Geology topics
Publications and source records attributed to T. E. C. Keith.
Three fossil fumaroles in rhyolite-rich ash-flow tuff from the lower Valley of Ten Thousand Smokes (VTTS) have been studied to ascertain chemical losses and gains and mineralogical evolution during fumarolic alteration and subsequent cooling and weathering processes. Major oxide and trace element data were interpreted using the isocon method of Grant (1986).
The great eruption of 1912 in the Aleutian Range of Alaska (Figure 1) is exceptional for both its size and relative simplicity. It was the largest eruption of this century and the largest rhyolitic outburst in almost 20 centuries. The 60-hour, 30-km 3 (ejecta volume) eruption produced extensive fallout deposits, an ash-flow sheet that gave rise to the Valley of Ten Thousand Smokes, and the 3-km-diameter Mt. Katmai caldera. Because magma reached the surface through uniform nonvolcanic basement and the vent underwent little or no collapse (unusual for such a large event), the site provides an ideal target for surface geophysical and subsurface coring exploration of structures and conditions produced by explosive volcanism [Panel on Volcanic Studies at Katmai, 1989]. A project to investigate upper crustal magmatic processes at Katmai is part of the U.S. Continental Scientific Drilling Program [Eichelberger and Hildreth, 1986]. The surface studies phase of this project was undertaken last summer and preliminary results are now emerging.
Shishaldin Volcano, in the central Aleutian volcanic arc, became seismically restless during the summer of 1998. Increasing unrest was monitored using a newly installed seismic network, weather satellites, and rare local visual observations. The unrest culminated in large eruptions on 19 April and 22-23 April 1999. The opening phase of the 19 April eruption produced a sub-Plinian column that rose to 16 km before rapidly dissipating. About 80 min into the 19 April event we infer that the eruption style transitioned to vigorous Strombolian fountaining. Exceptionally vigorous seismic tremor heralded the 23 April eruption, which produced a large thermal anomaly observable by satellite, but only a modest, 6-km-high plume. There are no ground-based visual observations of this eruption; however we infer that there was renewed, vigorous Strombolian fountaining. Smaller low-level ash-rich plumes were produced through the end of May 1999. The lava that erupted was evolved basalt with about 49% SiO2. Subsequent field investigations have been unable to find a distinction between deposits from each of the two major eruptive episodes.
Factor and canonical correlation analysis of geochemical data from eight fossil fumaroles suggest that six major factors controlled the formation and evolution of fumarolic encrustations on the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS). The six-factor solution model explains a large proportion (low of 74% for Ni to high of 99% for Si) of the individual element data variance. Although the primary fumarolic deposits have been degraded by secondary alteration reactions and up to 75 years of weathering, the relict encrustations still preserve a signature of vapor-phase element transport. This vapor-phase transport probably occurred as halide or oxyhalide species and was significant for As, Sb and Br. At least three, and possibly four, varied temperature leaching events affected the fumarolic deposits. High-temperature gases/liquids heavily altered the ejecta glass and mineral phases adjacent to the fumarolic conduit. As the fumaroles cooled. Fe-rich acidic condensate leached the ejecta and primary fumarolic deposits and resulted in the subsequent precipitation of Fe-hydroxides and/or Fe-oxides. Low- to ambient-temperature leaching and hydration reactions generated abundant hydrated amorphous phases. Up to 87% of the individual element data variance is apparently controlled by the chemistry of the ejecta on which the relict encrustations are found. This matrix chemistry factor illustrates that the primary fumarolic minerals surrounding the active VTTS vents observed by earlier workers have been effectively removed by the dissolution reactions. Element enrichment factors calculated for the VTTS relict encrustations support the statistical factor interpretations. On the average, the relict encrustations are enriched, relative to visibly unaltered matrix protolith, in As, Br, Cr, Sb, Cu, Ni, Pb, Fe, and LOI (an indirect measure of sample H2O content). ?? 1993.
