Reply on RC2

2021

Abstract. Zircon Raman dating based on irradiation damage is a debated concept but not an established geo-/thermochronological method. One issue is the temperature range of radiation-damage annealing over geological timescales. We conducted isochronal and isothermal annealing experiments on radiation-damaged zircons between 500 and 1000 ∘ C for durations between 10 min and 5 d to describe the annealing kinetics. We measured the widths ( Γ ) and positions ( ω ) of the ν1 (SiO 4 ), ν2 (SiO 4 ), and ν3 (SiO 4 ) internal Raman bands, and the external rotation Raman band at ∼974 , 438, 1008, and 356 cm −1 after each annealing step. We fitted a Johnson–Mehl–Avrami–Kolmogorov and a distributed activation energy model to the fractional annealing data, calculated from the widths of the ν2 (SiO 4 ), ν3 (SiO 4 ), and external rotation bands. From the kinetic models, we determined closure temperatures Tc for damage accumulation for each Raman band. Tc ranges from 330 to 370 ∘ C for the intern...

Contributions to Mineralogy and Petrology, 2002

American Journal of Science, 2005

Geochronology and thermochronology on detrital material provides unique constraints on sedimentary provenance, depositional ages, and orogenic evolution of source terrains. In this paper we describe a method and case-studies of measurement of both U/Pb and (U-Th)/He ages on single crystals of zircon that improves the robustness of constraints in each of these areas by establishing both formation and cooling ages of single detrital grains. Typically these ages correspond to crystallization and exhumation or eruption ages, and their combination can be used to more confidently resolve candidate source terrains, establish maximum depositional ages, and constrain the thermal histories of orogenic source regions. U/Pb dating is accomplished by laser-ablation ICP-MS in a small pit on the exterior of the crystal, and He dates are then determined on the bulk grain by conventional laser-heating and dissolution techniques. We present examples from Mesozoic aeolian sandstones, both modern and Paleogene fluvial sediments, and active margin turbidite assemblages from the Cascadia and Kamchatka margins. Important results include the fact that detritus from ancient orogens may dominate sediments thousands of kilometers away, crustal melting and exhumation appear to be spatially-temporally decoupled in at least two orogens, and first-cycle volcanic zircons older than depositional age are surprisingly rare in most settings except in the continental interior. In the case of the Kamchatkan, and possibly Olympic, turbidites, zircon He ages are partially reset. We present a method for estimating the extent of resetting of each grain and the thermal history of the sample, based on coupled (U-Th)/(He-Pb) age patterns among all the grains. introduction Detrital materials in sediments and sedimentary rocks are commonly used to reconstruct timing and rates of past orogenic episodes, constrain models of paleogeography and sediment transport, establish volcanic eruptive histories, and estimate depositional ages. Although detrital studies often focus on petrographic, compositional, or other characteristics of detrital material, it is the geochronology of specific detrital minerals that provides the fundamental interpretive basis for most geologic insights about the temporal evolution of source terrains, as well as age of host units themselves. Zircon is the most commonly dated phase in detrital geochronology because it: a) is resistant to chemical and physical weathering; b) is abundant in most crustal rocks; and c) has relatively high concentrations of U and Th and low common Pb. These features make it highly suitable for geochronology and thermochronology

Geostandards and Geoanalytical Research, 2009

... Urs Klötzli 1,* ,; Eva Klötzli 1 ,; Zekeriya Günes 1 ,; Jan Kosler 2. Article first published online: 18 FEB 2009. ... Ces biais étaient de l'ordre de quelques pourcents sur les âges U-Pb et Pb/Pb et semblaient varier indépendamment de l'âge du zircon ou de sa composition. ...

AGU Fall Meeting …, 2006

In this study we used LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) to determine U-Pb ages of 5 zircon samples of known age ($1800 Ma to $50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double-focusing ICP-MS coupled with a New Wave Research UP-213 laser system. The laser ablation pit sizes ranged from 30 to 40 mm in diameter. Laserinduced time-dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA-ICP-MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP-MS, can be used for the U-Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.

