Volcanic Gas Emissions From the Summit Craters and Flanks of Mt. Etna, 1987-2000

Tommaso Caltabiano1, Michael Burton1, Salvatore Giammanco2, Patrick Allard3, Nicola Bruno 1, Filippo Mure1 and Romolo Romano 4

In the last 13 years gas emissions from both the summit and the flanks of Mount Etna volcano have been monitored using remote sensing techniques (COSPEC, and FTIR since 2000) and on-site monitoring devices. The S 0 2 flux variations (600 to 25,000 Mg/day) indicated: (i) low values coinciding with deep seismicity prior to eruptions or/and preceding increases in summit volcanic activity; (ii) increasing trends tracking the ascent of fresh magma within the shallow feeding system and whose rate seems propor­tional to the speed of magma rise; (iii) decreasing trends related to progressive degassing of magma batches; (iv) an imbalance between the amount of magma erupted and that which contributed the S 0 2 emission (~ 13 % of the degassing magma having been erupt­ed during the studied period), implying that magma degassing is dominantly intrusive; (v) a seasonal component, probably due to variations in solar zenith angle, meteorologi­cal parameters and, possibly, tidal forces.FTIR monitoring allowed to recognize signifi­cant variations of SO/HCl and SO/HF ratios in the volcanic plume which, combined with COSPEC data, provided new insight into the dynamics of ascent and degassing of discrete magma bodies. Strong variations in C0 2 -rich soil degassing are interpreted as markers of gradual magma ascent from great depth (>10 km) to the upper (<5 km) feed­ing system of Mt. Etna. These changes appear to precede increases in S 0 2 plume flux at the craters and, so, provide additional constraints upon the interpretation of COSPEC data and the modeling of magma rise at that volcano.

1. INTRODUCTION

Magmatic gases play a key role in volcanic activity and can also provide important information on the evolution o f a mag­matic system. Measurements carried out on various types o f volcanic gas emissions (crater plumes, fumaroles, diffuse soil emanations, dissolved gases in ground waters) have shown their usefulness as indicators o f the state o f activity o f a vol­cano and o f magma dynamics in the feeding system [e.g.,

istituto Nazionale di Geofisica e Vulcanologia - Sezione di Catania, Catania, Italy. 2Istituto Nazionale di Geofisica e Vulcanologia - Sezione di Palermo, Palermo, Italy. 3Laboratoire Pierre Sue, CNRS-CEA, Gif/Yvette, France. 4Via Duca degli Abruzzi 2, Catania, Italy.

Mt. Etna: Volcano Laboratory Geophysical Monograph Series 143 Copyright 2004 by the American Geophysical Union 10.1029/143GM08

Delmelle and Stix, 2000 ] . Direct simultaneous sampling o f many different species o f volcanic gases for broad analysis with great accuracy in a laboratory is often prohibited by dif­ficult access or hazardous field conditions. This is the case on Mt. Etna, where, for those reasons, volcanic gas remote sens­ing methods have been used since 1975 [Hauletetal, 1977]. Here, we present and discuss data on both the volcanic plume and distal soil degassing at Mt. Etna that we obtained over the last 13 years by using different remote sensing techniques and monitoring devices.

Mt. Etna, the tallest active volcano in Europe, is known as one of the largest contributor of magmatic gases such as C 0 2 , S 0 2 , HC1 and HF to the atmosphere [e.g., Allard et al, 1991; Allard, 1997; Bruno etal, 1999; Caltabiano etal, 1994; D Alessan­dro etal, 1997; Francis etal, 1995; Francis etal, 1998; Ger-lach, 1991; Williams etal, 1992]. The remarkable combination o f high volcanic gas flux, good accessibility and extensive background knowledge at this volcano make it an ideal labo­ratory for performing long-term studies o f volcanic emissions

T 1— 14°45 15°15

Figure 1. Location of sampling sites of soil C 0 2 (P39 and P78) and of the roads (solid hard lines) along which the trav­erses for COSPEC measurements were carried out. The prevalent wind direction of the high altitude winds is marked by the arrow.

and for testing new gas monitoring techniques. Continuous plume emissions occur from the four summit craters o f Mt. Etna, Voragine, Bocca Nuova, North-East Crater and South-East Crater, which provide almost the totality of the emitted S 0 2 , HC1 and HF and the large majority o f the emitted C 0 2

[e.g., Allard et al, 1991; Bruno et al, 1999; Caltabiano et al, 1994; D'Alessandro et al, 1997; Francis et al, 1995; Francis et al, 1998]. Temporary emissions additionally take place dur­ing flank eruptions, from fractures and lateral vents. However, remarkable degassing of carbon dioxide also occurs from the flanks of the volcano, mainly through active faults [e.g., Allard etal, 1987,1991; Anzd etal, 1993; D 'Alessandro et al, 1997; Giammanco etal, 1995,1997,1998a, 1998b], as well as through groundwater transport [e.g. Anzd et al, 1989; Allard et al, \991;Aiuppa et a/., 2000] .

Measuring the concentrations and fluxes o f C 0 2 , S 0 2 , HC1 and HF has significant implications to the study o f magma ascent, transport and degassing in the plumbing system o f Mt. Etna and, therefore, to the forecasting o f its eruptions. Importantly volcanic gas emissions can provide insight into the degassing o f an up-rising magma. Carbon dioxide is, after

water vapor, the most abundant volatile species dissolved in Mt. Etna alkali basalt; its original content was assessed to range from about 0.7 wt. % [Armienti et al, 1994] to 1.2 wt. % [Allard, 1995]. Moreover, due to this high content and its very low solubility in basalt under decreasing pressure [Stolper and Holloway, 1988; Pan et al, 1991] carbon dioxide is one o f the first gases to be exsolved and released upon magma ascent. Clocchiatti et al [1992] found that gaseous C 0 2 already occurs in Mt. Etna magma at a confining pressure of about 800 MPa, which corresponds to lithostatic depths of between 21 and 24 km. Therefore, increases in volcanic C 0 2 emissions can be early indicators o f magma ascent. Sulfur dioxide is con­siderably more soluble than C 0 2 and therefore starts escaping from the melt at much lower confining pressures [e.g., Car­roll and Webster, 1994; Metrich et al, 1993, 2002] . The ini­tial concentration of sulfur dissolved in crystal melt inclusions o f historical Mt. Etna basalts averages 0.3 wt. % [Metrich and Clocchiatti, 1989; Clocchiatti etal, 1992; Metrich etal, 1993] and sulfur exsolution begins at pressures o f only 100-140 MPa (~ 3 - 4 km lithostatic depth). The solubility-pressure relationships o f HC1 and HF are less well known, however

recent data [Metrich et al, 1993, 2002] indicate that HC1 starts to degas efficiently at less than 50 MPa pressure (<1.5 km) and HF at still lower pressure.

