Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis

An Astronomically Tuned Neogene Time Scale (ATNTS2012) is presented, as an update of ATNTS2004 in GTS2004. The new scale is not fundamentally different from its predecessor and the numerical ages are identical or almost so. Astronomical tuning has in principle the potential of generating a stable Neogene time scale as a function of the accuracy of the La2004 astronomical solution used for both scales. Minor problems remain in the tuning of the Lower Miocene. In GTS2012 we will summarize what has been modified or added since the publication of ATNTS2004 for incorporation in its successor, ATNTS2012. Mammal biostratigraphy and its chronology are elaborated, and the regional Neogene stages of the Paratethys and New Zealand are briefy discussed. To keep changes to ATNTS2004 transparent we maintain its subdivision into headings as much as possible.

The Neogene Period

This paper reviews the basic principles of radiometric geochronology as implemented in a new software package called IsoplotR, which was designed to be free, flexible and future-proof. IsoplotR is free because it is written in non-proprietary languages (R, Javascript and HTML) and is released under the GPL license. The program is flexible because its graphical user interface (GUI) is separated from the command line functionality, and because its code is completely open for inspection and modification. To increase future-proofness, the software is built on free and platform-independent foundations that adhere to international standards, have existed for several decades, and continue to grow in popularity. IsoplotR currently includes functions for U-Pb, Pb-Pb, 40 Ar/ 39 Ar, Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, U-Th-He, fission track and U-series disequilibrium dating. It implements isochron regression in two and three dimensions, visualises multi-aliquot datasets as cumulative age distributions, kernel density estimates and radial plots, and calculates weighted mean ages using a modified Chauvenet outlier detection criterion that accounts for the analytical uncertainties in heteroscedastic datasets. Overdispersion of geochronological data with respect to these analytical uncertainties can be attributed to either a proportional underestimation of the analytical uncertainties, or to an additive geological scatter term. IsoplotR keeps track of error correlations of the isotopic ratio measurements within aliquots of the same samples. It uses a statistical framework that will allow it to handle error correlations between aliquots in the future. Other ongoing developments include the implementation of alternative user interfaces and the integration of IsoplotR with other data reduction software.

IsoplotR: A free and open toolbox for geochronology

Calibration of the geological time scale is achieved by independent radioisotopic and astronomical dating, but these techniques yield discrepancies of ∼1.0% or more, limiting our ability to reconstruct Earth history. To overcome this fundamental setback, we compared astronomical and 40 Ar/ 39 Ar ages of tephras in marine deposits in Morocco to calibrate the age of Fish Canyon sanidine, the most widely used standard in 40 Ar/ 39 Ar geochronology. This calibration results in a more precise older age of 28.201 ± 0.046 million years ago (Ma) and reduces the 40 Ar/ 39 Ar method9s absolute uncertainty from ∼2.5 to 0.25%. In addition, this calibration provides tight constraints for the astronomical tuning of pre-Neogene successions, resulting in a mutually consistent age of ∼65.95 Ma for the Cretaceous/Tertiary boundary.

https://science.sciencemag.org/content/sci/320/5875/500.full.pdf

Synchronizing Rock Clocks of Earth History

Age constraint on Burmese amber based on U–Pb dating of zircons

Eruption history of the Columbia River Basalt Group constrained by high-precision U-Pb and 40Ar/39Ar geochronology

Late Toarcian continental palaeoenvironmental conditions: An example from the Cañadón Asfalto Formation in southern Argentina

Production vs. preservation: a geochronologically-constrained test of the monterey hypothesis for miocene climate change

The thermal structure and compositional structure of arcs influence magmatic differentiation and lower-crustal foundering, two key processes impacting the evolution of the continental crust. Although many studies have proposed time scales of lithospheric recycling based on convective downwelling calculations, these models depend on the composition, density (𝜌), and thermal structure of the lower crust and mantle, which are difficult to quantify in active continental arcs. Here, we constrained these properties for the Andean Northern Volcanic Zone using direct petrologic observations from a unique suite of lower-crust and mantle xenoliths from Mercaderes, Colombia. Chemical abrasion−isotope dilution−thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb dates for zircons within the host tuff indicate the xenoliths erupted no earlier than 238 (±19) ka and thus capture a recent snapshot of the arc and subarc mantle. Equilibrium pressure-temperature (P-T) estimates for 81 xenoliths define three distinct thermal domains, interpreted as (1) a steep conductive geothermal gradient in the lower arc crust; (2) a convecting mantle wedge; and (3) cooled mantle in proximity to the subducting slab. Our results indicate the presence of an ∼10−14-km-thick, high-density lithospheric root that is ∼0.1 g/cm3 denser than the underlying mantle. Unlike records from exhumed paleo-arcs, Rayleigh-Taylor instability calculations using our P-T-𝜌 constraints are unrealistically short for the northern Andes. We suggest the presence of partial melts in this hot arc root as a potential source of buoyancy preventing or significantly slowing down foundering.

/pdf/to-sink-or-not-to-sink-the-thermal-and-density-structure-of-caegsyr8.pdf

To sink or not to sink: The thermal and density structure of the modern northern Andean arc constrained by xenolith petrology

(1) Lausanne, Institute of Earth Sciences, ISTE, Lausanne, Switzerland (thierry.adatte@unil.ch), (2) IDL-FCUL (Instituto Dom Luís, Faculdade de Ciências da Universidade de Lisboa), Campo Grande, Edifício C1, Piso 1, 1749-016 Lisbon, Portugal , (3) Department of Earth Sciences, Faculty of Science, University of Yaounde 1, P.O.Box 812 Yaounde Cameroon, (4) Department of Petroleum and Gas Exploration, Institute of Mines and Petroleum Industries, University of Maroua, P.O.Box 08 Kaele, Cameroon , (5) Department of Geosciences, Princeton University, Princeton, NJ 08544, USA, (6) Department of Geology, Amravati University, Amravati, India

Blair Schoene

Papers

Eruption history of the Columbia River Basalt Group constrained by high-precision U-Pb and 40Ar/39Ar geochronology

Late Toarcian continental palaeoenvironmental conditions: An example from the Cañadón Asfalto Formation in southern Argentina

Production vs. preservation: a geochronologically-constrained test of the monterey hypothesis for miocene climate change

To sink or not to sink: The thermal and density structure of the modern northern Andean arc constrained by xenolith petrology

Timing and tempo of Deccan volcanism: evidence from mercury anomalies