International Journal of Radiation Applications and Instrumentation. Part D. Nuclear Tracks and Radiation Measurements, 1987

, about II0 scientists met at the University of Cambridge, U.K. for the 5th International Specialist Seminar on Thermolumincscence (TL) and Electron-Spin-Resonance (ESR) Dating. These moderate-sized meetings have occurred every 2-3 years and provide an opportunity for investigators of widely varied scientific backgrounds (e.g. physics, chemistry, geology, geography, archaeology) to discuss the intricacies of these two related dating techniques. Together, the two methodologies embrace applications to a wide variety of materials, spanning the age range from a few hundred to perhaps several million years, and have the potential to provide absolute chronologies for a large variety of Earth surface processes which are otherwise undatable. At this most recent TL and ESR seminar, superbly organized by Ann G. Wintle of Cambridge University and located in the glorious King's College, about 70 oral and 60 poster presentations were made: about two-thirds dealing with TL, and one-third with ESR. Paralleling these scientific presentations were displays and expositions of new specialized equipment. For example, commercial automated multi-sample TL readers were exhibited by Daybreak Nuclear and Medical Inc. (Guilford, U.S.A.), Dr Botter-Jensen (Roskilde, Denmark), and Littlemore Engineering Ltd (Oxford, U.K.). An automated single-sample reader was displayed by Vana (Vienna, Austria): and I. K. Bailiff (Durham, U.K.) described an automated TL reader under development. Although most of the scientific presentations were of high quality, because of space restrictions only selected highlights are summarized below (Sections ! and 2 by Berger, Section 3 by lkeya and Section 4 by Singhvi).

Geochemistry, …, 2006

1] In this study we used LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometry) to determine U-Pb ages of 5 zircon samples of known age ($1800 Ma to $50 Ma) in order to determine the reproducibility, precision, and accuracy of this geochronologic technique. This work was performed using a ThermoFinnigan Element2 magnetic sector double-focusing ICP-MS coupled with a New Wave Research UP-213 laser system. The laser ablation pit sizes ranged from 30 to 40 mm in diameter. Laserinduced time-dependent fractionation is corrected by normalizing measured ratios in both standards and samples to the beginning of the analysis using the intercept method. Static fractionation, including those caused during laser ablation and due to instrumental discrimination, is corrected using external zircon standards. Total uncertainty for each laser analysis of an unknown is combined quadratically from the uncertainty in the measured isotope ratios of the unknown and the uncertainty in the fractionation factors calculated from the measurement of standards. For individual analyses we estimate that the accuracy and precision are better than 4% at the 2 sigma level, with the largest contribution in uncertainty from the measurement of the standards. Accuracy of age determinations in this study is on the order of 1% on the basis of comparing the weighted average of the LA-ICP-MS determinations to the TIMS ages. Due to unresolved contributions to uncertainty from the lack of a common Pb correction and from potential matrix effects between standards and unknowns, however, this estimate cannot be universally applied to all unknowns. Nevertheless, the results of this study provide an example of the type of precision and accuracy that may be possible with this technique under ideal conditions. In summary, the laser ablation technique, using a magnetic sector ICP-MS, can be used for the U-Pb dating of zircons with a wide range of ages and is a useful complement to the established TIMS and SHRIMP techniques. This technique is especially well suited to reconnaissance geochronologic and detrital zircon studies.

and sharing with colleagues.

2022

This supporting information contains the sample informations on the Myanmar data used in the main text, and the backgrounds of the modeling approach used for the inset in Figure 5.

The magmatic/igneous and metamorphic zircons were investigated with Raman spectrum microprobe analysis. We found notable differences between these two kinds of zircons exhibited by the variation trend of Raman peak intensity from core to rim of a crystal. In magmatic zircons, the intensity and the ratio (∆ = H/W) of Raman spectrum peaks (H = intensity; W = half-height width) gradually decrease from core to rim of a crystal, which is produced by an increase in metamictization degree and suggests an increase in U and Th concentrations from core to rim. In metamorphic zircons, there are two kinds of crystals according to their Raman spectra: the first group of zircons exhibits a variation trend opposite to those of magmatic zircons, tending to increase in the Raman peak intensity and ∆ value from core to rim of a crystal, which is produced by a decrease in metamictization degree and indicates a decrease of U and Th concentrations from core to rim of a crystal. The second group of zircons exhibits no change in Raman peak intensity and ∆ value through a crystal. The data of infrared and Raman spectra of these crystals show that they are well crystallized and have no lattice destruction induced by metamictization, and are thought to crystallize in high temperature stages of metamorphism. During these stages, the U and Th ions have been removed by metamorphic fluids from the parent rocks of these zircons. The other difference between magmatic and metamorphic zircons is the background level of their Raman spectra, which is high and sloped in magmatic zircons, but low and horizontal in metamorphic zircons. For the former, this may be caused by impurities with fluorescent radiation due to a higher crystallization temperature than the latter's. The differences between magmatic and metamorphic zircons can be used to identify the genesis of zircons and understand the origin and evolution history of their parent rocks.