2. METHODOLOGIES

2.1. COSPEC Measurements of S02 Plume Flux

Measurement o f sulfur dioxide in a volcanic plume is usu­ally performed by U V spectral absorption. The spectral con­tents o f the down-welling sky radiation is analyzed by mask correlation spectroscopy performed with an instrument denoted as Correlation SPECtrometer (COSPEC) [Millan andHoff, 1978; Millan, 1980]. On Mt. Etna a vertically point­ing Min i -COSPEC or C O S P E C V instrument (Barringer Research Inc., Toronto, Canada) was used to carry out meas­urements. The instrument receives the sky radiation in an opening angle o f 10 x 30 mrad and directs it to a grating spectrometer. In the exit plane o f the spectrometer there is a correlation mask arrangement that matches the S 0 2 absorp­tion spectrum in the 290-320 nm region. The instrument has a 4-step correlation mask that minimizes the effects o f the background changes. Furthermore, the instrument has an Automatic Gain Control (AGC) circuit that compensates for the background variations. The zero level o f the instrument is set while outside the volcanic plume, and the integrated over-head vertical concentration values are calibrated by inserting two quartz cells, with a nominal gas load o f 133 and 3 4 4 ppm-m (parts per mill ion meter) , respectively, and compensat ing the error due to non-l inear cal ibrat ion. C O S P E C furnishes an output voltage, the value o f which is proportional to the sulfur dioxide burden (in ppm-m) in the instrumental field o f vision.

C O S P E C measurements at Mt. Etna were made along traverses under the volcanic plume with the instrument pointing upward and usually mounted on a ground-based vehicle. The traverses were kept as straight as possible and the roads used were located at a distance o f 7-15 km from the summit craters (Figure 1). The best route was chosen for each measurement on the basis o f the wind direction prevailing that day. At Mt. Etna, winds at high altitudes are generally westerly. Traverses o f the volcanic plume were also sometimes performed by traversing the instrument from a fixed point (moving the instrument, or rotating its 4 5 degree mirror with a stepper motor, on a vertical or horizontal plane) or using aircraft or a boat. The S 0 2 burden obtained during the traverses is a con­centration path-length as a function o f distance (ppm-m). In order to determine the flux value, the S 0 2 burden is integrated along the traverse.

The integrated S 0 2 burden was multiplied by the wind speed at the plume height, assuming that wind speed is equivalent to the horizontal speed o f the volcanic plume (and then o f the sulfur dioxide) at the corresponding alti­tude. Normally, from 2 to 6 traverses per day were per­formed and the mean value was assumed to be the S 0 2

flux level on that day. Data correction took into account the winding o f the road used for measurements , the angle between the direction o f the road and the direction o f the plume axis, and the actual length o f that road. In addition, we filtered any noise induced by various obstacles (e.g., t rees , b r idges ) a long the t raverses that may al ter the C O S P E C signal, even i f for very short periods. Flux val­ues obtained in this way are normally expressed in mega-grams per day (Mg/d) . Data for wind speed, a parameter o f the utmost importance in evaluating gas fluxes in vol­canic plumes, were provided by the Italian Air Force Mete­orological Service, which uses soundings balloons to make such estimates. Wind speed was sometimes simultane­ously measured during some C O S P E C surveys with an anemometer at the altitude o f Mt. Etna's summit craters [Caltabiano et al., 1 9 9 4 ] , giving results comparable to those o f Air Force data (deviation o f about 15 % ) . The uncertainty in the wind speed at high altitudes was esti­mated by Malinconico [ 1 9 8 7 ] to be 15 to 2 5 %. Th i s results in an overall uncertainty ranging from 2 0 to 3 0 % on reported S 0 2 flux values, very near that reported from Stoiber et al. [ 1 9 8 3 ] . This includes also the uncertainty on the estimation o f the plume height and the different wind speed inside the plume, even i f the latter can affect sig­nificantly the flux estimate only under cases o f severe wind shear [Hoff and Millan, 1 9 8 1 ] .

2.2. FTIR Measurements of S02, HCl and HF Plume Concentrations

In this paper we also present data collected in 2 0 0 0 using an open-path Fourier transform infrared spectrometer (FTIR), that we used to measure the concentrations o f S 0 2 , HCl and HF within the bulk summit craters o f Mt. Etna. The data were collected with a Bruker 0PAG-22 F T I R spectrometer at 0.5 cm ' 1 resolution using medium Norton-Beer apodiza-tion. The detector was a LN 2 -cooled InSb photovoltaic semi­conductor with sensitivity between 1,500 and 6,000 c m - 1 . Spectra were collected in solar occultation mode [Francis et al, 1998] , measuring the absorption o f solar radiation pass­ing through the volcanic plume. Recorded infrared spectra were analyzed using a non-linear least squares fitting pro­gram based on the Rodgers [1976] optimal estimation algo­rithm and the Oxford R F M radiative transfer model [Edwards

5.0E+16 1.5E+17 2.5E+17 3.5E+17 4.5E+17 5.5E+17

HCl path amount (molecules-cm"2)

Figure 2. Results from FTIR spectra collected on 3rd August 2001 in solar occultation mode of the chemical compo­sition of volcanic gases emitted from Mt. Etna.

