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1. Timing of Dike and Sill Emplacement in the Inner Aureole of the Alta Stock, Utah Determined by Zircon and Monazite Petrochronology

   https://doi.org/10.1029/2024GC011946  

   Beno, C J; Stearns, M A; Bowman, J R; Bartley, J M; Fernandez, D P; Kylander‐Clark, A_R C (May 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Detailed geochronology from two compositionally distinct generations of dikes and sills intruded into the Alta metamorphic aureole, north‐central Utah, complement previous geochronologic studies from the Alta stock, providing information on the timing of magmatism and the nature of emplacement. Uranium/thorium‐lead dates and chemistry were measured in zircon and monazite from these intrusions and associated reaction selvages in hornfels by split‐stream laser ablation techniques. Concordant zircon U‐Pb dates (n = 532) define a dispersed population of dates that range from ∼38 to 32 Ma. Monazite Th‐Pb dates (n = 888) from granodioritic compositions range from ∼40 to 32 Ma. Evaluation of²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates with respect to common Pb, U and Th/U values allows rigorous evaluation of the effects of excess²⁰⁶Pb in these young monazites, yielding concordant²⁰⁸Pb/²³²Th and²⁰⁷Pb/²⁰⁶Pb‐corrected dates in monazites from the granodiorite, consistent with zircon dates from the same thin sections. Leucogranite sills and dikes, which cross‐cut the older granodiorite, have younger monazite dates from ∼33 to 28 Ma. Elevated heavy rare earth element concentrations and trends of larger negative Eu anomalies in the youngest monazites suggest crystallization from an evolved melt. Integration of these new geochronology results and field relationships with prior results from the Alta stock indicate the granodiorite represents the oldest material emplaced in the Alta system. Leucogranite aplite/pegmatite dikes and sills in the inner Alta aureole were emplaced during the final stage of Alta stock construction by injection of evolved water‐rich magmas. 

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2. Linking titanite U–Pb dates to coupled deformation and dissolution–reprecipitation

   Moser, A; Hacker, B; Gehrels, G; Seward, G; Kylander-Clark, ARC; Garber, Joshua M (March 2022, Contributions to Mineralogy and Petrology)

   Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains. 

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3. Linking titanite U–Pb dates to coupled deformation and dissolution–reprecipitation

   Moser, A; Hacker, B; Gehrels, G; Seward, G; Kylander‑Clark, R; Garber, J (March 2022, Contributions to Mineralogy and Petrology)

   Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mecha- nisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains. 

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4. Paleogene Paleohydrology of Ellesmere and Axel Heiberg Islands (Arctic Canada) From Palustrine Carbonates

   https://doi.org/10.1029/2023PA004609  

   Padgett, Ashly B.; Hyland, Ethan G.; West, Christopher K.; Burgener, Landon K.; Greenwood, David R.; Basinger, James F. (October 2023, Paleoceanography and Paleoclimatology)

   Abstract Ancient greenhouse periods are useful analogs for predicting effects of anthropogenic climate change on regional and global temperature and precipitation patterns. A paucity of terrestrial data from polar regions during warm episodes challenges our understanding of polar climate responses to natural/anthropogenic change and therefore our ability to predict future changes in precipitation. Ellesmere and Axel Heiberg Islands in the Canadian Arctic preserve terrestrial deposits spanning the late Paleocene to middle Eocene (59–45 Ma). Here we expand on existing regional sedimentology and paleontology through the addition of stable (δ¹³C, δ¹⁸O) and clumped (Δ₄₇) isotope analyses on palustrine carbonates. δ¹³C isotope values range from −4.6 to +12.3‰ (VPDB), and δ¹⁸O isotope values range from −23.1 to −15.2‰ (VPDB). Both carbon and oxygen isotope averages decrease with increasing diagenetic alteration. Unusually enriched carbon isotope (δ¹³C) values suggest that analyzed carbonates experienced repeated dissolution‐precipitation enrichment cycles, potentially caused by seasonal fluctuations in water availability resulting in summer carbonate dissolution followed by winter carbonate re‐precipitation. Stable isotopes suggest some degree of precipitation seasonality or reduction in winter water availability in the Canadian Arctic during the Paleogene. Clumped (Δ₄₇) temperature estimates range from 52 to 121°C and indicate low temperature solid‐state reordering of micritic samples and diagenetic recrystallization in sparry samples. Average temperatures agree with vitrinite reflectance data for Eureka Sound Group and underlying sediments, highlighting structural complexity across the region. Broadly, combined stable and clumped isotope data from carbonates in complex systems are effective for describing both paleoclimatic and post‐burial conditions. 

