Excessive Sulfur Dioxide Emissions from Chilean Volcanoes

Journal of Volcanology and Geothermal Research, 46 (1991) 323-329 323 Elsevier Science Publishers B.V., Amsterdam

Excessive sulfur dioxide emissions from Chilean volcanoes

R.J. Andres a, W.I. Rose a, P.R. Kyle b, S. deSilva c, 1, p. Francis c, 2, M. Gardeweg d and H. Moreno Roa e a Department of Geological Engineering, Geology and Geophysics, Michigan Tech Univ., Houghton, MI 49931, USA b Geoscience Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA c Lunar and Planetary Institute, 3303 NASA Road 1, Houston, TX 77058, USA d Servicio Nacional de Geologia y Mineria, Casilla 10465, Santiago, Chile e Departamento de Geologia y Geofisica, Universidad de Chile, Casilla 13518 - Correo 21, Santiago, Chile

(Received December 26, 1990; accepted in revised form March 7, 1991)

Introduction By calculating the amount of sulfur lost upon eruption from analyses of melt inclu- Sulfur dioxide (SO2) is the most abun- sions and volcanic glass, it is possible to calcu- dant sulfur species emitted during many vol- late the minimum volume of magma degassed canic eruptions (Gerlach and Nordlie, 1975; to produce the SO2 measured remotely. Of- Jaeschke et al., 1982). SO2 can be mea- ten the calculated volume of magma degassed sured remotely with a correlation spectrom- is greater than the observed volume of lava eter (Stoiber et al., 1983). erupted. This was observed in two Chilean Sulfur solubility studies have demonstrated volcanoes in November 1989. that release of significant amounts of sulfur Volcfin Lfiscar (23.37°S, 67.73°W, 5641 m) should occur at shallow depths for basaltic and Volcfin Lonquimay (38.38°C, 71.59°W, magmas (< 3 MPa or 150 m lithostatic) 2865 m) are located in the central and south- (Greenland et al., 1985; Gerlach, 1986; My- ern volcanic zones of the Andes, respectively. sen, 1988); shallower exsolution depths prob- In the Andes, volcanic activity is of the con- ably apply to more silicic magma composi- tinental margin, convergent plate type, re- tions. Pre-eruptive concentrations of sulfur lated to the subduction of the Nazca plate in magmas can be determined in melt inclu- beneath the South American plate (Jordan et sions which form as blebs of magma trapped al., 1983). in rapidly forming crystals (Anderson, 1974; Rose et al., 1978). Post-eruption sulfur con- Lfiscar centrations can be determined in volcanic glasses produced during the eruption as the Ninety-nine determinations of 802 fluxes lava quickly cools on the surface (Devine et in an ash-free plume from Volc~m L~iscar were al., 1984). made from ground sites with a correlation spectrometer on 15 and 16 November i989. Present addresses: Measured fluxes ranged from 540 to 6970 Department of Geology, Indiana State University, Terre Mg/d and averaged 2300 + 1120 (Rr) Mg/d Haute, IN 47809, USA 2 Hawaii Institute of Geophysics, University of Hawaii, Hon- (Table 1). Fast Fourier transforms (Ramirez, olulu, HI 96822, USA 1985) of the measured SO2 fluxes revealed

TABLE 1

SO2 fluxes measued at Volcfin Lfiscar

Lfiscar-1 Lfiscar-2 Lfiscar-3 Lfiscar-4 Date 15 Nov 89 15 Nov 89 16 Nov 89 16 Nov 89 Local time 1243-1323 1455-1609 1019-1027 1223-1347 # of scans 16 45 4 34 Location * 10 km W 10 km W 3 (2) km S 5.5 (3) km S Winds (m/s) 11 11 14 14 Min. flux (Mg/d) 540 750 2370 880 Mean flux (Mg/d) 1370 2940 3450 1750 Max. flux (Mg/d) 2090 6970 4470 2790 (1 ~) 376 1201 863 469

* Relative to vent. If plume was not measured as it left the vent, then distance to plume given in parentheses.

