Search

Explosive eruptions from subaerial arc volcanoes can have significant environmental impact because of the discharge of ash and volatiles directly into the atmosphere and oceans. The link between climate cooling and voluminous volcanic eruptions has remained speculative due to a lack of supporting evidence. A compilation of 2814 K-Ar and [sup.40]Ar/[sup.39]Ar age determinations from four circum-Pacific arcs indicates that periods of high volcanic output (i.e., flare-ups) have been episodic and, in some cases, synchronous. Peak periods and subsequent lulls in arc magmatism over the past 50 m.y. have occurred coeval with major fluctuations in global climate, including the Eocene-Oligocene transition, one of the most prominent global climate reorganizations in the Cenozoic. Hundreds of intermediate to silicic eruptions occurred during an extremely vigorous period of circum-Pacific volcanism beginning in the late Eocene, which likely led to the production of sulfur aerosols in the stratosphere and fertilization of surface waters of the Pacific Ocean. We provide a mechanism that may have been partly responsible for the climatic preconditioning that must have preceded and ultimately promoted Antarctic ice sheet growth ~34 m.y. ago.

Forty-three [sup.40] Ar/[sup.39] Ar age determinations of lava flows, domes, ignimbrites, and dikes, plus [sup.14]C dates from seven distal tephra layers, combined with stratigraphy, geochemistry, and Sr and Th isotope data, establish an eruptive chronology for the PuyehueCordon Caulle volcanic complex at 40.5[degrees] S in the Andean southern volcanic zone (SVZ). The complex preserves ~131 [km.sup.3] of lava and tephra that erupted from numerous vents widely separated in time and space. Approximately 80% of the total volume consists of basaltic to andesitic lava that formed two broad shield volcanoes between 314 and 70 ka. The modern Puyehue stratovolcano was built on the southerly shield during the past 69 k.y. following a hiatus of 25 k.y. Puyehue has erupted ~15 [km.sup.3] of basaltic to rhyolitic magma that spans the entire compositional range found in the southern SVZ and evolved via at least six phases including: (1) basaltic andesitic to dacitic lavas between 69 and 32 ka, (2) a shift to bimodal magma compositions that is first expressed by a rhyodacite mingled with inclusions of MgO-rich basaltic andesite at 34 ka, (3) dacitic to rhyolitic flows and domes from 19 to 12 ka, (4) basaltic to basaltic andesitic flows between 15 and 12 ka, (5) subsequent rhyolitic dome growth in several effusive and explosive stages between 7 and 5 ka, followed by (6) a powerful series of phreatomagmatic and sub-Plinian eruptions at ca. 1.1 ka that obliterated the preceding rhyolite domes and formed the present 2.5km-diameter, 280-m-deep summit crater. Along the Cordon Caulle fissure zone, ~5 [km.sup.3] of rhyodacitic to rhyolitic lavas, domes, and cones have formed during the past ~16.5 k.y., including explosive and effusive eruptions in 1921-1922 and 1960. Eruptive rates were nonuniform over time, with background growth at 0.04 [km.sup.3]/k.y, or less, punctuated by spurts at up to 0.90 [km.sup.3]/ k.y. The time-averaged rate, 0.42 [km.sup.3]/k.y., is nearly double that at the Tatara-San Pedro complex 500 km to the north during the past 300 k.y. These findings indicate that within a single arc the magmatic and eruptive fluxes at individual frontal volcanoes can be highly variable. The last three stratoeone-building events on Puyehue began during periods of deglaciation, suggesting a relationship between unloading of ice and ease of magma ascent. Puyehue basalt exhibits subtle changes in [sup.238]U-[sup.230]Th, [sup.87]Sr/[sup.86] Sr, and trace element composition over time that signal shifts in the composition and degree of melting of the mantle wedge, or the extent to which basalt was modified by assimilation of heterogeneous crustal melts. The complex has become exceptionally bimodal and more explosive over time with recent rhyolites evolving by extreme crystal fractionation of mafic magma and lesser volumes of andesite and dacite created via mixing of rhyolite and basalt. Despite the high flux of basalt during the past 300 k.y., no large silicic magma reservoir formed in the upper crust. Instead, [sup.238]U-[sup.230] Th data favor rapid ascent of several small bodies of basaltic and silicic magma from the lower crust, promoted perhaps by conduits that reflect strike-slip faulting beneath the complex. Keywords: Andes, volcanism, Puyehue, geochronology, [sup.40] Ar/ [sup.39] Ar dating.

