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The Wilkins Peak Member (WPM) of the Green River Formation in Wyoming, USA, comprises alternating lacustrine and alluvial strata that preserve a record of terrestrial climate during the early Eocene climatic optimum. We use a Bayesian framework to develop age-depth models for three sites, based on new 40Ar/39Ar sanidine and 206Pb/238U zircon ages from seven tuffs. The new models provide two- to ten-fold increases in temporal resolution compared to previous radioisotopic age models, confirming eccentricity-scale pacing of WPM facies, and permitting their direct comparison to astronomical solutions. Starting at ca. 51 Ma, the median ages for basin-wide flooding surfaces atop six successive alluvial marker beds coincide with short eccentricity maxima in the astronomical solutions. These eccentricity maxima have been associated with hyperthermal events recorded in marine strata during the early Eocene. WPM strata older than ca. 51 Ma do not exhibit a clear relationship to the eccentricity solutions, but accumulated 31%–35% more rapidly, suggesting that the influence of astronomical forcing on sedimentation was modulated by basin tectonics. Additional high-precision radioisotopic ages are needed to reduce the uncertainty of the Bayesian model, but this approach shows promise for unambiguous evaluation of the phase relationship between alluvial marker beds and theoretical eccentricity solutions. [ABSTRACT FROM AUTHOR]

The precision and accuracy of 40Ar/39Ar dates are ultimately linked to co‐irradiated reference materials of known age. Here we provide new data from the SK01 sanidine, which was analysed in three different laboratories to evaluate it as a 40Ar/39Ar reference material. Aliquots of 5 mg, incrementally heated in two laboratories, yielded indistinguishable results with a weighted mean age of 27.58 ± 0.06 Ma (95% confidence level). Single‐crystal step heating and single‐crystal total fusion analyses of SK01 sanidine were undertaken in the third laboratory to further test the intracrystalline homogeneity. For the seven step‐heating analyses, six crystals yielded nearly concordant age spectra with 40Ar/39Ar ages ranging from 26.853 ± 0.094 Ma to 26.963 ± 0.067 Ma, whereas one crystal gave an older age of 27.774 ± 0.071 Ma with a slightly discordant age spectrum. Twenty‐three single‐crystal total fusion analyses yielded 40Ar/39Ar ages ranging from 27.070 ± 0.108 Ma to 27.736 ± 0.062 Ma with a dispersion of ~ 3.8%. The older ages from single‐crystal total fusion dates are interpreted to reflect an inherited or excess argon component in some crystals. This is an initial characterisation of the SK01 sanidine, and additional work needs to be conducted to further evaluate the age dispersion so that it can be utilised as a 40Ar/39Ar reference material. [ABSTRACT FROM AUTHOR]

The [sup.40]Ar/[sup.39]Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra--marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between [sup.14]C and longer-lived isotopic systems that are too imprecise for dating young samples. However, [sup.40]Ar/[sup.39]Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, [sup.238]U-[sup.230]Th disequilibrium, to demonstrate that [sup.40]Ar/[sup.39]Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous [sup.40]Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in [sup.40]Ar/[sup.39]Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of [sup.40]Ar due to rapid loss to an 'infinite reservoir' such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. doi: 10.1130/G31064.1

New [sup.238]U-[sup.230]Th and Sr isotope data from [sup.40]Ar/[sup.39]Ar-dated mafic lavas of the composite cone, A.D. 1902 dacitic tephra, and 1922-present dacitic dome lavas from the Santa Maria--Santiaguito volcanic complex in northwestern Guatemala indicate that genesis of magma beneath the western end of the Central American volcanic arc requires sources much different than previously envisioned. Although U-series data from historical lavas along the Central American volcanic arc, including Guatemala, show variable but small degrees of either [sup.230]Th or [sup.238]U excess, 72-35 ka basalts and basaltic andesites from the Santa Maria cone have 15%-26% [sup.238]U excesses, among the largest measured in Central America. This implies that the mantle wedge beneath this sector of the arc was significantly modified by slab-derived fluids. A decrease in s7Sr]S6Sr and ([sup.238]U/[sup.230]Th) ratios over the past 72 k.y. as basaltic andesite gave way to dacite is consistent with fractional crystallization coupled with progressive assimilation of crust that has relatively unradiogenic, mid-oceanic ridge basalt--like Sr isotope composition. Overall, our U-series data along with published [sup.238]U/[sup.230]Th isotope results from Central America demonstrate that (1) slab-fluid flux can be high throughout Central America, including regions of relatively thick crust, and (2) the middle to lower crust beneath northwestern Guatemala may not be dominated by ancient metamorphic and granitic rocks. doi: 10.1130/G31062.1

