Geochronology – Methods and Case Studies. In situ U-Pb dating combined with SEM images on zircon crystals represent a powerful tool to reconstruct metamorphic and magmatic evolution of basements recording a long and complex geological history [ 1 – 3 ]. The development of high spatial and mass resolution microprobes e. The growth of zircon crystals, evidenced by their internal microtextures, can be easily revealed by SEM imaging by Cathodoluminescence CL and Variable Pressure Secondary Electrons VPSE detectors on separated grains or in situ within a polished thin rock section [ 6 , 4 , 7 ]. In acidic magmatic rocks abundant zircon crystals provide precise age data about magma emplacement and origin of source indicating the geodynamic context and the pertinence of terranes forming the continental crust. As regards the metamorphic context, zircon can potentially preserves multiple stages of metamorphic records owing its highly refractory nature, high closure temperature and slow diffusion rate of Pb, thus it is an ideal mineral for U-Pb dating of poly-metamorphic rocks [ 9 , 10 ]. In addition, in situ analyses of trace elements such as rare earth elements REE in zircon and between zircon and coexisting minerals is usefull to decipher the REE behavior and mineral chemistry during metamorphism and to determine metamorphic P-T conditions [ 8 , 11 , 12 ]. In particular, garnet is one of the most important rock-forming minerals in high-grade metamorphic rocks since it can be also used to constrain metamorphic conditions if its composition is combined with that of other major minerals such as pyroxene and amphibole [ 13 , 14 ].
He was involved in the first characterisation of a natural carbonate for use as a reference material, and in demonstrating the applicability of LA-ICP-MS U-Pb carbonate geochronology to a number of key applications, such as dating brittle deformation, ocean crust alteration, and paleohydrology. As well as providing deformation histories of basins and orogens, they are critical for understanding the formation, migration and storage of natural resources.
Determining the absolute timing of fault slip and fracture opening has lacked readily available techniques. Most existing methods require specific fault gouge mineralogy that is not always present, e. K-Ar illite dating.
Presented at the Third International Conference on Creationism, Pittsburgh, Pennsylvania, July 18–23, Published in: Proceedings of the First International.
In a single decaying system, the age determined from the exponential decay law is directly related to its linear Maclaurin approximation. This relationship can be additively extended to several decaying systems resulting in the same daughter element, by using proportionality functions, thus allowing an explicit formulation of the age as a function of element concentrations.
From the iteration data, the expressions of the two functions and the associated coefficients were determined by polynomial regression and mathematical programing on conveniently separated time and compositional intervals. Additional time- and composition-dependent age corrections optimized by mathematical programming of the residuals lead to an accuracy of 0. The error propagation can be traced through all the operations defined by explicit formulas according to simple error propagation rules, finally allowing the calculation of the standard error of the result.
The formulas and parameters derived can be used in a calculation spreadsheet. Tables, graphs and references. Fluid-induced disturbance of the monazite Th-Pb chronometer: In situ dating and element mapping in pegmatites from the Rhodope Greece, Bulgaria. Chemical Geology 3—4 : —, DOI
Heavy Metal Clocks, U-Pb and Th-Pb Dating Models: Radioactive Dating, Part 7
Lectures in Isotope Geology pp Cite as. Natural U consists of two radioactive parent-isotopes, U and U, the half-lives of which fall into the range of geologic time. Outside this range is the half-life of Th, which represents the only natural isotope of Th.
Thoriumlead and uraniumlead radiometric ages for ore-stage calcite show that Mississippi Valley-type (MVT) ore deposits can form in.
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. The results of this study suggest that alteration may disrupt the U—Pb system in perovskite.
The isotopic data suggest that the Kuruman kimberlite magmas were initially derived from a deeper sub-lithospheric source region, but their distinctive compositional characteristics were imposed through assimilation, first of metasomatized sub-continental lithospheric mantle and later of crustal material, during magma ascent.
Kimberlites are rare, small-volume, potassic and volatile-rich ultramafic rocks that have been the focus of many geochemical studies. In addition to being the carriers of diamonds and mantle xenoliths, kimberlite magmas can provide insights into deep Earth processes, including the links between metasomatism and low-degree melting.
Developing state-of-the-art analytical techniques for high-precision U-Pb dating
The actinide elements U and Th are among the heaviest naturally-occurring elements. They are radioactive and decay by alpha-decay down a complex decay series to Pb, and also by fission. U has two main radioactive isotopes, U and U, that decay to isotopes of Pb, Pb and Pb respectively. Th also has one common long-lived radioactive isotope, Th, that decays to Pb. The half-life of U is much shorter that the age of Earth, so most has since decayed away.
Such a sophisticated ion probe, which can attain a high sensitivity at a high mass resolution, based on a double focusing high mass-resolution spectrometer, designed by Matsuda , was constructed at the Australian National University. Since its installation, our focus has been on the in-situ U—Pb dating of the mineral apatite, as well as zircon, which is a more common U-bearing mineral. In this paper, we review the methodology associated with in-situ apatite dating and our contribution to Earth and Planetary Science over the past 16 years.
Since the monumental studies reported in , 1 an enormous amount of work related to the Pb—Pb dating of meteorites has been reported via the use of TIMS Thermal Ionization Mass Spectrometry. After this, following cleaning with ethanol, to minimize surface contaminating Pb, the thin section was gold-coated to prevent a charge from developing on the sample surface during SHRIMP analyses. The positive secondary ions were thereby extracted and detected on a single electron multiplier by peak switching.
A similar equation also describes the U—Pb secondary ion emission data from other minerals such as monazite, titanite, baddeleyite, rutile. In the case of ion microprobe U—Pb zircon dating, the expected initial lead based on the U—Pb evolution model is subtracted from the observed lead signals. Moreover, it should be noted that it would be a demerit where only one or two in some cases age information data sets from a number of apatite grains are obtained by an isochron method in comparison with the conventional calibration for zircon.
