Roman J. Motyka

Ukinrek Maars, Alaska, I. April 1977 eruption sequence, petrology and tectonic setting

Abstract During ten days of phreatomagmatic activity in early April 1977, two maars formed 13 km behind the Aleutian arc near Peulik volcano on the Alaska Peninsula. They have been named “Ukinrek Maars”, meaning “two holes in the ground” in Yupik Eskimo. The western maar formed at the northwestern end of a low ridge within the first three days and is up to 170 m in diameter and 35 m in depth. The eastern maar formed during the next seven days 600 m east of West Maar at a lower elevation in a shallow saddle on the same ridge and is more circular, up to 300 m in diameter and 70 m in depth. The maars formed in terrain that was heavily glaciated in Pleistocene times. The groundwater contained in the underlying till and silicic volcanics from nearby Peulik volcano controlled the dominantly phreatomagmatic course of the eruption. During the eruptions, steam and ash clouds reached maximum heights of about 6 km and a thin blanket of fine ash was deposited north and east of the vents up to a distance of at least 160 km. Magma started to pool on the floor of East Maar after four days of intense phreatomagmatic activity. The new melt is a weakly undersaturated alkali olivine basalt (Ne = 1.2%) showing some transitional character toward high-alumina basalts. The chemistry, an anomaly in the tholeitic basalt-andesite-dominated Aleutian arc, suggests that the new melt is primitive, generated at a depth of 80 km or greater by a low degree of partial melting of garnet peridotite mantle with little subsequent fractionization during transport. The Pacific plate subduction zone lies at a depth of 150 km beneath the maars. Their position appears to be tectonically controlled by a major regional fault, the Bruin Bay fault, and its intersection with cross-arc structural features. We favor a model for the emplacement of the Ukinrek Maars that does not link the Ukinrek conduit to the plumbing system of nearby Peulik volcano. The Ukinrek eruptions probably represent a genetically distinct magma pulse originating at asthenospheric depths beneath the continental lithosphere.

Geochemistry, isotopic composition, and origin of fluids emanating from mud volcanoes in the Copper River basin, Alaska

Abstract Analyses of waters and gases from the Copper River basin, Alaska, show marked differences in the fluid chemistries of two groups of mud-volcano saline springs. The Tolsona group discharges Na-Ca rich, HCO3-SO4 poor saline waters accompanied by small amounts of CH4, N2, and He. Tolsona gases have 3 He 4 He ratios of 0.75 to 2.7 ( R R atm ), δ13C-CH4 values of − 33 to − 22 per mil, and δ15N values of − 1.8 to + 1.6 per mil. The Klawasi group discharges Ca poor, Na-HCO3 rich saline waters with large amounts of CO2 and 3 He 4 He ratios of 2.6 to 4.1 ( R R atm ), δ13CCO2 values of − 4.8 to − 3.1 per mil, δ13CCH4 values of − 23 to − 18 per mil, and δ15N of − 3.5. Spring waters from both groups differ substantially from seawater composition. Compared to the Tolsona waters, the Klawasi waters are strongly enriched in Li, Na, K, Mg, HCO3, SO4, B, SiO2, As and 18O and strongly depleted in Ca, Sr and deuterium. The differences in fluid chemistry are attributed to the interaction of CO2 with Tolsona-type formation waters. The CO2 is thought to be derived from a deep-seated magmatic intrusive and contact decarbonation of limestone beds underlying the Klawasi area.

Taku Glacier, Southeast Alaska, U.S.A.: Late Holocene History of a Tidewater Glacier

Taku Glacier is the largest glacier draining the Juneau Icefield, and reaches tidewater near the mouth of the Taku River. Taku Glacier historically calved icebergs into a 100-m-deep tidal basin but...

Stable-isotope evidence for a magmatic component in fumarole condensates from Augustine Volcano, Cook Inlet, Alaska, U.S.A.

Abstract D/H and 18 O 16 O ratios have been determined for fumarole condensates from Augustine Volcano, an active calc-alkaline stratovolcano in Lower Cook Inlet, Alaska. The isotopic data for the condensates form a linear δ D-δ 18 O array from low-temperature fluids ( δ D ⋍ −150‰ , δ 18 O ⋍ −19‰ ) to high-temperature (>450°C) fluids collected at the volcano summit which are enriched in both D and 18 O ( δ D ⋍ −35‰ , δ 18 O ⋍ +3.5‰ ). Several lines of evidence suggest that the D-and 18 O-rich condensates likely are “magmatic” fluids released into the hydrothermal system during and immediately after the 1976 eruption. Prior to 1976, the Augustine hydrothermal system was dominated completely by local meteoric waters. Between 1976 and 1982, fumarole condensates were observed to be variable mixtures of the “magmatic” fluid and meteoric water, with the proportion of the former systematically decreasing as the hydrothermal system cooled following the 1976 eruption.