et al, 1996] , using spectral line data from the HITRAN 96 database [Rothman et al, 1998] , Solar spectra were ana­lyzed by simulating the complete atmospheric transmittance, based on a 50 layer FASCODE standard atmosphere, together with an idealized volcanic plume layer with one pressure and temperature. Each spectrum was analyzed to produce a path amount o f S 0 2 , HCl and HF, three gases which have negligible concentrations in the unpolluted free troposphere but which are abundant within volcanic emissions. The nature o f the F T I R is such that all detectable wavelengths are measured simultaneously. This means that, despite the swiftly changing concentrations o f gas in the line o f measurement

between the spectrometer and the sun, each measured spec­trum records the instantaneous composition o f all the meas­urable volcanic gases. Unlike COSPEC, F T I R measurements do not allow to measure a gas flux: they are performed at a fixed point and with a fixed viewing, so that they provide only the concentrations and the molar ratios o f different gas species. Molar ratios are determined by measuring 50 or more spectra o f a particular gas plume, and then plotting retrieved amounts o f S 0 2 against HCl and HF. The gradient o f these plots are the molar ratios o f S 0 2 / H C 1 and S 0 2 / H F . An example o f data collected on the 3 r d August 2 0 0 0 is shown in Figure 2. Each point shows the retrieved S 0 2 path

1987 1888 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998

Time (Years)

9 20000 9

1987 1988 1 1997 1998 1999 2000

Time (Years)

Figure 4. Band pass filtering on S 0 2 flux signal, (a) The thick curve in the upper part of the graph highlights the 1-year component, (b) The thick curve in the upper part of the graph highlights the 4-year component. Superimposed filtered signals are shown in arbitrary units. The grey bands indicate Mt. Etna's eruptive activities.

amount plotted against retrieved HCl path amount for a single spectrum. Note the wide variation in gas amount but the very steady S 0 2 / H C 1 ratio during the measurement period.

2.3. Monitoring of C02 Soil Emanations

C 0 2 emissions were measured in the soil using the method proposed by Gurrieri and Valenza [1988] and also described in Giammanco et al. [1995, 1997, 1998a, 1998b]. A special cylindrical probe is inserted into the soil to a depth o f 50 cm, and is connected to an IR spectrophotometer (accuracy within ± 5 % ) . The probe has an open outlet that allows mixing between the soil, atmosphere and air, and the gas mixture is pumped to the detector for enough time (usually a few minutes) to let the C 0 2 concentration reach a constant value. C 0 2 con­centration values measured in that way are called "dynamic"

concentrations. It was experimentally found that they are directly proportional to the C 0 2 efflux through the soil accord­ing to the relation 0 = k-Cd, where 0 is the C 0 2 flux through the soil (in g c m - 2 s _ 1 ) , Cd is the "dynamic concentration" o f C 0 2 (in ppmv) as read by the IR spectrophotometer and k (in g ppmv 1 cm - 2 s"1) is an empirical constant whose value depends on several factors: the features o f the sampling system, the pumping flow rate, the depth of probe insertion into the soil and the physical characteristics o f the soil. I f all o f latter parame­ters are assumed to be constant at a given site, then changes o f C 0 2 dynamic concentrations at that site may be taken as rep­resentative o f real variations in the soil gas efflux. Further­more, any possible error in the C 0 2 efflux measurements is much smaller than the relative differences in C 0 2 dynamic concentration observed at each site during the period o f inves­tigation, as already shown by Giammanco et al. [1995; 1998a] for soil C 0 2 data previously collected on Mt. Etna.

1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000

Time (Years)

Figure 5. High pass filtering on S 0 2 flux signal. In the upper part of the graph filtered signal is plotted. Superimposed filtered signal is shown in arbitrary units. The grey bands indicate Mt. Etna's eruptive activities.

3. R E S U L T S

3.1. SO2 Plume Flux

Mt. Etna, together with Kilauea in Hawaii, is one o f the few active volcanoes worldwide whose S 0 2 flux emissions have been the most intensively surveyed with COSPEC. On Mt. Etna the COSPEC methodology has been applied intermit­tently between 1975 and 1986 [Haulet et al., 1977; Allard et al., 1991] and routinely since 1987 [Caltabiano et al., 1994; Bruno et al., 1999]. The measurements were carried out on a weekly basis since 1987 and two to three times a week since October 1996.

Figure 3 shows the S 0 2 flux values measured during the period 1 9 8 7 - 2 0 0 0 . The uncertainty o f reported values is

about 20 -25 %. During 13 years considered in this study, a total o f 933 S 0 2 flux measurements were made. From this data set we highlight the following main features o f the vol­canic S 0 2 flux from Mt. Etna during that period: a wide range o f variation, from a minimum value o f 6 7 0 Mg/d (on April 2 9 , 1999) to a maximum o f about 2 6 , 6 0 0 Mg/d (on September 19, 1989) ; a mean flux value o f 5 ,560 Mg/d ± 3,580 ( l a ) over the whole period; fluxes greater than 10,000 Mg/d measured prevalently during eruptive activities and a higher frequency o f high S 0 2 peaks during two periods: 1 9 8 9 - 1 9 9 3 and 1 9 9 7 - 2 0 0 0 .

We have applied a linear interpolation to the entire tem­poral sequence o f S 0 2 flux data to obtain an equally spaced time series with one point a day. In this way we could cal­culate a moving average o f the signal. The exceptionally

1993 1994 1995 Figure 6. Comparison between the S 0 2 flux (dashed line) and the tremor spectra amplitude (solid line expressed in arbitrary units) at Mt. Etna in the period 1993-1995. Tremor was recorded at Serra Pizzuta Calvarina seismic station (alti­tude of 1600 m a.s.l.). The grey bands indicate Mt. Etna's eruptive activities (modified from Bruno et al. [1999]).

Figure 7. Graph showing the cumulative volume of degassing magma at Mt. Etna calculated for a mean original S con­tent of 3000 ppm in the magma (thick curve, left axis) and the volume of erupted magma (thin curve, right axis).

long measurement period allowed us to analyze the behav­ior o f the volcanic gas discharge to a frequency with Nyquist period o f about 6 years. B y applying a 30 day moving aver­age to the interpolated time series, we filtered out the high frequency oscillations in the flux due to short term variations caused by errors in the measurements and other rapid changes in the gas emission. Low pass filters with different cut-off frequencies were applied to the signal and the most important results are shown in Figure 4 . This figure high­lights both a 1- year component (Figure 4a) and a longer (4-year) component in the S 0 2 flux signal (Figure 4b) .

