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1. Interpreting and reporting 40Ar/39Ar geochronologic data

   https://doi.org/10.1130/B35560.1  

   Schaen, Allen J.; Jicha, Brian R.; Hodges, Kip V.; Vermeesch, Pieter; Stelten, Mark E.; Mercer, Cameron M.; Phillips, David; Rivera, Tiffany A.; Jourdan, Fred; Matchan, Erin L.; et al (July 2020, GSA Bulletin)

   null (Ed.)

   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. 

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2. Cenozoic tectono-thermal history of the southern Talkeetna Mountains, Alaska: Insights into a potentially alternating convergent and transform plate margin

   https://doi.org/doi.org/10.1130/GES02008.1  

   Terhune, P.; Benowitz, J.; O'Sullivan, P.; Gillis, R.; Freymuller, J. (January 2019, Geosphere)

   The Mesozoic–Cenozoic convergent margin history of southern Alaska has been dominated by arc magmatism, terrane accretion, strike-slip fault systems, and possible spreading-ridge subduction. We apply 40Ar/39Ar, apatite fission-track (AFT), and apatite (U-Th)/He (AHe) geochronology and thermochronology to plutonic and volcanic rocks in the southern Talkeetna Mountains of Alaska to document regional magmatism, rock cooling, and inferred exhumation patterns as proxies for the region’s deformation history and to better delineate the overall tectonic history of southern Alaska. High-temperature 40Ar/39Ar thermochronology on muscovite, biotite, and K-feldspar from Jurassic granitoids indicates postemplacement (ca. 158–125 Ma) cooling and Paleocene (ca. 61 Ma) thermal resetting. 40Ar/39Ar whole-rock volcanic ages and 45 AFT cooling ages in the southern Talkeetna Mountains are predominantly Paleocene–Eocene, suggesting that the mountain range has a component of paleotopography that formed during an earlier tectonic setting. Miocene AHe cooling ages within ~10 km of the Castle Mountain fault suggest ~2–3 km of vertical displacement and that the Castle Mountain fault also contributed to topographic development in the Talkeetna Mountains, likely in response to the flat-slab subduction of the Yakutat microplate. Paleocene–Eocene volcanic and exhumation-related cooling ages across southern Alaska north of the Border Ranges fault system are similar and show no S-N or W-E progressions, suggesting a broadly synchronous and widespread volcanic and exhumation event that conflicts with the proposed diachronous subduction of an active west-east–sweeping spreading ridge beneath south-central Alaska. To reconcile this, we propose a new model for the Cenozoic tectonic evolution of southern Alaska. We infer that subparallel to the trench slab breakoff initiated at ca. 60 Ma and led to exhumation, and rock cooling synchronously across south-central Alaska, played a primary role in the development of the southern Talkeetna Mountains, and was potentially followed by a period of southern Alaska transform margin tectonics. 

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3. Zircon (U-Th)/He thermochronology of Grand Canyon resolves 1250 Ma unroofing at the Great Unconformity and <20 Ma canyon carving

   https://doi.org/10.1130/G48699.1  

   Thurston, Olivia G.; Guenthner, William R.; Karlstrom, Karl E.; Ricketts, Jason W.; Heizler, Matthew T.; Timmons, J. Michael (November 2021, Geology)

   Abstract Our study used zircon (U-Th)/He (ZHe) thermochronology to resolve cooling events of Precambrian basement below the Great Unconformity surface in the eastern Grand Canyon, United States. We combined new ZHe data with previous thermochronometric results to model the <250 °C thermal history of Precambrian basement over the past >1 Ga. Inverse models of ZHe date-effective uranium (eU) concentration, a relative measure of radiation damage that influences closure temperature, utilize He diffusion and damage annealing and suggest that the main phase of Precambrian cooling to <200 °C was between 1300 and 1250 Ma. This result agrees with mica and potassium feldspar 40Ar/39Ar thermochronology showing rapid post–1400 Ma cooling, and both are consistent with the 1255 Ma depositional age for the Unkar Group. At the young end of the timescale, our data and models are also highly sensitive to late-stage reheating due to burial beneath ∼3–4 km of Phanerozoic strata prior to ca. 60 Ma; models that best match observed date-eU trends show maximum temperatures of 140–160 °C, in agreement with apatite (U-Th)/He and fission-track data. Inverse models also support multi-stage Cenozoic cooling, with post–20 Ma cooling from ∼80 to 20 °C reflecting partial carving of the eastern Grand Canyon, and late rapid cooling indicated by 3–7 Ma ZHe dates over a wide range of high eU. Our ZHe data resolve major basement exhumation below the Great Unconformity during the Mesoproterozoic (1300–1250 Ma), and “young” (20–0 Ma) carving of Grand Canyon, but show little sensitivity to Neoproterozoic and Cambrian basement unroofing components of the composite Great Unconformity. 

