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Sediments deposited into foreland basins can provide valuable insights related to the geological evolution of their hinterlands. Located in the peripheral foreland of the South Sistan Suture Zone (SE Iran), the Karvandar Basin exhibits a several‐kilometer‐thick shallow‐marine to continental clastic sedimentary sequence forming elongated sub‐circular synclines. These synclines overlie a mud‐dominated formation with exotic volcanic blocks that hosts one of Iran's largest mud volcano, known as Pirgel. In this study, we present a ∼3.5‐km‐thick magnetostratigraphic section and U‐Pb zircon ages of interlayered tuffs that constrain a depositional age of the Karvandar Basin of ∼24–17 Ma. Sandstone and microconglomerate framework analyses and paleocurrent directions suggest a first‐cycle active volcanic arc source to the northeast of the basin. We interpret the mud‐dominated lithology with volcanic blocks as an olistostrome originating from a similar source as the overlying clastic sequence. The deposition of the olistostrome is dated at ∼24.5 Ma by a U‐Pb calcite age from a coral block. The absence of large‐scale anticlines and the occurrence of angular unconformities suggest that the sub‐circular synclines in the Karvandar Basin formed by gravity‐driven downbuilding into the unconsolidated fluid‐saturated olistostrome, resembling salt‐related minibasins. Integrated results indicate that a late Oligocene to early Miocene Makran volcanic arc represents the source of the clastic sequence. Hence, our results provide new constraints on the initiation of arc volcanism related to the Makran subduction zone, predating earliest reported ages from the Mirabad pluton (19 Ma) to the northeast of the Karvandar Basin by ∼5 Myr. Key Points: A ∼3.5‐km‐thick section in the Karvandar foreland basin was sampled for magnetostratigraphy and sandstone framework analysisThe shallow‐marine to continental sequence (24–18 Ma) consists of first‐cycle volcanic lithics and overlays an olistostrome depositThe late Oligocene to early Miocene sequence represents the erosion product of a volcanic arc related to the Makran subduction [ABSTRACT FROM AUTHOR]

Mount Elbrus, Europe's tallest and largely glaciated volcano, is made of silicic lavas and is known for Holocene eruptions, but the size and state of its magma chamber remain poorly constrained. We report high spatial resolution U–Th–Pb zircon ages, co-registered with oxygen and hafnium isotopic values, span ~ 0.6 Ma in each lava, documenting magmatic initiation that forms the current edifice. The best-fit thermochemical modeling constrains magmatic fluxes at 1.2 km3/1000 year by hot (900 °C), initially zircon-undersaturated dacite into a vertically extensive magma body since ~ 0.6 Ma, whereas a volcanic episode with eruptible magma only extends over the past 0.2 Ma, matching the age of oldest lavas. Simulations explain the total magma volume of ~ 180 km3, temporally oscillating δ18O and εHf values, and a wide range of zircon age distributions in each sample. These data provide insights into the current state (~ 200 km3 of melt in a vertically extensive system) and the potential for future activity of Elbrus calling for much-needed seismic imaging. Similar zircon records worldwide require continuous intrusive activity by magmatic accretion of silicic magmas generated at depths, and that zircon ages do not reflect eruption ages but predate them by ~ 103 to 105 years reflecting protracted dissolution–crystallization histories. [ABSTRACT FROM AUTHOR]

Mount Elbrus, Europe's tallest and largely glaciated volcano, is made of silicic lavas and is known for Holocene eruptions, but the size and state of its magma chamber remain poorly constrained. We report high spatial resolution U–Th–Pb zircon ages, co-registered with oxygen and hafnium isotopic values, span ~ 0.6 Ma in each lava, documenting magmatic initiation that forms the current edifice. The best-fit thermochemical modeling constrains magmatic fluxes at 1.2 km3/1000 year by hot (900 °C), initially zircon-undersaturated dacite into a vertically extensive magma body since ~ 0.6 Ma, whereas a volcanic episode with eruptible magma only extends over the past 0.2 Ma, matching the age of oldest lavas. Simulations explain the total magma volume of ~ 180 km3, temporally oscillating δ18O and εHf values, and a wide range of zircon age distributions in each sample. These data provide insights into the current state (~ 200 km3 of melt in a vertically extensive system) and the potential for future activity of Elbrus calling for much-needed seismic imaging. Similar zircon records worldwide require continuous intrusive activity by magmatic accretion of silicic magmas generated at depths, and that zircon ages do not reflect eruption ages but predate them by ~ 103 to 105 years reflecting protracted dissolution–crystallization histories. [ABSTRACT FROM AUTHOR]