3- to 8-minute periodicities which agree with were made from ground sites with a corre- the 3- to 8-minute periodicity observed in lation spectrometer on 19 and 21 November visible puffing. These are the first SO2 mea- 1989. Measured fluxes ranged from 320 to surements taken at Lfiscar and whether they 14,910 Mg/d and averaged 2380 + 2720 (lcr) are representative of the long-term degassing Mg/d. When the data set is segmented based of the volcano is unknown. Historic activ- upon collection periods the fluxes are much ity is characterized by brief explosions such better constrained (Table 2). These fluxes are as those of September 1986 (Glaze et al., 5 to 45 times greater than those measured on 1989), July 1988 (SEAN, 1988), and Febru- 13 July 1989 (Table 2). Long-term (monthly) ary 1990 (GVN, 1990a). Since February 1989, field observations indicate that increased lava an andesitic (SiO2 = 60-61%) lava dome has flow rates or changes in lava composition grown in the crater (SEAN, 1989a). did not cause the increased SO2 fluxes, however, field observations do not preclude short- Lonquimay term variations in these parameters. Also, the spatially more distant July measurements Fifty-four determinations of 802 fluxes may underestimate SO2 fluxes due to sepa- from the Navidad cone of Volcfin Lonquimay ration of SO2 from the visible steam plume

TABLE 2

SO2 fluxes measured at Volcfin Lonquimay

Lonquimay- 1 Lonquimay-2 Lonquimay-3 Lonquimay-4 Date 13 Jul 89 19 Nov 89 19 Nov 89 21 Nov 89 Local time 1339-1513 1205-1238 1255-1339 1200-1340 # of scans 161 8 27 19 Location * 9 (5.8) km SE 1.3 km SE 1.3 km SE 1.3 km SE Winds (m/s) * 8 26 26 14 Min. flux (Mg/d) 80 5130 420 320 Mean flux (Mg/d) 170 7760 1900 790 Max. flux (Mg/d) 300 14910 3680 1510 (lcr) 48 3510 820 370

*Relative to vent. If plume was not measured as it left the vent, then distance to plume given in parentheses. For Lonquimay-2, 3, 4, this value represents the rise rate of the vertically rising plume. EXCESSIVE SULFUR DIOXIDE EMISSIONS FROM CHILEAN VOLCANOES 325