A cyclic component with a 1-year period is probably due to the seasonal influences o f solar zenith angle, meteorologi­cal parameters and tidal forces. In contrast, the 4-year period component may be truly volcanic in origin and related to long-term periodical supply, replenishment or/and long-term peri­odic movements (overturns?) o f magma within the feeding conduit system o f the volcano. B y applying a high pass filter to the flux signal (Figure 5) in order to eliminate the low fre­quencies, the remaining high-frequency oscillations are in general positively correlated with eruptive phenomena. Fur­thermore, i f the envelope o f the high pass filtered signal is taken into account, it is possible to see that the signal is mod­ulated also by very low frequencies (i.e., cyclic components with period longer than 4 years). According to this procedure, Figure 5 highlights two periods, 1987-1993 and 1997-2000, when peaks o f S 0 2 flux were significantly higher than in the intermediate period. These two periods were also characterized by more intense volcanic activity, which strengthen the hypoth­

esis that long period S 0 2 emissions provide information on deep magma supply.

As regards the relationship between the volcanic S 0 2 flux and seismic activity on Mt. Etna, we find a broad positive correlation with the evolution o f the seismic tremor, as illus­trated in the example o f Figure 6. This correlation is partic­ularly clear during periods o f strong eruptive activity, as described in Leonardi et al [1999] . This evidence implies that volcanic tremor and S 0 2 emissions are produced by a common physical mechanism, related to shallow magma degassing. Other correlations can be found between S 0 2 flux changes and volcanic earthquakes heralding eruptive activity. In fact, drastic drops o f the S 0 2 flux were observed in coin­cidence with deep earthquakes recorded about 4 to 6 weeks prior to eruptions and located mainly beneath the western flank o f Mt. Etna. In particular, the deep seismic events o f August 3, 1989 and October 23 , 1991 (h > 7-10 km accord­ing to Castellano et al [1995]) and the minimum S 0 2 flux val­ues o f August 9, 1989 (970 Mg/d) and November 8, 1991 (840 Mg/d) can be temporally correlated with the onset o f 1989 and 1991 -1993 eruptions, respectively. It is interesting to note how the occurrence o f a deep seismic sequence on June 19-20, 1988, in the absence o f a well defined flux min­imum (2,940 Mg/d on June 2 9 , 1 9 8 8 ) did not precede a major eruption, but only a brief lava fountaining activity (August 1988). Therefore, minimum values o f S 0 2 flux, particularly i f coupled with deep seismicity, seem to be an indicator o f impending eruptive activity and/or upward magma migration [Caltabiano et al, 1994; Bruno et al, 1999]. Further study o f

x 3 05

CN

o c o

20000

15000

10000

5000

1 9 9 3 1 9 9 4 1 9 9 5

Figure 8. The 1993-1995 degassing cycle with increasing (about 100 t/d2) and decreasing (about -5 t/d2) trends. The grey bands indicate Mt. Etna's eruptive activities.

the temporal relationships between such seismic events and degassing trends on Mt. Etna is required to fully understand the operating mechanisms.

Knowing the initial and residual sulfur content in Mt. Etna basalts, the measured S 0 2 flux values can be used to assess the amount o f degassing magma sustaining the crater plume emissions o f Mt. Etna [Allard, 1997; Bruno et al., 1994 , 1999]. Because nearly all sulfur is lost from the melt upon eruption [e.g. Metrich et al, 1993] , the weight loss o f S dur­ing degassing is equivalent to the initial sulfur content o f the melt (0.3 wt. % ) and the volume o f degassed magma is then derived as:

V magma (m 3 ) = 0.5 O S O 2 / [ 0 . 0 0 3 * p m * ( l - x c ) ] (1)

where 0>SO2 is the S 0 2 flux (the factor 0.5 accounting for the twice molar weight o f S 0 2 compared to S) , p m is the density o f molten Mt. Etna basalt (2 ,650 kg/m 3 ) and x c is the crystal content o f the basalt (~ 4 0 % on average). Thus, the mean S 0 2 flux o f 5 ,450 Mg/d in 1987-2000 leads to compute a total discharge o f ~ 2 6 T 0 9 kg o f sulfur dioxide produced by bulk degassing o f 2.7 km 3 o f basalt over 13 years. The aver­age volume o f magma degassed rate, 0.21 km 3/yr, is compa­rable to the rate previously assessed for the period 1975-1995 o f 0.28 km 3/yr [Allard, 1997] . Comparing with the quantity o f magma actually erupted during 1 9 8 7 - 2 0 0 0 (~ 0.36 km 3 ) results in an important observat ion that ~ 13 % o f the degassing magma was erupted (Figure 7) . This implies a sep­arated conduit flow o f gas and melt on average and a storage

J J A Year 2000

Figure 9. Time series of FTIR measurements collected in 2000, showing the variation in retrieved SO/HO (open cir­cles) and S0 2 /HF (filled circles) molar ratios. The grey bands indicate Mt. Etna's eruptive activities.

or/and recycling o f most (~ 87 % ) o f the degassed magma within or/and beneath the volcano, as previously shown by Allard [1997] and in Bruno et al [ 1999 ] . With respect to these average figures, the S 0 2 flux minima can be used to estimate the amount o f degassing magma that was "missing" in the conduit system and that could not contribute to the background degassing. In the case o f minimum flux values

which preceded the September-October 1989 and December 1991-March 1993 eruptions, these missing relative quantities o f magma were about 6.5 million and 4 million cubic meters, respectively [Bruno etal, 1999].

Furthermore, the different rate o f increasing S 0 2 flux trends before the eruptions may be indicative o f the rise rate o f magma as well as the type o f resulting eruptive activity [Bruno et al,

5E+11

•§ 3E+11 3 a

J 2E+11

" " " J l \ Z . 0 200 400 600 800 1000 1200 1400 1600

Per iod (days)

k b)

. *A S ^ J * 0 200 400 600 800 1000 1200 1400 1600

Per iod (days)

Figure 11. Periodograms for soil C 0 2 data measured at site P78 (a) from 1994 to 1997 and (b) from 1998 to 2000. Ordinate values in arbitrary units. The arrows highlight the annual cycle.

1999]. As observed by Bruno et al. [1999], the 1989 eruption was preceded by a high increasing trend o f plume S 0 2 flux (about 155 Mg/d). The eruption was characterized by strong eruptive emissions o f S 0 2 during a relatively brief interval (13 days). In the case of the 1991-1993 eruption, the rate of increase was notably lower (about 50 Mg/d) and was followed by low eruptive S 0 2 emissions over a long period (473 days).

