich vapor phase from greater depth between eruptions. Our results suggest that either ‘fluxing’ from deeper magma is perhaps not necessary to obtain high CO2 in co-existing vapor phase in a silicic magma, or that fluxed CO2 would have to accumulate in the mid-crust and affect large magma batches equally.

degassed primarily as a closed system with minor amounts of open system degassing observed up to 1 year following final dome extrusion. SO2 was the dominant sulfur-containing gas species, making up on average 92% of the total sulfur degassing throughout the eruption, though higher CO2 emission rates were accompanied with a slightly higher proportion of H2S relative to total sulfur, suggesting that H2S may be more dominant at depth. Magmas were vapor saturated with a C- and S-rich volatile phase, and regardless of magma composition erupted, the volatile composition had a stable CO2/SO2 ratio of ~ 2.4. Primary volatile contents calculated from degassing and erupted magma volumes range from 0.9 to 2.1 wt.% CO2 and 0.27–0.56 wt.% S; whole-rock normalized values are slightly lower (0.8–1.7 wt.% CO2 and 0.22–0.47 wt.% S) and are similar to what was calculated for the 1989–90 eruption. Such contents argue that primary arc magmas are rich in CO2 and S, as is expected based on melt inclusion analyses that do not follow typical open- or closed-degassing trends from equilibrium compositions at other arc volcanoes. Primary melt contents of 1.25 wt.% CO2 and 0.35 wt.% S resulted in magma volume trends that were consistent with erupted magma volume trends. Assuming these values are correct, up to ~ 30% additional magma degassed over that which erupted in the year following final dome emplacement.

Acknowledgments 6. Conclusions We presented 35 airborne measurements of volcano gas emissions for CO2, SO2, and H2S emission rates spanning ~2 years around the 2009 eruption of Redoubt Volcano. Pre-eruptive degassing was characterized by high CO2 emission rates (between 3630 and 9020 t/d in the final 6 weeks prior to eruption), whereas SO2 was only observed in trace amounts (≤180 t/d or 4% overall). Despite such high CO2 emission rates, only 14% of the total CO2 was emitted prior to eruption. Emission rates of all gasses were highest during the eruptive phase, starting with the March 15 phreatic eruption. The highest emission rates measured were 33,110, 16,650, and 1230 t/d of CO2, SO2, and H2S, respectively, and were measured within 9 h of the April 4 explosion and during a significant seismic swarm and increased dome growth on May 4. Overall the magmatic system

The authors are very grateful for the dedicated and careful work from the pilots of Security Aviation (Steve Jones and Jerry Morris) and Rick Wessels for making COSPEC measurements. Helpful reviews were provided by Jake Lowenstern and one anonymous reviewer. This work was funded by the US Geological Survey Volcano Hazards Program.

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Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012

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C. Werner et al. / Journal of Volcanology and Geothermal Research xxx (2012) xxx–xxx

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Please cite this article as: Werner, C., et al., Degassing of CO2, SO2, and H2S associated with the 2009 eruption of Redoubt Volcano, Alaska, J. Volcanol. Geotherm. Res. (2012), doi:10.1016/j.jvolgeores.2012.04.012