(Andres et al., 1991). Lonquimay was un- logic method and the measured SO2 emis- dergoing a continuous strombolian/vulcanian sion rate to calculate the rates of extrusion at eruption from the summit of the Navidad Lfiscar and Lonquimay gives 9.3 x 105 m3/d cone and emission of an andesitic blocky lava for Lfiscar and 9.7 x 105 m3/d for Lonquimay. flow from the base of the cone from Decem- These calculated rates are 150 (Lfiscar) and ber 1988 to January 1990 (GVN, 1990b). 65 (Lonquimay) times too high--observed extrusion rates are 6300 m3/d for Lfiscar Excessive sulfur emissions (SEAN, 1989b) and 15,000 m3/d for Lon- quimay (SEAN, 1989c). The SO2 fluxes of Lfiscar and Lonquimay Examination of lava thin sections from are similar to some other volcanoes, e.g. both Chilean volcanoes has shown no ev- Mount Etna (Haulet et al., 1977) or Mount idence of unusual sulfur enrichment, such St. Helens in 1980 (Casadevall et al., 1983), as abundant sulfide or sulfate phases, which which are characterized as open-vent volca- might explain the high SO2 fluxes. Micro- noes because there is a magma-air interface probe studies of melt inclusions and intersti- across which magma erupts or gas passes. tial glass in Lfiscar bombs give a pre-eruptive However, the Chilean volcanoes have higher volatile content of < 30 ppm sulfur (< 0.003 SO2 fluxes than many other open-vent volca- wt.% S). Preliminary microprobe data on noes which fall in the range of 5-2,500 Mg/ melt inclusions in highly fretted calcic plagio- d (Stoiber et al., 1983). At Kilauea, the SO2 clase crystals have < 200 ppm sulfur (< 0.02 emission rate appears to correlate well with wt.% S). One hundred percent degassing of the lava extrusion rate, assuming a nearly a Lfiscar magma containing 200 ppm sulfur complete degassing of erupted magma upon would imply a magma extrusion rate more eruption (Greenland et al., 1985; Andres et than 350 times that observed. The paucity of al., 1989). But, the rates at Lfiscar and Lon- large phenocrysts with melt inclusions has not quimay are hard to explain if one assumes allowed comparable microprobe analyses of that they result solely from degassing of Lonquimay lavas. erupted magma. Liscar and Lonquimay add to a growing By dividing the average mass of sulfur list of volcanoes which have excessive sulfur emitted by the time-averaged dome growth emission rates (Table 3B). Most of these vol- rate (for L~iscar) or lava flow rate (for Lon- canoes occur at convergent plate boundaries. quimay), a weight percent of sulfur (wt.% Table 3B shows that estimated sulfur emis- S) lost from the magma upon eruption can sion rates based on erupted magma volumes be calculated. This calculation assumes only and petrologic data (Table 3A) are underesti- erupted magma contributes sulfur. Lfiscar mates. would have lost 6.9 wt.% S and Lonquimay Two of these volcanoes, El Chich6n and 3.1 wt.% S. These sulfur losses are 50 to 100 Ruiz, have had their excessive sulfur emis- times higher than sulfur losses calculated for sions studied in more detail. Luhr et al. various magmatic eruptions using a "petro- (1984) suggested that magmatic anhydrite logic" method of estimation (Devine et al., and a sulfur-rich vapor phase contributed to 1984) (Table 3A). The petrologic method of the excess sulfur emissions at El Chich6n. At estimating sulfur loss compares the concen- Ruiz, Fournelle (1990) also proposed a role trations of sulfur in pre-eruption melt inclu- for anhydrite while Sigurdsson et al. (1990) sions and post-eruption volcanic glasses. Us- gave evidence for a sulfur-rich vapor phase. ing the average basaltic andesite sulfur loss Williams et al. (1990) indicate changing pres- (0.0495 wt.% S) determined by the petro- sure and temperature on a buried caldera sys- 326 R.J. ANDRES ET AL.

TABLE 3 Weight percent sulfur lost upon eruption A. Petrologic method (Devine et al., 1984)

wt.% S lost # of eruptions basalt 0.04-0.08 5 basaltic andesites 0.04-0.06 2 phonolite 0.007 1 dacitic 0-0.006 6 Agung, 1963 0.04 1 Mt. St. Helens, 1980 0.004 1 El Chich6n, 1982 0.003 3

B. Other methods (arranged by decreasing wt.% S lost)

magma mass S mass wt.% S ref. * (Mg) (Mg) lost Etna, 1975, basalt 4.0 x 104 7.5 x 103 15 1 L~iscar, 1989, andesite 3.7 × 106 2.7 × 105 6.9 This paper Lonquimay, 1989, andesite 1.1 x 105 3.5 x 103 3.1 This paper Ruiz, 1985, dacitic andesite 3.5 x 107 3.3 x 105 0.93 2, 3 Fuego, 1974, hi-Al basalt 2.2 × 108 1.6 x 106 0.72 4 El Chich6n, 1982, trachyandesite 1.2 × 109 6.7 × 106 0.56 5, 6 2.5 × 106 0.21 f Pacaya, 1972, basalt 2.6 x 104 130 0.50 7 Agung, 1963, basaltic andesite 5.0 x 109 0.6 x 107 0.12 8 Mt. St. Helens, 1980, dacite 7.4 x 108 0.3 x 106 0.04 9, 6 2.4 x 105 0.03 10 Mauna Loa, 1984, basalt 5.4 x 108 1.2 x 105 0.02 11

