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Date Author Loehn, Clayton William. Metadata Show full item record. Representing diverse igneous and metamorphic lithologies, these grains yielded conventional isotopic ages ranging in age from Neoarchean to Devonian.
This website contains adult material, all members and persons appearing on this site have contractually represented to us that they are 18 Monazite+dating+methods+used+in+archaeology years of age or older U.S.C. RECORD-KEEPING REQUIREMENTS COMPLIANCE STATEMENT. In order to evaluate the electron microprobe (EMP) method for chemical dating of monazite, we chemically analyzed selected suites of monazite grains that were previously dated by standard U/Pb isotopic methods at three laboratories each equipped with a sensitive high resolution ion microprobe (SHRIMP). Monazite Age Dating. Introduction. Determining the age of a rock or rock unit (either relative or absolute) is one of the most fundamental tasks in geology. Because of this routine need, monazite age dating rapidly became a commonly used tool for those involved in field geology. Even though monazite typically occurs only as an.
Chemical dating was performed at Virginia Tech using a Cameca SX EMP in which the analytical routines and settings were specifically optimized for monazite geochronology, including correction of analytical peaks for all major spectral interferences and correction of peak intensities for local background emission. Placement of cross-grain analytical traverses was based on backscattered electron BSE images together with wavelength-dispersive WD generated X-ray maps for Y, Th, U, and Ca, which revealed the internal compositional complexity of each grain.
Single or fractions of crystals are selected for dating, usually by thermal ionization mass spectrometry TIMS. That means one age is generated for a single monazite crystal or for a group of crystals. The age information obtained is obviously inconsistent and inaccurate, because even a single monazite crystal contains zones of different ages. Also, the mechanical separation for monazite often destroy the associated textual and spatial information of monazite, which is crucial in interpreting relationship between domains and geological environment.
Monazite dating methods
For the above reasons, the demand for in situ analysis is increasing. In situ means analyzing monazite at its original place without monazite separation refer to in situ such that the texture and zonation pattern are kept intact in order to reveal a more comprehensive geological history of the host rock.
Direct sampling techniques, high spatial resolution and precision are the requirements for an in situ analysis. With technological advancement, more and more measurement tools such as laser ablation inductively coupled plasma mass spectrometry LA-ICPMS and laser microprobe mass spectrometer LMMS become capable for such analysis. Below shows a general procedure for monazite dating.
The characteristics and procedure are different for each measurement tool, especially sample preparation and dating method. Details of some common measurement tools are described in the section: Measurement tools. Sample preparation: Thin sections of limestone rocks.
Monazite identification: Illustration showing backscattered electron image of a rock sample with monazite at the centre with white colour.
Compositional mapping: Illustration showing X-ray Th composition map of a monazite grain. Brighter colour represents higher concentration. Quantitative dating: Histogram of age measured, showing two age zonations in monazite. Illustration of age map of a monazite grain.
Brighter colour corresponds to older age. In both conventional and in-situ dating, a thin section of the rock in interest is prepared. First, a thin layer of rock is cut by a diamond saw and ground to become optically flat.
Then, it is mounted on a slide made of glass or resin, and ground smooth using abrasive grit. The two images are usually superimposed to reflect sample texture and monazite locations at the same time. Monazite grains which show useful relations with microtextures or host minerals are selected for compositional mapping. Major elemental and sometimes trace elemental maps are created at high magnification by electron microprobe X-ray mapping to show composition zonation patterns.
Maps of elemental Y, Th, Pb, U have been proven useful in identifying composition domains in monazite. Estimated ages are calculated across the compositional map by analysing the concentration of Th, Ph and U by total-Pb dating method. The result is then used to generate an age map which approximately identifies all the age domains. A number of spots within an age domains are selected and further dated accurately with the measurement tools by isotopic dating method.
The results are then analysed statistically to give an accurate age of each age domain. Employment of different analysis techniques conventional or in situ analysis provides selection of different measurement techniques. Choice between these techniques in turn affects the resolution, precision, detection limits and costs of monazite geochronology.
Since this method involves the chemical separation of monazite isotope dilutionit is regarded as a conventional analysis technique.
Major methods of isotopic dating
Generally, it takes several hours for a U-Pb measurement. The precision of date is nearly 0. It is regarded as the most precise method in monozite geochronology. Monazite mineral grains selected are carefully hand-picked for dating. They are spiked with a tracer solution and dissolved in HF or HCl.
Using ion exchange chemistry, U, Th and Pb are separated from other elements. The purposes of the separation are 1 potential isobaric interference should be removed before analysis because of the high-sensitivity and low-mass resolution nature of TIMS; 2 ionization of the interested elements maybe impeded by other elements, which results in reduced signal size and precision.
The separated U, Th and Pb samples are put carefully onto a metal filament, which is usually made from Re. The elements are heated and ionize to the respective ions, which accelerate under strong magnetic field and are measured by a detector. The tracer solution is a solution with a known amount of U and Pb tracer isotopes. Due to elemental fractionation, both elements cannot be measured simultaneously by TIMS.
