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Canadian Journal of Earth Sciences
Stella Poma 1 , Eduardo O. Mcnaughton curtin. Two episodes of different age and genesis have been identified.
In many cases U/Pb ages from apatite agree with zircon in rapidly cooling between U/Pb apatite and zircon ages are observed due to Pb loss or by Ar/Ar dating of muscovite and hornblende minerals (Dallmeyer ).
Continue to access RSC content when you are not at your institution. Follow our step-by-step guide. Zircon has been widely used as a geochronometer with the U—Pb decay system but rarely with the Th—Pb system. As a one-dimensional system, a series of consistent Th—Pb ages can be used to date a geological event. In contrast, a wide variation in Th—Pb ages could result from Pb loss or multiple growth events, making it difficult to link to specific geological events.
The results demonstrated that these seven U—Pb zircon standards have similar absolute concentrations of common lead. The radiogenic Pb concentrations depending on the Th content and age determine the proportion of common lead and define the extent of variation in the Th—Pb system under certain analytical conditions. This relationship could be used as a criterion to evaluate whether it is a single population or not based on Th—Pb dating results of unknown zircons.
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U–Pb geochronology and Hf isotope data from the Late Cretaceous Mawat ophiolite, NE Iraq
The main products of volcanic activity in the teschenite-picrite association TPA are shallow, sub-volcanic intrusions, which predominate over extrusive volcanic rocks. They comprise a wide range of intrusive rocks which fall into two main groups: alkaline teschenite, picrite, syenite, lamprophyre and subalkaline dolerite. The weighted average age for all three samples is
Multiphase U-Pb Geochronology and Shock Analysis of apatite, titanite and However, dating impact structures can be chal- While partial Pb-loss is rec-.
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. Note that the time offset between the data and log is approximately seconds.
You can should zoom in to verify that the automatic synchronization was successful.
Canadian Journal of Earth Sciences
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.
Both Pb and U were loaded with a silica gel – H3PO4 emitter solution on single and leached to reduce the effects of Pb – loss, after the methods described by be accounted for when comparison is made between highprecision U – Pb dates.
Email Address. Sign In. To provide better constraints on the sources of the kimberlite magmas and the timing of magmatism, we have carried out in situ U—Pb dating and Sr—Nd isotopic analysis of groundmass perovskite from four of the Kuruman kimberlites Bathlaros, Elston, Helpmekaar and Zero. We also have measured the Sr-isotope composition of groundmass apatite and carbonate in several of the kimberlites, for comparison with published whole-rock isotopic data.
Attempts to date perovskite using in situ laser ablation inductively coupled plasma mass spectrometry LA-ICP-MS were hindered by extensive Pb loss, even in apparently unaltered grains, yielding anomalously young emplacement ages. The smaller sampling volume of secondary ion mass spectrometry and the ability to measure Pb, thus allowing more precise common-Pb corrections, produces concordant ages. The results of this study suggest that alteration may disrupt the U—Pb system in perovskite.
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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. We apply this method to apatite-bearing gabbroic rocks from layered intrusions Bushveld, Bjerkreim-Sokndal, Hasvik, and Skaergaard ranging in age from ca.
Uranium–lead dating, abbreviated U–Pb dating, is one of the oldest and most refined of the Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming.
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. However, there are several lines of evidence that indicate radiogenic Pb can be inherited during crystallization of the mineral grains, and that open-system behavior is common, with radiogenic Pb lost by diffusion due to the way the Pb is held in the crystal lattice.
Wetherill 4 and Wasserburg 5 subsequently derived mathematical equations to describe this steady loss and demonstrated its consistency with published U-Pb age data. Wasserburg 5 also proposed that Pb loss by diffusion resulted from radiation damage to crystal lattices. In fact, it has now been confirmed that radiation damage can drastically increase the rate of Pb diffusion. This is dramatically illustrated by the contact metamorphic effects of a Tertiary granite stock on zircon crystals in surrounding regionally metamorphosed Precambrian sediments and volcanics.
