Magmatic gas scrubbing: implications for volcano monitoring

Rb. Symonds et al., Magmatic gas scrubbing: implications for volcano monitoring, J VOLCANOL, 108(1-4), 2001, pp. 303-341
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
0377-0273 → ACNP
Year of publication
303 - 341
SICI code
Despite the abundance Of SO2(g) in magmatic gases, precursory increases in magmatic SO2(g) are not always observed prior to volcanic eruption, probabl y because many terrestrial volcanoes contain abundant groundwater or surfac e water that scrubs magmatic gases until a dry pathway to the atmosphere is established. To better understand scrubbing and its implications for volca no monitoring, we model thermochemically the reaction of magmatic gases wit h water. First, we inject a 915 degreesC magmatic gas from Merapi volcano i nto 25 degreesC air-saturated water (ASW) over a wide range of gas/water ma ss ratios from 0.0002 to 100 and at a total pressure of 0.1 MPa. Then we mo del closed-system cooling of the magmatic gas, magmatic gas-ASW mixing at 5 .0 MPa, runs with varied temperature and composition of the ASW, a case wit h a wide range of magmatic-gas compositions, and a reaction of a magmatic g as-ASW mixture with rock. The modeling predicts gas and water compositions, and, in one case, alteration assemblages for a wide range of scrubbing con ditions; these results can be compared directly with samples from degassing volcanoes. The modeling suggests that CO2(g) is the main species to monito r when scrubbing exists; another candidate is HS(g), but it can be affected by reactions with aqueous ferrous iron. In contrast, scrubbing by water wi ll prevent significant SO2(g) and most HCl(g) emissions until dry pathways are established, except for moderate HCl(g) degassing from pH < 0.5 hydroth ermal waters. Furthermore, it appears that scrubbing will prevent much, if any, SO2(g) degassing from long-resident boiling hydrothermal systems. Seve ral processes can also decrease or increase H-2(g) emissions during scrubbi ng making H-2(g) a poor choice to detect changes in magma degassing. We applied the model results to interpret field observations and emission r ate data from four eruptions: (1) Crater Peak on Mount Spurr (1992) where, except for a short post-eruptive period, scrubbing appears to have drastica lly diminished pre-, inter-, and post-eruptive SO2(g) emissions, but had mu ch less impact on CO2(g) emissions. (2) Mount St. Helens where scrubbing Of SO2(g) was important prior to and three weeks after the 18 May 1980 erupti on. Scrubbing was also active during a period of unrest in the summer of 19 98. (3) Mount Pinatubo where early drying out prevented SO2(g) scrubbing be fore the climactic 15 June 1991 eruption. (4) The ongoing eruption at Popoc atepetl in an and region of Mexico where there is little evidence of scrubb ing. In most eruptive cycles, the impact of scrubbing will be greater during pre - and post-eruptive periods than during the main eruptive and intense passi ve degassing stages. Therefore, we recommend monitoring the following gases : CO2(g) and H2S(g) in precursory stages; CO2(g), H2S(g), SO2(g), HCl(g), a nd HF(g) in eruptive and intense passive degassing stages; and CO2(g) and H 2S(g) again in the declining stages. CO2(g) is clearly the main candidate f or early emission rate monitoring, although significant early increases in the intensity and geographic distribution of H2S(g) emissions should be tak en as an important sign of volcanic unrest and a potential precursor. Owing to the difficulty of extracting SO2(g) from hydrothermal waters, the emerg ence of > 100 t/d (tons per day) of SO2(g) in addition to CO2(g) and H2S(g) should be taken as a criterion of magma intrusion. Finally, the modeling s uggests that the interpretation of gas-ratio data requires a case-by-case e valuation since ratio changes can often be produced by several mechanisms; nevertheless, several gas ratios may provide useful indices for monitoring the drying out of gas pathways. Published by Elsevier Science B.V.