Canadian Journal of Earth Sciences

The new mass spectrometer is equipped with a highly sensitive and linear Daly ion counting system and high-resistance 10E12 Ohm Faraday cups for low-level multicollection of small Pb and U ion beams. It is operational since September and delivers U-Pb dates of 0. Our precision and reproducibility of zircon U-Pb dating using chemical-abrasion, isotope-dilution, thermal ionisation mass spectrometry. For most recent developments see Schaltegger et al. The intercalibration is regularly checked every 12 months, and continues to yield the same perfect coincidence since the Triton TIMS has been upgraded with 1E13 Ohm resistance amplifiers in These results have been presented at Goldschmidt Conference in Boston Schaltegger, and are subject to a scientific publication in an international journal. Davies J. Goldschmidt Conference , Prague, Schaltegger U.

Reference materials

Apatite geochronology is a versatile method for providing medium temperature history constraints of magmatic and metamorphic rocks. Magmatic apatite often shows a sufficient spread in data to obtain a precise and accurate lower intercept age. If this is not the case, the initial Pb isotopic composition needs to be estimated to obtain accurate and precise age information from apatite.

Two approaches are common, one being the estimation of common Pb from a Pb evolution model and the other being the measurement of a coexisting mineral phase that tends to incorporate Pb but not U, e. The resulting age information is accurate and precise despite using plagioclase rather than K-feldspar, as is normally used, to define initial Pb isotope compositions.

now have the tools not just to date the age of formation of rocks, but also later the Pb loss, which are given by the intercepts of the line with Concordia.

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U–Pb geochronology and Hf isotope data from the Late Cretaceous Mawat ophiolite, NE Iraq

Abstract Zircon U—Pb geochronology has become a keystone tool across Earth science, arguably providing the gold standard in resolving deep geological time. The development of rapid in situ analysis of zircon via laser ablation and secondary ionization mass spectrometry has allowed for large amounts of data to be generated in a relatively short amount of time and such large volume datasets offer the ability to address a range of geological questions that would otherwise remain intractable e.

The ease of acquisition, while bringing benefit to the Earth science community, has also led to diverse interpretations of geochronological data. In this work we seek to refocus U—Pb zircon geochronology toward best practice by providing a robust statistically coherent workflow. We discuss a range of data filtering approaches and their inherent limitations e.

1) prevalence of the Pb loss effect implies that crustal zircon grains would U-Pb Dating): Implication for deep geodynamics of modern oceans.

Stella Poma 1 , Eduardo O. Mcnaughton curtin. Two episodes of different age and genesis have been identified. Hf isotope signature of the units indicates mantle sources as well as crustal components. The tectonic setting and age of the Gondwanan magmatism in NW Argentina allow to differentiate: a. Permian intra-plate magmatism developed under similar conditions to the upper section of the Choiyoi magmatism exposed in the Frontal Cordillera and San Rafael Block, Argentina; b.

Triassic magmatism belonging to a poorly known subduction-related magmatic arc segment of mostly NS trend with evidence of porphyry type mineralization in Chile, allowing to extend this metallotect into Argentina. It comprises epizonal plutonic and volcanic rocks, the latter including pyroclastic facies. The Cordillera Frontal outcrops are the most voluminous and have been the subject of study over the last 40 years e. This region is characterized by felsic rocks with subordinated basic and mesosilicic rocks.

Isolated Permian-Triassic plutons have been recognized in the Puna region of Salta province, Argentina, suggesting that the magmatic event reached that latitude Zappettini and Blasco, ; Page and Zappettini, ; Poma et al. The aim of this paper is to contribute to the knowledge of the Gondwanan magmatism through the presentation and interpretation of chemical and isotopic data of previously poorly known units that represent the northernmost outcrops identified in Argentina.