* 1, Haulet et al. (1977); 2, Krueger et al. (1990); 3, Naranjo et al. (1986); 4, Rose et al. (1982); 5, Sigurdsson et al. (1984); 6, Evans and Kerr (1983); 7, Stoiber and Jepsen (1973); 8, Cadle et al. (1976); 9, Casadevall et al. (1983); 10, Stoiber et al. (1981); 11, Casadevall et al. (1984). A. Krueger, pers. commun. tern beneath Ruiz as the origin of the ex- (1) The excess sulfur comes from cessive sulfur emissions. However, the unique unerupted magma (Rose et al., 1982; Casade- pieces of evidence which led to these site spe- vall et al., 1983; Meeker, 1988; Andres et al., cific causes of excessive sulfur emissions are 1991). Convection or shallow intrusion of the not present in the Chilean volcanoes. magma could allow sulfur exsolution without The absence of the unique pieces of ev- magma eruption. Convection could continu- idence which led to the mechanisms men- ally bring undegassed magma to the surface tioned above, indicates a new model is while carrying away the degassed magma back needed to explain the excessive sulfur emis- into the magma chamber. At depth, the de- sions of the Chilean volcanoes. This model gassed magma could absorb more sulfur and must be consistent with what has been ob- other previously exsolved volatiles and recir- served at Ldscar and Lonquimay. Several fac- culate to the surface to degas again. tors, alone or in concert, may account for the (2) The excess sulfur may be derived from excessive sulfur emissions: an unerupted basaltic intrusion. Basalts have EXCESSIVE SULFUR DIOXIDE EMISSIONS FROM CHILEAN VOLCANOES 327 higher sulfur-bearing capacities than more silicic magmas (Wendlandt, 1982; Carroll and Rutherford, 1985). Upon intrusion, the basalt cools, crystallizes, and releases its volatiles to the magma it has intruded. Also, the basalt intrusion could add the thermal energy to initiate or continue the eruption (Sparks et al., 1977). Previous eruptions of Lfiscar have shown evidence of basaltic intrusions (Deru- elle, 1985). (3) The excess sulfur may be brought to the surface in a distillation process. The exsolution and upward migration of less soluble species, e.g. CO2, may transport more soluble ' ~ BASALT species, e.g. SO2, to the surface. Such a mechanism has been proposed for the transport of more soluble HC1 in less soluble water va- , J por (Anderson, 1974). This process does not require magma movement. We suggest that the generation of basalt at convergent plate boundaries and its in- Fig. 1. Schematic diagram showing basalt intruding an an- trusion into previously existing, more sili- desitic magma chamber. Cooling and crystallization of the cic magma chambers (Eichelberger, 1975; intruded basalt releases heat and volatiles into the overly- Sakuyama, 1984) may be the fundamental ing andesitic magma. Excessive sulfur dioxide emissions result cause of excess sulfur at the Chilean volca- from the increased convection rates in the andesite and upward distillation of volatiles from the basalt and andesite. noes (Fig. 1). Basaltic intrusions would not only add volatiles to the overlying, more silicic magma but would also increase magma the phenocrysts residing in the more silicic convection rates and enhance the distillation magma if the two magmas do not physically of sulfur by other gases (primarily CO2) from mix. Third, the heat added by the basalt may intermediate depths (1-6 km). This model is cause the resorption of phenocrysts rather similar to that of Melson et al. (1990) as sug- than their growth. Without crystal growth, en- gested for Ruiz, except this model highlights trapment of new melt inclusions which show the role of distillation in bringing volatiles to elevated sulfur content will not occur. the surface. The general nature of this model may also make it applicable to other volca- Conclusions noes where unique pieces of evidence do not indicate one of the mechanisms suggested for There exists a growing list of volcanoes E1 Chich6n or Ruiz. which have measured sulfur emissions in ex- There are several reasons why the petro- cess of those expected based upon reason- logic method could be insensitive to basaltic able magma sulfur content and the volume of intrusions which do not physically intermin- erupted material (Table 3B). Most of these gle with the overlying, more silicic magma. volcanoes occur at convergent plate bound- First, the melt inclusions may have been aries. A general model invoking unerupted trapped prior to the basaltic intrusion. 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