[sup.40]Ar/[sup.30]Ar dating of 15 lava flows indicates that Volcan de Santa Maria grew episodically to 8 [km.sup.3] in size at an average rate of 0.12 [km.sup.3]/ka between 103 and 35 ka. The composite cone grew in four phases, including two periods of intense activity at ca. 72 ka and ca. 35 ka, during which 1.5 [km.sup.3] and 3 [km.sup.3] of basaltic to andesitic magmas were erupted. There is no evidence of further volcanism after ca. 35 ka until the great dacitic eruption in 1902. The average eruptive rate is 0.16 [km.sup.3]/ka, if products of the 1902 eruption and subsequent Santiaguito dome are included. Whereas the Mono Lake excursion is not clearly recorded at Volcan de Santa Maria, as had been inferred from earlier studies, virtual geomagnetic poles (VGPs) of the 35 ka cone-forming lavas exhibit high-amplitude paleosecular variation that may correspond in time to the Mono Lake excursion. Two older packages of lava flows are each associated with a distinctive cluster of VGPs, which supports the [sup.40]Ar/[sup.39]Ar age model and the conclusion that cone building was episodic. During the final 60% of cone growth, lavas evolved from basaltic to andesitic (51%-57% Si[O.sub.2]) with time, but with a regression to slightly less evolved compositions during the onset of the final cone-building phase. Despite the relatively small volume of Santa Maria, cone-growth processes and geochemical evolution through time mirror observations at other currently active volcanoes along the Central American volcanic arc, and may prove useful as an analogy in assessing long-term hazards posed by other predominantly basaltic-andesitic composite volcanoes. 10.1130/B26569.1

The western Idaho shear zone is one of several Cretaceous high-strain zones in the Cordillera that are thought to have been associated with the northward translation and/or docking of terranes presently in British Columbia. Located in west-central Idaho, this zone of intense deformation consists of a mid-crustal exposure of a lithospheric-scale, intra-arc dextral shear zone that overprints the Salmon River suture zone along the western edge of the Idaho batholith. U/Pb zircon geochronology constrains the main phase of deformation to between ca. 105 and 90 Ma. Cessation of movement on the shear zone occurred by 90 Ma, as determined by dating of the syntectonic Payette River tonalite and a crosscutting pegmatite dike in the Little Goose Creek complex. The [sup.40]Ar/[sup.39]Ar thermochronology indicates that the shear zone passed through both the hornblende (~550[degrees]C) and biotite (~325[degrees]C) closure temperatures between 85 and 70 Ma. The [sup.40]Ar/[sup.39]Ar biotite dates from an outcrop-scale, crosscutting shear zone are indistinguishable from that of the host rock, indicating that deformation occurred above the closure temperature of biotite. Apatite fission-track analysis suggests that exhumation to shallow crustal levels occurred ca. 40 Ma during mid-Tertiary regional exhumation or renewed tectonic activity along the Salmon River suture zone. Taken together, the [sup.40]Ar/[sup.39]Ar results and apatite fission-track analyses indicate a two-stage uplift history for the western Idaho shear zone. Overall, the geochronology indicates that dextral transpressional movement on the western Idaho shear zone was temporally distinct from the Early Cretaceous suturing event. Additionally, the first stage of exhumation recorded by the western Idaho shear zone immediately followed transpressional deformation. Cessation of displacement on the western Idaho shear zone by ca. 90 Ma indicates that the exhumation did not solely occur as a result of ductile deformation on the shear zone itself. Moreover, dextral strike-slip movement on the western Idaho shear zone had also ceased by 90 Ma, indicating that terrane translation models for the Cordillera can only use the western Idaho shear zone to accommodate northward translation up to ca. 90 Ma. Lastly, the timing of movement on the western Idaho shear zone and contractional deformation recorded in the Insular terrane suggests a correlation between these events. This hypothesis implies that the deformation recorded in the western Idaho shear zone may have been linked to the oblique collision of the Insular superterrane with North America. Keywords: western Idaho shear zone, transpression, Late Cretaceous, Idaho batholith, U/Pb, [sup.40]Ar/[sup.39]Ar, apatite fission-track analysis.

