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Crystal mush systems, often referenced in the context of large silicic magma bodies, involve the reactivation of a near- solidus crystal mush by heat input from mafic injections. This model suggests that interstitial melt is extracted from the mush, leading to the generation of high-silica rhyolites and granites. Such processes have been well-documented in various tectonic settings and contribute to both large-scale eruptions and the formation of granitic plutons. However, in the Mineral Mountains, Utah, the zircon and whole rock geochemical record indicate a different scenario. The presence of sector-zoned zircons and the absence of highly evolved central domains indicative of extraction from a mush suggest rapid magma generation from partial melting of solid granitoids rather than from a long-lived crystal mush. Fractional crystallization and equilibrium partial melting models support derivation from the granitoid bodies, rather than from a common shared parental rhyolitic magma or from coeval basalts. The proposed model, presented here, for rhyolite formation in the Mineral Mountains involves episodic injections of mafic magma into the crust, leading to localized partial melting of different granitoid lithologies. Partial melting up to 30% can produce isolated, ephemeral pools of high-silica melt, which crystallize zircons rapidly and ascend to form rhyolitic domes. This process is distinct from the long-lived crystal mush model, explains the lack of intermediate compositions, and the confinement of mafic eruptions to lower elevations. By integrating geochemical data, zircon morphology, and fractionation modeling, this study provides a comprehensive framework for understanding the magmatic processes at play in the Mineral Mountains. 

Formation and evolution of the basal layer in large landslides has important implications for processes that reduce frictional resistance to sliding. In this report, we show that zircon geochronology and tectonic provenance can be used to investigate the basal layer of the gigantic-scale Markagunt gravity slide of Utah, USA. Basal layer and clastic injectite samples have unique tectonic chronofacies that identify the rock units that were broken down during emplacement. Our results show that basal material from sites on the former land surface is statistically indistinguishable and formed primarily by the breakdown of upper plate lithologies during sliding. Decapitated injectites have a different tectonic chronofacies than the local basal layer, with more abundant lower plate-derived zircons. This suggests clastic dikes formed earlier in the translation history from a structurally deeper portion of the slide surface and a compositionally different basal layer before being translated to their current position. 

Abstract The physical processes that facilitate long‐distance translation of large‐volume gravity slides remain poorly understood. To better understand these processes and the controls on runout distance, we conducted an outcrop and microstructural characterization of the Sevier gravity slide across the former land surface and summarize findings of four key sites. The Sevier gravity slide is the oldest of three mega‐scale (>1,000 km²) collapse events of the Marysvale volcanic field (Utah, USA). Field observations of intense deformation, clastic dikes, pseudotachylyte, and consistency of kinematic indicators support the interpretation of rapid emplacement during a single event. Furthermore, clastic dikes and characteristics of the slip zone suggest emplacement involved mobilization and pressurized injection of basal material. Across the runout distance, we observe evidence for progressive slip delocalization along the slide base. This manifests as centimeter‐ to decimeter‐thick cataclastic basal zones and abundant clastic dikes in the north and tens of meters thick basal zones characterized by widespread deformation of both slide blocks and underlying rock near the southern distal end of the gravity slide. Superimposed on this transition are variations in basal zone characteristics and slide geometry arising from interactions between slide blocks during dynamic wear and deposition processes and pre‐existing topography of the former land surface. These observations are synthesized into a conceptual model in which the presence of highly pressurized fluids reduced the frictional resistance to sliding during the emplacement of the Sevier gravity slide, and basal zone evolution controlled the effectiveness of dynamic weakening mechanisms across the former land surface. 

Abstract 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.