• Application Of Argon-40/Argon-39 Chronostratigraphy To Geologic Problems In Yellowstone Caldera And Cook Inlet Basin

      Dallegge, Todd A.; Layer, Paul (2002)
      The 40Ar/39Ar dating method is a useful tool in addressing problems concerning stratigraphic distribution of rock units. This tool is used where previously K-Ar-dated units are problematic, and where further chronostratigraphic control is needed for lavas of the Central Plateau Member (CPM) of the Plateau Rhyolite in Yellowstone National Park and volcanic ash dispersed in the sedimentary units of the Kenai Group in Cook Inlet Basin. Sanidine 40Ar/39Ar ages for CPM rhyolite lavas indicate that xenocrysts, excess argon, and incomplete degassing affected the K-Ar age interpretations. With the 40Ar/39Ar dating method these effects are removed, revealing four apparent eruptive episodes at approximately 135, 122, 107, and 90 ka. The 40Ar/ 39Ar eruption ages are 10's ky younger than U-Th ages from zircon indicating significant residence time of the CPM magma chamber prior to eruption. 40Ar/39Ar dating using multiple analytical runs and statistical assessment of the data produced satisfactory results from the potassium-poor, plagioclase-bearing, tephras of the Kenai Group. The results indicate that excess argon, argon loss, and xenocrystic contamination exist. The chronostratigraphic framework indicates that faulting offsets strata of widely different ages causing repetition of the stratigraphy. Age data indicate that the Sterling and Beluga Formations are time-equivalent strata representing lateral facies variations rather than distinct time-stratigraphic formations. Based on crosscutting relations and structural folding, the dated horizons suggest that structural deformation is no older than early Pliocene in age. Based on the structural complexities affecting the Kenai Group strata, the previously measured coal resource estimates for the Kenai Peninsula are overestimated by 23% in the Clam Gulch area and 50% in the Diamond Creek area. Desorbed gas contents, ranging between 66 and 123 scf/t (daf) from cuttings collected from the Beaver Creek and Kenai Gas Fields, are applied to coal volumes to estimate coalbed methane gas-in-place. CBM in-place estimates suggest values of 93 Tcf for the Kenai Peninsula and 272 Tcf for Cook Inlet Basin. Hypothetical recovery rates suggest 4.7 and 13.6 Tcf for the Kenai Peninsula and Cook Inlet Basin, respectively. The hypothetical CBM recovery rates could continue natural gas supplies to the Anchorage area for an additional 60 years beyond the estimated shortened supply date of 2011.* *This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirement: Adobe Acrobat.
    • Experimental And Petrologic Constraints On Magma Movement, Storage, And Interactions At Two Volcanoes In Katmai National Park, Alaska

      Coombs, Michelle Lynn; Eichelberger, John C. (2001)
      Between 1953 and 1974, ~0.5 km3 of lava and tephra erupted from a new vent on the southwest flank of Trident volcano in Katmai National Park, Alaska, forming an edifice now known as Southwest Trident. The eruption commenced soon after mixing of dacite and andesite magmas at shallow crustal levels. The dacite lava flows contain andesitic enclaves as well as compositional banding. Dacite phenocryst melt inclusions and phase equilibria experiments on the andesite imply that the two magmas last resided at a water pressure of 90 MPa, and contained ~3.5 wt % H2O, equivalent to 3 km depth. Diffusion profiles in phenocrysts suggest that mixing preceded eruption of the earliest lava by approximately one month. The enclaves in the dacite had experienced a complex history by the time they were erupted. Quantitative analysis of groundmass microphenocrysts in enclaves from the lava shows that the enclaves underwent a textural maturation. I have run experiments that replicate the path taken by andesite during magma mixing in which the andesite was annealed at 1000�C, cooled at various rates to 890�C, held for residence time t, and then quenched. The andesite experimentally cooled at the slower rates (2�C/h and 10�C/h) most resembles enclave groundmass. This is consistent with cooling of the andesite below an andesite-dacite interface, suggesting that pre-enclave formation crystallization caused vapor exsolution and enclave flotation. Decompression experiments on the dacite suggest an average ascent time for the eruption of 30 hours. The high silica rhyolite erupted during the June 1912 eruption of Katmai is notable both for its large volume and evolved composition. Hydrothermal, water-saturated experiments constrain the magma's pre-eruptive storage condition to a region in P-T space between 800�C and 100 MPa and 850�C and 40 MPa. Amphibole is only present in the rhyolite of Novarupta dome, the last product of the eruption. Novarupta dome rhyolite probably was stored under the same conditions but underwent magma mixing with andesite and dacite prior to effusion.
    • Geology and timing of zinc-lead-silver mineralization, northern Brooks Range, Alaska