Precise dating of K-rich minerals in volcanic rocks via the [sup.40]Ar/[sup.39]Ar geochronometer has become crucial to resolving many geological problems. In some cases, ash beds containing biotite phenocrysts but lacking sanidine are the only datable horizons at key stratigraphic intervals, necessitating comparison of sanidine and biotite ages within the same chronostratigraphic framework. To assess the integrity of biotite and evaluate the accuracy of ages obtained from this often problematic mineral, incremental heating [sup.40]Ar/[sup.39]Ar experiments were performed on large, millimeter-sized, euhedral biotite crystals from four key Eocene tuffs in the Green River and Wind River Basins and are compared with sanidine [sup.40]Ar/[sup.39]Ar ages. Unaltered biotite crystals with homogenous K-compositions from two tuff beds yielded concordant plateau ages that are indistinguishable from cogenetic sanidine ages. In contrast, biotite crystals from two other tuffs yielded discordant spectra, with relatively young initial steps followed by older, down-stepping apparent ages. Plateau ages from the discordant experiments are older than the sanidine ages by I to 14%. Integrated (total fusion) ages from these experiments exhibit scatter toward both younger and older ages that correlate with the degree of spectral discordance. Electron microprobe transects reveal that biotite crystals yielding discordant age spectra contain 1-10 [micro]m thick K-depleted ( Keywords: Biotite, sanidine, [sup.40]Ar/[sup.39]Ar, recoil, Bridger Formation, Wagon Bed Formation, Wind River Formation, EPMA, tephra, turf, volcanic ash

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.

Numerous [sup.40]Ar/[sup.39]Ar experiments on sanidine and biotite from 22 ash beds and 3 volcaniclastic sand beds from the Greater Green River, Piceance Creek, and Uinta Basins of Wyoming, Colorado, and Utah constrain ~8 m.y. of the Eocene Epoch. Multiple analyses were conducted per sample using laser fusion and incremental heating techniques to differentiate inheritance, [sup.40]Ar loss, and [sup.39]Ar recoil. When considered in conjunction with existing radioisotopic ages and lithostratigraphy, biostratigraphy, and magnetostratigraphy, these new age determinations facilitate temporal correlation of linked Eocene lake basins in the Laramide Rocky Mountain region at a significantly increased level of precision. To compare our results to the geomagnetic polarity time scale and the regional volcanic record, the ages of Eocene magnetic anomalies C24 through C20 were recalibrated using seven [sup.40]Ar/[sup.39] Ar ages. Overall, the ages obtained for this study are consistent with the isochroneity of North American land-mammal ages throughout the study area, and provide precise radioisotopic constraints on several important biostratigraphic boundaries. Applying these new ages, average sediment accumulation rates in the Greater Green River Basin, Wyoming, were approximately three times faster at the center of the basin versus its ramp-like northern margin during deposition of the underfilled Wilkins Peak Member. In contrast, sediment accumulation occurred faster at the edge of the basin during deposition of the balanced filled to overfilled Tipton and Laney Members. Sediment accumulation patterns thus reflect basin-center-focused accumulation rates when the basin was underfilled, and supply-limited accumulation when the basin was balanced filled to overfilled. Sediment accumulation in the Uinta Basin, at Indian Canyon, Utah, was relatively constant at ~150 mm/k.y. during deposition of over 5 m.y. of both evaporative and fluctuating profundal facies, which likely reflects the basin-margin position of the measured section. The most rapid sediment accumulation for the entire system (>1 m/k.y.) occurred between 49.0 and 47.5 Ma, when volcaniclastic materials from the Absaroka and/or Challis volcanic fields entered the Green River Formation lakes from the north. Our new ages combined with existing paleomagnetic and biostratigraphic control permit the first detailed synoptic comparison of lacustrine depositional environments in all the Green River Formation basins. Coupled with previously published paleocurrent observations, our detailed correlations show that relatively freshwater lakes commonly drained into more saline downstream lakes. The overall character of Eocene lake deposits was therefore governed in part by the geomorphic evolution of drainage patterns in the surrounding Laramide landscape. Freshwater (overfilled) lakes were initially dominant (53.5-52.0 Ma), possibly related to high erosion rates of remnant Cretaceous strata on adjacent uplifts. Expansion of balanced-fill lakes first occurred in all Green River Formation basins at 52.0-51.3 Ma and again between 49.6 and 48.5 Ma. Evaporative (underfilled) lakes occurred in various basins between 51.3 and 45.1 Ma, coincident with the end of the early Eocene climatic optima and subsequent onset of global cooling defined from marine record. However, evaporite intervals in the different depocenters were deposited at different times rather than being confined to a single episode of arid climate. Evaporative terminal sinks were initially located in the Greater Green River and Piceance Creek Basins (51.3-48.9 Ma), then gradually migrated southward to the Uinta Basin (47.1-45.2 Ma). This history is likely related to progressive southward construction of the Absaroka Volcanic Province, which constituted a major topographic and thermal anomaly that contributed to a regional north to south hydrologic gradient. The Greater Green River and Piceance Creek Basins were eventually filled from north to south with Absaroka-derived detritus at sedimentation rates 1-2 orders of magnitude greater than the underlying lake deposits. Keywords: At-At, Absaroka, Uinta Basin, Piceance Creek Basin, land-mammal ages, lake type, Laramide, Eocene, Green River Formation