Radioisotopic dating of subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provides insight into the timing of arc formation in the middle Eocene. Twenty-eight [sup.40]Ar/[sup.39Ar] ages constrain the duration of arc magmatism to the last 46 m.y. Basaltic lavas from the Finger Bay volcanics, the oldest exposed rocks in the arc, gave an isochron age of 37.4 [+ or -] 0.6 Ma, which is 12-17 m.y. younger than a widely cited age of 55-50 Ma. Three main pulses of arc-wide magmatism occurred at 38-29, 16-11, and 6-0 Ma, which coincide with periods of intense magmatism in other western Pacific island arcs. Using the geochronology and volumetric estimates of crust generated and eroded over the last 46 m.y., we calculate a time-averaged magma production rate for the entire arc that exceeds previous estimates by almost an order of magnitude. Keywords: Aleutian Arc, magma production, [sup.40]Ar/[sup.39Ar dating, arc growth.

New [sup.40]Ar/[sup.39]Ar dating coupled with detailed field mapping, stratigraphy, and chemical analyses have established an eruptive chronology that reveals and constrains the compositional and volumetric evolution of Seguam Island in the Aleutian Island arc, Alaska. Sixty new [sup.40]Ar/[sup.39]Ar ages from lavas, domes, and pyroclastic deposits were obtained using furnace incremental-heating techniques on replicate samples of whole-rock and ground-mass separates, and they constrain the duration of Pleistocene to Holocene subaerial volcanism to 318 k.y. The [sup.40]Ar/[sup.39]Ar plateau ages indicate that over 85% of the complex, ~68 [km.sup.3] of material, was erupted almost continuously between 318 ka and 9 ka. At ca. 9 ka, a stratocone on the eastern half of the island partially collapsed producing a 4-km-wide crater. Rhyolitic dome-forming eruptions followed from vents in the newly created crater, and were likely contemporaneous with 8.0 [km.sup.3] of basaltic and basaltic andesitic effusions from Pyre Peak, and a 1.4 [km.sup.3] basaltic eruption from a monogenetic cone on the far eastern end of the island. Geochemical changes over the last 318 k.y. are subtle. Most notably, the earliest eruptions from 318 to 142 ka produced no andesite, and basalt from this period has larger ranges in Zr/Rb and La/Yb than younger basalts. Small volumes of dacitic to rhyolitic magma were produced from basalt by a monotonic crystal-liquid fractionation process that varied only slightly in successive eruptive phases over 318 k.y. We identified minor geochemical changes in magma composition during each of the three main stages of volcanism, but overall the monotonic variations in major- and trace-element compositions of basaltic andesitic to rhyolitic lavas are consistent with an origin via closed-system fractional crystallization of basalt. Using the [sup.40]Ar/[sup.39]Ar geochronology and estimates of individual flow volumes, we calculated a time-averaged eruptive rate at Seguam that is similar to growth rates of other well-dated arc volcanoes in the Cascades and Chilean Southern volcanic zone but less than that of Mount Katmai and Mount Mageik, which are located on the Alaska Peninsula. The eruptive flux at Seguam has been highly variable, fluctuating more than an order of magnitude, from 0.07 [km.sup.3]/k.y. during the early history of bimodal volcanism to 1.18 [km.sup.3]/k.y. over the past 9 k.y. Keywords: Aleutian island arc, [sup.40]Ar/[sup.39]Ar geochronology, magmatic evolution, Seguam Island, eruptive rates.

Author(s): Brad S. Singer (corresponding author) [1]; Kenneth A. Hoffman [2]; Robert S. Coe [3]; Laurie L. Brown [4]; Brian R. Jicha [1]; Malcolm S. Pringle [5]; Annick Chauvin [6] [...]

