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Abstract The thermal and compositional structure of arcs influence magmatic differentiation and lower-crustal foundering, two key processes impacting the evolution of the continental crust. Although many studies have proposed time scales of lithospheric recycling based on convective downwelling calculations, these models depend on the composition, density (ρ), and thermal structure of the lower crust and mantle, which are difficult to quantify in active continental arcs. Here, we constrained these properties for the Andean Northern Volcanic Zone using direct petrologic observations from a unique suite of lower-crust and mantle xenoliths from Mercaderes, Colombia. Chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates for zircons within the host tuff indicate the xenoliths erupted no earlier than 238 (±19) ka and thus capture a recent snapshot of the arc and subarc mantle. Equilibrium pressure-temperature (P-T) estimates for 81 xenoliths define three distinct thermal domains, interpreted as (1) a steep conductive geothermal gradient in the lower arc crust; (2) a convecting mantle wedge; and (3) cooled mantle in proximity to the subducting slab. Our results indicate the presence of an ~10–14-km-thick, high-density lithospheric root that is ~0.1 g/cm3 denser than the underlying mantle. Unlike records from exhumed paleoarcs, Rayleigh-Taylor instability calculations using our P-T-ρ constraints are unrealistically short for the northern Andes. We suggest the presence of partial melts in this hot arc root as a potential source of buoyancy preventing or significantly slowing down foundering. 

Abstract. Chemical abrasion is a technique that combines thermal annealing and partialdissolution in hydrofluoric acid (HF) to selectively removeradiation-damaged portions of zircon crystals prior to U–Pb isotopicanalysis, and it is applied ubiquitously to zircon prior to U–Pb isotopedilution thermal ionization mass spectrometry (ID-TIMS). The mechanics ofzircon dissolution in HF and the impact of different leaching conditions onthe zircon structure, however, are poorly resolved. We present amicrostructural investigation that integrates microscale X-ray computedtomography (µCT), scanning electron microscopy, and Ramanspectroscopy to evaluate zircon dissolution in HF. We show that µCTis an effective tool for imaging metamictization and complex dissolutionnetworks in three dimensions. Acid frequently reaches crystal interiors viafractures spatially associated with radiation damage zoning and inclusionsto dissolve soluble high-U zones, some inclusions, and material aroundfractures, leaving behind a more crystalline zircon residue. Other acid pathsto crystal cores include the dissolution of surface-reaching inclusions andthe percolation of acid across zones with high defect densities. In highlycrystalline samples dissolution is crystallographically controlled withdissolution proceeding almost exclusively along the c axis. Increasing theleaching temperature from 180 to 210 ∘C results indeeper etching textures, wider acid paths, more complex internal dissolutionnetworks, and greater volume losses. How a grain dissolves strongly dependson its initial radiation damage content and defect distribution as well asthe size and position of inclusions. As such, the effectiveness of anychemical abrasion protocol for ID-TIMS U–Pb geochronology is likelysample-dependent. We also briefly discuss the implications of our findingsfor deep-time (U-Th)/He thermochronology. 

High‐resolution three‐dimensional discrete element method (DEM) simulations of sandbox‐scale models of accretionary wedges suggest thrusts follow a variety of propagation processes and orientations depending on a number of factors. These include the stage of development of the wedge (precritical vs. critical), basal friction, and type of thrust (forward vs. backward‐vergent). In terms of propagation processes, two clear mechanisms are identified. The first involves propagation from the décollement to the wedge top, similar to the standard model of thrust propagation seen in many kinematic models, and in the second, thrusts grow downward from an initial nucleation point just below the top surface of the wedge as well as upward from the décollement joining in the middle. In terms of orientation, forward‐vergent thrusts initially form at Roscoe (θ_R = 45° − Ψ/2) or Arthur orientations (θ_A = 45° − (ϕ + Ψ)/4), and over greater shortening, rotate into Coulomb orientations (θ_C = 45° − ϕ/2). To arrive at these results, a wide array of continuum parameters and fields were extracted from the DEM simulations, including stress, strain, strain rate, kinetic energy, Mohr‐Coulomb parameters, and proximity to yielding using the Drucker‐Prager criterion to visualize thrust nucleation and propagation. Lastly, the advantages and disadvantages of these continuum proxies for discerning failure in the granular assembly are considered, and the spatial and temporal relationship between proximity to yielding and strain localization (both pre‐peak and subsequent persistent shear banding) in the granular model of an accretionary wedge is explored.

