Aleutian Arc Geothermal Fluids: Chemical Analyses of Water and Gas Samples Collected During Volcano Hazards Investigations
Click on the links below for detailed information on each volcano:
As part of ongoing efforts by the Alaska Volcano Observatory (AVO) to study and monitor volcanoes of the Aleutian Arc, samples of water and gas are occasionally collected from thermal springs, fumaroles, gas vents, and other features. These samples are analyzed in one or more laboratories at the U.S. Geological Survey (USGS). Some of the analytical results are eventually included in publications that summarize the field work or present major conclusions, but some data remains unpublished, especially results from single "grab" samples that are collected during the performance of other work. Such data could be useful for purposes such as constraining the strength of magmatic degassing, evaluating geothermal resources, or simply establishing baseline hydrologic conditions at remote volcanoes that are seldom visited. This report contains the chemical and isotopic data from thermal waters and gases collected from the Aleutian Arc (Figure 1) over the past 20 years, where such data remain unpublished or only published in part. We discuss some interesting features in the data, and offer brief overviews on what the data indicate about subsurface conditions. We emphasize however, that the datasets are small and the inferences drawn from them are preliminary and should not be considered as peer reviewed. One main goal in providing these overviews is to stimulate future research.
For completeness, this report also summarizes the focused investigations of thermal fluids and gases carried out in the past 20 years at Aleutian Arc volcanoes where the data have already been published (e.g., Chiginagak, Akutan, Ukinrek Maars). Results from ongoing investigations focused on the Katmai area (Lopez and others, in prep.) are not included. Brief statements about the volcanoes and their recent eruptive histories are mostly taken from the Alaska Volcano Observatory website (AVO, 2015) as of 1 April 2015, but derive in many cases from Miller and others (1998) or Wood and Kienle (1990).
A general summary of collection and analytical protocols was given by Bergfeld and others (2013). Gas samples discussed herein were collected in evacuated glass sample bottles without added caustic. Bubbling gases were collected from springs using an inverted funnel and water displacement. Fumarole gases were collected through a metal tube into a T-handle glass bottle with a downstream pinch clamp. Samples for dissolved inorganic carbon (DIC) and its isotopes were collected by injecting water into a septum-equipped evacuated glass bottle using a syringe. Helium isotope samples were collected in copper tubes sealed with refrigeration clamps.
Some water samples were collected according to established field protocols, including: on-site measurement of pH; filtration of water for chemical analysis through 0.45 μm membrane filters; acidification of cation samples; collection of isotope and alkalinity samples in tightly sealed glass bottles (Bergfeld and others, 2013). However, for some of the water samples, field measurements were minimal and consist of temperature and location, and only raw (unfiltered) water samples were collected. Unless otherwise specified, the pH, specific conductance, and alkalinity values presented in the tables are laboratory measurements, and filtration as needed prior to chemical analysis was performed in the laboratory. Lack of field processing probably has minimal negative impact on conservative species like Na, Cl and most major ions, but trace and redox-sensitive species like Fe or nutrients like NO3 and PO4 may have large uncertainties. A few samples were collected raw and acidified, which allows suspended particles to dissolve after collection and can greatly increase the concentrations of many of the metal species.
Chemical analyses of gas and water samples were carried out at USGS laboratories in Menlo Park, California, using methods described in Bergfeld and others (2013). Gases were analyzed by using gas chromatographs equipped with a thermal-conductivity detector. Water samples were analyzed for anions by ion chromatography except for HCO3, which was analyzed by titration and should be considered as "total alkalinity as HCO3" in all tables attached to this report. Cations were analyzed by argon plasma optical-emission spectrometry, and the results given in mg/L and μg/L. For some samples, major cation and trace element concentrations were determined at the USGS Minerals Program laboratories in Denver, Colorado, by inductively coupled plasma mass spectrometry (ICP-MS) using procedures described by Lamothe and others (1999). These results are usually given in ppm and ppb. Analytical results presented herein preserve the units of the reporting laboratory. For low salinity waters (dissolved solids <5000 mg/L), mg/L nearly equals ppm, and μg/L nearly equals ppb. Stable isotope analyses of waters, steam, DIC, CO2, and CH4 were performed by mass spectrometry at the USGS Stable Isotope Laboratory in Reston, Virginia, and are reported in per mil (Revesz and others, 2008a,b). Noble-gas ratios were determined by mass spectrometry at the USGS Noble Gas Laboratory in Denver, Colorado, using methods described in Hunt and others (2013). Reported 3He/4He ratios are corrected for air contamination and are given as RC/RA values, where RA is the 3He/4He ratio in air.
