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We present data that distinguishes the long-known Littlefield Rhyolite of eastern Oregon (northwestern United States) into two distinct, voluminous, Snake River-type, high-temperature rhyolite lava packages that erupted in short sequence over <100 k.y., with minimum volumes of 100 and 150 km3 respectively, contemporaneous with flood basalt volcanism of the Grande Ronde Basalt phase of the Columbia River Basalt Group. Contemporaneity of rhyolites with flood basalts is exceptionally demonstrated within the Malheur Gorge by intercalated mafic units belonging to the Grande Ronde Basalt that are stratigraphically constrained by underlying and overlying Littlefield Rhyolite flows, and the underlying Dinner Creek Tuff (unit 1). Our new ages of 16.11 Ma and 16.02 Ma for the lower and upper Littlefield Rhyolite, respectively, provide a narrow age constraint on the controversial lower age of Grande Ronde Basalt volcanism. Petrological data on local, intercalated Fe-rich andesitic (icelanditic) lavas provide further evidence for coeval existence of rhyolitic and mafic magmas, and additionally provide location evidence for storage sites of Grande Ronde Basalt magmas. Based on these data in addition to similar data on the nearby Dinner Creek Tuff rhyolite center, as well as the locations of other rhyolite centers that fall within the same period of intense rhyolite volcanism of ca. 16.1 Ma, we infer that Grande Ronde Basalt crustal magma reservoirs were widespread in this area of eastern Oregon. We further infer that the main eruptions of stored flood basalt magmas followed the magmas' lateral transport from these reservoirs to the well-known dike swarms located at the periphery of the rhyolite distribution area where local eruptions of rhyolites are notably absent. Our study highlights the interplay of mafic and crustally derived rhyolite magmas, with implications for other continental flood basalt provinces that are less well preserved than the Columbia River Basalt province. [ABSTRACT FROM AUTHOR]

Miocene rhyolitic volcanism of eastern Oregon, USA, can be divided into two main episodes. Mantle plume upwelling is thought to have generated Columbia River Basalt Group (CRBG) lavas and coeval >16.5-15 Ma silicic volcanism trending north--south from northeast Oregon to northern Nevada. Rhyolite volcanism of the 12-0 Ma High Lava Plains province has been ascribed to either buoyancy-driven westward plume spreading or to slab rollback and mantle convection spanning from southeast Oregon to Newberry Volcano to the west. The apparent ca. 15-12 Ma eruptive hiatus suggests that rhyolites of these provinces were a product of separate processes, yet this gap was based on incomplete data. The lack of data on ~33 of the total ~50 rhyolitic eruptive centers in the area where the two provinces overlap (117-119°W, 43-44°N) yields only tenuous relationships between these two provinces. We acquired 40Ar/39Ar ages for 29 previously unanalyzed rhyolite centers that confirm the existence of a rhyolitic eruptive episode concurrent with CRBG flood basalt volcanism. Rhyolite eruptions gradually initiated at ca. 17.5 Ma, and our new ages indicate that peak intensity of the first eruptive episode occurred between 16.3 Ma and 14.4 Ma. We refine the ca. 15-12 Ma rhyolitic eruptive hiatus to 14.4-12.1 Ma, where strong recommencement of rhyolite eruptions began with Beatys Butte at 12.05 Ma. We find two prominent fluxes in rhyolitic eruptive activity after 12.1 Ma as opposed to one continuous, age-progressive trend, at 12.1-9.6 Ma and 7.7-5.1 Ma, which are separated by an ~2 m.y. period of decreased rhyolite volcanism. Rhyolite eruptions were scarce after 5.1 Ma, at which point most eruptions were associated with Newberry Volcano. Periodicity of rhyolite volcanism along the High Lava Plains demands more punctuated basalt inputs than what continuous partial melting from west-spreading plume material should generate. Our new data suggest that regional rhyolite eruptions are a series of episodic events related to the arrival and storage of mafic mantle magmas. Paucity in rhyolite eruptions from 14.4 Ma to 12.1 Ma is related to decreased flux of CRBG flood basalt magmas at ca. 15 Ma. Strong recommencement of rhyolite volcanism at 12.1 Ma is related to continued Northwest Basin and Range extension and a peak rotation rate of Siletzia affecting regional lithosphere weakened by CRBG volcanism. Waning rhyolitic eruptive activity from ca. 9.6 Ma to 7.7 Ma reflects a regional transition in the primary mode of accommodation of extension from Northwest Basin and Range normal faulting to extension and shearing of the Brothers Fault Zone. Rhyolite volcanism between 7.7 Ma and 5.1 Ma was driven by continued regional extension in an area less affected by CRBG magmatism. Post-5.1 Ma rhyolite eruptions occurred within crust not influenced by CRBG magmatism but impacted by both regional extension and the Cascadia subduction zone. [ABSTRACT FROM AUTHOR]

Focuses on the geochemical composition and stratigraphy of tephra layers in antarctic blue ice from Transantarctic Mountains, Antarctica. Stratigraphic sequence of tephra based on the global positioning system mapping; Findings of the petrographic observations conducted at the site; Instruments used in the determination of trace element composition of tephra shards.