We present new zircon U-Pb SHRIMP and Hf isotope data and we explore the nature and characteristics of the magma sources to better constrain a petrogenetic model and characterize the crustal components of the Puna region. In central to southern Chile this scenario is related to the prior collision of Chilenia Willner et al.

Uranium–lead dating

This is a step-by-step guide to reducing simple U-Pb data using iolite v4. You should have already installed iolite v4 before beginning this tutorial. To follow along, you can download the examples files here. Occasionally, longer delays were used between spots to help define the background. If nothing shows up in the data files list after trying to import Zircon. When synchronizing the log file, it should look as below.

Geochemistry, Nd isotopes and U–Pb SHRIMP zircon dating of Neoproterozoic volcanic rocks the young ages but these probably reflect recent Pb loss or per-.

Monazite is an underutilized mineral in U—Pb geochronological studies of crustal rocks. It occurs as an accessory mineral in a wide variety of rocks, including granite, pegmatite, felsic volcanic ash, felsic gneiss, pelitic schist and gneiss of medium to high metamorphic grade, and low-grade metasedimentary rocks, and as a detrital mineral in clastic and metaclastic sediments. In geochronological applications, it can be used to date the crystallization of igneous rocks, determine the age of metamorphism in metamorphic rocks of variable metamorphic grade, and determine the age and neodymium isotopic characteristics of source materials of both igneous and sedimentary rocks.

It is particularly useful in the dating of peraluminous granitic rocks where zircon inheritance often precludes a precise U—Pb age for magmatic zircon. The U—Pb systematics of the mineral are not without complexity, however. Being a mineral that favors incorporation of Th relative to U, it can contain considerable amounts of excess Pb derived from initially incorporated Th, an intermediate decay product of U.

Monazite is known to be capable of preserving inheritance in a manner similar to that of zircon, and it can lose Pb during episodic or prolonged heating events of uppermost amphibolite and granulite facies metamorphic grades. Examples of U—Pb systematics from most of the above situations are presented in this paper to illustrate both the utility and complexity of monazite in geochronological studies in an attempt to encourage more widespread application of this dating method.

Nadia Mohammadi , Christopher R.

High precision U-(Th)-Pb chronology

Thank you for visiting nature. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser or turn off compatibility mode in Internet Explorer. In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript. Zircon of crustal origin found in mantle-derived rocks is of great interest because of the information it may provide about crust recycling and mantle dynamics.

CL images of representative zircons for in situ SIMS U-Pb dating. Table S1), probably due to Pb loss (25), the remaining 27 analyses show.

In the laboratory, rock samples are crushed and the zircon grains are separated from the other minerals by heavy liquid and other mineral separation techniques. After being mounted, the crystals can be analyzed using an instrument such as a SHRIMP Sensitive High mass Resolution Ion MicroProbe which focuses a very narrow ion beam onto the grains so that mass spectrometers can measure the ratios of the isotopes vaporized from the targeted spot.

In this way, even different growth zones in individual crystals can be analyzed and thus “dated. An alternative procedure is to take all the zircon grains liberated from a rock sample, and if they are of uniform composition, chemically digest them into solution for standard mass spectrometer analysis. This dating method has become very popular for dealing with Precambrian terranes where it can often be difficult to resolve relationships between rock units and the geological history.

But just how good is this dating method? It must be assumed that when the zircon grains crystallized, no radiogenic Pb was in them, and that all the radiogenic Pb now measured was derived by radioactive decay from U and Th.

U-Pb Zircon & Apatite dating

Official websites use. Share sensitive information only on official, secure websites. Zircon geochronology is a critical tool for establishing geologic ages and time scales of processes in the Earth’s crust.

recognise discrete Pb-loss and multiple stages of zircon growth or isotopic Please cite this article as: Crowley, Q.G., et al., High-precision U–Pb dating of.

