Quantifying long-term reproducibility of zircon reference materials by U-Pb LA-ICP-MS dating

This study was supported by ETH Research Grant ETH‐34 15‐2. Open access funding provided by Eidgenossische Technische Hochschule Zurich.Inter-session excess variance of U/Pb and Pb/Pb ratios in LA-ICP-MS zircon dating is the largest contributor to systematic errors, which in turn limit the accuracy of age determinations. Quantifying long-term excess variance of reference materials allows for the estimation of excess variance in samples, but such compilations are not available in the literature. Here, we present the results of over 100 measurement sessions over 6 years for seven common zircon reference materials and calculate a characteristic excess variance for each. For 206Pb/238U ages, these values (2s) are as follows: AusZ7-1 = 1.7% (115 sessions), AusZ7-5 = 0% (74), OD-3 = 0.6% (19), Temora2 = 1.2% (86), Plešovice = 1.1% (100), 91500 = 1.0% (146) and Mud Tank = 3.0% (12). For 207Pb/206Pb ages, smaller excess variances are observed: Temora2 = 0.3%, Plešovice = 0.4%, Mud Tank = 1.7% and 91500 = 0.4%. These values are well-constrained estimates of inter-session excess variance for the ETH Zürich LA-ICP-MS laboratory using a sector-field ICP-MS, and may provide either a first-order estimate for other laboratories or a value to compare against their own reference material compilations.Publisher PDFPeer reviewe

Quantifying Long-Term Reproducibility of Zircon Reference Materials by U-Pb LA-ICP-MS Dating

Inter-session excess variance of U/Pb and Pb/Pb ratios in LA-ICP-MS zircon dating is the largest contributor to systematic errors, which in turn limit the accuracy of age determinations. Quantifying long-term excess variance of reference materials allows for the estimation of excess variance in samples, but such compilations are not available in the literature. Here, we present the results of over 100 measurement sessions over 6 years for seven common zircon reference materials and calculate a characteristic excess variance for each. For Pb-206/U-238 ages, these values (2s) are as follows: AusZ7-1 = 1.7% (115 sessions), AusZ7-5 = 0% (74), OD-3 = 0.6% (19), Temora2 = 1.2% (86), Plesovice = 1.1% (100), 91500 = 1.0% (146) and Mud Tank = 3.0% (12). For Pb-207/Pb-206 ages, smaller excess variances are observed: Temora2 = 0.3%, Plesovice = 0.4%, Mud Tank = 1.7% and 91500 = 0.4%. These values are well-constrained estimates of inter-session excess variance for the ETH Zurich LA-ICP-MS laboratory using a sector-field ICP-MS, and may provide either a first-order estimate for other laboratories or a value to compare against their own reference material compilations.ISSN:1639-4488ISSN:1751-908

U-Th Zircon Dating by Laser Ablation Single Collector Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS)

Zircon crystals in the age range of ca. 10–300 ka can be dated by 230Th/238U (U-Th) disequilibrium methods because of the strong fractionation between Th and U during crystallisation of zircon from melts. Laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of nine commonly used reference zircons (at secular equilibrium) and a synthetic zircon indicates that corrections for abundance sensitivity and dizirconium trioxide molecular ions (Zr2O3 +) are critical for reliable determination of 230Th abundances in zircon. When corrected for abundance sensitivity and interferences, mean activity ratios of (230Th)/(238U) for nine reference zircons analysed on five different days averaged 0.995 ± 0.023 (95% confidence weighted by data-point uncertainty only, MSWD = 1.6; n = 9), consistent with their U-Pb ages &gt; 4 Ma that imply equilibrium for all intermediate daughter isotopes (including 230Th) within the 238U decay chain. U-Th zircon ages generated by LA-ICP-MS without mitigating (e.g., by high mass resolution) or correcting for abundance sensitivity and molecular interferences on 230Th are potentially unreliable. To validate the applicability of LA-ICP-MS to this dating method, we acquired data from three late Quaternary volcanic units: the 41 ka Campanian Ignimbrite (plutonic clasts), the 161 ka Kos Plateau Tuff (juvenile clasts) and the 12 ka Puy de Dôme trachyte lava (all eruption ages by Ar/Ar, with zircon U-Th ages being of equal or slightly older). A comparison of the corrected LA-ICP-MS results with previously published secondary ion mass spectrometry (SIMS) data for these rocks shows comparable ages with equivalent precision for LA-ICP-MS and SIMS, but much shorter analysis durations (~ 2 min vs. ~ 15 min) per spot with LA-ICP-MS and much simpler sample preparation. Previously undated zircons from the Yali eruption (Kos-Nisyros volcanic centre, Greece) were analysed using this method. This yielded a large age spread (~ 45 to &gt; 300 ka), suggesting significant antecryst recycling. The youngest zircon age (~ 45 ± 10 ka) provides a reasonable maximum estimate for the eruption age, in agreement with the previously published age using oxygen isotope stratigraphy (~ 31 ka).</p

The origin of placental mammal life histories

Funding was provided by the University of Edinburgh, the Royal Society (grant NIF\R1\191527), National Science Foundation (grants DEB 1654949 and EAR 1654952), European Research Council (ERC) starting grants (nos. 756226 and 805246) under the European Union’s Horizon 2020 Research and Innovation Programme, a Philip Leverhulme Prize and a SNSF Mobility Fellowship (grant P2EZP2_199923).After the end-Cretaceous extinction, placental mammals quickly diversified , occupied key ecological niches and increased in size , but this last was not true of other therians. The uniquely extended gestation of placental young may have factored into their success and size increase, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30-75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals.PostprintPeer reviewe