Since the publication of 40Ar/39Ar dates from Cretaceous bentonites in the Western Interior Basin by J.D. Obradovich in 1993 and in Japan by J.D. Obradovich and colleagues in 2002, improvements in the 40Ar/39Ar method have included a shift to astronomically calibrated ages for standard minerals and development of a new generation of multi-collector mass spectrometers. Thus, the 40Ar/39Ar chronometer can yield results that are synchronous with U-Pb zircon dates and astrochronologic age models for Cretaceous strata. Ages determined by Obradovich have ± 2σ analytical uncertainties of ± 400 ka (excluding J value or systematic contributions) that have been used to discriminate stratigraphic events at ca. 1 Ma resolution. From among several dozen sanidine samples, 32 of which were dated by Obradovich in 1993, we present new multicollector 40Ar/39Ar ages that reduce the average analytical uncertainties by nearly an order of magnitude. These new ages (where the uncertainties also include the contribution of the neutron fluence J value) include: • Topmost Bentonite, Mowry Shale, Kaycee, Wyoming, USA, 97.52 ± 0.09 Ma • Clay Spur Bentonite, Mowry Shale, Casper, Wyoming, 98.17 ± 0.11 Ma • Arrow Creek Bentonite, Colorado Shale, Montana, USA, 99.12 ± 0.14 Ma • Upper Newcastle Sandstone, Black Hills, Wyoming, 99.49 ± 0.07 Ma • Middle Newcastle Sandstone, Black Hills, Wyoming, 99.58 ± 0.12 Ma • Shell Creek Shale, Bighorn Basin, Crow Reservation, Wyoming, 99.62 ± 0.07 Ma • Shell Creek Shale, Bighorn Basin, Greybull, Wyoming, 99.67 ± 0.13 Ma • Shell Creek Shale, Bighorn Basin, Lander, Montana, 100.07 ± 0.07 Ma • Muddy Sandstone, Wind River Basin, Wyoming, 101.23 ± 0.09 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 101.36 ± 0.11 Ma • Vaughn Member, Blackleaf Formation, Sweetgrass Arch, Montana, 102.68 ± 0.07 Ma • Taft Hill Member, Blackleaf Formation, Sweetgrass Arch, Montana, 103.08 ± 0.11 Ma • Base of the Skull Creek Shale, Black Hills, Wyoming, 104.87 ± 0.10 Ma • Thermopolis Shale, Bighorn Basin, Wyoming, 106.37 ± 0.11 Ma A new U-Pb zircon age of 104.69 ± 0.07 Ma from the Skull Creek Shale at Dinosaur Ridge, Colorado, USA, is close to the new 40Ar/39Ar age of the Skull Creek Shale in the Black Hills, Wyoming, but 5 m.y. is missing in the unconformity between the Skull Creek Shale of the Black Hills and the overlying Newcastle Sandstone. Considering the average total uncertainties that include decay constant and standard age or tracer composition for the 40Ar/39Ar (± 0.19 Ma) and the U-Pb (± 0.13 Ma) ages does not alter this finding. Moreover, the lower Thermopolis Shale in the Bighorn Basin is 1.5 Ma older than the Skull Creek Shale in the Black Hills. The 100.07 ± 0.07 Ma Shell Creek Bentonite in Montana is close to the Albian--Cenomanian boundary age of 100.2 ± 0.2 Ma of Obradovich and colleagues from Hokkaido, Japan, and 100.5 ± 0.5 Ma adopted in the 2012 geological time scale of J.G. Ogg and L.A. Hinnov. Our findings indicate that correlations based on similarity of lithology, without independent radioisotopic ages or detailed biostratigraphic constraints, can be problematic or invalid. There is much more time missing in unconformities than has been previously recognized in these important, petroleumbearing reservoir strata. [ABSTRACT FROM AUTHOR]

We studied in detail the Gioia Sannitica active normal fault (GF) along the Southern Matese Fault system in the southern Apennines of Italy. The current activity of the fault system and its potential to produce strong earthquakes have been underestimated so far, and are now defined. Precise mapping of the GF fault trace on a 1:20,000 geological map and several point data on geometry, kinematics and throw rate are made available in electronic format. The GF, and in general the entire fault system along the southern Matese mountain front, is made of slowly-slipping faults, with a long active history revealed by the large geologic offsets, mature geomorphology, and complex fault pattern and kinematics. Present activity has resulted in Late Quaternary fault scarps resurrecting the foot of the mountain front, and Holocene surface faulting. The slip rate varies along-strike, with maximum Late Pleistocene – Holocene throw rate of ~0.5 mm/yr. Activation of the 11.5 km-long GF can produce up to M 6.1 earthquakes. If activated together with the 18 km-long Ailano-Piedimonte Matese fault (APMF), the seismogenic potential would be M 6.8. The slip history of the two faults is compatible with a contemporaneous rupture. The observed Holocene displacements on the GF and APMF are compatible with activations during some poorly known historical earthquakes, such as the 1293 (M 5.8), 1349 (M 6.8; southern prolongation of the rupture on the Aquae Iuliae fault?) and CE 346 earthquakes. A fault rupture during the 847 poorly-constrained historical earthquake is also compatible with the dated displacements. [ABSTRACT FROM AUTHOR]

