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 ]. Relatively to REE partition in metamorphic rocks garnet, pyroxene, amphibole and zircon being competitors for REE partition, represent a usefull tool to outline continental crust evolution. In this paper we present the geochronological and chemistry data collected in the last ten years in Calabria and Peloritani sectors of Italy, utilizing the new analytical techniques, usefull to reconstruct the magmatic and metamorphic history of a key sector of the South European Variscan Belt in the peri-Mediterranean area.
U-series and U-Pb carbonate geochronology
U-Pb dating of speleogenetic dolomite: A new sulfuric acid speleogenesis chronometer. Victor J. Sulfuric acid speleogenesis SAS produces sulfate, carbonate, and oxide byproducts. We applied U-Pb analyses of a dolomite crust sample from Carlsbad Cavern. A model age of 4. U-Pb dating of speleogenetic dolomite is a new way of measuring the timing of SAS.
PbPb Dating. The first attempt to date the CBa Gujba chondrites was reported by Krot et al. () where they applied the absolute Pb.
Ephesians This final article of the series examines the common-lead method of radioactive dating, sometimes referred to as the Pb-Pb method. This method reaches the pinnacle of radioisotope dating methods in terms of complication and convolution. In an attempt to solve this problem, the isochron equation for U is divided by the isochron equation for U to yield an isochron equation that only involves Pb isotope concentrations on one side of the equation:.
The result is a transcendental equation that cannot be solved for t time. Now we must make some adjustments to the equation in order for it to be practically useful—adjustments that involve dubious assumptions. First, we assume, as a corollary to the closed system assumption, that Pb and Pb concentrations only change via decay of U and U respectively. We are then left with the following equation for dating rocks based solely on their radiogenic Pb and Pb concentrations:.
However, a critical question arises: How do we know the initial concentrations of the radiogenic Pb and Pb concentrations in the rock we are dating? They then measure the present radiogenic lead ratio in a rock sample and interpolate the age of the sample from a table of age versus the radiogenic lead ratio constructed from Equation 2.
Do you tell your age? – High-precision U–Pb dating
Toggle navigation. Have you forgotten your login? Conference papers. Et Al.
All. All; News; Master’s Programmes; Researchers; Publications; Jobs. Section for Geology. About the Department · Employees Toggle submenu. Section for.
These samples were used to determine Fission-Track and crystallization ages. HeFTy software was employed to interpret the cooling histories of the samples using forward and inverse models. The inverse model was an approach of reproducing the observed data, and it was carried out only for fission-track data from the apatite grains. And it was constructed after generating a number of forward models, where in each of these models the predicted apatite fission-track parameters were compared to the measured values.
U-Pb Zircon & Apatite dating
The CB chondrites are metal-rich meteorites with characteristics that sharply distinguish them from other chondrite groups. Their unusual chemical and petrologic features and a young formation age of bulk chondrules dated from the CB a chondrite Gujba are interpreted to reflect a single-stage impact origin. Here, we report high-precision internal isochrons for four individual chondrules of the Gujba chondrite to probe the formation history of CB chondrites and evaluate the concordancy of relevant short-lived radionuclide chronometers.
All four chondrules define a brief formation interval with a weighted mean age of Formation in a debris disk mostly devoid of nebular gas and dust sets an upper limit for the solar protoplanetary disk lifetime at 4.
will also shed light on U–Pb dating applied to less commonly encountered and dated minerals. 2. Basics of U–Pb geochronology. The U–Pb system.
Moorbath et al. Here I report a Pb—Pb isochron age for a relatively young carbonate sequence in Taiwan and demonstrate that the long-lived U—Pb decay scheme can be used to determine events as young as Mesozoic age. Odin, G. Numerical Dating in Stratigraphy , — Wiley, Chichester, Clauer, N. Google Scholar. Bonhomme, M.
Moorbath, S. Nature , —
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. The age determinations of geological events are a significant tool in basic and applied earth science studies. The complexity of geological processes registered in the minerals can be dated with these techniques.
Zircon U-Pb dating of Mesozoic volcanic and tectonic events in north-west Palmer Land and south-west Graham Land, Antarctica – Volume 21 Issue 6 – P.T. Leat.
Tomoharu Miyamoto , Takeru Yanagi. U-Pb and Rb-Sr radiometric dating has been done on zircon samples and mineral concentrates, respectively, separated from psammitic schists of the Sangun metamorphic rocks in central Kyushu, Japan. Based on their shapes and colors, zircon grains were grouped into six. Rounded and short prismatic zircons define, independently of their colors, a common discordia with an upper intercept at 1. Elongated zircons define another discordia with an upper intercept at 0.
A Rb-Sr internal isochron defined by the mineral concentrates gives an age of
Pb-Pb Isochron Dating
Lead—lead dating is a method for dating geological samples, normally based on ‘whole-rock’ samples of material such as granite. For most dating requirements it has been superseded by uranium—lead dating U—Pb dating , but in certain specialized situations such as dating meteorites and the age of the Earth it is more important than U—Pb dating. There are three stable “daughter” Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb, Pb, and Pb.
These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U, U and Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate.
In uranium-lead (U-Pb) dating of zircon, the zircon is found to exclude initial lead almost completely. Minerals too are predictable chemical compounds that can.
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. The age of apatite grains can be calculated by plotting their U-Pb isotopic composition to form a discordia line.
Apatite has a lower closure temperature than zircon, i. Therefore, they provide different information about the source of sandstones than zircons such as low grade metamorphic rocks. This provides further information about sediment input pathways to sedimentary basins and, when combined with detrital zircon analysis, provides a powerful tool to identify the provenance of sediments. U-Pb Dating of Apatite The age of apatite grains can be calculated by plotting their U-Pb isotopic composition to form a discordia line.