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Posted by: Kazile Posted on: 09.06.2020

Jercinovic University of Massachusetts. Assumption: No non-radiogenic lead in monazite or at least very little. If you can precisely measure U, Th, and Pb in ppm , you can solve this equation iteratively for lead to obtain an age. Map thin section with the microprobe Ce, Fe, Y to find all monazite crystals. See an abstract of Williams et al.

Both the oldest and youngest points appear to be outliers and should probably be ignored in any case.

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The best hope for finding older monazites in Proterozoic rocks from this and other ranges is likely to be within the cores of M1 garnets and other porphyroblasts. This will undoubtably be a very fruitful avenue of research for years to come. All of the participants at the workshop very much appreciate the time and energy that Mike Jercinovic put into it. We look forward to utilizing and improving this exciting new technique! Thanks Mike. We are a research and service division of:.

Jercinovic University of Massachusetts Method Overview Assumption: No non-radiogenic lead in monazite or at least very little If you can precisely measure U, Th, and Pb in ppmyou can solve this equation iteratively for lead to obtain an age Map thin section with the microprobe Ce, Fe, Y to find all monazite crystals Map monazite grains Th, Y, U to see chemical domains Measure major elements for matrix corrections Spot analyses Measure: U, Th, Pb, Y to correct for peak interference with Pb-a minor correction Solve the age equation Samples: See an abstract of Williams et al.

Data analysis download a Kb Excel file The probe data files have been organized into a spreadsheet modeled on the spreadsheet that Mike supplied that computes the ages. Several improvements have been made: Error estimates analytical precision based on counting statistics have been automated. The standard error of the mean didn't appear to work very well for these analyses because of ambiguity regarding mixed populations of ages in the sample monazites.

The age is calculated using macros which iterate the age equations until a solution is reached to 0. Be sure to read the comments on the macros by editing them you may need to configure Excel to use the 'solver' module. One macro iterates for one column of ages and should be used on the 'All data' sheet while the other iterates two columns of ages on the error sheets - e. All of the worksheets in the ARRerror-estimate. This will allow rapid age and error calculations by entering new probe analyses into the 'Data from probe files' columns highlighted in blue on the 'All data' worksheet.

Several sort columns A,B,C can be used to sort the data in spreadsheet in different ways be careful. One mode of monazite formation is crystallization from an igneous melt. The concentric zoning pattern reflects the changing composition of the melt which affects the composition of the crystallizing monazite. Sector zoning is also associated with the crystallization of monazite in a melt. However, some elements may have a tendency to crystallize onto a specific crystal face.

This results in uneven growth and composition. Core-rim zoning is usually associated with the fluid-assisted dissolution-precipitation in metamorphic reactions, forming successive rims each with a new composition.

Mottled and patchy zoning patterns are more complex zonations.

Electron microprobe dating of monazite Jean-Marc Montel *, Suzanne Foret, Michle Veschambre, Christian Nicollet, Ariel Provost C.N.R.S. URA I0 'Magmas et Volcans', DJpartement des Sciences de la Terre, UniversitJ Blaise Pascal, 5 Rue Kessler, F, Clermont-Ferrand Cedex, France. M. Dating emilie janots 1, chemical. Emilie janots', sedimentary rocks. Eine in-situ-ablation von monazit in situ monazite dating of detrital monazite dating of monazite allows the 17 ma. It is used to date and specific reaction history of rock major element influences on monazite using the. In-Situ microprobe dating schemes. 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.

The interpretations are usually not simple. Isotopic dating and chemical dating are the two typical methods used in monazite geochronology. Both methods make use of the radioactive nature of Th and U in monazite. Isotopic dating requires measuring the isotopic concentration of radioactive U and Th, and radiogenic Pb in monazite. By treating each decay chain in the U-Th-Pb system independently, three classic isochron equations can be obtained:.

Combinations of the use of the above equations, such as U-Th-Pb dating, U-Pb dating and Pb-Pb dating, require different levels of analysis techniques and offer variable levels of precision and accuracy.

Chemical dating requires measuring the elemental abundances of U, Th and Pb but not isotopes. U-Th-total Pb dating, also known as electron microprobe U-Th-Pb dating, measures the elemental abundances of the three elements by an electron microprobeand calculates the age t by the below equation. For chemical dating results to be valid, the following assumptions are required: [2] [21].

The first assumption tends to be true since monazite is very unlikely to incorporate Pb during its growth. The non-radiogenic Pb content in many laboratory tests was found to be very low, nearly always less than 1 ppm. That means the system is either reset totally or unaffected totally by geological processes, there is no partial resetting of the system.

Minor errors may arise due to negligible disturbance during mass transfer. Thus, Pb accumulates at a high rate by radioactive processes. In less than hundreds of years, it reaches a level high enough to be measured accurately by an electron microprobe. Age and compositional zonation as well as the texture of monazite provide evidence on the successive growth of the crystal during discrete geological events.

