Interpreting and reporting ⁴⁰Ar/³⁹Ar geochronologic data

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

Deformation-related volcanism in the Pacific Ocean linked to the Hawaiian-Emperor bend

Ocean islands, seamounts and volcanic ridges are thought to form above mantle plumes. Yet, this mechanism cannot explain many volcanic features on the Pacific Ocean floor and some might instead be caused by cracks in the oceanic crust linked to the reorganization of plate motions. A distinctive bend in the Hawaiian–Emperor volcanic chain has been linked to changes in the direction of motion of the Pacific Plate, movement of the Hawaiian plume, or a combination of both. However, these links are uncertain because there is no independent record that precisely dates tectonic events that affected the Pacific Plate. Here we analyse the geochemical characteristics of lava samples collected from the Musicians Ridges, lines of volcanic seamounts formed close to the Hawaiian–Emperor bend. We find that the geochemical signature of these lavas is unlike typical ocean island basalts and instead resembles mid-ocean ridge basalts. We infer that the seamounts are unrelated to mantle plume activity and instead formed in an extensional setting, due to deformation of the Pacific Plate. 40Ar/39Ar dating reveals that the Musicians Ridges formed during two time windows that bracket the time of formation of the Hawaiian–Emperor bend, 53–52 and 48–47 million years ago. We conclude that the Hawaiian–Emperor bend was formed by plate–mantle reorganization, potentially triggered by a series of subduction events at the Pacific Plate margins

Trace-element fingerprinting of the 69–36 ka Colli Albani eruptive units: A preliminary dataset for archaeological and tephra studies in central-southern Italy

Trace element compositions determined by ICP-MS have been acquired for six eruptive units from the most recent explosive activity of the Colli Albani Volcanic District, central Italy, documented at the Albano maar by seven eruptive units dated at ~69 ka (Albano 1–3) and ~41 ka through ~36 ka (Albano 4–7). Ratios of selected immobile trace elements (e.g. Zr/Y vs Nb/Y) have been used to build an original reference dataset of 15 samples for discriminating the six main units of the Albano maar eruptive succession, albeit it has to be considered preliminarysincescarcityofoutcrops andincompleteoccurrenceoftheseeruptiveunitshinderedtheacquisition of a larger dataset. In order to test its applicability, we have used it as an integrative tool to identify a weathered tephra layer (CIS-1) which has been here dated by 40Ar/39Ar method at 70 ± 2 ka, previously attributed to an undefined Albano unit, occurring in the stratified Paleolithic site of Cava Muracci. The new 40Ar/39Ar and trace element data, alongside the previous major element composition, indicate that tephra CIS-1 correlates with Albano 3 (~69 ka). The results of this study, besides providing a chronological constraint for a local archaeological site, demonstrate how trace element discrimination diagrams combined with other tephrochronological constraints can provide a useful indication for tephrostratigraphic purposes. Although more reference data are needed totestthefull soundnessofthetrace elementratiosfor fingerprintingindividual Albano units,the results also support this method as a new promising tool for the development and application of the tephrochronology, which would offer the great advantage to be potentially applied even to strongly altered and unsuitable volcanic deposits

MIS 9 to MIS 5 terraces along the Tyrrhenian Sea coast of Latium (central Italy): Assessing interplay between sea-level oscillations and tectonic activity

We present a review of the geomorphology of the Tyrrhenian Sea coast of central Italy integrated by a novel structural-geomorphological study coupled with statistical analysis of topographic culminations and comparison with a Digital Elevation Model, aimed at reconstructing a suite of paleo-surfaces corresponding to remnant portions of marine terraces. We performed geochronological, sedimentological, micromorphological and mineralogical investigations on the deposits forming the different paleo-surfaces between Civitavecchia and Anzio towns, in order to provide chronostratigraphical, paleogeographical and paleoenvironmental constraints. Using the newly achieved dataset we correlate these paleo-surfaces with the coastal terraces formed during past sea-level highstands, as recognized by previous studies, and we refine their correlation with the Marine Isotope Stage (MIS) timescale. In particular, we have extended our geomorphological analyses landward in the area between the Tiber River mouth and Anzio, in order to include the oldest paleo-surface developed above the deposits of the last large explosive eruption at 365 ± 4 ka in the Colli Albani Volcanic District. Results of this study allow us to recognize a set of higher paleo-surfaces at elevation ranging 108 thought 71 m a.s.l., which we interpret as one tectonically displaced, widespread coastal terrace originated during the MIS 9.1 highstand. We correlate the previously identified paleo-surfaces of 66–62 m a.s.l. and 56–52 m a.s.l. with the equivalent coastal terraces developed during the sea-level highstands of sub-stages 7.5 and 7.3/7.1. Moreover, based on data from literature on relative elevation of maximum sea level during the highstands of MIS 11 through MIS 5.1, we assess the regional uplift and the concurrent tectonic displacements that have occurred since 900 ka in this area

The 40K/40Ar and 40Ar/39Ar Methods

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