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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. 

Rhyolitic melt that fuels explosive eruptions often originates in the upper crust via extraction from crystal-rich sources, implying an evolutionary link between volcanism and residual plutonism. However, the time scales over which these systems evolve are mainly understood through erupted deposits, limiting confirmation of this connection. Exhumed plutons that preserve a record of high-silica melt segregation provide a critical subvolcanic perspective on rhyolite generation, permitting comparison between time scales of long-term assembly and transient melt extraction events. Here, U-Pb zircon petrochronology and 40 Ar/ 39 Ar thermochronology constrain silicic melt segregation and residual cumulate formation in a ~7 to 6 Ma, shallow (3 to 7 km depth) Andean pluton. Thermo-petrological simulations linked to a zircon saturation model map spatiotemporal melt flux distributions. Our findings suggest that ~50 km 3 of rhyolitic melt was extracted in ~130 ka, transient pluton assembly that indicates the thermal viability of advanced magma differentiation in the upper crust. 

Abstract The Eocene Huitrera Formation of northwestern Patagonia, Argentina, is renowned for its diverse, informative, and outstandingly preserved fossil biotas. In northwest Chubut Province, at the Laguna del Hunco locality, this unit includes one of the most diverse fossil floras known from the Eocene, as well as significant fossil insects and vertebrates. It also includes rich fossil vertebrate faunas at the Laguna Fría and La Barda localities. Previous studies of these important occurrences have provided relatively little sedimentological detail, and radioisotopic age constraints are relatively sparse and in some cases obsolete. Here, we describe five fossiliferous lithofacies deposited in four terrestrial depositional environments: lacustrine basin floor, subaerial pyroclastic plain, vegetated, waterlogged pyroclastic lake margin, and extracaldera incised valley. We also report several new 40Ar/39Ar age determinations. Among these, the uppermost unit of the caldera-forming Ignimbrita Barda Colorada yielded a 40Ar/39Ar age of 52.54 ± 0.17 Ma, ∼6 m.y. younger than previous estimates, which demonstrates that deposition of overlying fossiliferous lacustrine strata (previously constrained to older than 52.22 ± 0.22 Ma) must have begun almost immediately on the subsiding ignimbrite surface. A minimum age for Laguna del Hunco fossils is established by an overlying ignimbrite with an age of 49.19 ± 0.24 Ma, confirming that deposition took place during the early Eocene climatic optimum. The Laguna Fría mammalian fauna is younger, constrained between a valley-filling ignimbrite and a capping basalt with 40Ar/39Ar ages of 49.26 ± 0.30 Ma and 43.50 ± 1.14 Ma, respectively. The latter age is ∼4 m.y. younger than previously reported. These new ages more precisely define the age range of the Laguna Fría and La Barda faunas, allowing greatly improved understanding of their positions with respect to South American mammal evolution, climate change, and geographic isolation. 

The rhyolite‐producing Laguna del Maule volcanic field (LdMVF), Chile, has had numerous post‐glacial eruptions that produced large explosions and voluminous lava flows. During the Holocene ∼60 m of surface uplift is recorded by paleo‐shorelines of the fresh‐water Laguna del Maule, with an inflation source near the Barrancas volcanic complex. Rhyolites from the Barrancas complex erupted over ∼14 ka including some of the youngest (1.4 ± 0.6 ka) lava flows in the field. New gravity data collected on the Barrancas complex reveals a residual gravity low (−6 mGal, “Barrancas anomaly”) that is distinct from the pronounced gravity low (−19 mGal; “Lake anomaly”) associated with present‐day ground uplift to the northwest. Three‐dimensional inversion of the Barrancas anomaly indicates the presence of a magma body with a maximum density contrast with the host rock of −250 kg/m³centered at a depth of ∼3 km below surface. Nearby Miocene high‐silica granites represent frozen remnants of highly evolved rhyolitic magma. Comparison of the densities measured from samples of these plutons with the geophysical model densities, and integration of thermodynamic modeling of silicic melt evolution, provide constraints on our interpretation. We propose a magma body, containing <30% melt phase and low volatile content, exists beneath Barrancas. The Barrancas and Lake gravity lows represent magma in different physical states, associated with past and present‐day storage beneath LdMVF. The gravity model mirrors geochemical observations which independently indicate that at least two distinct rhyolites were generated and stored as discrete magma bodies within the broader LdMVF.

 

Interpreting unrest at silicic volcanoes requires knowledge of the magma storage conditions and dynamics that precede eruptions. The Laguna del Maule volcanic field, Chile, has erupted ~40 km³of rhyolite over the last 20 ka. Astonishing rates of sustained surface inflation at >25 cm/year for >12 years reveal a large, restless system. Integration of geochronologic, petrologic, geomorphic, and geophysical observations provides an unusually rich context to interpret ongoing and prehistoric processes. We present new volatile (H₂O, CO₂, S, F, and Cl), trace element, and major element concentrations from 109 melt inclusions hosted in quartz, plagioclase, and olivine from seven eruptions. Silicic melts contain up to 8.0 wt. % H₂O and 570 ppm CO₂. In rhyolites melt inclusions track decompression‐driven fractional crystallization as magma ascended from ~14 to 4 km. This mirrors teleseismic tomography and magnetotelluric findings that reveal a domain containing partial melt spanning from 14 to 4 km. Ce and Cl contents of rhyolites support the generation of compositionally distinct domains of eruptible rhyolite within the larger reservoir. Heat, volatiles, and melt derived from episodic mafic recharge likely incubate and grow the shallow reservoir. Olivine‐hosted melt inclusions in mafic tephra contain up to 2.5 wt. % H₂O and 1,140 ppm CO₂and proxy for the volatile load delivered via recharge into the base of the silicic mush at ~14 to 8 km. We propose that mafic recharge flushes deeper reaches of the magma reservoir with CO₂that propels H₂O exsolution, upward accumulation of fluid, pressurization, and triggering of rhyolitic eruptions.

 

Despite the hazard posed by explosive silicic eruptions, the magma storage conditions and dynamics that precede these events remain controversial. The Laguna del Maule volcanic field, central Chile, is an exceptional example of postglacial (younger than ca. 20,000 years) rhyolite volcanism and sustained unrest driven by a large, shallow, active silicic magma system. New zircon petrochronologic data reveal that compositionally distinct domains developed concurrently within the Laguna del Maule magma reservoir, which produced two episodes of concentrated rhyolitic eruptions at 23–19 and 8–2 ka. Zircon crystallization ages record 160 kyr of magma emplacement resulting in a several hundreds of cubic kilometers reservoir that has been imaged geophysically. The average magma emplacement rate inferred from the zircon geochronology and tomographically defined magma volume is consistent with those required by thermal models to maintain a shallow silicic system. Ti‐in‐zircon temperatures of crystal cores and rims and hiatuses in crystal growth indicates most of this volume persisted in a near‐solidus state. However, consistent patterns of trace element zoning in crystal interiors and crystallization rates derived from a model of diffusion‐limited zircon growth suggest the erupted rhyolite magma batches originated from long‐lived hot zones of extractable mush embedded within the larger, cool reservoir of rigid mush. These contrasting, coeval magma storage conditions obviate a simple hot versus cold storage dichotomy for large silicic magma systems.