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5. A window to the Early Cretaceous North American Climate and Environment: The ‘Lake Carpenter’ lacustrine strata of the Cedar Mountain Formation

   Suarez, M.B.; Al-Suwaidi, A.; Kirkland, J.I.; Snell, K.; Moller, A.; McLean, N.; Suarez, C.A.; Montgomery, E.; Paige, M. (January 2023, The Anatomical record Supplement)

   The Cedar Mountain Formation is thought to span a significant portion of the lower Cretaceous and the base of the upper Cretaceous (Valanginian to Cenomanian). As such, the Cedar Mountain Formation is important for understanding the transition of terrestrial ecosystems from those characterized by pre-angiosperm ecosystems of the Jurassic to the angiosperm-dominated ecosystems that characterized the height of dinosaur diversity in the later part of the Cretaceous. Lacustrine strata offer unique opportunities to shed light on environmental and climate conditions of the past. This study presents results from a multi-proxy study of lacustrine strata in the Cedar Mountain Formation termed “Lake Carpenter.” The sequence of strata is about ~30 m thick and located near Arches National Park. The lower ~7 m is characterized by dark organic-rich mudstones, shales, and tan limestones and dolostone. The middle portion between about 7 and 25m consists of more massive carbonate-rich strata with abundant aquatic fossils including ostracodes, charophytes, and fish scales. The upper portion to about 30 m consists of green to tan mudstones with carbonate nodules and increases in siliciclastic content. Carbonate mineralogies include calcite, high-magnesium calcite, and dolomite (including dolomicrites) based on XRD analyses. To put the lacustrine sequence into stratigraphic context, bulk organic C isotope values were utilized to construct a chemostratigraphic record. The carbon isotope values range from -32.3‰ to -21.1‰ vs. VPDB. Zircons from four suspected volcanic ash layers were analyzed for U-Pb using LA-ICP-MS. One of these produced concordant Cretaceous dates. The youngest zircons from this sample was analyzed using CA-ID-TIMS and produced a date of 115.65 ± 0.18 Ma. Based on the chemostratigraphic record and the U-Pb date, the deposition of the lacustrine sequence occurs in the mid to late Aptian and spans a time that is thought to have coincided with a cold snap based on marine records. Carbonate analyses of the carbonates within the lacustrine sequence ranges from -9.2‰ to +5.4‰ vs. VPDB for carbon and -9.3 to -0.3‰ vs. VPDB for oxygen. Overall, carbonate isotope data is positively covariant and along with the minerology, seems to suggest that the lake was a closed-basin, alkaline lake and would have likely experience significant evaporation. To investigate paleotemperature, selected samples were analyzed for clumped isotope values (47) to determine temperature of formation. Preliminary temperature estimates of calcite formation range from 27°C to 41°C. Estimates for dolomite range from 19°C to 21°C. Lacustrine carbonate formation typically is biased toward spring and summer and as such some of these temperatures (particularly the values for dolomites) seem slightly lower than expected for a greenhouse climate but may be consistent with a “cold-snap” during the late Aptian. Palustrine carbonates from the type section of the Ruby Ranch Member range 19.8°C to 44.5°C (Suarez et al. 2021) and suggests the lacustrine strata records a similar range in temperatures during the Aptian Stage in this part of North America. REFERENCES CITED: Suarez, MB, Knight, J, Snell, KE, Ludvigson, GA, Kirkland, JI, Murphy, L 2020. Multiproxy paleoclimate estimates of the continental Cretaceous Ruby Ranch Member of the Cedar Mountain Formation. In: Bojar, A-V, Pelc., A, Lecuyer, C, editors. Stable Isotopes Studies of Water Cycle and Terrestrial Environments. Geol Soc, London, Spec Pub, 507: https://doi.org/10.1144/SP507-2020-85 KEYWORDS: Early Cretaceous, lacustrine, stable isotopes, paleoclimate Presentation Mode: Invited Speaker 

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