Decreasing trends in S 0 2 flux can also be useful for the understanding of magma transport and degassing in the conduit feeding system of Mt. Etna. Different rates of S 0 2 flux decrease can be related to progressive degassing of a batch of magma fill­ing the upper conduits o f the volcano, depending on both its vol­ume and the rate o f magma overturn. An example o f this can be found in the behavior o f S 0 2 plume emissions during the period August 1993 to June 1995 (Figure 8). At the beginning of this non-eruptive period the average rate of S 0 2 increase, likely due to the arrival of new gas-rich magma in the upper conduit sys­tem, was about 100 Mg/d. After late October 1993 the S 0 2

fluxes showed a decreasing trend with a rate of about 5 Mg/d, on which marked oscillations (values ranging from 4,000 to 12,000 Mg/d) were superimposed between November 1993 and early May 1994. Despite the low sampling frequency, we find that the periodicity of these oscillations increased from an initial weekly value in November-December 1993, to once

every two weeks in January-March 1995 and to greater values afterwards. Such an increase in the periodicity of S 0 2 flux oscillations may be attributed to a progressive slowing down of convective currents in the magma emplaced in the central conduit system of the volcano, whose existence is supported by the overall balance between the degassing and erupted magma volumes.

3.2. S02, HCl and HF Plume Contents and Halogen Fluxes

The initiation o f routine F T I R measurements o f S 0 2 , HCl and HF concentrations in the summit crater plume o f Mt. Etna in 2000 coincided with two periods o f eruptive activ­ity located, first, at South-East Crater (SEC) then at B o c c a Nuova (BN) . In the first six months o f 2000 a series o f 64 periodical lava fountaining events, often accompanied by lava flows, occurred at S E C . One ultimate, less intense, explosive event occurred at that crater by the end o f August. Then, from late November to early December lava was emit­ted from the north-east flank o f SEC cone with a very low effusion rate and almost no explosive activity. At the Bocca Nuova central crater, quiescent fuming persisted from early March to early September. On the 10th of September sustained ash emissions began and developed until the 2 7 t h September, when Strombolian eruptive activity resumed. The intensity of this activity at BN increased to a peak in October, and then declined unsteadily to finally finish on the 7 t h December.

Figure 9 shows the variations of S0 2 /HC1 and S0 2 /HF during that period. There are several features of interest in these data. Firstly, these very first long-term FTIR measurements on Mt. Etna reveal strong temporal variations in the chemical compo­sition of the volcanic plume. It thus seems clear that there are processes occurring within the magmatic system which can strongly affect the chemical composition of released gas. The overall form of the time series is of a smooth rise in both S0 2/HC1 and S 0 2 / H F ratios starting from August 2000 and peaking in late September-early October that preceded the Strombolian eruptive resurge at BN. The S 0 2 / H F ratio increases by a factor of approximately 10 between August and October; while S0 2/HC1 increases by a factor of 3 over the same period. After another peak in late October, both ratios decreased gradually before returning to background values in early December as the Strom­bolian activity stopped.

It is interesting to compare these results with the contempo­raneous evolution of the S 0 2 flux. Because both data types were not always collected on the same day, due to logistical chal­lenges, and because FTIR were not as frequent as COSPEC measurements in 2000, we have applied a linear interpolation of COSPEC data to derive the S 0 2 flux for each day of FTIR

data sampling. Such an approximation is considered reasonable since the S 0 2 flux maintained high values well above its mean variability during the period under investigation. By dividing the interpolated S 0 2 flux by the S0 2 /HC1 and S 0 2 / H F ratios we determine the fluxes of HCl and HF. These plots demonstrate a fascinating variation of the HCl flux during Strombolian erup­tive activity at BN around October 2000 (Figure 10), whose significance is discussed thereafter including other data types.

3.3. CO2 Soil Degassing

The existence of diffuse soil emanations of magma-derived carbon dioxide through the flanks of Mt. Etna was discovered in the 1980's [Carbonnelle and Zettwoog, 1982; Allard et al, 1987,1991] and since August 1989 soil C 0 2 surveys have been carried out regularly (about once per month) [e.g. Giammanco et al, 1995; 1998a]. These surveys are focused on two most actively degassing areas located, respectively, in the central part o f the eastern flank, near the town of Zafferana Etnea, and on the lower southwestern flank o f the volcano, around the town o f Paterno [Giammanco et al, 1995]. These areas are characterized by the strongest gas concentration anomalies in soils, as well as by the highest content of magmatic gas dissolved into local ground­waters [Anzd et al, 1989; Giammanco et al, 1996; Allard et al, 1997]. Other areas o f Mt. Etna have also been regularly sur­veyed later for soil C 0 2 emissions, and anomalies were found there along active volcano-tectonic structures.

Apart from the diffuse degassing, the two areas above men­tioned are also characterized by a few sites with very high focused degassing. Temporal variations o f gas emission from these sites may not be coherent with those o f the rest o f the respective areas. Because o f their high flux, gases may not be subject to strong interaction with relatively shallow fluids (e.g., ground water; Giammanco et al. [1998a]). For the pur­poses o f this study we selected two o f these sites (Figure 1), both characterized by a very high emission o f soil C 0 2 . Site P78, on the eastern flank o f Mt. Etna, is located at 320 m a.s.l. about 3 km east o f the town of Zafferana Etnea, while site P39, on the Southwestern flank, is located at 115 m a.s.l.. about 2 km southwest o f the town o f Paterno. Besides C 0 2 , the gas emanations contain subordinate amounts o f CH 4 , He and sometimes H 2 and CO, especially at P39 [Giammanco et al, 1998a]. Recent geochemical studies on the gases emit­ted at sites P39 and P78 indicate that these soil emanations are dominantly magmatic in origin [D 'Alessandro and Parello, 1995; Giammanco et al, 1998a]. In particular, the chemical and the isotopic features o f gases emanating at site P39, on a NE-SW-directed regional fault [Gurrieri et al, 1998] , evi­dence their connection with the mantle source o f Mt. Etna basalts [Giammanco et al, 1998a; Italiano et al, 1999; Rasa