Abstract: Supergene and hypogene jarosite have been dated successfully, but accurate dating of weathering-derived jarosite and its application to landscape-evolution studies has been limited because of difficulties in obtaining high-purity mineral separates. Hydrofluoric acid (HF) can remove potassium-bearing silicates from supergene samples, but its effects on jarosite crystal chemistry and age dating are unquantified. Three experiments were conducted to determine whether HF treatment removes silicates without altering the potassium or argon composition of the jarosite prior to dating using the 40Ar/39Ar method. In the first two experiments, pure hypogene jarosite from Peña Blanca Mexico (PB; 9.42±0.22Ma) and a mixture of 85% PB:15% Fish Canyon sanidine (FC-2; 28.02Ma) were crushed and treated with 25% HF for 0, 30, 240, and 480min. Jarosite partially dissolves during HF treatment with grains becoming increasingly pitted and rounded with time in acid, but PB''s potassium concentration and apparent age remained constant regardless of treatment time. FC-2 was absent from all treated samples, suggesting that 30min in HF is sufficient to remove mechanically mixed sanidine crystals from jarosite. Methods developed during experiments conducted on PB jarosite were applied to fine-grained supergene jarosite from the Red River Valley (RRV) in Northern New Mexico. RRV jarosite is mixed with Oligocene (~24.86Ma) potassium-bearing silicates that make up 50% or more of each sample. Four RRV jarosite samples that yielded age spectra with clear signs of contamination by older phases were treated with HF for 30min and re-dated. Despite treatment, back-scattered electron images show that silicates, including sanidine and illite, continued to comprise approximately 30% of HF-treated RRV samples, and the age spectra produced during laser step heating continued to show signs of contamination in higher-wattage steps. However, the integrated ages of these samples were consistently less than 1Ma, at least 7Ma younger than the expected integrated age if the 30:70 mixture of RRV silicates and jarosite degassed completely. The young ages suggest that young jarosite controls the apparent age of these samples and that these ages can be interpreted as the maximum timing of supergene jarosite formation. [Copyright &y& Elsevier]

Abstract: Six new 40Ar/39Ar and three cosmogenic 36Cl age determinations provide new insight into the late Quaternary eruptive history of Erebus volcano. Anorthoclase from 3 lava flows on the caldera rim have 40Ar/39Ar ages of 23±12, 81±3 and 172±10 ka (all uncertainties 2σ). The ages confirm the presence of a second, younger, superimposed caldera near the southwestern margin of the summit plateau and show that eruptive activity has occurred in the summit region for 77±13 ka longer than previously thought. Trachyte from “Ice Station” on the eastern flank is 159±2 ka, similar in age to those at Bomb Peak and Aurora Cliffs. The widespread occurrences of trachyte on the eastern flank of Erebus suggest a major previously unrecognized episode of trachytic volcanism. The trachyte lavas are chemically and isotopically distinct from alkaline lavas erupted contemporaneously in the summit region <5 km away. The three 36Cl ages are the first exposure ages reported for rocks from Erebus volcano. At an assumed erosion rate of 1.5 mm/ka 36Cl cosmogenic ages are 6.0±0.5 ka for the Ice Tower Ridge flow, 4.9±0.4 ka for Lower Hut flow, and 6.5±0.6 ka for the Northeast flow. The 36Cl ages overlap within analytical uncertainty or are slightly younger than 40Ar/39Ar ages for these lava flows, and confirm the overall young age (<25 ka) of the summit lava flows. One interpretation of these uniformly young ages is that older eruptions infilled the topographic depression caused by caldera formation and subsequently was covered by younger lavas. [Copyright &y& Elsevier]

We have determined precise eruption ages for andesites from Ruapehu volcano in the Tongariro Volcanic Centre of the Taupo Volcanic Zone (TVZ) using 40Ar/39Ar furnace step-heating of separated groundmass concentrates. The plateau ages indicate several eruptive pulses near 200, 134, 45, 22 and <15 ka and, based on our and previous field mapping confirm the lavas of the Te Herenga Formation as the oldest exposed part of the volcanic edifice. The pulse at ∼134 ka includes an entire >300-m section of lavas in Whangaehu gorge as well as some lavas in Ohinepango and Waihianoa catchments on eastern Ruapehu, and this suite of lavas belongs to the Waihianoa Formation. This pulse of activity is not represented on nearby Tongariro volcano, indicating that the two volcanoes have independent magmatic systems. A younger group of lavas yields dates between 50 and 20 ka and includes lava flows from the Turoa skifield and in the Ohinepango and Mangatoetoenui catchments and is consistent with two pulses of magmatism around the time of the last glacial maximum, relating it broadly to the Mangawhero Formation. Syn- and post-last glacial activity lavas, with ages <15 ka are assigned to the Whakapapa Formation, and include the voluminous flows of the Rangataua Member on southern Ruapehu. Magma flux, integrated over 1000-yr periods, averages 0.6 km3 ka−1 assuming a volcano lifespan of 250 ka. Fluxes for the Te Herenga, Waihianoa and Mangawhero Formations are consistent at 0.93, 0.9 and 0.88 km3 ka−1, respectively. These fluxes are broadly comparable with those measured at other modern andesite arc volcanoes (e.g. Ngauruhoe, 0.88; Merapi, 1.2 and Karymsky 1.2 km3 ka−1). The relatively low flux (0.17 km3 ka−1) calculated for the Whakapapa Formation may derive from underestimates of erupted volume arising from an increase in phreatomagmatic explosive eruptions in postglacial times. However, using volume estimates for the 1995–1996 eruptions and a recurrence interval of 25 yr has yielded an integrated 1000-yr flux of 0.8 km3 ka−1 in remarkable agreement to estimates for the prehistoric eruptions. Overall, Ruapehu shows consistency in magma flux, but at time scales of the order of one hundred to some thousands of years, field evidence suggests that short bursts of activity may produce fluxes up to twenty times greater. This is significant from the perspective of future activity and hazard prediction. [Copyright &y& Elsevier]

Determines the paleomagnetic and geochronologic data on the structural evolution of Silver Peak extensional complex in Nevada. Horizontal tilting of mafic dikes; Association of the vertical rotation of crustal-scale blocks with displacement transfer in the Central Walker Lane; Presence of Paleozoic sedimentary rocks in late Neogene.