Dense, glassy pyroclasts found in products of explosive eruptions are commonly employed to investigate volcanic conduit processes through measurement of their volatile inventories. This approach rests upon the tacit assumption that the obsidian clasts are juvenile, that is, genetically related to the erupting magma. Pyroclastic deposits within the Yellowstone-Snake River Plain province almost without exception contain dense, glassy clasts, previously interpreted as hyaloclastite, while other lithologies, including crystallised rhyolite, are extremely rare. We investigate the origin of these dense, glassy clasts from a coupled geochemical and textural perspective combining literature data and case studies from Cougar Point Tuff XIII, Wolverine Creek Tuff, and Mesa Falls Tuff spanning 10 My of silicic volcanism. These results indicate that the trace elemental compositions of the dense glasses mostly overlap with the vesiculated component of each deposit, while being distinct from nearby units, thus indicating that dense glasses are juvenile. Textural complexity of the dense clasts varies across our examples. Cougar Point Tuff XIII contains a remarkable diversity of clast appearances with the same glass composition including obsidian-within-obsidian clasts. Mesa Falls Tuff contains clasts with the same glass compositions but with stark variations in phenocryst content (0 to 45%). Cumulatively, our results support a model where most dense, glassy clasts reflect conduit material that passed through multiple cycles of fracturing and sintering with concurrent mixing of glass and various crystal components. This is in contrast to previous interpretations of these clasts as entrained hyaloclastite and relaxes the requirement for water-magma interaction within the eruptive centres of the Yellowstone-Snake River Plain province. [ABSTRACT FROM AUTHOR]

During its late‐stage evolution, the European Alpine orogen witnessed a northwest‐directed propagation of its deformation front along an evaporitic basal décollement into the foreland. This resulted in the decoupling of the northern Alpine Molasse Basin from its basement and the formation of the Jura fold‐and‐thrust belt. Here, we present the first absolute age and temperature constraints on deformation along this basal décollement using carbonate U‐Pb LA‐ICP‐MS dating and clumped isotope thermometry. We analyzed calcite veins associated with a thrust fault branching off from the basal décollement in the distal Molasse Basin and slickenfibers from thrusts and strike‐slip faults in the eastern Jura Mountains. Our U‐Pb data provide evidence for tectonic activity related to Alpine contraction between 14.3 and 4.5 Ma ago. According to the oldest deformation ages, the propagation of Alpine deformation into the distal foreland along the basal décollement occurred earlier than previously inferred by biostratigraphy, at Middle Miocene (Langhian) times at the latest. Younger deformation ages between 11.3 and 4.5 Ma correspond very well in time with shortening in the Subalpine Molasse and the Central Alps, proving simultaneous tectonic activity along both thrust fronts; i.e., the Jura Mountains and the Subalpine Molasse. Clumped isotopes reveal vein calcite precipitation at temperatures between 53 and 104 °C from fluids with oxygen isotope compositions between −6.2‰ and +9.5‰ reflecting distinct burial settings. Combined, our U‐Pb and clumped isotope data show that the burial conditions in the studied area remained constant between 14.3 and 4.5 Ma indicating that large‐scale foreland erosion did not initiate before 4.5 Ma. Key Points: Propagation of Alpine deformation along the basal décollement into the distal foreland initiated at 14.3 Ma at the latestDeformation occurred over a time period of at least ∼10 Ma (14.3–4.5 Ma) with contemporaneous activity of thrusts and strike‐slip faultsConstant in situ temperatures and fluid sources show that large‐scale foreland erosion did not initiate before 4.5 Ma [ABSTRACT FROM AUTHOR]