1994 1995 1996 1997 1998 1999 2000

Figure 12. C 0 2 dynamic concentration values (ppmv) measured at the sampled sites: (a) raw values from site P78; (b) filtered and nor­malized P78 values (see text for details on the filtering and nor­malization procedure); (c) values from site P39. Figures (b) and (c) also show the long-term trends calculated for the periods 1994-1997 and 1998-2000.

et al, 1995]. Site P78 is part o f an anomalous degassing zone that is aligned on a W N W - E S E fault system [Anzd et al, 1993; Giammanco et al, 1 9 9 5 ] . Gases from this site are assumed to be connected with the shallower magma conduit system o f Mt. Etna [Giammanco et al, 1998a; Bruno et al, 2001] . Correlation between 5 1 3 C and C 0 2 dynamic concen­tration values at site P78 highlighted that when C 0 2 flux is lower, the C isotope composition becomes more negative, thus pointing to a greater interaction with shallow non-mag-matic fluids, in this case cold ground water and biogenic C 0 2

[Giammanco et al, 1998a]. At high C 0 2 fluxes, 8 1 3 C values are similar to those o f gases collected in fumaroles close to the summit craters. This indicates rapid gas flow with limited

a )

* P78 J 1 1 J 1 1 1 1 —

c )

120000 > '

a

60000

6 mm o

Figure 13. Temporal behavior during year 2000 of: (a) C 0 2 dynamic concentrations measured at site P39 and calculated for P78 after filtering; (b) summit craters S 0 2 flux and thorium content in emitted lavas; (c) summit craters HCl and HF fluxes calculated from S 0 2 flux using S02/HC1 and S0 2/HF ratios. The grey bands indicate strombolian and effusive activ­ities at BN and SEC during the second half of year 2000 due to arrival of new magma.

interaction with shallow fluids and suggests free gas trans­port along discrete conduits in which any ground water is saturated with C 0 2 , as already envisaged by Sorey et al. [1998] at Mammoth Mountain.

Soil C 0 2 data collected at both sites cover the period 1 9 9 4 - 2 0 0 0 . In general, C 0 2 flux values at site P39 were about one order o f magnitude higher than those at site P78. Wide changes were observed at both sites, and in particular site P78, where soil C 0 2 values varied from about 180,000

ppmv to almost the air concentration (about 360 ppmv). The temporal variations o f C 0 2 emissions at each site were ana­lyzed to evaluate any possible cyclic oscillation due to envi­ronmental parameters. A 90-days running average was calculated for both sequences o f data, and the resulting time-averaged series was analyzed by Fast Fourier Transform. No significant cyclic component was found at site P39 while the time series at site P78 first show a weak cyclic component for the period January 1994-December 1997 (Figure 11a),

then a stronger one for the period January 1998 to December 2000 (Figure 1 lb ) . It is noteworthy that soil C 0 2 concentra­tions measured since early 1998 were significantly lower than the previous ones, which may explain a greater sensitivity o f soil degassing to environmental factors. The C 0 2 values from 1998 to 2 0 0 0 were compared with the most relevant meteorological data available (rainfall, air temperature and barometric pressure). A significant positive correlation ( R = 0.59) was found only between C 0 2 degassing and air temperature values during this period, as already observed dur­ing previous investigations on soil C 0 2 emissions in Mt. Etna area [Giammanco et al., 1995; Bruno et al, 2 0 0 1 ] . Simolar to Giammanco et al. [1995] in the present investi­gation found a linear relationship between diffuse C 0 2 emis­sions at P78 and air temperature during the period January 1998-December 2000 :

C d ( t ) = 2,276.7 t- 11,878 (2)

where C d ^ is the calculated C 0 2 dynamic concentration (ppmv) that depends on air temperature t (in °C). We used the above relationship to remove the seasonal influence from the data set and to evaluate the variations in diffuse C 0 2 degassing possibly linked to the volcanic activity, using the simple bal­ance equation:

C d ( v ) = C d m e a s " C d ( t ) + C d ( 2 1 ° C ) (3)

where C d m e a s is the C 0 2 dynamic concentration (ppmv) measured in each survey; Cd ( t ) is the dynamic concentration calculated using equation (2) ; and Cd ( 2 1 o C ) is the normal­ization factor o f the C 0 2 emissions calculated at 21 °C, which is the mean temperature value measured at site P78 during that period. Figure 12a and Figure 12b respectively show the raw data and the normalized C 0 2 dynamic con­centrations measured at site P78 and Figure 12c shows the values at site P 3 9 .

An interpretative model o f C 0 2 release from uprising magma at Mt. Etna and its mechanism o f transport to the surface [Giammanco et al, 1995] suggests that transient increases in C 0 2 output from sites o f fault-driven focused degassing on the flanks o f the volcano may indicate increases in C 0 2 partial pressure at depth, related to the input o f gas-rich magma in the feeding conduit system [Harris and Rose, 1996] .

Anomalous transient decreases in soil C 0 2 emissions at Mt. Etna have been interpreted by Giammanco et al [1995] as a sign o f upward migration o f the gas source (i.e., a C0 2 -over-saturated magma). At some point, shallow magma up-rise leads to a greater gas pressure gradient between the "source"

t

Plume SO ?

P78 CO,

P39 C O 2

Plume SO 1

tune

Figure 14. Schematic model of the theoretical behavior of gas release from ascending magma within Mt. Etna's feeder system and of the inferred position of magma reservoirs beneath the volcano, based on soil C 0 2 and plume S 0 2 data collected between January and August 2000. Horizontal lines with numbers indicate the estimated depth of exsolution of C 0 2 (1), S 0 2 (2) and halogens (3) from upris­ing magma. Areas colored in light grey show zones filled with gas-depleted magma; areas in dark grey show volumes of rock intruded by gas-rich magma; small arrows indicate inferred directions of the flux vectors of magmatic gas; big arrows indicate directions of gas-rich magma motion into Mt. Etna's feeder system. Inbox A indi­cates the portion of magma feeder system modeled in Figure 15.

o f gas (i.e., the magma) and the top o f the volcano, compared to the gradient existing between the magma and the peripheral areas. This contrast favors magma degassing along the direc­tion source-summit o f the volcano. Such a phenomenon may explain in part the average decrease in soil C 0 2 efflux recorded clearly at site P78 (Figure 12b) and less markedly at site P39 since late 1997. Dynamic C 0 2 concentrations at site P39 actu­ally showed a slight increasing trend from 1994 to late 1997, and then a decreasing trend (Figure 12c). The broad decrease in flank C 0 2 emissions observed at Mt. Etna during the period corresponds to a broad increase in S 0 2 flux along with an increase in both the frequency and the intensity o f eruptive events at the summit craters. This suggests a possibly more effi­cient mechanism of magma transport to the surface in the main feeder conduits, with shorter intervals of pounding at the shal­low levels probably intercepted by the faults that outcrop in the area of site P78.