Meteoric waters from cold springs and streams outside of the 1912 eruptive deposits filling the Valley of Ten Thousand Smokes (VTTS) and in the upper parts of the two major rivers draining the 1912 deposits have similar chemical trends. Thermal springs issue in the mid-valley area along a 300-m lateral section of ash-flow tuff, and range in temperature from 21 to 29.8??C in early summer and from 15 to 17??C in mid-summer. Concentrations of major and minor chemical constituents in the thermal waters are nearly identical regardless of temperature. Waters in the downvalley parts of the rivers draining the 1912 deposits are mainly mixtures of cold meteoric waters and thermal waters of which the mid-valley thermal spring waters are representative. The weathering reactions of cold waters with the 1912 deposits appear to have stabilized and add only subordinate amounts of chemical constituents to the rivers relative to those contributed by the thermal waters. Isotopic data indicate that the mid-valley thermal spring waters are meteoric, but data is inconclusive regarding the heat source. The thermal waters could be either from a shallow part of a hydrothermal system beneath the 1912 vent region or from an incompletely cooled, welded tuff lens deep in the 1912 ash-flow sheet of the upper River Lethe area. Bicarbonate-sulfate waters resulting from interaction of near-surface waters and the cooling 1953-1968 southwest Trident plug issue from thermal springs south of Katmai Pass and near Mageik Creek, although the Mageik Creek spring waters are from a well-established, more deeply circulating hydrothermal system. Katmai caldera lake waters are a result of acid gases from vigorous drowned fumaroles dissolving in lake waters composed of snowmelt and precipitation. ?? 1992.
Fumarolic gas samples collected in 1978 and 1979 from the stratovolcanoes Mount Griggs, Mount Mageik, and the 1953-68 SW Trident cone in Katmai National Park, Alaska, have been analysed and the results presented here. Comparison with recalculated analyses of samples collected from the Valley of Ten Thousand Smokes (VTTS) in 1917 and 1919 demonstrates differences between gases from the short-lived VTTS fumaroles, which were not directly magma related, and the fumaroles on the volcanic peaks. Fumarolic gases of Mount Griggs have an elevated total He content, suggesting a more direct deep crustal or mantle source for these gases than those from the other volcanoes. ?? 1992.
Samples from a fossil fumarole originating in the 1912 ash-flow tuffin the Valley of Ten Thousand Smokes have been analyzed to ascertain chemical changes resulting from high-temperature fumarolic alteration and subsequent cooling and weathering of the protolith. Samples of the underlying, dominantly leached, dacite-rich portion of the ash-flow tuff adjacent to the fumarolic conduit and samples of encrusted fallout from the shallow part of the fossil fumarole were interpreted using the isocon method of Grant (1986). The results show that, relative to unaltered l9l2 dacite, chosen as a standard composition for the protolith in this fossil fumarole, mass was conserved during the alteration reactions for most of the system, but mass gains of l4–2D% were determined for three samples in the leached ash-flow tuff Relative to unaltered dacite protolith, significant enrichments occurred in SO 3 , LOI (~H 2 O), Cl, F, Zn, Pb, Cu, Sn, Cr, Ni, As, Sb, Au, Br in various parts of the fossil fumarole. Some of these were during the high-temperature part of the alteration, and some were during cooling processes when acid alteration becomes prominent. The REEs indicate some depletion in highly altered samples relative to dacite protolith and differential mobility of Eu 2+ relative to trivalent REEs. This is manifested by positive Eu anomalies in REE patterns normalized against REE in the dacite protolith. Mineral phases introduced in the alteration assemblages include alunite reflecting high SO 3 , activity, hydrated aluminum hydroxy-fluoride (a ralstonite-like phase) and fluorite reflecting high F activity, smectite, magnetite, hematite, and goethite reflecting oxidation and hydration reactions. Opal and a portion of the α-cristobalite reflect SiO 2 , mobility; however, the abundance of α-cristobalite is formed from pumice leached during high-temperature vapor-phase processes and devitrification of the altered glass.
Isotopic compositions were determined for hydrothermal quartz, calcite, and siderite from core samples of the Newberry 2 drill hole, Oregon. The ??15O values for these minerals decrease with increasing temperatures. The values indicate that these hydrothermal minerals precipitated in isotopic equilibrium with water currently present in the reservoirs. The ??18O values of quartz and calcite from the andesite and basalt flows (700-932 m) have isotopic values which require that the equilibrated water ??18O values increase slightly (- 11.3 to -9.2???) with increasing measured temperatures (150-265??C). The lithic tuffs and brecciated lava flows (300-700 m) contain widespread siderite. Calculated oxygen isotopic compositions of waters in equilibrium with siderite generally increase with increasing temperatures (76-100??C). The ??18O values of siderite probably result from precipitation in water produced by mixing various amounts of the deep hydrothermal water (- 10.5 ???) with meteoric water (- 15.5 ???) recharged within the caldera. The ??13C values of calcite and siderite decrease with increasing temperatures and show that these minerals precipitated in isotopic equilibrium with CO2 of about -8 ???. The ??18O values of weakly altered (<5% alteration of plagioclase) whole-rock samples decrease with increasing temperatures above 100??C, indicating that exchange between water and rock is kinetically controlled. The water/rock mass ratios decrease with decreasing temperatures. The ??18O values of rocks from the bottom of Newberry 2 show about 40% isotopic exchange with the reservoir water. The calculated ??18O and ??D values of bottom hole water determined from the fluid produced during the 20 hour flow test are -10.2 and -109???, respectively. The ??D value of the hydrothermal water indicates recharge from outside the caldera. ?? 1987.