      Werdon, Melanie Beth; Newberry, Rainer J. (1999)
      The north-central and northwestern Brooks Range of Alaska hosts widespread Carboniferous Zn-Pb-Ag +/- Ba shale-hosted massive sulfide (Sedex) deposits, and Zn-Pb-Ag +/- Cu vein-breccia and disseminated sulfide occurrences. The Sedex deposits are hosted by black carbonaceous shale and siliceous mudstone of the Mississippian to Pennsylvanian Kuna Formation and are spatially associated with minor (e.g. Red Dog) to locally abundant (e.g. Drenchwater) volcanic and hypabyssal intrusive rocks. The vein-breccia and disseminated sulfide occurrences show no obvious igneous association and are hosted by a deformed but only weakly metamorphosed package of Upper Devonian to Lower Mississippian mixed continental and marine elastic rocks (the Endicott Group). Textural, mineralogical, isotopic, chemical, and fluid inclusion data indicate that sulfides, quartz, and lesser carbonates in the Kady vein-breccia and disseminated sulfide prospect were deposited from slightly acidic, low salinity, carbon-destructive, relatively oxidized, low temperature (<250�C) hydrothermal fluids, under evolving chemical conditions (i.e. decreasing temperature and pressure, and increasing pH, fo2, fs2). The lack of known Sedex mineralization in the north-central Brooks Range and the presence of sulfide mineralization within the Endicott Group suggests that Kady represents the hydrothermal fluid pathway below a failed or non-existent Sedex system. Trace element analyses of volcanic rocks and 40Ar/ 39Ar laser step-heating ages indicate the following geologic history for the north-central and northwestern Brooks Range: within-plate alkaline volcanic rocks at Red Dog and Drenchwater were emplaced from approximately 344 Ma to 336 Ma in a continental extensional environment. This presumably set up an elevated geothermal gradient, which heated basinal fluids. Sedex mineralization is estimated to have formed between 337 and ~314 Ma by basinal dewatering. 40Ar/39Ar ages of recrystallized white mica in Upper Devonian sandstone adjacent to large sulfide-bearing vein-breccia zones fall within the independently estimated time frame for Sedex mineralization. Tholeiitic gabbro magmatic activity occurred around 276 +/- 15 Ma. The transition with time from within plate alkaline to tholeiitic magmatism suggests progressive episodic extension in a continental basin.
    • Hydrothermal history of the Long Valley Caldera, California: Life after collapse