0 HF

o s o 2 /

H C l / "

s o

HF

Figure 15. Schematic model of magma degassing in the upper con­duit system of Mt. Etna, as interpreted from FTIR and COSPEC data, (a) Initial condition: low S 0 2 flux; (b) indicates the opening of the upper conduit due to increased gas flux or magma ascent, result­ing in simultaneous increases in S 0 2 , HCl and HF flux; (c) the magma batch rises to sufficient depth to allow greater degassing leading to coalescence, fast bubbles rise and increasing SO/HCl molar ratio; (d) the magma batch continues to degas until the volatile components are exhausted and fluxes return to background values.

3.4. Interpretation of Magma Degassing Processes at Mt. Etna

Several models of magma ascent and emplacement at Mt. Etna have been proposed over the last two decades [e.g. Wadge and Guest, 1981; Armienti et al. 1994, Allard, 1997 etc.]. Com­bining data for the S 0 2 plume flux and soil C 0 2 emissions obtained between July 1997 and March 1999, Bruno et al. [2001] proposed a model o f the dynamics o f magma ascent at various levels in the feeder system o f Mt. Etna. Although acquired with different techniques and different sampling fre­quencies, these data allowed the identification of clear episodes o f magma rise from depth. Generally, each episode started with an increase in soil C 0 2 emission at site P39, on the south­western flank o f the volcano. Then, a similar increase in soil C 0 2 degassing was subsequently observed at site P78, on the eastern flank, later followed by an increase o f the S 0 2 flux at the summit craters. The end o f each episode o f anomalous degassing was invariably marked by an increase in volcanic activity (Strombolian explosions, lava fountains, lava flows emissions, or a combination o f them) at the craters. Such a sequence o f events heralded the arrival o f a new batch o f magma at the surface, losing volatiles as a function o f their sol­

ubility versus depth behavior upon ascent. The time lag between the observed degassing anomalies at the different sampling locations on Mt. Etna was coherent with increases in gas pressure in magma bodies emplaced at increasingly shallower depth. In reality, magma "reservoirs" at Mt. Etna rather consist o f a plexus o f dykes and sills that are tem­porarily filled with new gas-rich melt before its rise towards the surface.

The novelty o f routine FTIR survey of S0 2 /HC1 and S 0 2 / H F ratios within the volcanic plume o f Mt. Etna in 2000 pro­vides additional constraints to interpret the dynamics o f magma rise and degassing at this volcano. In the following dis­cussion we combine the data collected during 2 0 0 0 with COSPEC, F T I R and C 0 2 soil monitoring (Figure 13) to ten­tatively explain the variations observed in the measured param­eters in relationship with volcanic activity at the summit of Mt. Etna.

During that year a complete sequence o f signals from the above different gas emissions o f Mt. Etna was obtained. Soil C 0 2 degassing (Figure 13a) was relatively elevated at site P39 during the first months o f 2 0 0 0 , and then decreased after early April. According to the interpretations o f soil C 0 2 emis­sions at that site, this pattern would indicate a higher gas pres­sure (i.e., input o f fresh magma) in the deeper parts o f the plumbing system (depth > 10 km, according to Bruno et al. [2001]) , followed by a pressure drop in that reservoir due to magma ascent to shallower crustal levels. Conversely, soil C 0 2 emissions at site P78 markedly increased since mid-April 2000 , after having been very stable during the previ­ous months o f the year. This may indicate gas/magma pressure increase in a shallower reservoir, likely located at depth o f 5 to 10 km [Bruno et al, 2 0 0 1 ] . The subsequent return o f C 0 2

flux values at site P78 to their pre-April level could be attrib­uted to a decrease in C 0 2 pressure in this reservoir between mid-July and mid-August 2000 , implying magma migration towards even shallower portions o f the feeder conduits. Indeed, the approach o f a volatile-rich magma batch in the upper con­duit system is attested by a rise in S 0 2 flux during August 2000 (Figure 13b), and we assume that this was the same magma body previously responsible for degassing C 0 2 through the flanks o f Mt. Etna. B y combining the interpolated COSPEC flux data with S0 2 /HC1 and S O / H F molar ratios it was possible to calculate the flux o f HCl and HF. This reveals that the fluxes o f HCl and HF (Figure 13c) increased at almost the same moment as the S 0 2 flux. In order to understand this observation we propose a conceptual model o f the degassing process (Figures 14 and 15). Three factors are o f primary importance; (i) the degree o f chemical equilibrium that exists between the exsolved gas phase (bubbles) and volatiles dis­solved in the melt; (ii) the relative solubilities o f the volatile

species; and (iii) the intensity o f magma convection. As we have seen, the long-term voluminous degassing o f Mt. Etna in the absence o f proportional lava effusion is strong evi­dence that magma convection is a dominant process within the volcano [Allard, 1997; Bruno et al, 1999] . As magma rises within the system dissolved volatile species become satu­rated and form gas bubbles. The relative velocity o f bubble rise compared with rising magma, together with kinetics o f volatile diffusion into the bubbles, will determine the degree o f chem­ical equilibrium between bubbles and melt. I f bubbles rise at the same rate as magma and in perfect equilibrium, we may determine the final chemical composition o f emitted gases by comparing the proportions o f S 0 2 , HCl and HF in the most primitive (undegassed) crystal melt inclusions with the residual amounts o f these species in glassy pyroclasts ejected from the B N , assuming that all the volat i le loss is via degassing. The original concentrations o f S 0 2 , HCl and HF in Etnean basalts average about 0.3, 0.2 and 0.1 wt. % respec­tively. Examination o f glassy pyroclasts demonstrates that erupted lavas have lost at least 95 % of their S content and 50 % o f their original CI content [Allard, 1997; Metrich et al, 2002] . This implies that the S O / H C l molar ratio o f bubbles rising at the same rate as (and in chemical equilibrium with) magma would be around 3. If, rather, the bubbles rise too quickly to allow achievement o f chemical equilibrium, then the gas phase at the surface will be correspondingly enriched in the more insoluble species ( S 0 2 ) , resulting in higher S O / H C l and S O / H F molar ratios. To the contrary, S0 2 /HC1 molar ratios less than 3 will indicate degassing o f a magma body that has already been depleted in sulfur during previous activity.