The tracer solution is therefore used to measure ratios of sample isotope to tracer isotopes. The ratios are converted to moles of sample isotopes for dating. The following measurement techniques applies in in situ analysis which involves direct sampling of monazite grains using an incident ion beam or a laser.
SIMS is a mass spectrometry method to measure small-scale elemental and isotopic variations of samples. The secondary ions liberated from the mineral are accelerated, analysed and measured in the mass spectrometer.
Sample are analysed in rotation with standard with known elemental or isotopic ratios in order to measure the ratios in the sample for dating. Since it enables relatively short and cheap yet high-spatial-resolution analysis, it has become the most utilized method of monazite geochronology. Mineral sample surface is sputtered by a laser inside a sample cell. The ablated particles are collected and incorporated into a carrier gas. The resulting aerosols is analyzed by a mass spectrometer for dating.
A solid-state or gas-source laser with short wavelength is commonly used as the laser ablation system in geochronology. EMPA is employed in monazite geochronology especially in in situ chemical dating total-Pb dating. The high content of U, Th and Pb in monazite match with the requirement arose from the relatively higher lower detection limit.
It can achieve a precision of myr in Pb-rich monazite, and myr in Pb-poor monazite. Monazite geochronology can reveal complex geological history recorded in the monazite mineral grains. The characteristic composition and age zonations are the basic for carrying out such analysis, with each domain representing a past geological event with a certain age.
The most important issue in monazite geochronology is to relate textures and compositions in each domain to the associated geological events. Even for a single monazite grain may reveal complex history, in which events maybe inter-related or even happen at the same time, making it hard to clearly separate each event for discussion.
The below section aims to provide briefly how composition and age data are interpreted to link different types of events. Understanding the igneous petrology of monazite is important to date crystallisation age of igneous rocks. Monazite is commonly present as accessory mineral in low-CaO peraluminous granitoidsfrom dioritesmicaceous granites to pegmatites. The reason of the low CaO content is probably that melts with high CaO content promotes the formation of apatite and allanite but not monazite.
It is commonly formed from the magmatism involving carbonatic melts but not mafic plutons or lavas. Those rocks usually host economic REE ore depositsmaking monazite geochronology important in mining exploration.
The simplest monazite zonation showing successive crystallisation of melts is concentric zonation, with new monazite crystallised as rims by rims surrounding the core. The rims often shows compositional variation due to the preferential incorporation of certain elements in the crystal lattice. For example, considering a closed system, Th is preferentially incorporated into the monazite mineral structure, leaving Th-depleted melt.
Therefore, older monazite is rich in Th while younger monazite contains less Th. This results in a rimward decrease of Th in a concentric zoning pattern. Investigating composition and age variation of these rims help to constrain the timing and rate of crystallisation as well as the composition of the melt, especially for rocks where zircon is not present for zircon dating. Monazite - cheralite - huttonite system. Monazite geochronology can also reveal igneous differentiation events such as magma mixing, where the magma chamber is evolved into different composition.
Isomorphous substitution is one of the examples. It is a form of substitution where one element is replaced by another without changing the crystal structure.
In the case of monazite, the rare earth elements are replaced by Ca and Th. The level of substitution usually depends composition of melt and thus the geological environment. Illustration showing clusters formed by multiple crystals. Edited after Schandl Hydrothermal process is usually coupled with igneous process.
Monazite geochronology helps studying the development from igneous process to Hydrothermal process, and revealing later hydrothermal alternation, which is vital in the study of ore formation. Although it is hard to distinguish between magmatic monazite and hydrothermal monazite, analysing the texture and the occurrence pattern of monazite may help distinguishing them.
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Hydrothermal monazites tend to appear in clusters formed by multiple crystals, while igneous monazites tend to appear homogenous throughout the rock. Also, hydrothermal monazites usually contain low ThO 2 content. These distinctive features can be easily identified with textual and compositional analysis in monazite geochronology. Monazite geochronology is generally regarded as a powerful tool to reveal metamorphic history.
Metamorphism is the mineral change of textural change in preexisting rocks in response to a change in environment with different temperatures and pressures. The mineral assemblage formed under metamorphism depends on the composition of the parent rock protolith and more importantly, the stability of different minerals in varying temperature and pressure P-T.
INVESTIGATION OF THE MONAZITE CHEMICAL DATING TECHNIQUE Clayton W. Loehn ABSTRACT In order to evaluate the electron microprobe (EMP) method for chemical dating of monazite, we chemically analyzed selected suites of monazite grains that were previously dated by standard U/Pb isotopic methods at three laboratories each equipped with a. Jan 05, Hydrothermal monazite-(Ce) crystals from sub-horizontal and younger sub-vertical Alpine fissures located in the Mont Blanc and Aiguilles Rouges Massifs were dated by in-situ SIMS Th-U-Pb methods to identify major growth phases during deformation. Dating - Dating - Major methods of isotopic dating: Isotopic dating relative to fossil dating requires a great deal of effort and depends on the integrated specialized skills of geologists, chemists, and physicists. It is, nevertheless, a valuable resource that allows correlations to be made over virtually all of Earth history with a precision once only possible with fossiliferous .
