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. McFarlane , David R.
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.
Apollo 12 breccia Impact-induced partial Pb loss in zircon and its Apollo 12, Impacts, Zircon, U-Pb dating, SIMS, Partial resetting.
Climate change. Geology of Britain. Quantifying geological time is central to our understanding of the evolution of the earth system. By establishing the exact timing of geological events, such as climatic and evolutionary transitions, we can test hypotheses that invoke cause and effect relationships between different components of the earth system, and quantify rates of processes. Radioisotopic dating provides the means to obtain precise and accurate temporal constraints on nearly all types of geological archives, from the ocean sediments that record Cenozoic climate change, to xenoliths from the lower crust that record plate tectonic processes and detrital minerals that track landscape evolution.
These complementary techniques allow us to tackle a wide variety of geochronological problems on materials spanning nearly the entire age of the earth. Our facilities include multiple modern mass spectrometers and lasers, imaging capabilities, and clean labs for low blank high precision dating. We are the only laboratory in the UK that undertakes high precision U- Th -Pb analysis of zircon and other accessory minerals by isotope dilution, and we maintain a strong international reputation in this field.
We exploit this technique for a variety of problems where the highest age resolution is required e.
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. However, for zircons compromised by open system behavior, achieving robust dates can be difficult.
Isotopic and chemical alteration of zircon by metamorphic fluids: U-Pb age depth-profiling Application of various dating techniques and interpretations of quantitatively the extent of Pb loss affecting the weakly discordant – Ma.
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.
Canadian Journal of Earth Sciences
diffusion, can each affect the topology of a U-Pb date profile. Furthermore, the loss of Pb from both apatite and rutile (Fig. 1b); in this case.
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.
Amelin, Y. Geochimica et Cosmochimica Acta, 66, — Auwera, J. Contributions to Mineralogy and Petrology, , 60— Brown, L. Geophysical Journal International, , —
Dubious Radiogenic Pb Places U-Th-Pb Mineral Dating in Doubt
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.
In situ U–Th–Pb dating using High-Resolution-SIMS Perovskite from the Bathlaros kimberlite shows extensive Pb loss; the Pb-corrected.
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.
As a result, newly-formed zircon deposits will contain no lead, meaning that any lead found in the mineral is radiogenic. Since the exact rate at which uranium decays into lead is known, the current ratio of lead to uranium in a sample of the mineral can be used to reliably determine its age. The method relies on two separate decay chains , the uranium series from U to Pb, with a half-life of 4. Uranium decays to lead via a series of alpha and beta decays, in which U with daughter nuclides undergo total eight alpha and six beta decays whereas U with daughters only experience seven alpha and four beta decays.
The existence of two ‘parallel’ uranium—lead decay routes U to Pb and U to Pb leads to multiple dating techniques within the overall U—Pb system. The term U—Pb dating normally implies the coupled use of both decay schemes in the ‘concordia diagram’ see below. However, use of a single decay scheme usually U to Pb leads to the U—Pb isochron dating method, analogous to the rubidium—strontium dating method. Finally, ages can also be determined from the U—Pb system by analysis of Pb isotope ratios alone.
U–Pb geochronology and Hf isotope data from the Late Cretaceous Mawat ophiolite, NE Iraq
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.
The final collision of the CAOB: Constraint from the zircon U–Pb dating of the the age-peak shift that is generally associated with minor amounts of Pb-loss.
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.
U-Pb Zircon & Apatite dating
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 Available data suggest that volcanic activity in the Silesian Basin took place from to Ma with the the main magmatic phase constrained to Ma.
resolution U-Pb dating represented by chemical abrasion – isotope dilution – from modern day (Pb-loss) or reverse (U-loss or unsupported radiogenic Pb).
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.
Similarly, the currently available zircon data set for all lunar samples may be distorted by partial Pb loss, resulting in meaningless and misleading age distribution patterns.
High precision U-(Th)-Pb chronology
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.
If only U is lost from the system substantially after the time of formation of the mineral and Pb is unchanged, the Pb isotopes would reflect a history.
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. The corresponding age equations then become: 5. This built-in redundancy provides a powerful internal quality check which makes the method arguably the most robust and reliable dating technique in the geological toolbox. The initial Pb composition can either be determined by analysing the Pb composition of a U-poor mineral e.
Note that isotopic closure is required for all intermediary isotopes as well. Initially, the U-Pb method was applied to U-ores, but nowadays it is predominantly applied to accessory minerals such zircon and, to a lesser extent, apatite, monazite and allanite. Note that these are only a function of time. Equations 5. The Pb-Pb method has the following advantages over conventional U-Pb dating: There is no need to measure uranium.