The observation o f relatively constant S0 2 /HC1 and S 0 2 / H F ratios in August 2000 , together with a rise in the flux o f S 0 2

from 1,400 to over 5,000 Mg/d during the same period, indi­cates that the fluxes o f HCl and HF rose simultaneously with S 0 2 . This coincident increase in the fluxes o f S 0 2 , HCl and HF may be explained either by increased gas flow across a con­duit o f constant width or by an increase in the cross-sectional area of the feeding conduit at the magma/atmosphere interface. This may have occurred due to a rise in the level o f magma within the feeding system. In early September the S 0 2 flux continued to rise, eventually reaching a peak o f over 8,000 Mg/d on 21 September, while during the same period the S0 2 /HC1 molar ratios rose from 2.2 to 4.8, indicating a relative decrease in the flux o f HCl. In the same time, the S 0 2 / H F ratio increased by a factor o f ~ 6. Such a trend is consistent with intense degassing o f S 0 2 and H 2 0 from an ascending volatile-rich magma body leading to the coalescence and so to the fast (separated) ascent o f gas bubbles enriched in S 0 2 with respect to both HCl and HF. Simultaneously, forceful ash emissions

occurred at Bocca Nuova crater from the 1 0 t h to the 2 7 t h Sep­tember, consistent with larger bubbles o f gas producing more vigorous explosions upon arrival at the top o f the magma col­umn. On the 2 8 t h September strombolian activity was first observed at the BN, indicating the arrival o f magma at the sur­face. This activity continued during October before decreasing in intensity during November, in coincidence with sharply decreasing trends in S 0 2 flux and S0 2 /HC1 molar ratio. These observations clearly indicate that the rising magma body had become depleted in volatiles, and could therefore no longer sustain explosive activity.

In summary we interpret the strong variations in gas emis­sions from Mt. Etna during 2000 as the rise o f discrete body o f magma, seen first by increased C 0 2 emissions on the flanks o f the volcano, and afterwards by a strong increase in the fluxes o f S 0 2 (measured), HCl and HF (calculated) due to effective widening o f the conduit system. Intense degassing leading to bubble coalescence produced both increases in S0 2 /HC1 and S 0 2 / H F molar ratios and explosive gas bursting initially responsible for ash emissions and later for strombo­lian eruptive activity. As the discrete magma body became gradually depleted in volatiles, both the S 0 2 flux and the eruptive activity reduced and S0 2 /HC1 S 0 2 / H F molar ratios progressively decreased towards their pre-eruptive level. Data on thorium content o f Mt. Etna's eruptive products collected during the same period (Figure 13b) [Clocchiatti and Joron, 2000 , data are available from the World Wide Web server for the Club-Internet France Node at http://perso.club-inter­net, fr/rivierec/archives.htm] provide tentative support for the above hypothesis; decreases in thorium content in erupted lavas indicate arrival o f new, undifferentiated magma at the top o f the magmatic column.

4. CONCLUSIONS

Multi-parametric monitoring o f volcanic activity, using dis­tal C 0 2 , S 0 2 , HCl and HF emissions, provides valuable indi­cations on the evolution o f volcanic systems towards eruptive conditions months in advance. Magma contains dissolved volatiles that reach saturation pressure when magma is depres-surized, or crystallization o f the magma increases the rela­tive volatiles abundance within the melt. After exsolution the liquid volatile phase may rise to the surface with the same velocity as surrounding magma or separation can occur, with bubbles o f gas arriving at the surface before the magma batch from which it was exsolved. The gas species we measured allowed us to trace magma motion inside Mt. Etna's plumb­ing system from its deepest to its shallowest parts with remark­able detail o f time and depth. We observed variations in these gas species which are consistent with induced degassing o f the

most soluble species in the uppermost part o f the conduit sys­tem due to the large increase in flux from depth creating an increased overturn o f magma in this part o f the magma feed­ing system. Estimates o f the volume o f magma involved in the degassing process, obtained from S 0 2 flux data, allow us to discriminate between steady-state degassing due to magma convection within the main volcanic storage system and that due to excess magma that is moving towards the shallowest portions o f the feeder conduits. Sustained high-flux degassing without voluminous production o f lava on volcanoes such as Mt. Etna requires a convective supply of volatile-rich magma from depth [Kazahaya et al, 1994; Allard, 1997]. The rate o f convection and volatile content o f supplied magma control the chemical composition o f gas emitted by the volcano. Changes in magma dynamics lead to variations in the final chemical composition o f volcanic gas emissions. Therefore, monitoring volcanic gases can reveal changes in the mag­matic system that may have implications for eruptive activity and calculation o f the volume o f excess degassing magma can be used to estimate the magnitude of possible future erup­tive events. Finally, we have demonstrated that the combina­tion o f F T I R molar ratios and C O S P E C S 0 2 flux measurements has allowed an unprecedented insight into the dynamics o f magma ascent and degassing at this volcano. This heralds an exciting new phase in geochemical monitor­ing o f volcanoes, which we believe will provide a deeper con­ceptual understanding o f magma dynamics in the future.

Acknowledgements. We would like to thank Giuseppe Salerno for his help in the processing of COSPEC data and we are also indebted to all the scientists and technicians that gave their help in field data acquisition during the long period of this study. Special thanks are due to the officers and staff of the Meteorological Service of the Italian Air Force for the information on high-altitude winds. We also thank PR. Kyle and G. Chiodini for their careful reviews of the manuscript.

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R. Romano, Via Duca degli Abruzzi 2,1-95100, Catania, Italy. T. Caltabiano, M. Burton, N. Bruno and F. Mure, Istituto Nazionale

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