The crucial advantages of this method are that it is not necessary to know the isotopic composition of common Pb in contrast to the conventional Concordia diagram, and that both U and U decay schemes are used simultaneously, yielding a smaller justifiable age uncertainty for the case of U—Pb systematics. In this case, both the upper and lower intercept ages are geologically valid, corresponding to a formation age and an alteration age, respectively.
Although the upper intercept age is not a geological age in case II, the lower intercept ages are geochronologically valid in both cases I and II. In following chapters, we overview our research findings obtained using these methods.
High resolution (5 μm) U–Th–Pb isotope dating of monazite with excimer laser ablation (ELA)-ICPMS
Mark Harrison, Elizabeth J. Reviews in Mineralogy and Geochemistry ; 48 1 : — The dominant occurrence of phosphate minerals in crystalline rocks is as accessory phases, most notably apatite, monazite, and xenotime. Because these minerals tend, to varying degrees, to partition U and Th into their structures they can often contain the majority of those elements in a rock.
These three phases, again to varying degrees, tend not to incorporate significant amounts of Pb during crystallization and thus were early candidates for utilization as U-Th-Pb geochronometers.
Minerals used in U-Th-Pb dating. Mineral. Formula. U content. (ppm). Th/U. Common Pb. (ppm). Rock. Type. Zircon. Zr SiO4. 1 – >10,
Climate change. Geology of Britain. U-series and U-Pb capability for carbonate geochronology has been developed in the geochronology and tracers facility to support NERC climate research, benefitting from extensive knowledge transfer from our U- Th -Pb geochronology facility. Sea floor geochronology and tracers is a recently developed but rapidly growing area for the facility. This science area is focused on the chronology of sea floor deposits that can be dated by U-Th methods e.
An issue with such projects is access to samples, and we are working with partners in Norway and the US to build collaboration and access to unique sample sets, and to include other UK interested parties. Press Office.
Geochronology – Confirming the existence of an Ancient ‘lost continent’ (LG-SIMS)
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,
ubiquity of minerals containing high U and Th concentrations (i.e., zircon, titanite, monazite, baddeleyite, apatite, rutile, etc.), has led to the use of. U-Th-Pb geo/.
Chemical Geology , , pp. View at publisher. LA-ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but also particularly into the reference material. Common Pb correction of the age standard can be undertaken using either the Pb, Pb or Pb no Th methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments.
This downhole fractionation model is applied to the unknowns and sample-standard bracketing using a user-specified interpolation method is used to calculate final isotopic ratios and ages. Pb and Pb no Th corrected concordia diagrams and Pb, Pb and Pb no Th -corrected age channels can be calculated for user-specified initial Pb ratio s. All other conventional common Pb correction methods e. The secondary Durango
EPMA u-th-pb monazite dating of metamorphic rocks from the mogok metamorphic belt, central myanmar
This page provides a short tutorial leading through some steps that are required for obtaining U-Pb zircon ages using the UCLA ims ion microprobe:. Pb isotope intensity of zircon during O – ion bombardement is significantly enhanced if the sample surface is saturated with regard to oxygen. During zircon analysis, oxygen gas is leaked through a valve into the ims sample chamber.
U-Th-Pb Geochron analytical procedure used in the. SHRIMP comparison of SHRIMP zircon dating with conventional zircon ages and 40Ar/39Ar analysis. In.
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.
Mineral U-Th/Pb dating
All publications more feeds DOI: BibTeX file. We present results obtained with a confocal 3D micro-XRF set-up for chemical age dating using the U, Th and Pb concentrations of monazite within rock thin sections.
Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) allanite U-Th-Pb dating has yielded Th-Pb isochron ages of ± Ma (MSWD.
The giant Beiya Au skarn deposit and Machangqing porphyry Cu-Mo- Au deposit are located in the middle part of the Jinshajiang—Ailaoshan alkaline porphyry metallogenic belt. The Beiya deposit is the largest Au skarn deposit in China, whilst the Machangqing deposit comprises a well-developed porphyry-skarn-epithermal Cu-Mo- Au mineral system. In this paper, we present new allanite U-Th-Pb ages and trace element geochemical data from the two deposits and discuss their respective skarn metallogenesis.
The Beiya and Machangqing alkali porphyry-related mineralization are synchronous and genetically linked to the magmatic hydrothermal activities of the Himalayan orogenic event. Such differences in the ore-forming fluids may have contributed to the different metallogenic scales and metal types in the Beiya and Machangqing deposit.
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4.10 – U–Th–Pb Geochronology
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Xenotime is an ideal mineral for U-Th-Pb isotopic dating because of its relatively high U and Th contents, but typically low con- centration of common Pb. These.
The Cretaceous age of this granite and its Mo-W mineralization is shown using two independent methods: U-Pb on zircon and Re-Os on molybdenite. The studied zircons have a typical homogeneous character with oscillatory zoning and scarce restite cores. Two molybdenite-bearing samples of very different character affirm a genetic relation between W-Mo mineralization and the Rochovce granite.
One sample, a quartz-molybdenite vein from the exocontact altered quartz-sericite schist of the Ochtina Formation , provides a Re-Os age of The second molybdenite occurs as 1—2 mm disseminations in fine-grained granite, and provides an age of Both Re-Os ages are identical within their 2-sigma analytical uncertainty and suggest rapid exhumation as a consequence of post-collisional, orogen-parallel extension and unroofing.
The Rochovce granite represents the northernmost occurrence of Cretaceous calc-alkaline magmatism with Mo-W mineralization associated with the Alpine-Balkan-Carpathian-Dinaride metallogenic belt. Impact factor for year – 1.