The origin of placental mammal life histories

Funding was provided by the University of Edinburgh, the Royal Society (grant NIF\R1\191527), National Science Foundation (grants DEB 1654949 and EAR 1654952), European Research Council (ERC) starting grants (nos. 756226 and 805246) under the European Union’s Horizon 2020 Research and Innovation Programme, a Philip Leverhulme Prize and a SNSF Mobility Fellowship (grant P2EZP2_199923).After the end-Cretaceous extinction, placental mammals quickly diversified , occupied key ecological niches and increased in size , but this last was not true of other therians. The uniquely extended gestation of placental young may have factored into their success and size increase, but reproduction style in early placentals remains unknown. Here we present the earliest record of a placental life history using palaeohistology and geochemistry, in a 62 million-year-old pantodont, the clade including the first mammals to achieve truly large body sizes. We extend the application of dental trace element mapping by 60 million years, identifying chemical markers of birth and weaning, and calibrate these to a daily record of growth in the dentition. A long gestation (approximately 7 months), rapid dental development and short suckling interval (approximately 30-75 days) show that Pantolambda bathmodon was highly precocial, unlike non-placental mammals and known Mesozoic precursors. These results demonstrate that P. bathmodon reproduced like a placental and lived at a fast pace for its body size. Assuming that P. bathmodon reflects close placental relatives, our findings suggest that the ability to produce well-developed, precocial young was established early in placental evolution, and that larger neonate sizes were a possible mechanism for rapid size increase in early placentals.PostprintPeer reviewe

Decadal-to-centennial increases of volcanic aerosols from Iceland challenge the concept of a Medieval Quiet Period

Existing global volcanic radiative aerosol forcing estimates portray the period 700 to 1000 as volcanically quiescent, void of major volcanic eruptions. However, this disagrees with proximal Icelandic geological records and regional Greenland ice-core records of sulfate. Here, we use cryptotephra analyses, high-resolution sulfur isotope analyses, and glaciochemical volcanic tracers on an array of Greenland ice cores to characterise volcanic activity and climatically important sulfuric aerosols across the period 700 to 1000. We identify a prolonged episode of volcanic sulfur dioxide emissions (751–940) dominated by Icelandic volcanism, that we term the Icelandic Active Period. This period commences with the Hrafnkatla episode (751–763), which coincided with strong winter cooling anomalies across Europe. This study reveals an important contribution of prolonged volcanic sulfate emissions to the pre-industrial atmospheric aerosol burden, currently not considered in existing forcing estimates, and highlights the need for further research to disentangle their associated climate feedbacks

Evidence for a spike in mantle carbon outgassing during the Ediacaran period

© 2017 The Author(s). Long-term cycles in Earth\u27s climate are thought to be primarily controlled by changes in atmospheric CO2 concentrations. Changes in carbon emissions from volcanic activity can create an imbalance in the carbon cycle. Large-scale changes in volcanic activity have been inferred from proxies such as the age abundance of detrital zircons, but the magnitude of carbon emissions depends on the style of volcanism as well as the amount. Here we analyse U-Pb age and trace element data of detrital zircons from Antarctica and compare the results with the global rock record. We identify a spike in CO2-rich carbonatite and alkaline magmatism during the Ediacaran period. Before the Ediacaran, secular cooling of the mantle and the advent of cooler subduction regimes promoted the sequestration of carbon derived from decarbonation of subducting oceanic slabs in the mantle. We infer that subsequent magmatism led to the extensive release of carbon that may at least in part be recorded in the Shuram-Wonoka carbon isotope excursion. We therefore suggest that this pulse of alkaline volcanism reflects a profound reorganization of the Neoproterozoic deep and surface carbon cycles and promoted planetary warming before the Cambrian radiation

Zircon geochronology and geochemistry to constrain the youngest eruption events and magma evolution of the Mid-Miocene ignimbrite flare-up in the Pannonian Basin, eastern central Europe

A silicic ignimbrite flare-up episode occurred in the Pannonian Basin during the Miocene, coeval with the syn-extensional period in the region. It produced important correlation horizons in the regional stratigraphy; however, they lacked precise and accurate geochronology. Here, we used U–Pb (LA-ICP-MS and ID-TIMS) and (U–Th)/He dating of zircons to determine the eruption ages of the youngest stage of this volcanic activity and constrain the longevity of the magma storage in crustal reservoirs. Reliability of the U–Pb data is supported by (U–Th)/He zircon dating and magnetostratigraphic constraints. We distinguish four eruptive phases from 15.9 ± 0.3 to 14.1 ± 0.3 Ma, each of which possibly includes multiple eruptive events. Among these, at least two large volume eruptions (&gt;10 km3) occurred at 14.8 ± 0.3 Ma (Demjén ignimbrite) and 14.1 ± 0.3 Ma (Harsány ignimbrite). The in situ U–Pb zircon dating shows wide age ranges (up to 700 kyr) in most of the crystal-poor pyroclastic units, containing few to no xenocrysts, which implies efficient recycling of antecrysts. We propose that long-lived silicic magma reservoirs, mostly kept as high-crystallinity mushes, have existed in the Pannonian Basin during the 16–14 Ma period. Small but significant differences in zircon, bulk rock and glass shard composition among units suggest the presence of spatially separated reservoirs, sometimes existing contemporaneously. Our results also better constrain the time frame of the main tectonic events that occurred in the Northern Pannonian Basin: We refined the upper temporal boundary (15 Ma) of the youngest counterclockwise block rotation and the beginning of a new deformation phase, which structurally characterized the onset of the youngest volcanic and sedimentary phase