 

Abstract Plutons offer an opportunity to study the extended history of magmas at depth. Fully exploiting this record requires the ability to track changes in magmatic plumbing systems as magma intrudes, crystallizes, and/or mixes through time. This task has been difficult in granitoid plutons because of low sampling density, poorly preserved or cryptic intrusive relationships, and the difficulty of identifying plutonic volumes that record the contemporaneous presence of melt. In particular, the difficulty in delineating fossil magma reservoirs has limited our ability to directly test whether or not high-SiO2 rhyolite is the result of crystal-melt segregation. We present new high-precision U-Pb zircon geochronologic and geochemical data that characterize the Miocene Searchlight pluton in southern Nevada, USA. The data indicate that the pluton was built incrementally over ~1.5 m.y. with some volumes of magma completely crystallizing before subsequent volumes arrived. The largest increment is an ~2.7-km-thick granitic sill that records contemporaneous zircon crystallization, which we interpret to represent a fossil silicic magma reservoir within the greater Searchlight pluton. Whole-rock geochemical data demonstrate that this unit is stratified relative to paleo-vertical, consistent with gravitationally driven separation of high-SiO2 melt from early-formed crystals at moderate crystallinity. Zircon trace-element compositions suggest that our geochronologic data from this unit record most of the relevant crystallization interval for differentiation and that this process occurred in <150 k.y. 

Abstract Silicic magmas within large igneous provinces (LIPs) are understudied relative to volumetrically dominant mafic magmas despite their prevalence and possible contribution to LIP-induced environmental degradation. In the 66 Ma Deccan LIP (India), evolved magmatism is documented, but its geographic distribution, duration, and significance remain poorly understood. Zircons deposited in weathered Deccan lava flow tops (“red boles”) offer a means of indirectly studying potentially widespread, silicic, explosive volcanism spanning the entire period of flood basalt eruptions. We explored this record through analysis of trace elements and Hf isotopes in zircon crystals previously dated by U–Pb geochronology. Our results show that zircon populations within individual red boles fingerprint distinct volcanic sources that likely developed in an intraplate setting on cratonic Indian lithosphere. However, our red bole zircon geochemical and isotopic characteristics do not match those from previously studied silicic magmatic centers, indicating that they must derive from yet undiscovered or understudied volcanic centers associated with the Deccan LIP. 

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. 

We document the performance of new ATONA (‘aA to nA’) amplifiers installed on an Isotopx Phoenix thermal ionisation mass spectrometer (TIMS) at Princeton University and evaluate their suitability for high-precision analyses of Pb and U isotopes in pg- to ng-size samples characteristic for U–Pb geochronology. The new amplifiers are characterised by low and stable noise levels comparable to 10 12 to 10 13 ohm resistors, response time <0.5 s, exceptional gain stability <1 ppm and a vast dynamic range theoretically allowing to quantify signals from aA (10 −18 A) to nA (10 −9 A) level. We measured a set of Pb standards, synthetic U–Pb solutions and natural zircons at currents of 2 × 10 −16 to 2 × 10 −12 A (corresponding to intensities of 20 μV to 200 mV relative to a 10 11 ohm amplifier) to assess the utility of ATONA in replacing ion counting for the smallest samples. The results show a clear precision benefit of using ATONA-Faraday detection over Daly ion counting for ion currents of >10 −14 A (1 mV relative to a 10 11 ohm amplifier or ca. 60 kcps). As such currents are routinely achievable for major Pb peaks of interest ( 205–208 Pb) in natural samples containing more than ca. 10 pg Pb* (radiogenic Pb), we expect ATONA-Faraday detection to find broad applications in U–Pb geochronology. Its practical use for low-blank, radiogenic samples continues to require ion counting for 204 Pb, either with a fixed Faraday–ion counter gain or using a dynamic two-step ( e.g. FaraDaly) method. Routine adoption of ATONA-Faraday collection in place of ion counting for most major Pb and U isotopes has the potential to increase sample throughput and precision, both improving the accessibility of isotope dilution (ID)-TIMS geochronology and pushing this technique towards better reproducibility.