Location data in all tables is given in decimal degrees. The reference datum was not always recorded but assuming WGS84 seems to match aerial imagery for most features.
This report presents the first published chemistry data on waters or gases from three Aleutian Arc volcanoes: Semisopochnoi, Little Sitkin, and Tana. Water chemistry at Little Sitkin and Tana shows that the hydrothermal systems are vapor-dominated, and high concentrations of ammonium reveal a significant interaction with buried organic matter. Gas geothermometry implies a reservoir temperature of 235°C at Little Sitkin, which would make this system one of the hottest in Alaska. This report also updates the record of fluid chemistry at several volcanoes; notably, several new gas analyses from Augustine and a new stream sample at Redoubt, or fills in results for constituents not analyzed in previous studies, such as carbon isotopes.
The carbon isotope data presented in the tables here, or in the recent reports cited, can be combined with results from the two major compilations of carbon isotope values of fluids from Aleutian Arc volcanoes in the literature (Motyka and others, 1993; Symonds and others, 2003b) to fill in the pattern for the entire active part of the arc, from Spurr at the eastern end to Kiska at the western end. The combined carbon isotope dataset is given in Table 15 and plotted in Figure 23. For plotting purposes, values from multiple sites were averaged (for example several sites at Makushin; 3 sites at Atka) so that a single value for each volcano is plotted for each set of researchers. Where 2 or 3 groups of researchers give results, the agreement between the groups is generally fair, worst case being Makushin where the one data point from Symonds and others (2003b) is significantly heavier than all 9 values given by Motyka and others (1993). A few widely divergent values were not given in Table 15 or plotted in Figure 23. These include an extremely negative value from Akutan (-18.1‰) and a positive value from Augustine (+2.3‰) in Motyka and others (1993) and values obtained on cold dilute waters at Augustine and Ukinrek Maars.
Most geoscientists accept the view that much of the carbon emitted by island arc volcanoes derives from subducted sediments and carbonate minerals, and that the mix between them is a controlling factor in the isotopic composition of the emitted CO2 (Sano and Marty, 1995). The relative importance of shallow crustal carbon sources and losses is still debated, but was particularly emphasized by Symonds and others (2003a). Figure 23 suggests that the carbon isotope composition does vary in a systematic way along the arc, shifting from heavier values toward the east (Spurr excepted) to lighter values toward the center. The shift occurs close to the transition zone from a continental to an oceanic arc. However, a shift back to heavier values at the western end is very apparent, especially considering the new data, and this shift does not correspond to an obvious change in geologic setting. The shift begins hundreds of km east of the intersection of the arc with the Bowers Ridge. Thus the cause of the carbon isotope variations along arc remains uncertain. Nevertheless, the pattern is intriguing and worthy of further investigation. Carbon isotope data are still lacking for known geothermal systems on many of the Aleutian Arc volcanoes. Searches for magmatic CO2 in soil gases, as recently found at Kasatochi and Kiska, have rarely been done in the Aleutian Arc.
Work presented or described herein highlights the fact that thermal springs are not necessarily the most useful features in volcano monitoring. Thermal springs such as those at Aniakchak and ELVC often exhibit long-term stability in composition through the years (Tables 2 & 5). Gas samples from acid-sulfate areas and summit fumaroles, such as those at Augustine (Table 3), are more likely to respond quickly to magmatic unrest. Under certain conditions, streams draining the summit areas can provide useful information on magmatic gas flux, as at Chiginagak and Redoubt. Opportunities to monitor similar streams should not be overlooked, especially in cases when direct sampling in the summit area is precluded by inaccessibility or hazard issues.
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