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1. Initial 40 Ar‐ 39 Ar Ages of the Paleocene‐Eocene Boundary Impact Spherules

   https://doi.org/10.1029/2019GL082473  

   Schaller, Morgan F. ; Turrin, Brent D. ; Fung, Megan K. ; Katz, Miriam E. ; Swisher, Carl C. ( August 2019 , Geophysical Research Letters)

   We report⁴⁰Ar‐³⁹Ar step‐heating ages of Paleocene‐Eocene (P‐E) boundary impact spherules from Atlantic Margin coastal plain and open ocean sites. We test the hypothesis that the P‐E spherules are reworked from an earlier event (e.g., K‐Pg impact at ~66 Ma), which predicts a cooling age discordant from their depositional age of 55.93 ± 0.05 Ma at the P‐E boundary. Isochrons from the step‐heating analysis yield⁴⁰Ar‐³⁶Ar intercepts in excess of the modern in most cases, indicating that the spherules have excess radiogenic Ar (⁴⁰Ar*), typical of impact glasses incompletely degassed before solidification. The weighted mean of the isochron‐corrected plateau age is 54.2 ± 2.5 Ma (1σ), and their isochron age is 55.4 ± 4.0 Ma, both indistinguishable from their P‐E depositional age, not supporting the K‐Pg reworking hypothesis. This is consistent with all other stratigraphic and geochemical evidence for an impact at the P‐E boundary and ejecta distribution by air fall.

    

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2. Preservation of ancient impact ages on the R chondrite parent body: 40 Ar/ 39 Ar age of hornblende‐bearing R chondrite LAP 04840

   https://doi.org/10.1111/maps.12692  

   Righter, K. ; Cosca, M. A. ; Morgan, L. E. ( August 2016 , Meteoritics & Planetary Science)

   The hornblende‐ and biotite‐bearing R chondriteLAP04840 is a rare kind of meteorite possibly containing outer solar system water stored during metamorphism or postshock annealing deep within an asteroid. Because little is known regarding its age and origin, we determined⁴⁰Ar/³⁹Ar ages on hornblende‐rich separates of the meteorite, and obtained plateau ages of 4340(±40) to 4380(±30) Ma. These well‐defined plateau ages, coupled with evidence for postshock annealing, indicate this meteorite records an ancient shock event and subsequent annealing. The age of 4340–4380 Ma (or 4.34–4.38 Ga) for this and other previously dated R chondrites is much older than most impact events recorded by ordinary chondrites and points to an ancient event or events that predated the late heavy bombardment that is recorded in so many meteorites and lunar samples.

    

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3. Modelling white mica pressure‐temperature‐time (P‐T‐t) paths using thermobarometric and 40 Ar/ 39 Ar thermochronologic data

   https://doi.org/10.1111/ter.12381  

   Gonzalez, Joseph P. ; Baldwin, Suzanne L. ( March 2019 , Terra Nova)

   White micas are major rock forming minerals in igneous and metamorphic rocks, and their chemical and isotopic variations can be used to determine pressure, temperature and time (P‐T‐t) histories. We apply⁴⁰Ar/³⁹Ar multi‐diffusion domain modelling to white micas from blueschists blocks in serpentinite matrix mélange from the exhumed Baja California subduction complex. Thermal history models yielded T‐t paths suggesting that⁴⁰Ar* resides within multiple diffusion domains with varying⁴⁰Ar* retentivity. Modelled white mica thermal histories and thermobarometric data were used to forward model continuous P‐T‐t paths. P‐T‐t paths are consistent with previous studies and are interpreted to constrain blueschist block exhumation paths within the Baja accretionary wedge. Our P‐T‐t models use temperature controlled⁴⁰Ar/³⁹Ar step heat data in which argon loss by volume diffusion can be demonstrated, and for which the white mica petrogenesis is known.

    

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4. No Cretaceous‐Paleogene Boundary in Exposed Rajahmundry Traps: A Refined Chronology of the Longest Deccan Lava Flows From 40 Ar/ 39 Ar Dates, Magnetostratigraphy, and Biostratigraphy

   https://doi.org/10.1029/2020GC009149  

   Fendley, Isabel M. ; Sprain, Courtney J. ; Renne, Paul R. ; Arenillas, Ignacio ; Arz, José A. ; Gilabert, Vicente ; Self, Stephen ; Vanderkluysen, Loÿc ; Pande, Kanchan ; Smit, Jan ; et al ( September 2020 , Geochemistry, Geophysics, Geosystems)

   Deccan Traps flood basalt volcanism affected ecosystems spanning the end‐Cretaceous mass extinction, with the most significant environmental effects hypothesized to be a consequence of the largest eruptions. The Rajahmundry Traps are the farthest exposures (~1,000 km) of Deccan basalt from the putative eruptive centers in the Western Ghats and hence represent some of the largest volume Deccan eruptions. Although the three subaerial Rajahmundry lava flows have been geochemically correlated to the Wai Subgroup of the Deccan Traps, poor precision associated with previous radioisotopic age constraints has prevented detailed comparison with potential climate effects. In this study, we use new⁴⁰Ar/³⁹Ar dates, paleomagnetic and volcanological analyses, and biostratigraphic constraints for the Rajahmundry lava flows to ascertain the timing and style of their emplacement. We find that the lower and middle flows (65.92 ± 0.25 and 65.67 ± 0.08 Ma, ±1σsystematic uncertainty) were erupted within magnetochron C29r and were a part of the Ambenali Formation of the Deccan Traps. By contrast, the uppermost flow (65.27 ± 0.08 Ma) was erupted in C29n as part of the Mahabaleshwar Formation. Given these age constraints, the Rajahmundry flows were not involved in the end‐Cretaceous extinction as previously hypothesized. To determine whether the emplacement of the Rajahmundry flows could have affected global climate, we estimated their eruptive CO₂release and corresponding climate change using scalings from the LOSCAR carbon cycle model. We find that the eruptive gas emissions of these flows were insufficient to directly cause multi‐degree warming; hence, a causal relationship with significant climate warming requires additional Earth system feedbacks.

    

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5. The Ocean's Global 39 Ar Distribution Estimated With an Ocean Circulation Inverse Model

   https://doi.org/10.1029/2019GL082663  

   Holzer, Mark ; DeVries, Timothy ; Smethie, Jr., William ( July 2019 , Geophysical Research Letters)

   ³⁹Ar with its 269‐year half‐life has great potential for constraining ocean ventilation and transport. Here we estimate the distribution of³⁹Ar using a steady ocean circulation inverse model. Our estimates match available³⁹Ar measurements to within an absolute error of ∼9% modern argon without major biases. We find that³⁹Ar traces out the world ocean's ventilation pathways and that the³⁹Ar age Γ^Arand the ideal mean age have broadly similar large‐scale patterns. At the surface,³⁹Ar is close to saturated except at high latitudes. Undersaturation imparts a finite³⁹Ar age to surface waters relative to the atmosphere, with peak values exceeding 100 years in Antarctic waters. This reservoir age is propagated into the interior with Antarctic Bottom Water, elevating Γ^Arby ∼50 years in the deep Pacific and Indian oceans. Our estimates identify the large‐scale gradients and uncertainty patterns of³⁹Ar, thus providing guidance for future measurements.

    

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