New 40Ar/39Ar and whole-rock geochemical data are used to develop a detailed eruptive chronology for Akutan volcano, Akutan Island, Alaska, USA, in the eastern Aleutian island arc. Akutan Island (166°W, 54.1°N) is the site of long-lived volcanism and the entire island comprises volcanic rocks as old as 3.3 Ma. Our current study is on the 225 km² western half of the island, where our results show that the focus of volcanism has shifted over the last ~700 k.y., and that on occasion, multiple volcanic centers have been active over the same period, including within the Holocene. Incremental heating experiments resulted in 56 40Ar/39Ar plateau ages and span 2.3 Ma to 9.2 ka. Eruptive products of all units are primarily tholeiitic and medium-K, and range from basalt to dacite. Rare calc-alkaline lavas show evidence suggesting their formation via mixing of mafic and evolved magmas, not via crystallization-derived differentiation through the calc-alkaline trend. Earliest lavas are broadly dispersed and are almost exclusively mafic with high and variable La/Yb ratios that are likely the result of low degrees of partial mantle melting. Holocene lavas all fall along a single tholeiitic, basalt-to-dacite evolutionary trend and have among the lowest La/Yb ratios, which favors higher degrees of mantle melting and is consistent with the increased magma flux during this time. A suite of xenoliths, spanning a wide range of compositions, are found in the deposits of the 1.6 ka caldera-forming eruption. They are interpreted to represent completely crystallized liquids or the crystal residuum from tholeiitic fractional crystallization of the active Akutan magma system. The new geochronologic and geochemical data are used along with existing geodetic and seismic interpretations from the island to develop a conceptual model of the active Akutan magma system. Collectively, these data are consistent with hot, dry magmas that are likely stored at 5--10 km depth prior to eruption. The prolonged eruptive activity at Akutan has also allowed us to evaluate patterns in lava-ice interactions through time as our new data and observations suggest that the influence of glaciation on eruptive activity, and possible magma composition, is more pronounced at Akutan than has been observed for other well-studied Aleutian volcanoes to the west. [ABSTRACT FROM AUTHOR]

As increasing atmospheric carbon dioxide (CO2) and temperatures accompany modern climate change, ancient hothouse periods become a focal point for understanding ecosystem function under similar conditions. The early Eocene exhibited high temperatures, high CO2 levels, and similar tectonic plate configuration to today, so it has been invoked as an analog to modern climate change. During the early Eocene, the greater Green River Basin (GGRB) of southwest Wyoming was covered by an ancient hypersaline lake (Lake Gosiute; Green River Formation) and associated fluvial and floodplain systems (Wasatch and Bridger Formations). The volcaniclastic Bridger Formation was deposited by an inland delta that drained from the northwest into freshwater Lake Gosiute and is known for its vast paleontological assemblages. The Blue Rim escarpment exposes approximately 100 meters of the lower Bridger Formation, which includes plant and mammal fossils, paleosols and organic remains suitable for geochemical analyses, as well as ash beds and volcaniclastic sandstone beds suitable for radioisotopic dating. New 40Ar/39Ar ages from the middle and top of the Blue Rim escarpment constrain age of its strata to ~49.5-48.5 Ma ago, during the “falling limb” of the early Eocene climatic optimum. Using several geochemical tools, we reconstructed provenance and parent material in both the paleosols and the associated sediments and found no change in sediment input source despite significant variation in sedimentary facies and organic carbon burial. We also reconstructed environmental conditions at the time, including temperature and precipitation (from paleosols) and the isotopic composition of CO2 from plants found in the floral assemblages, before comparing them to reconstructions for the same time made using leaf physiognomic techniques and marine proxies. The paleosol-based reconstructions (near the base of the section) of precipitation (608-1167 mm yr-1) and temperature (10.4 to 12.0 °C) were within error of, although lower than, those based on floral assemblages, which were stratigraphically higher in the section. Geochemistry and detrital feldspar geochronology indicate a consistent provenance for Blue Rim sediments, sourcing predominantly from the Idaho paleoriver, which drained the active Challis volcanic field. Thus, because there was neither significant climatic change nor significant provenance change, variation in sedimentary facies and organic carbon burial likely reflected localized geomorphic controls, and the relative height of the water table. The ecosystem can be characterized as a wet, subtropical forest throughout the interval based upon the floral humidity province and Holdridge life zone schemes. Given the mid-paleolatitude position of the Blue Rim Escarpment, those results are consistent with marine proxies that indicate that globally warm climatic conditions continued beyond the peak warm conditions of the early Eocene climatic optimum. The reconstructed atmospheric δ13C value (-5.3 to -5.8 ‰) closely matches both the independently reconstructed value from marine microfossils (-5.4 ‰), as well as the isotopic composition of the mantle (-5.4 ‰), suggesting that the warm conditions were maintained by volcanic outgassing. [ABSTRACT FROM AUTHOR]