A set of mineral assemblage that formed under similar temperature and pressure is called metamorphic facies. Actually, most mineral changes during rock burial, uplift, hydrothermal processes and deformation are associated with metamorphic reactions. Monazite is commonly found in many metamorphic rocks, especially in those formed from pelites and sandstones.
The zonation in monazite reflects the successive monazite forming events. They may be formed from reactions along a single pressure-temperature P-T loop in a phase diagramor reactions without changing P-T. For a metamorphic event, monazite is formed by the reactions with more than one P-T loop. We can then put time constrains on the P-T loops, forming a comprehensive pressure-temperature-time loops revealing the metamorphic history of the rocks.
P-T path associated with generation of monazite inclusion bearing porphyroblast and matrix. Different porphyroblasts like garnet and quartz are often formed during metamorphism in different ranges of P-T.
Monazite grains are often found as inclusion in porphyroblasts. Porphyroblast garnet is formed during high grade metamorphism while porphyroblast cordierite is formed during exhumation afterwards. Both porphyroblasts contain monazite inclusions which dated Ma and Ma respectively. And matrix monazite is dated Ma. Thus, it is interpreted that high grade metamorphism occurred after Ma and before Ma, while exhumation after Ma, and the final annealing cooling and coarsening of minerals at Ma.
Within the same setting as above, monazite inclusions in garnet maybe either younger, older than or have similar ages with the matrix monazite. Both of them may even have a wide range of ages with no systematic distribution. These scenarios are interpreted to represent different metamorphic paths and conditions, giving varying or complex sequences of metamorphic reactions.
Elemental fractionation refers to the difference between the amount of element incorporated into the solid mineral phase and the amount of element stayed in the liquid fluid phase. Minerals have the characteristic of preferential intake of certain elements during its growth. For example, as monazite grows in size, it preferentially incorporate Th in the crystal structure.
It results in less available Th in the environment for future growth. Thus, younger monazite tends to have lower Th contents. It provides one of the reasons for the compositional variation of monazite.
When considering the whole system of metamorphic rocks, there are also other minerals which shows elemental fractionation. The interplay between fractionations in monazite and these minerals has a great impact on the compositional zonation of monazite.
The interplay is often caused by the formation and breakdown of the minerals, which is in turn a result of different stages in P-T paths. Dating fractionating zonation thus help putting time constraint on metamorphism. P-T path corresponding to formation of low-Y core and high-Y rim of monazite. The mostly studied system is the yttrium Y fractionation between monazite and silicates garnet and xenotime. All three minerals preferentially fractionate Y, yet they form and break down at different stage of metamorphism.
Xenotime has the highest fractionating power, then garnet and then monazite. In a simplified case of a clockwise P-T path involving garnet and monazite, garnet grows along prograde path with Y continuously incorporated, thus the Y content in monazite formed at this stage prograde should decrease progressively with higher grade.
However, as temperature increases to a certain point, partial melting anatectic of monazite occurs and it dissolves along the rim, releasing Y into the melts. As the system later cools and melt crystallises, regrowing monazite will have higher Y content. Partial melting usually happen during peak metamorphism highest temperature in P-T pathbut age and chemical information during this stage are not recorded since the monazite is melting. However, the ages of last prograde growth rim lowest Y and the first post-anatectic growth rim highest Y usually bracket the time of partial melting.
Another scenario involves the formation or breakdown of garnet, influencing the Y and HREE heavy rare earth elements content in the environment, thus the content of growing monazite. Basically, monazite growth before garnet formation has a higher Y and HREE content than those during or after garnet formation.
As garnet start breaking down in the later stage of metamorphism, the monazite forms rims rich in Y and HREE.
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The extent of fractionation of Y between garnet and monazite is also found to be related to temperature. It is thus used as a thermometer, providing the temperature constrain on the P-T path.
Timing deformation events is one of the important components in tectonic study. Large scaled cross-cutting relationships between rocks, dikes and plutons easily provide certain but relatively broad time constrain on deformation. In contrast, monazite can itself be participated in deformation fabric, reaction and fracture, thus studying microfabrics and microtextures of monazite offers a more straightforward method of dating a deformation event. Deformation events may trigger metamorphic reactions which produce monazite.
For example, a metamorphic reaction associated with the movement in the Legs Lake shear zone partly replaced garnet with cordierite. This reaction also generated new monazite with high content of Y, and dated around Ma. The age is treated as the timing of shearing. One point to notice is those monazite forming reactions may happen a bit later than the shearing after the rocks have been in re-equilibrium in response to a new pressure environment.
That means monazite age may not be equivalent to shearing age, yet it provides a more precise age than the other methods. Monazite grain is aligned with foliation S1. New monazite overgrowth grows along S1 direction. Edited after Mccoy, Monazite mineral itself can form fabric caused by deformation.
Monazite may be present as elongate grains aligned in foliation. It can be interpreted that the monazite is formed before the shearing and align during shearing, or formed at the same time of shearing. It thus provides an upper limit of the shearing age.
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For example, if the monazite is dated Ma, the age of shearing cannot be older than Ma. However, it can also be interpreted that the monazite grew along the foliation of other minerals long after the shearing. This problem can be solved by analysing the compositional domains of monazite.