      Mcconnell, Vicki Sue (1995)
      Drilling of the Long Valley Exploratory Well (LVEW) on the resurgent dome in the 760 ka Long Valley Caldera opened a window to view the geologic history of the central caldera. Stratigraphic relationships indicate piston/cylinder (Valles-type) collapse for this caldera, and a resurgent structure intimately linked to post-caldera-collapse rhyolitic intrusions. Samples recovered from this and other wells proximal to the resurgent dome were characterized through isotope microanalytical techniques, petrographic and microprobe study, and analysis of fluid inclusions within alteration minerals. This work revealed the complexity of primary magmatic and secondary hydrothermal activity involved in the formation of a resurgent dome. Measurements of the $\rm\delta\sp{18}O$ composition of silicate components forming the intracaldera lithologies display disequilibrium within samples as a result of variable exchange with hydrothermal fluids. A maximum calculated temperature of $350\sp\circ\rm C$ at 1800 m depth in LVEW indicates paleohydrothermal temperatures exceeded the known present-day hydrothermal conditions by more than $100\sp\circ\rm C.$ Contouring of $\rm\delta\sp{18}O$ values from wells on a line crossing the caldera define a pattern of convective flow with upwelling beneath the resurgent dome. Although surface volcanism at the LVEW site ended about 650 ka, laser probe $\rm\sp{40}Ar/\sp{39}Ar$ microanalysis of samples from sill-like intrusions into the intracaldera ignimbrite reveals intrusive events at ${\sim}650$ ka, ${\sim}450$ ka, and ${\sim}350$ ka. Sanidine phenocrysts from the Bishop Tuff at 1772 and 1792 m depths and whole rock samples of the Mesozoic metavolcanic basement rocks at 1957 m depth record times of disturbance by hydrothermal pulses at ${\sim}530$ ka and ${\sim}350$ ka. Repeated emplacement of intrusions into the centrally located caldera ignimbrite was a primary process of resurgence. In turn, the feeders for the intrusions and the intrusions themselves supplied heat for resurgent-dome-centered hydrothermal flow. After approximately 300 ka, all activity shut off in the central caldera only to resume at ${\sim}40$ ka in response to renewed Holocene volcanic activity in the West Moat. Geophysical evidence of recent intrusive activity beneath the resurgent dome indicates this shallow magma emplacement mechanism is not totally extinct in the central caldera. Most likely a new cycle of volcanism and hydrothermal circulation is underway as the caldera matures.
    • Mid -Cretaceous plutonic -related gold deposits of interior Alaska: Characteristics, metallogenesis, gold-associative mineralogy and geochronology

      Mccoy, Daniel Thomas; Newberry, Rainer J. (2000)
      Mid-Cretaccous gold deposits in interior Alaska are hosted in or near apices of low magnetite plutons that formed in a broad continental arc. Ore is hosted in (1) anastomosing quartz veins with potassic or albitic envelopes, (2) planar veins and shear zones with sericitic alteration, and (3) pyroxene-rich skarn deposits. This study was undertaken to constrain the fluid and metal source and composition, formation conditions, gold associative mineralogy, age relationships, and areal extent of this mineralizing event Techniques included reflected light petrographic, 40Ar/39Ar step-heating, stable isotope, fire assay, Mossbauer spectroscopy, electron microprobe, and scanning ion mass spectroscopy analysis. Results suggest ages between 85 Ma and 107 Ma with a 0 to 2 million-year differential between magmatic biotite and hydrothermal veins in the same deposits. Deposits are 10 to 20 million years younger than local metamorphism. Fluid calculated stable isotopic ratios (delta13C = -9 to -10 per mil; delta18O = 5--10 per mil; deltaD = -47 to -100; delta34S = -5 to +5 per mil) suggest gold precipitated from magmatic fluids. Fluid inclusions in ore-bearing quartz contain high CO2 with trapping temperatures and pressures of 270� to 570�C and 0.5 to 2 kb respectively. The Fort Knox and Pogo deposits have a strong Au-Bi association and high relative amounts of potassic; and albitic alteration with mineralogical evidence for the original existence of maldonite (Au2Bi) or Au-Bi melt subsequently overprinted by native gold + bismuthinite. The True North deposit has a strong Au-As association and no Au-Bi association. It lacks potassic or albitic alteration and contains only sub-micron gold, approximately half chemically bound to arsenopyrite or arsenian pyrite. The Dolphin and Ryan Lode deposits are intermediate in Au-Bi association, gold-associative mineralogy and alteration features. Arsenopyrite geothermometry yield temperatures between 300� and 630�C for albitic and potassic alteration and between 250� and 420�C for sericitic alteration. 40Ar/39Ar dating and metal ratios suggest that gold mineralization is (1) solely mid-Cretaceous in the Fairbanks mining district, (2) mid-Cretaceous and late Cretaceous in the Kantishna mining district, and (3) mid-Cretaceous and early Tertiary in the Livengood, district.
    • Origin, character, application and correlation of tephra partings in tertiary coal beds of the Kenai Peninsula, Alaska