The 163 k.y. history as well as the chemical and 46 km3 volumetric evolution of Volcan Parinacota are described in detail by new mapping, stratigraphy, and 57 [sup.40]Ar/[sup.39]Ar ages determined from groundmass or sanidine crystals in basaltic andesitic to rhyolitic lavas. A more precise chronology of eruptions and associated eruptive volumes of this central Andean volcano, which was built upon 70-km-thick crust, provides a more complete view of how quickly volcanic edifices are built in this setting and how their magmatic systems evolve during their lifetime. Development of the complex involved initial eruption of andesitic lava flows (163-117 ka) followed by a rhyodacite dome plateau (47-40 ka) synchronous with the onset of the building of a stratocone (52-20 ka), which was later destroyed by a debris avalanche ~ 3 times larger than that at Mount St. Helens in 1980. Dome plateau emplacement occurred faster and later than has previously been published, implying a compressed duration of cone building and introducing a preceding 65 k.y. hiatus. Debris avalanche timing is refined here to be older than 10 but younger than 20 ka. Rapid postcollapse rebuilding of the volcanic edifice is delineated by 16 groundmass and whole-rock [sup.40]Ar/[sup.39]Ar ages, which include some of the youngest lava flows dated by this method. Increase in cone-building rate and a continued trend toward more mafic compositions following collapse imply an inter-relationship between the presence of the edifice and flux of magma from the feeding reservoir. Cone-building rates at Parinacota are similar to those at other well-dated volcanoes on thinner crust; however, the distributed basaltic volcanism prevalent in those other arcs is virtually absent both at Parinacota and elsewhere in the Central Volcanic Zone. This suggests that while the hydrous, calc-alkaline magmas that make up the central volcanoes are not significantly retarded by thick crust, primitive, dry basalts might be. Keywords: Andes, Central Volcanic Zone, Parinacota, [sup.40Ar]/[sup.39]Ar geochronology, eruptive rates.

Cosmogenic surface-exposure [sup.10]Be dating of Last Glacial Maximum (LGM) moraines indicates that glaciers in the southwestern Uinta Mountains remained at their maximum positions until ca. 16.8 [+ or -] 0.7 ka, ~2 k.y. after glaciers in the neighboring Wind River Range and Colorado Rockies began to retreat. The timing of the local LGM in the southwestern Uintas overlaps with both the hydrologic maximum of Lake Bonneville and preliminary estimates of the local LGM in the western Wasatch Mountains. This broad synchroneity indicates that Lake Bonneville and glaciers in northern Utah were responding to similar climate forcing. Furthermore, equilibrium line altitudes (ELAs) for reconstructed LGM alpine glaciers increase with distance from the Lake Bonneville shoreline, rising from ~2600 m to ~3200 m over the 120 km length of the glaciated Uintas. This pronounced ELA gradient suggests that the magnitude of the latest Pleistocene glacial advance in the western Uintas was due, at least in part, to enhanced precipitation derived from Lake Bonneville; thus, the lake acted as a local amplifier of regional climate forcing. This relationship underscores the sensitivity of alpine glaciers to moisture availability during the latest Pleistocene, and further demonstrates the importance of local moisture sources on glacier mass balance. Keywords: Last Glacial Maximum, alpine glaciers, climate, Rocky Mountains, late Pleistocene.

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.