Fumarolic emissions from Mount St. Augustine, Alaska : 1979-1984 degassing trends, volatile sources and their possible role in eruptive style

Gas samples were collected from high-temperature, rooted summit vents at Mount St. Augustine in 1979, 1982, and 1984. All of the gas samples exhibit various degrees of disequilibrium. Thermodynamic restoration of the analyzed gases permits partial or complete removal of these disequilibrium effects and allows inference of equilibrium gas compositions. Long-term (1979–1984) degassing trends within resampled or adjacent vents are characterized by increases (from 97.4 to 99.8 mole%) in the H2O fraction and major decreases in the residual gases. Over this same period total gas HCl contents decreased by a factor of 3 to 10 while dry gas (H2O-free recalculated) HCl contents increased by a factor of 1.6 to 3. Dry gas mole proportions at these sites changed from being CO2-dominated (≈46% CO2, 24% H2 in 1979) to H2-dominated (≈49% H2, 22% CO2 in 1984). The overall trends in gas chemistry and the stable isotope patterns in gases and condensates from the summit fumaroles can be explained by progressive magmatic outgassing coupled with increasing proportions of seawater in the fumarole emissions.Studies of the gaseous emissions following the 1976 and 1986 Mount St. Augustine eruptions confirmed the Cl- and S-rich nature of the Mount St. Augustine emanations. Seawater, possibly derived from magmatic assimilation or dehydration of near-surface seawater-bearing sediments, could supply a portion of the outgassed Cl and S. Continued seawater influx through subvolcanic fractures or permeable sediments would recharge the seawater-depleted zone and provide a near-surface Cl and S source for the next eruptive cycle,Various lines of evidence support a phreatomagmatic component in the 1976 and 1986 Mount St. Augustine eruptions. We suggest that seawater may interact with magma or volcanic gases during the early explosive phase of Mount St. Augustine eruptions and that it continues to influence high-temperature fumarole emissions as the volcanic system cools.

The geyser bight geothermal area, Umnak Island, Alaska

The Geyser Bight geothermal area contains one of the hottest and most extensive areas of thermal springs in Alaska, and is the only site in the state with geysers. Heat for the geothermal system is derived from crustal magma associated with Mt. Recheshnoi volcano. Successive injections of magma have probably heated the crust to near its minimum melting point and produced the only high-SiO[sub 2] rhyolites in the oceanic part of the Aleutian arc. At least two hydrothermal reservoirs are postulated to underlie the geothermal area and have temperatures of 165 and 200 C, respectively, as estimated by geothermometry. Sulfate-water isotope geothermometers suggest a deeper reservoir with a temperature of 265 C. The thermal spring waters have relatively low concentrations of Cl (600 ppm) but are rich in B (60 ppm) and As (6 ppm). The As/Cl ratio is among the highest reported for geothermal waters. 41 refs., 12 figs., 8 tabs.

Glacier-volcano interactions in the North Crater of Mt Wrangell, Alaska

Abstract Glaciological and related observations from 1961 to 2005 at the summit of Mt Wrangell (62.00° N, 144.02°W; 4317m a.s.l.), a massive glacier-covered shield volcano in south-central Alaska, show marked changes that appear to have been initiated by the Great Alaska Earthquake (Mw = 9.2) of 27 March 1964. The 4×6 km diameter, ice-filled Summit Caldera with several post-caldera craters on its rim, comprises the summit region where annual snow accumulation is 1–2m of water equivalent and the mean annual temperature, measured 10 m below the snow surface, is –20°C. Precision surveying, aerial photogrammetry and measurements of temperature and snow accumulation were used to measure the loss of glacier ice equivalent to about 0.03 km3 of water from the North Crater in a decade. Glacier calorimetry was used to calculate the associated heat flux, which varied within the range 20–140Wm–2; total heat flow was in the range 20–100MW. Seismicity data from the crater’s rim show two distinct responses to large earthquakes at time scales from minutes to months. Chemistry of water and gas from fumaroles indicates a shallow magma heat source and seismicity data are consistent with this interpretation.

Alaska geothermal bibliography

The Alaska geothermal bibliography lists all publications, through 1986, that discuss any facet of geothermal energy in Alaska. In addition, selected publications about geology, geophysics, hydrology, volcanology, etc., which discuss areas where geothermal resources are located are included, though the geothermal resource itself may not be mentioned. The bibliography contains 748 entries.

Geological, geochemical, and geophysical survey of the geothermal resources at Hot Springs Bay Valley, Akutan Island, Alaska

An extensive survey was conducted of the geothermal resource potential of Hot Springs Bay Valley on Akutan Island. A topographic base map was constructed, geologic mapping, geophysical and geochemical surveys were conducted, and the thermal waters and fumarolic gases were analyzed for major and minor element species and stable isotope composition. (ACR)