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4. Deep-time thermal history of the Great Unconformity in the Grand Canyon, USA: Combined zircon (U-Th)/He and K-feldspar 40Ar/39Ar thermochronometers

   https://doi.org/10.1130/B37358.1  

   Thurston, OG; Guenthner, WR; Karlstrom, KE; Heizler, MT; Ricketts, JW; McDannell, KT (May 2024, Geological Society of America Bulletin)

   Deep-time thermochronology by the zircon (U-Th)/He (ZHe) method is an emerging field of study with promise for constraining Precambrian rock thermal and exhumation histories. The Grand Canyon provides an opportunity to further explore this method because excellent geologic constraints can be integrated with multiple thermochronometers to address important questions about the spatial variability of basement erosion below the sub-Cambrian Great Unconformity composite erosional surface. In this study, we synthesize new ZHe results (n = 26) and published (n = 77) ZHe data with new K-feldspar 40Ar/39Ar data and models (n = 4) from Precambrian basement rocks of the Grand Canyon, USA. We use HeFTy and QTQt thermal history modeling to evaluate the ability of the individual ZHe and K-feldspar 40Ar/39Ar thermochronometric data sets to resolve Precambrian thermal histories and compare those results with jointly modeled data using the QTQt software. We also compare Precambrian basement thermal histories of the eastern and western Grand Canyon, where the eastern Grand Canyon has ∼4 km of Grand Canyon Supergroup strata deposited and preserved, and the western Grand Canyon, where the Supergroup was either never deposited or not preserved. In all locations, models constrained only by ZHe data have limited resolving power for the past ∼600 m.y., compared to models that combine K-feldspar 40Ar/39Ar and ZHe data, which extends the recorded history into the Mesoproterozoic. Our model results suggest that two regional basement unroofing events occurred. A ca. 1350−1250 Ma cooling event is interpreted to record basement exhumation from depths of ∼10 km, and a second cooling episode (∼200−100 °C total) records exhumation from a depth of ∼3 km to 7 km to near-surface conditions between ca. 600 Ma and 500 Ma. Easternmost Grand Canyon models suggest that the preserved maximum ∼4 km thickness of the Grand Canyon Supergroup (with burial heating at ∼100 °C) approximates the total original Mesoproterozoic and Neoproterozoic stratal thickness. Whether these Supergroup rocks were present and then eroded in the western Grand Canyon, as suggested by regional geologic studies, or were never deposited is not constrained by thermochronological data. 

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5. Short communication: Cosmogenic noble gas depletion in soils by wildfire heating

   https://doi.org/10.5194/gchron-6-71-2024  

   Balco, Greg; Hidy, Alan J; Struble, William T; Roering, Joshua J (January 2024, Geochronology)

   Measurements of cosmic-ray-produced beryllium-10, neon-21, and helium-3 in quartz in a soil profile from a forested landscape in the Oregon Coast Range show that the cosmogenic noble gases 21Ne and 3He are depleted relative to 10Be in the shallow subsurface. The noble gases are mobile in mineral grains via thermally activated diffusion and 10Be is not, implying that noble gas depletion is the result of surface heating by wildfires and subsequent mixing of partially degassed quartz downward into the soil. Cosmogenic noble gas depletion by wildfire heating of soils is a potential means of estimating wildfire intensity and/or frequency over pre-observational timescales. 

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