The 40Ar/39Ar dating method is among the most versatile of geochronometers, having the potential to date a broad variety of K-bearing materials spanning from the time of Earth's formation into the historical realm. Measurements using modern noble-gas mass spectrometers are now producing 40Ar/39Ar dates with analytical uncertainties of ~0.1%, thereby providing precise time constraints for a wide range of geologic and extraterrestrial processes. Analyses of increasingly smaller subsamples have revealed age dispersion in many materials, including some minerals used as neutron fluence monitors. Accordingly, interpretive strategies are evolving to address observed dispersion in dates from a single sample. Moreover, inferring a geologically meaningful "age" from a measured "date" or set of dates is dependent on the geological problem being addressed and the salient assumptions associated with each set of data. We highlight requirements for collateral information that will better constrain the interpretation of 40Ar/39Ar data sets, including those associated with single-crystal fusion analyses, incremental heating experiments, and in situ analyses of microsampled domains. To ensure the utility and viability of published results, we emphasize previous recommendations for reporting 40Ar/39Ar data and the related essential metadata, with the amendment that data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR) by both humans and computers. Our examples provide guidance for the presentation and interpretation of 40Ar/39Ar dates to maximize their interdisciplinary usage, reproducibility, and longevity. [ABSTRACT FROM AUTHOR]

We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track. [ABSTRACT FROM AUTHOR]

Calc-alkaline plutons are the major crustal building blocks of continental margins, but are rarely exposed in oceanic island arcs. Two of the best examples are the ∼10 km wide Hidden Bay and Kagalaska plutons that intrude Eocene mafic volcanic–sedimentary rocks on Adak and Kagalaska islands in the central Aleutian arc. Twenty new Ar/Ar and U/Pb ages, coupled with published ages, show that the Hidden Bay pluton was intruded in multiple stages from ∼34·6 to 30·9 Ma, whereas the Kagalaska pluton was intruded at ∼14 Ma. The plutons largely consist of medium- to high-K2O hornblende-bearing cumulate diorite (53–55 wt % SiO2) and hornblende–biotite granodiorite (57–64 wt %), with lesser amounts of gabbro (50–52 wt % SiO2), leucogranodiorite (67–69 wt % SiO2) and aplite (76–77 wt % SiO2) that can generally be linked to each other by crystal fractionation. The compositions of these plutons are generally similar to those of continental plutons, except for more oceanic-like large ion lithophile element and isotopic signatures (87Sr/86Sr = 0·703–0·7033; ɛNd = 9·4–7·7) that reflect oceanic- rather than continental-type crustal contaminants. Chemical similarities between the Hidden Bay homogeneous gabbros and high-Al basalts in Adak Pleistocene-Holocene volcanoes indicate little temporal evolution in the general character of the mantle-derived basalts. Rather than a unique arc setting and distinctive magmas, formation of the Aleutian calc-alkaline plutons seems to require a sufficient crustal thickness (∼37 km) and a high enough water content to stabilize pargasitic hornblende amphibole in a relatively closed magma system that favors increasing K, Ti and H2O at the end of a magmatic cycle. This termination of magmatism coincides with a northward migration of the magmatic front that is inferred to be associated with fore-arc subduction erosion. In accord with Adak region crustal architecture based on seismic data, crystallization models for the plutons suggest that mantle-generated hydrous arc basalts fractionated olivine and clinopyroxene in the lower crust to form high-Al basaltic composition magmas that rose into the mid-crust, where gabbro and diorite crystallized to form the magmas that buoyantly rose into the upper crust and crystallized to form the volumetrically dominant granodiorite (58–63 wt % SiO2). The most important temporal changes in chemistry can be explained by fore-arc crust incorporated into the mantle wedge by fore-arc subduction erosion creating 'adakitic' signatures at times of northward arc migration and a change to a more continental subducted sediment component at the time of Plio-Pleistocene glaciation. [ABSTRACT FROM AUTHOR]