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.
Improving U Th Pb Electron Microprobe mineral dating
Symposium on Geol. Resources of Madagascar – A good example of the usefulness of the in-situ dating method. Madagascar displays a huge portion of the lower crust and its study is fundamental for the understanding of the evolution of this part of the crust. Unfortunately, our knowledge of this domain is limited. At this time, numerous works structural, geochemical, petrological are in progress in Madagascar, but there are only a few geochronological data.
Continue to access RSC content when you are not at your institution. Follow our step-by-step guide. We analysed standard zircon crystals using a zircon crystal Thompson Mine and Monangotory standard monazites, dated using a monazite crystal More easily obtaining high resolution age data is useful for the precise determination of the U—Pb age. If you are not the author of this article and you wish to reproduce material from it in a third party non-RSC publication you must formally request permission using Copyright Clearance Center.
Go to our Instructions for using Copyright Clearance Center page for details. Authors contributing to RSC publications journal articles, books or book chapters do not need to formally request permission to reproduce material contained in this article provided that the correct acknowledgement is given with the reproduced material. If the material has been adapted instead of reproduced from the original RSC publication “Reproduced from” can be substituted with “Adapted from”.
Passarelli; Miguel A. Basei; Oswaldo Siga Jr. Sproesser; Vasco A. It provides reliable and accurate results in age determination of superposed events. However, the open-system behavior such as Pb-loss, the inheritance problem and metamictization processes allow and impel us to a much richer understanding of the power and limitations of U-Pb geochronology and thermochronology.
Since , the Interdepartmental Laboratory of Isotopic Geology focus the study of the Earth’s geologic processes, dealing with themes such as plate tectonics, plutonism, volcanism, sedimentary rocks, tectono-thermal evolution, and more recently environmental studies.
This paper outlines an advanced procedure involving the chemical Th-U-total Pb isochron method (CHIME) dating of monazite using a field-emission electron.
Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks. Over naturally-occurring isotopes are known. Some do not change with time and form stable isotopes i. The unstable or more commonly known radioactive isotopes break down by radioactive decay into other isotopes.
Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces. These are released as radioactive particles there are many types. This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable. This radioactivity can be used for dating, since a radioactive ‘parent’ element decays into a stable ‘daughter’ element at a constant rate. For geological purposes, this is taken as one year.
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 method does not provide the detail of isotopic methods, but results can be Electron microprobe U–Th–Pb monazite dating of the Transamazonian.
Monazite rim formation was facilitated via dissolution—reprecipitation of Neoproterozoic monazite. The monazite rims record garnet growth as they are depleted in Y 2 O 3 with respect to the Neoproterozoic cores. Rims are also characterized by relatively high SrO with respect to the cores. Results of the zircon depth-profiling revealed igneous zircon cores with crystallization ages typical for SNC metasediments. These results show that both monazite and zircon experienced dissolution—reprecipitation under high-pressure conditions.
Caledonian monazite formed coeval with garnet growth during subduction of the Vaimok Lens, whereas zircon rim formation coincided with monazite breakdown to apatite, allanite and clinozoisite during initial exhumation. To understand the subduction—exhumation dynamics of continental lithosphere that reached ultra- high pressure [ U HP] depths, we must extract information regarding the timing and conditions of prograde, peak and retrograde metamorphism.
Problematically, continental lithosphere that experienced U HP metamorphism is commonly overprinted by exhumation-related structures and retrogressive metamorphism. Moreover, peak-temperatures of rocks that experienced U HP metamorphism are typically reached during decompression. Robust geochronometers, such as monazite and zircon, are refractory minerals that can retain their geochronological and geochemical signatures through high temperature conditions, retrogressive metamorphism and deformation e.
Engi ; Foster et al.
Electron Microprobe Dating of Monazite
Geochronology is the science of providing ages of events in the history of the Earth and extraterrestrial material and of determining the temporal rates of geological processes by using a number of different dating methods. The ages can be absolute e. Most absolute dating methods rely on the analysis of radioactive isotopes and their radiogenic decay products.
A number of radioactive isotopes from different elements, such as uranium, thorium, rhenium, samarium, lutetium, rubidium and potassium are used for this purpose. Techniques exist to date practically all geological materials, from billions of years in age to historical records. For instance:.
We have developed UPb and PbPb dating method of monazite by using a Cameca NanoSIMS NS50 ion microprobe. A ~4 nA O– primary beam.
Chemical U-Th-Pb dating of monazite by 3D-Micro x-ray fluorescence analysis with synchrotron radiation. The relative detection limits particularly for Pb are below 10 ppm for counting times of s. Therefore, this 3D micro Reference materials GM3, F6, can be reproduced within error. The average 3D micro-XRF dates reproduce the reference ages with discrepancies between Ediacaran times witnessed a hemisphere-scale orogenesis forming the extensive Pan-African mountain ranges and resulting in the final assembly of Gondwana supercontinent.
The Elat metamorphic basement S Israel located at the northernmost tip of a major Pan-African orogenic suture, the Arabian Nubian Shield ANS , comprises amphibolite facies schists and gneisses and was most likely shaped by this major continental collision. However the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are non-conclusive and a major emphasis was given to pre-Ediacaran island-arc related tectonics.
This is mostly because U-Pb dating of zircon, widely used in Elat and elsewhere, is very successful in constraining the ages of the igneous and sedimentary protoliths, but is ‘blind’ to metamorphism at grades lower than granulite. The earliest three phases were defined as ductile and penetrative, but some of the available geochronological data apparently contradict field relations. The corresponding REE patterns of the dated monazite grains indicate that porphyroblast growth, either garnet or staurolite.
Dating metamorphic events appears to be unsatisfactorily addressed by most of the widely-employed and otherwise accurate and productive isotopic techniques, because the phases and systems investigated do not directly relate to the metamorphic events themselves.
Investigation of the monazite chemical dating technique
U-Pb analytical methods for monazite dating are similar to those for zircon, with only a few modifications. Monazite can be dissolved in 12N HCl at ‘°C in.
These metrics are regularly updated to reflect usage leading up to the last few days. Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts. The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric. Find more information on the Altmetric Attention Score and how the score is calculated.
The U—Pb geochronologic analysis of accessory minerals has played an important role in Earth and solar system science in constraining the ages of a wide variety of rocks and minerals. Currently, the significant matrix effects observed between different accessory minerals and the lack of high-quality standards for many minerals of interest are the major limitations of its geochronological applications.