      Reinink-Smith, Linda Margareta; Hopkins, David M. (1989)
      Volcanic and non-volcanic partings occur in coal beds of the Neogene Beluga and Sterling Formations along the shores of the Kenai lowland, Alaska. The partings were systematically characterized to determine their potential geological applications: Two-thirds of the partings originated as air-fall tephra. Of these, partly altered, Pliocene tephra typically contain volcanic glass + feldspar $\pm$ montmorillonite $\pm$ quartz $\pm$ kaolinite $\pm$ opal-CT. Highly altered Miocene partings are characterized by feldspar $\pm$ kaolinite $\pm$ montmorillonite $\pm$ quartz $\pm$ crandallite $\pm$ altered volcanic glass, where crandallite appears to have formed by replacement of volcanic glass prior to clay formation. About one-third of the partings are of detrital origin and contain detrital chlorite + illite + smectite + quartz $\pm$ feldspar $\pm$ siderite $\pm$ kaolinite. A Pliocene pumice parting near the top of the Sterling Formation was correlated from the northwestern to the southeastern Kenai lowland on the basis of similar glass morphologies, an absence of opaque minerals, and geochemical similarities. A crystal-tuff near the middle of the section could be traced across the Kenai lowland as one or two ash-falls, based on inertinite contents of adjacent coal, mineralogy, and geochemistry. Some other prominent tephras could not be correlated. The tephra partings are time-equivalent to DSDP cores from the Gulf of Alaska and along the Aleutian Island chain. Tephras occur every 125-500 yr in the lower part of the Beluga Formation, and their deposition probably coincides with a volcanic pulse 10.5 m.y. ago. This pulse is not well recorded in nearby DSDP cores. In the upper part of the Beluga Formation, during volcanic quiescence, tephras are recorded at an average rate of one every 9,000 yr. Time equivalent DSDP cores show a near absence of tephras. A volcanic pulse occurred during the deposition of the lower Sterling Formation, about 7.5 m.y. ago, with intervals between volcanism which averages 11,000 yr or longer. Volcanic sources appear to have been distant, which is consistent with an absence of tephra layers in a Gulf of Alaska core. About 5 m.y. ago, concurrent with the deposition of the upper Sterling Formation, the thicknesses of the tephra layers dramatically increase and the frequency increases to an average of one tephra every 2,000 years. This increase is recorded in DSDP cores as well.
    • The 1996 Eruption Of Karymsky Volcano, Kamchatka: Detailed Petrological Study Of A Single Basalt -Triggered Eruption Cycle

      Izbekov, Pavel Edgarovich (2002)
      The current activity at Karymsky Volcano, Kamchatka, began on January 2, 1996, with simultaneous eruptions from two vents located 6 km apart: Karymsky summit vent, which erupted andesite, and a newly formed vent within Academy Nauk caldera, which erupted basalt. Detailed petrologic study of volcanic ash, bombs, and lavas of Karymsky erupted during 1996--1999 provides evidence for basaltic replenishment at the beginning of the eruptive cycle, as well as a record of compositional variations within the Karymsky magma reservoir induced by basaltic recharge. Shortly after the beginning of eruption the composition of matrix glasses of Karymsky tephra became more mafic, and then, within two months, gradually returned to its original state and remained almost constant the following three years. Further evidence for basaltic replenishment includes the presence of xenocrysts of basaltic origin in andesites erupted from Karymsky. A conspicuous portion of plagioclase phenocrysts in Karymsky andesites contain calcic cores, the composition and texture of which mimic those in Academy Nauk basalt. The earlier portions of andesite also contain rare xenocrysts of olivine, which occur as relicts in plagioclase-pyroxene aggregates. The compositions of olivine xenocrysts match those of olivines in Academy Nauk basalt. Compositional variations of glass and the presence of xenocrysts indicate that Karymsky magma reservoir was recharged by basalt at the onset of the 1996 eruptive cycle. The mixing of basalt with host andesite was both thorough and rapid, perhaps due to a modest contrast in temperature, viscosity, and density between the magmas. Academy Nauk basalt contains granophyre xenoliths, the whole-rock compositions of which are identical to that of dacites erupted twice at 40,000 yr. BP and 7,900 yr. BP, and formed the neighboring Academy Nauk and Karymsky calderas. According to hydrothermal experiments and petrologic observations both dacites last equilibrated at 3--4 km depth. At the same depth granophyre phase assemblage is reproduced by isobaric crystallization of dacites, thus implying that the granophyres represent a crystallized silicic reservoir, which produced dacites 40,000 yr. BP and formed Academy Nauk caldera. In 1996 this crystallized body was sampled by ascending basalt, which erupted in the northern part of the caldera.
    • The Physical Volcanology And Petrology Of The 3400 Ybp Caldera -Forming Eruption Of Aniakchak Volcano, Alaska