Zircon crystals are often chemically and physically inhomogeneous, 7 reflecting growth during crystallization from magma.
U-Pb Zircon & Apatite dating
Uranium—lead dating , abbreviated U—Pb dating , is one of the oldest  and most refined of the radiometric dating schemes. It can be used to date rocks that formed and crystallised from about 1 million years to over 4. The method is usually applied to zircon. This mineral incorporates uranium and thorium atoms into its crystal structure , but strongly rejects lead when forming.
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.
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, Geostandards Newsletter 19, The application of laser ablation-inductively coupled plasma-mass spectrometry to in situ U—Pb zircon geochronology.
A reconnaissance ion-probe study of hafnium isotopes in zircons. Geochimica et Cosmochimica Acta 55, A new isotopic and trace element standard for the ion microprobe: preliminary TIMS U-Pb and electron microprobe data, current research. Radiogenic Age and Isotopic Studies: Report Geological Survey of Canada, Ottawa, Canada. Plesovice zircon – a new natural reference material for U-Pb and Hf isotopic microanalysis.
Chemical Geology, , ,
Developing state-of-the-art analytical techniques for high-precision U-Pb dating
Cara L. Donnelly, William L. To provide better constraints on the sources of the kimberlite magmas and the timing of magmatism, we have carried out in situ U—Pb dating and Sr—Nd isotopic analysis of groundmass perovskite from four of the Kuruman kimberlites Bathlaros, Elston, Helpmekaar and Zero. We also have measured the Sr-isotope composition of groundmass apatite and carbonate in several of the kimberlites, for comparison with published whole-rock isotopic data.
Attempts to date perovskite using in situ laser ablation inductively coupled plasma mass spectrometry LA-ICP-MS were hindered by extensive Pb loss, even in apparently unaltered grains, yielding anomalously young emplacement ages. The smaller sampling volume of secondary ion mass spectrometry and the ability to measure Pb, thus allowing more precise common-Pb corrections, produces concordant ages.
susceptible to Pb loss than zircon (e.g., ref. 1–5). However, despite great potential of this geochronometer for dating silica undersaturated igneous rocks, there.
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.
Dubious Radiogenic Pb Places U-Th-Pb Mineral Dating in Doubt
U-Pb dating by zircon dissolution method using chemical abrasion. Nine Temora II zircon grains were analyzed by the laser ablation method yielding an age of Zircon grains of a same population were separated for chemical abrasion before dissolution and mass spectrometry analyses.
Coupled Plasma Mass Spectrometry (LA-ICP-MS); Pb-loss. 1. Introduction. U–Pb geochronology is a widely used radiometric dating tool.
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.
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. As our in situ analytical techniques become progressively more precise, appropriate statistical handing of U—Pb datasets will become increasingly pertinent.
Unless compromised by metamictization or low-temperature dissolution–precipitation (causing Pb loss and discordance), concordant zircon U-Pb dates are thus.
It consists of many metre-to kilometre-sized tectonic slices of serpentinized dunite, peridotite, gabbro, basaltic rocks and associated oceanic metasediments. Felsic intrusions crosscut the ophiolite. We present U—Pb zircon and monazite ages and Hf zircon isotopes from two crosscutting felsic dykes and a gabbro from the mantle section of the ophiolite.
Zircons from the felsic dykes contain spongy domains and xenotime and monazite inclusions. They give ages from to 46 Ma. The age range is interpreted to be caused by secondary processes such as radiogenic Pb mobility and Pb loss. The monazite age of The gabbro zircons give ages between 81 to 38 Ma of which the two oldest grains give the weighted average age of Two mafic units of different ages were identified: the older unit is cut by the 95 Ma felsic dykes and the younger one is represented by the 81 Ma gabbro located within a thrust zone.
The youngest ages of 40 Ma are considered to be related to crustal extension. During the closure of the Neo-Tethyan ocean numerous oceanic fragments ophiolites were obducted in the Tethyan suture zone.