At present, Chemostrat can determine U-Pb ages for zircon and apatite crystals. Zircon is a robust mineral and so the crystals preserve the age at which they formed or underwent high grade metamorphism. Consequently, U-Pb zircon geochronology can be employed to constrain the age of the basement rocks and in turn can help to identify sediment dispersal patterns and to correlate sandstones.

If the analysed zircon crystal has not suffered either Pb loss or U gain, it will plot on the concordia line from which its age can be deduced. Sandstones frequently contain detrital zircon grains and if these grains are undisturbed and concordant, their ages provide some clue as to their provenance. Generally at least fifty grains from each sandstone sample need to be analysed in order to obtain reliable data.

Developing state-of-the-art analytical techniques for high-precision U-Pb dating

These terms should be limited to synthetic or transformed and homogenized natural materials with certified elemental or isotopic compositions. Chemical Geology , Precise U—Pb ages of Duluth Complex and related mafic intrusions, northeastern Minnesota: geochronological insights into physical, petrogenetic, paleomagnetic and tectonomagmatic processes associated with the 1. Journal of Geophysical Research 98,

sup Pb/.sup U dates, indicating that their discordance is dominated by recent Pb loss due to fast pathway diffusion or volume diffusion. Hence.

Apollo 12 breccia is composed of two portions, one grey in colour, the other black. The grey portion of the breccia consists mainly of felsite thought to have formed during a single crystallisation event, while the black part is characterized by presence of lithic fragments of noritic rocks and individual plagioclase crystals. In this study, U-Pb analyses of Ca-phosphate and zircon grains were conducted in both portions of the breccia. Moreover, some grains exhibit recrystallisation features and potentially formation of neoblasts.

The latter process requires high temperatures above degrees C leading to the decomposition of the primary zircon grain and subsequent formation of new zircon occurring as neoblasts. This age was interpreted to date the Imbrium impact. Given the brecciated nature of this part of the sample, the interpretation of these ages as representing igneous crystallisation or resetting by impact events remains ambiguous since there is no direct link to their source rocks via textural relationships or crystal chemistry.

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U and Th are found on the extremely heavy end of the Periodic Table of Elements. Furthermore, the half life of the parent isotope is much longer than any of the intermediary daughter isotopes, thus fulfilling the requirements for secular equilibrium Section 2. We can therefore assume that the Pb is directly formed by the U, the Pb from the U and the Pb from the Th.

The ingrowth equations for the three radiogenic Pb isotopes are given by: 5. The corresponding age equations are: 5. This assumption cannot be made for other minerals, young ages, and high precision geochronology.

and sedimentary systems. U-Th-Pb geochronology is also used as the standard by has experienced recent “Pb-loss” (preferential loss of Pb from the target.

Bomparola, C. Ghezzo, E. Belousova, W. Griffin, Suzanne Y. A detailed in situ isotopic U—Pb, Lu—Hf and geochemical study of zircon populations in a composite sequence of foliated to massive Cambro-Ordovician intrusions in the Deep Freeze Range North Victoria Land, Antarctica , has highlighted great complexity in zircon systematics. In contrast, zircons from undeformed samples display a limited range of U—Pb ages and trace-element compositions.

The observed isotopic and chemical variations in zircon are attributed to a sub-solidus recrystallization under hydrous conditions and varying temperature, in a setting characterized by a transpressional to extensional stress regime. Zircon is chemically and physically resistant to modification under most geological conditions; this characteristic makes it one of the most important tools to determine the age, origin and thermal history of crustal igneous and metamorphic rocks.

However, many recent studies e. Black et al. These studies clearly demonstrate that zircon does not always behave as a closed system. The interpretation of zircon U—Pb ages may therefore contain ambiguities. A number of processes such as igneous and metamorphic overgrowth, inheritance, annealing after radiation damage, hydrothermal alteration, precipitation and dissolution can severely affect the chemical and isotopic systems, leading to mixtures of age components within single zircon grains.

Decay scheme of K-Ar, U-Pb, Rb-Sr and Sm-Nd isotopic systems