O.M. was partly supported by the ERC Grant agreement 787399-SEISMAZE and by the PAUSE program (Grant #C7H-PUB23A59." Finally, in the Acknowledgements section, "The authors acknowledge the support of NSF grant EAR 1822977. [Extracted from the article]

The Heise volcanic field is the second youngest caldera complex of the Yellowstone-Snake River Plain province (USA) and represents a polycyclic caldera system with rhyolitic volcanism extending over more than 2 Myr. The products of the Heise volcanic field include four regionally extensive ignimbrites, including the Blacktail Creek and Kilgore tuffs, which both have volumes estimated at >1000 km³, separated by sequences of smaller volume tuffs, lavas and sedimentary deposits. Rhyolites from the Heise volcanic field are both normal-δ18O and low-δ18O, making it a key locality for investigating rhyolite petrogenesis. However, the occurrence of abundant young basaltic lava has limited our ability to fully characterise this volcanic centre, particularly in terms of post-caldera volcanism. Here we describe rhyolitic samples from both a >700 m thick section of drillcore within the Snake River Plain and the exposed outflow stratigraphy on the margins of the plain. Based on a combination of bulk-rock and mineral geochemical, isotopic, and geochronological evidence, we conclude that the rhyolites from the drillcore are not exposed at the surface, nor are the surficial rhyolites found in the drillcore. High-precision isotope dilution thermal ionisation mass spectrometry U-Pb geochronology dates the rhyolite at the base of the drillcore to 4.0248 ± 0.0011 Ma, ~0.4 Myr younger than the youngest caldera-forming ignimbrite at Heise, the 4.48Ma Kilgore Tuff, whereas U-Pb secondary ionisation mass spectrometry dates the uppermost portion of rhyolite in the drillcore to 3.86 ± 0.19 Ma. The combined geochemistry and stratigraphic relations suggest that the drillcore penetrates the intracaldera stratigraphy. The intracaldera rhyolites are compositionally and mineralogically similar to the outflow stratigraphy with high-temperature magmas (>800°C) persisting for the full >3 Myr history of the Heise centre. The δ18O values of pyroxene, sanidine, and quartz from the unaltered drillcore samples are consistent with high-temperature equilibrium and return magma δ18O values that are low (4.1-6.0‰ based on Δ18O melt-sanidine of 0.6‰) but somewhat higher than the value for the preceding Kilgore Tuff magma of 3.3‰. Buried deep within the drillcore are also hydrothermally altered rhyolites with bulk δ18O ranging from-3.5‰ to +1.0‰ (SMOW) with complex X-ray diffraction spectra revealing the presence of epidote, quartz and chlorite. These altered samples are, however, not markedly different in bulk major or trace elemental geochemistry from the unaltered Heise rhyolites. Rhyolite-MELTS models using these hydrothermally altered samples as potential assimilants can reproduce the compositions, mineralogy, and crystallinity of the low-δ18O Kilgore Tuff with 40-50% assimilation while also satisfying the mass balance constrained on the basis of δ18O. These results support a cannibalisation model for Heise volcanism while highlighting that the lowest δ18O rhyolites may require large amounts of extremely 18O-depleted hydrothermally alteredmaterial available for assimilation. [ABSTRACT FROM AUTHOR]

Chemical Abrasion Isotope-Dilution Thermal Ionization Mass Spectrometry (CA-ID-TIMS) is known as a high precision technique for resolving lead loss and improving the interpretation of U-Pb zircon age data. Here, we argue that combining CA with the widely applied Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) improves the precision and accuracy of zircon dates, while removing the substantial parts with lead loss, reducing data scatter, and providing meaningful geological interpretations. The samples are magmatic rocks chosen from different geological time periods (one Paleozoic, one Mesozoic and three Cenozoic). All zircon separates are analysed by LA-ICP-MS before and after CA, and age data are compared with CA-ID-TIMS 206Pb/238U dates that are considered as the most accurately obtainable age. All CA-treated zircon crystals show up to 50% less data scatter compared to the non-CA treated zircon grains and thus reduction of the calculated uncertainties is apparent. The obtained wt average LA-ICP-MS 206 Pb/238U ages of the CA-treated zircon grains are up to 4-6% higher than those of the non-CA treated crystals, exceeding the analytical uncertainties of the LA- ICP-MS dating technique of 1-2%. The damaged crystal parts, caused by U-decay, with lead loss are removed, so that we can exclude younging from the possible geological scenarios. CA-LA-ICP-MS age data are in good agreement with the CA-ID-TIMS dates and suggest advantages of using CA-LA-ICP-MS in order to define accurate ages. The use of the CA technique for very young zircons (~0.2 Ma, Kos rhyolitic tuff, Greece) seems optional; as the obtained mean 206Pb/238U ages of non-CA and CA treated zircons coincide within the uncertainty. The negligible time to produce the lattice damage (based on alpha decay or spontaneous fission) makes lead loss less important for age dating and data interpretation of very young zircons (<1 Ma). [ABSTRACT FROM AUTHOR]