Chemical and mineralogical studies of fresh and hydrothermally altered rhyolitic material in Upper and Lower Geyser Basins, Yellowstone National Park, show that all the altered rocks are enriched in Cs and that Cs is selectively concentrated in analcime. The Cs content of unaltered rhyolite lava flows, including those from which the altered sediments are derived, ranges from 2.5 to 7.6 ppm. The Cs content of analcime-bearing altered sedimentary rocks is as high as 3000 ppm, and in clinoptilolite-bearing altered sedimentary rocks Cs content is as high as 180 ppm. Altered rhyolite lava flows which were initially vitrophyres, now contain up to 250 ppm Cs, and those which were crystallized prior to hydrothermal alteration contain up to 14 ppm. Mineral concentrates of analcime contain as much as 4700 ppm Cs. The Cs must have been incorporated into the analcime structure during crystallization, rather than by later cation substitution, because analcime does not readily exchange Cs. The Cs Cl "> CsCl of the fluids circulating through the hydrothermal system varies, suggesting that Cs is not always a conservative ion and that Cs is lost from upflowing thermal waters due to water-rock interaction resulting in crystallization of Cs-bearing analcime. The source of Cs for Cs enrichment of the altered rocks is from leaching of rhyolitic rocks underlying the geyser basins, and from the top of the silicic magma chamber that underlies the area. Analcime is an important natural Cs sink, and the high Cs concentrations reported here may prove to be an important indicator of the environment of analcime crystallization.
A rhyolitic ash-flow tuff in a hydrothermally active area within the Yellowstone caldera was drilled in 1967, and cores were studied to determine the nature and distribution of primary and secondary mineral phases. The rocks have undergone a complex history of crystallization and hydrothermal alteration since their emplacement 600,000 years ago. During cooling from magmatic temperatures, the glassy groundmass underwent either devitrification to alkali feldspar + ??-cristobalite ?? tridymite or granophyric crystallization to alkali feldspar + quartz. Associated with the zones of granophyric crystallization are prismatic quartz crystals in cavities similar to those termed miarolitic in plutonic rocks. Vapor-phase alkali feldspar, tridymite, magnetite, and sporadic ??-cristobalite were deposited in cavities and in void spaces of pumice fragments. Subsequently, some of the vapor-phase alkali feldspar crystals were replaced by microcrystalline quartz, and the vapor-phase minerals were frosted by a coating of saccharoidal quartz. Hydrothermal minerals occur primarily as linings and fillings of cavities and fractures and as altered mafic phenocrysts. Chalcedony is the dominant mineral related to the present hydrothermal regime and occurs as microcrystalline material mixed with various amounts of hematite and goethite. The chalcedony displays intricate layering and was apparently deposited as opal from silica-rich water. Hematite and goethite also replace both mafic phenocrysts and vapor-phase magnetite. Other conspicuous hydrothermal minerals include montmorillonite, pyrite, mordenite, calcite, and fluorite. Clinoptilolite, erionite, illite, kaolinite, and manganese oxides are sporadic. The hydrothermal minerals show little correlation with temperature, but bladed calcite is restricted to a zone of boiling in the tuff and clearly was deposited when CO2 was lost during boiling. Fractures and breccias filled with chalcedony are common throughout Y-5 and may have been produced by rapid disruption of rock caused by sudden decrease of fluid pressure in fractures, most likely a result of fracturing during resurgent doming in this part of the Yellowstone caldera. The chalcedony probably was deposited as opal or ??-cristobalite from a pre-existing silica floc that moved rapidly into the fractures and breccias immediately after the sudden pressure drop. ?? 1978.
Explore the source record for details and available documents.