      Dreher, Scott Travis; Eichelberger, John C. (2002)
      Zoned ignimbrites are often interpreted as overturned "snapshots" of a compositionally zoned magma chamber. While ignimbrite zonation requires the spatial and temporal coexistence of diverse magmas, the origin of both the zonation and the diversity are poorly understood. While many studies have developed in situ differentiation models, these models do not explain composition gaps, commonly observed in the ignimbrites. Aniakchak Caldera on the Alaska Peninsula was formed approximately 3,400 years ago. The eruption began with a plinian phase that deposited a thin rhyodacitic pumice fall deposit, and valley-filling rhyodacitic ignimbrites. A second phase is represented by a massive ignimbrite sheet (probably resulting from column collapse) comprising both rhyodacite and andesite. The eruption culminated in the collapse of the volcanic edifice and the deposition of a lithic-rich andesitic ignimbrite. The andesite (~60 wt. % SiO2) emplaced in the caldera-forming eruption, appears to have been produced by magma mixing. Although crystal-poor (~8 wt. % crystals), the andesite contains a bimodal plagioclase population and rare grains of both quartz and olivine. Self-contamination, wherein residual liquids from a crystallizing marginal boundary layer are reincorporated into the hybrid magma during successive basalt or rhyodacite injections, probably led to deviations from a binary mixing trend. The rhyodacite (~70 wt. % SiO2), the most silicic magma erupted at Aniakchak is also crystal-poor (~2 wt. % crystals), but contains a single plagioclase population. The crystal poor nature, simple mineralogy, and silicic composition of the rhyodacite imply an uncomplicated history. Geochemical models indicate partial melting of mafic plutons and fractional crystallization of a basaltic magma contributed to the rhyodacite erupted in caldera-forming eruption. It appears that the andesitic composition was maintained by repeated injections of small volumes of both basaltic and rhyodacitic magmas. Each successive mixing event induced an increment of self-contamination in the andesite chamber. Increased pressure associated with an unusually large influx of rhyodacite, however, immediately induced an eruption, before significant mixing occurred. After the rhyodacitic magma was evacuated, and now destabilized the roof collapsed, leading to the eruption of the residence andesite.
    • The Weathering Of Placer Gold And The Quaternary Geology Of Valdez Creek, Clearwater Mountains, Alaska

      Teller, Steven D.; Hopkins, David M. (1995)
      Placer gold grains collected from six paleochannels in the Valdez Creek drainage, south-central Alaska, were deposited during successive interglacial/interstadial intervals since the mid-Early Pleistocene. Statistical analysis of grain size, shape, grain surface characteristics, and the gold content of the interior and exterior of the gold grains determined by electron microprobe analysis demonstrates that the grains were affected by both mechanical and chemical weathering, and that the weathering increased with time. Etch pits, observed under a scanning electron microscope, are a ubiquitous feature of the grain surfaces. Grain surfaces average 26.7% richer in gold than the interior of the grains. The gold content of the surface of the grains increases with age. No high gold fineness rims were observed in cross section on the grains. This evidence indicates that the gold grains experienced corrosion. <p>