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

The Makhonjwa Mountains, traditionally referred to as the Barberton Greenstone Belt, retain an iconic Paleoarchean archive against which numerical models of early earth geodynamics can be tested. We present new geologic and structural maps, geochemical plots, geo- and thermo-chronology, and geophysical data from seven silicic, mafic to ultramafic complexes separated by major shear systems across the southern Makhonjwa Mountains. All reveal signs of modern oceanic back-arc crust and subduction-related processes. We compare the rates of processes determined from this data and balance these against plate tectonic and plume related models. Robust rates of both horizontal and vertical tectonic processes derived from the Makhonjwa Mountain complexes are similar, well within an order of magnitude, to those encountered across modern oceanic and orogenic terrains flanking Western Pacific-like subduction zones. We conclude that plate tectonics and linked plate-boundary processes were well established by 3.2–3.6 Ga. Our work provides new constraints for modellers with rates of a ‘basket’ of processes against which to test Paleoarchean geodynamic models over a time period close to the length of the Phanerozoic.

Paleoarchean bedrock lithologies across the Makhonjwa Mountains of South Africa and Swaziland linked to geochemical, magnetic and tectonic data reveal early plate tectonic genes flanking subduction margins

Radiometric Constraints on the Timing, Tempo, and Effects of Large Igneous Province Emplacement

Zircon is the preeminent chronometer of deep time on Earth, informing models of crustal growth and providing our only direct window into the Hadean Eon. However, the quantity of zircon crystallised per unit mass of magma is highly variable, complicating interpretation of the terrestrial zircon record. Here we combine zircon saturation simulations with a dataset of ∼52,000 igneous whole rock geochemical analyses to quantify secular variation in relative zircon abundance throughout Earth history. We find dramatically increasing zircon abundance per mass of magma through geologic time, suggesting that the zircon record underestimates past crustal volume even if preservation bias is eliminated. Similarly, zircons were even less likely to crystallise from low-silica magmas in early Earth history than they are today, together suggesting that the observed Hadean zircon record may require a larger volume of generally felsic Hadean crust than previously expected.

https://eartharxiv.org/repository/object/1196/download/2714/

Temporal variation in relative zircon abundance throughout Earth history

Geochronology can resolve dispersed zircon dates in plutonic rocks when magma cooling time scales exceed the temporal precision of individual U-Pb analyses; such age heterogeneity may indicate protracted crystallization between the temperatures of zircon saturation (Tsat) and rock solidification (Tsolid). Diffusive growth models predict asymmetric distributions of zircon dates and crystallization temperatures in a cooling magma, with volumetrically abundant old, hot crystallization at Tsat decreasing continuously to volumetrically minor young, cold crystallization at Tsolid. We present integrated geochronological and geochemical data from Bergell Intrusion tonalites (Central Alps, Europe) that document zircon compositional change over hundreds of thousands of years at the hand-sample scale, indicating melt compositional evolution during solidification. Ti-in-zircon thermometry, crystallization simulation using MELTS software, and U-Pb dates produce zircon mass-temperature-time distributions that are in excellent agreement with zircon growth models. These findings provide the first quantitative validation of longstanding expectations from zircon saturation theory by direct geochronological investigation, underscoring zircon’s capacity to quantify supersolidus cooling rates in magmas and resolve dynamic differentiation histories in the plutonic rock record. INTRODUCTION Zircon is an inimitable chronicler of the temporal, thermal, and compositional evolution of many crustal magmatic systems. U-Pb zircon geochronology by chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS; Mattinson, 2005) is the most robust method available for constraining the tempo of pluton assembly and the longevity of magma reservoirs in deep time, having been applied to discern the incremental nature of composite intrusive suites (Coleman et al., 2004). As a result of steadily improved analytical precision, CA-ID-TIMS can resolve heterogeneous U-Pb zircon dates, or zircon spectra, in many igneous rocks at the hand-sample scale (Miller et al., 2007; Schoene et al., 2012; Broderick et al., 2015). Such studies demonstrate that zircon age distributions can reflect protracted zircon crystallization time scales, supplanting convention that zircon retains a singular emplacement age of its host rock (e.g., see discussion in Samperton et al., 2015). Therefore, zircon age information combined with complementary petrologic data from zircon and other phases can be used to constrain the time scales of magma emplacement, calculate supersolidus cooling rates, and track magma compositional evolution. An important step in using zircon to further quantify magmatic processes is to compare zircon geochronology of natural systems with theoretical and experimental models for zircon saturation and growth (Watson, 1996; Harrison et al., 2007; Ferry and Watson, 2007; Boehnke et al., 2013; Bindeman and Melnik, 2016). Ab initio diffusive saturation calculations predict noninstantaneous, nonlinear volumetric zircon growth during monotonic magma cooling, with an asymmetric distribution of zircon mass crystallized as a function of time (t) or temperature (T) (Watson, 1996). This model predicts high initial crystallization rates at the system’s zircon saturation temperature (Tsat) and continuous, near-exponentially decreasing growth to the solidus temperature (Tsolid). While the treatment of Watson (1996) has been employed to describe zircon dissolution during thermal rejuvenation (Frazer et al., 2014), no attempt has been made to quantitatively compare theoretical expectations with measured zircon age spectra. If a zircon population is the product of crystallization in a cooling, closed system, then the following criteria may be observed: (1) resolvable dispersion in U-Pb zircon dates, reflecting protracted crystallization time scales; (2) trends in zircon composition through time, reflecting evolving melt composition during fractional crystallization; (3) a systematic decrease in Ti-in-zircon crystallization temperature with time, reflecting magma undercooling (Ferry and Watson, 2007); and (4) asymmetric distributions of zircon mass crystallized as functions of time and temperature (Watson, 1996). While such features have been previously documented individually (e.g., Ickert et al., 2011; Tierney et al., 2016), no prior study has demonstrated these criteria simultaneously. As such, the geochronological, experimental, and modeling perspectives of zircon crystallization have yet to be concisely unified, and our ability to link U-Pb geochronology, petrology, and numerical modeling in magmatic systems thus remains limited. Here we address this shortcoming through geochronological, geochemical, and thermometric characterization of zircon from mid-crustal granitoids. PREVIOUS GEOCHRONOLOGY AND HYPOTHESIS We use the Bergell Intrusion, Central Alps, Europe (Fig. 1), as an excellent locale to test the four criteria presented here. Thermal insulation of *Current address: Nuclear and Chemical Sciences Division, Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, California 94550, USA; E-mail: samperton1@llnl.gov. †Current address: Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA. GEOLOGY, November 2017; v. 45; no. 11; p. 983–986 | Data Repository item 2017333 | doi:10.1130/G38645.1 | Published online 23 August 2017 © 2017 eological Society of A erica. For permission to copy, contact editing@geosociety.org. Insubric line Enga dine line tona lite gran odio rite hybr id. San Martino