Zircon Pb-U dating has become a key technique for answering many important questions in geosciences. This paper describes a new LA-ICP-MS approach. We show, using previously dated samples of a large quaternary rhyolitic eruption in the Kos-Nisyros volcanic centre (the 161 ka Kos Plateau Tuff), that the precision of our LA-ICP-MS method is as good as via SHRIMP, while ID-TIMS measurements confirm the accuracy. Gradational age distribution over >140 ka of the Kos zircons and the near-absence of inherited cores indicate near-continuous crystallisation in a growing magma reservoir with little input from wall rocks. Previously undated silicic eruptions from Nisyros volcano (Lower Pumice, Nikia Flow, Upper Pumice), which are stratigraphically constrained to have happened after the Kos Plateau Tuff, are dated to be younger than respectively 124 ± 35 ka, 111 ± 42 ka and 70 ± 24 ka. Samples younger than 1 Ma were corrected for initial thorium disequilibrium using a new formula that also accounts for disequilibrium in 230Th decay. [ABSTRACT FROM AUTHOR]

Many studies now acknowledge the occurrence of systematic discrepancies between U-Pb ages determined in zircons in situ by LA-ICP-MS and the benchmark analytical method ID-TIMS. In this study, we present detailed investigations into the ablation characteristics of zircons that suggest an underlying mechanism responsible for these age biases relative to ID-TIMS. Confocal laser scanning microscopy of laser ablation pits reveals that there are small but significant differences in the amount of material removed by the laser between different zircons. Based on numerous pit depth and LA-ICP-MS 206Pb/238U ratio measurements of a suite of natural zircon reference materials and samples, we demonstrate that a systematic age bias is strongly correlated with the offset in ablation rates between the primary reference material and sample zircons. We offer further insights concerning the effects of thermal annealing on the ablation behaviour of zircons and demonstrate that, although there is a change in laser ablation rates for annealed zircons, the variations between different zircons are not eliminated. Finally, we show that slight variations in laser focus also influence the ablation behaviour of zircons and may further degrade the accuracy of U-Pb age determinations. [ABSTRACT FROM AUTHOR]

To reflect magmatic conditions, volcanic rocks must retain their compositions through eruption and post-eruptive cooling. Mostly, this is the case. However, welded ignimbrites from the Yellowstone-Snake River Plain magmatic province reveal systematic modification of the lithium (Li) inventory by post-eruptive processes. Here we show that phenocrysts from slowly cooled microcrystalline ignimbrite interiors consistently have significantly more Li than their rapidly quenched, glassy, counterparts. The strong association with host lithology and the invariance of other trace elements indicate that Li remains mobile long after eruption and readily passes into phenocrysts via diffusion as groundmass crystallisation increases the Li contents of the last remaining melts. Li isotopic measurements reveal that this diffusion during cooling combined with efficient degassing on the surface may significantly affect the Li inventory and isotopic compositions of volcanic rocks. Utilisation of Li for petrogenetic studies is therefore crucially dependent on the ability to ‘see through’ such post-eruptive processes. Lithium, an increasingly economically important element, is also used to trace the cycling of materials through the Earth system. Here the authors show that post-eruptive processes such as degassing and groundmass crystallisation control the inventory of lithium in volcanic deposits. [ABSTRACT FROM AUTHOR]