The scope of information that can be obtained largely depends on the analysis techniques employed in geochronology. Conventionally, monazite is separated from samples by dissolution and chemical methods. 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, mechanical separation for monazite often destroys the associated textural and spatial information in the monazite crystals, which is crucial in interpreting relationships between domains and geological environments. For the above reasons, the demand for in-situ analysis is increasing. In-situ means analyzing monazite grains in their original host rocks without 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.

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 are capable of such analysis. Shown below is a general procedure for monazite dating. The characteristics and procedures are different for each measurement tool, especially sample preparation and dating methods.

In both conventional and in-situ dating, a thin section of the rock of interest is prepared. Then, it is mounted on a slide made of glass or resin, and ground smooth using abrasive grit.

May 05, Monazite fractions from these rocks, precisely dated with concordant ID-TIMS U-Pb ages (Friedl et al., ), were subjected to a systematic microprobe analysis and it soon became clear that the EMP dating method worked well. To our own surprise the TIMS monazite ages could all be reproduced within a moderate uncertainty of ojasjobz.com by: 6. The chemical Th-U-total Pb isochron method (CHIME) of dating by electron microprobe is an established and widely used technique for monazite geochronology (e.g. Suzuki and Adachi, , Montel et al., , Cocherie and Albarede, , Jercinovic and Williams, , Pyle et al., , Williams et al., , Williams et al., ). Of. Monazite is a suitable chronometer for studying magmatic and polymetamorphic events due to its high Th and U contents and negligible common Pb content (Parrish, ). U-Th-Pb dating of monazite with the electron microprobe microanalyser (EPMA) has been recently.

The two images are usually superimposed to reflect sample texture and monazite locations at the same time. Monazite grains which show useful relationships 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 compositional zonation patterns. Estimated ages are calculated across the compositional map by analysing the concentration of Th, Pb and U by the total-Pb dating method. A number of spots within an age domain are selected and further dated accurately with the measurement tools by isotopic dating method.

The choice of various conventional or in-situ analysis techniques affects the resolution, precision, detection limits and cost of monazite geochronology. Since this method involves the chemical separation of monazite isotope dilutionit is regarded as a conventional analysis technique.

Generally, it takes several hours for a U-Pb measurement.

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The precision of date is nearly 0. It is regarded as the most precise method in monazite geochronology. Monazite mineral grains 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 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 their respective ions, which are accelerated under a 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.

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The ratios are converted to moles of sample isotopes for dating. The following measurement techniques apply to 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.

Electron Microprobe Dating of Monazite. We are a research and service division of: Preliminary results of a workshop at NMT given by Michael J. Jercinovic (University of Massachusetts) Method Overview. Assumption: No non-radiogenic lead in monazite or at least very little. A new electron microprobe dating method. Microprobe monazite dating has been increasingly used to constrain the timing of deformation and metamorphism because of the potential to date very small monazite domains (down to 5 ?m or less.

The secondary ions liberated from the mineral are accelerated, measured and analyzed in the mass spectrometer. Sample are analysed in rotation with a standard of known elemental or isotopic ratios in order to determine 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.

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The mineral sample surface is sputtered by a laser inside a sample cell. The ablated particles are collected and incorporated into a carrier gas.

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The resulting aerosols are analyzed by a mass spectrometer for dating. A solid-state or gas-source laser with a short wavelength is commonly used as the laser ablation system in geochronology. EMPA is employed in monazite geochronology especially for in-situ chemical dating total-Pb dating.

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Monazite geochronology can reveal complex geological history recorded in the monazite mineral grains. The characteristic composition and age of each domain or zone represent a past geological event with a certain age. The key challenge in monazite geochronology is to correctly relate textures and compositions in each domain to the associated geological events which formed them. Even a single monazite grain may reveal a complex history, in which geologic events maybe inter-related or coeval, making discrimination difficult.

The section below aims to briefly explain how composition and age data are interpreted to link different types of events. Understanding the igneous petrology of monazite is important to be able to date the crystallisation age of igneous rocks. Monazite is commonly present as an accessory mineral in low-CaO peraluminous granitoidsfrom dioritesmicaceous granites to pegmatites.

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, in which new monazite layers are crystallized rim-by-rim around the pre-existing core.

The rims often show compositional variations 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 a Th-depleted melt. Therefore, older monazite near the core of a grain is rich in Th while younger monazite contains less, resulting 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.

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Monazite geochronology can also reveal igneous differentiation events such as magma mixing, where the magma chamber is evolved into a different composition.

Isomorphous substitution is one of the examples. It is a form of substitution in which 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 upon melt composition and the geological environment. Hydrothermal processes are usually coupled with igneous processes.

Monazite geochronology helps studying the evolution from igneous processes to hydrothermal processes, [32] and revealing later hydrothermal alteration, [33] 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 pattern of monazite may help distinguish them. Also, hydrothermal monazites usually contain low ThO 2 content.