Zircon age-temperature-compositional spectra in plutonic rocks

One model for rhyolite generation in the upper crust is via extraction of interstitial melt from crystal-rich magma reservoirs. Although silicic magma reservoirs may grow incrementally over c. 10–10 yr timescales, they can be remobilized prior to eruption much more rapidly (c. 10–10 yr). This process implies the formation of cumulate residues with a composition complementary to the extracted melt, but the predicted cumulates have so far eluded widespread identification. The 7 2– 6 2 Ma Risco Bayo–Huemul plutonic complex comprises 150 km that is subdivided into at least seven gabbroic to granitic domains emplaced at <7 km depth. Distinct Ti, Zr, Nb, and rare earth element variations in amphibole document pulsed emplacement of the gabbroic to granodioritic Risco Bayo pluton 800 kyr prior to the adjacent quartz monzonite to high-silica granite of the Huemul pluton. The quartz monzonite is inferred to be a silicic cumulate on the basis of whole-rock mass balance and enrichments in Ba and Zr concentrations. Here, we combine fine-scale textural analysis using energy-dispersive X-ray spectrometry (EDS) phase mapping with in situ mineral compositions to explore the silicic cumulate hypothesis. Quartz monzonite textures are porphyritic and comprise 58–64 modal % of partially interlocking, 2–5 mm long euhedral plagioclase, together with euhedral biotite, orthoclase, and amphibole. The finer-grained interstitial matrix is composed of anhedral orthoclase, plagioclase, and quartz. Calculations using the compositions of the interstitial phases suggest that the matrix represents a highly evolved melt similar in major and trace element chemistry to coeval, high-silica granite inferred to be extracted and frozen rhyolite. The high-silica granites are equigranular and contain dense concentrations of miarolitic cavities implying, together with Al-in-Hbl barometry for the quartz monzonite, emplacement and volatile saturation within the upper crust at the granite minimum. Plagioclase, orthoclase, and biotite in high-silica granite are depleted in Ba, Sr, and Eu, similar to their bulk-rock compositions, and support an origin as highly fractionated products of melt extraction. Miarolitic cavities within the high-silica granite overlying the granite domain suggest that volatiles played an important role in the upward percolation of melt. Our observations indicating transport of volatiles and melt through an underlying crystal mush, including accumulation of vapor bubbles at the roof of the magma VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 1991 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 10, 1991–2016 doi: 10.1093/petrology/egy087 Advance Access Publication Date: 7 September 2018

Textural and Mineralogical Record of Low-pressure Melt Extraction and Silicic Cumulate Formation in the Late Miocene Risco Bayo–Huemul Plutonic Complex, Southern Andes

Blair Schoene

Papers

Paleoarchean bedrock lithologies across the Makhonjwa Mountains of South Africa and Swaziland linked to geochemical, magnetic and tectonic data reveal early plate tectonic genes flanking subduction margins

Radiometric Constraints on the Timing, Tempo, and Effects of Large Igneous Province Emplacement

Temporal variation in relative zircon abundance throughout Earth history

Zircon age-temperature-compositional spectra in plutonic rocks

Textural and Mineralogical Record of Low-pressure Melt Extraction and Silicic Cumulate Formation in the Late Miocene Risco Bayo–Huemul Plutonic Complex, Southern Andes