Monazite geochronology is generally regarded as a powerful tool to reveal metamorphic history. Metamorphism is the mineralogical and textural changes in preexisting rocks in response to a change in environment to different temperatures and pressures. The mineral assemblage formed by metamorphism depends on the composition of the parent rock protolith and more importantly, the stability of different minerals at varying temperature and pressure P-T. A set of mineral assemblages that form under similar temperature and pressure is called a metamorphic facies.

Most mineral changes during rock burial, uplift, hydrothermal processes and deformation are associated with metamorphic reactions.

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Monazite is commonly found in many metamorphic rocks, especially in those formed from pelites and sandstones. They may be formed from reactions along a single pressure-temperature P-T loop in a phase diagramor reactions without changing P-T.

Radioactive Monazite Tested!

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.

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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. Both porphyroblasts contain monazite inclusions which were dated at 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 occurred after Ma, and the final annealing cooling and coarsening of minerals happened at Ma.

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Within the same setting as above, monazite inclusions in garnet maybe either younger than, 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. Elemental fractionation refers to the difference between the amount of an element incorporated into the solid mineral phase and the amount left in the fluid phase.

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Minerals display preferential intake of certain elements during growth. For example, as monazite grows in size, it preferentially incorporates Th in its crystal structure, resulting in less available Th in the fluid for future monazite growth. Thus, younger monazite tends to have lower Th content.

When considering the whole system of metamorphic rocks, there are other minerals which show elemental fractionation. The interplay between fractionation in monazite and these other minerals has a great impact on the compositional zonation of monazite. The mostly studied system is yttrium Y fractionation between the phosphate monazite and the silicates garnet and xenotime.

All three minerals preferentially fractionate Y, yet they form and break down at different stages 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 a prograde path with Y continuously being incorporated, thus the Y content in monazite formed at this stage prograde should decrease progressively with higher grade.

Microprobe monazite dating

However, as temperature increases to a certain point, partial melting anatectic of monazite occurs around its rim, releasing Y into the melts. As the system later cools and melt crystallizes, regrown monazite will have higher Y content.

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.

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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.

The extent of fractionation of Y between garnet and monazite is also found to be related to temperature.

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It is thus used as a thermometer, providing temperature constraints on the P-T path. Timing deformation events is one of the important components in a tectonic study. Large scale cross-cutting relationships between rocks, dikes and plutons provide certain but relatively broad time constraints on deformation.

Monazite can be incorporated into deformation fabrics, reaction textures and fractures; thus, studying microfabrics and microtextures of monazite offers a more straightforward method of dating a deformation event.

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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. The age is interpreted as the timing of shearing. Monazite-forming reactions may happen a bit later than shearing after the rocks have been in re-equilibrium in response to a new pressure environment.

Monazite can form in fabrics caused by deformation. Monazite may be present as elongate grains aligned in foliation.

It can be interpreted that either the monazite formed before the shearing and was aligned during shearing, or formed at the same time as the shearing. 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. Monazite along existing foliation would have a tendency to grow at the two ends along the foliation. Fractures and offsets in a single monazite crystal have been observed mimicking bookshelf faulting in a larger-scale fracturing event.

Moreover, new monazite may later grow and fill up the space created by the fracture, enclosing the time constraint completely.

Detrital monazite grains are produced by the weathering and erosion of pre-existing rocks and then transported into [[sedimentary basins]. The detrital monazite contains zonation patterns which preserve the geological history of the source region.

Investigating detrital monazite in the basin not only helps in reconstructing the metamorphic, tectonic and hydrothermal history of the source region, but also finding the depositional age, structural evolution and sediment sources of the basin.

Diagenetic monazite is the monazite that formed during or after the lithification of sedimentary rocks. Monazite has been observed to grow on other minerals or in pore spaces during diagenesis of sediments. U-Th-Pb data and monazite ages can be used as a valuable tool for prospecting. From Wikipedia, the free encyclopedia. Successive growth of monazite grain by fluid-assisted dissolution-precipitation.

A Reaction ceased due to recrystallisation of precipitating phase dark orange. B Reaction ceased due to change in reaction system blue. Concentric zoning: monazite grows with new successive layers with different compositions. Sector zoning: different elements crystallised preferentially at different faces of the crystal.

Core-rim zoning: altered rim formed surrounding the original core under a dissolution-precipitation reaction. Zoning patterns of monazite.

Intensity of colour represents the concentration of a certain element. Edited after Williams, [2]. Further information: Uranium-lead dating. Monazite identification: Illustration showing a backscattered electron image of a rock sample with monazite at the center with white color.

Edited after Williams, Compositional mapping: Illustration showing X-ray Th composition map of a monazite grain. Brighter color represents higher concentration. Quantitative dating: Histogram of ages measured, showing two age zonations in monazite. Illustration of an age map of a monazite grain.



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