28 research outputs found
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Analytical perils (and progress) in electron microprobe trace element analysis applied to geochronology: Background acquisition, interferences, and beam irradiation effects
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Electron microprobe age mapping and dating of monazite: Techniques and applications
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Microprobe monazite geochronology: putting absolute time into microstructural analysis
High-resolution compositional mapping and dating of monazite on the electron microprobe is a powerful addition to microstructural analysis and an increasingly important tool for tectonic analysis. Microprobe monazite geochronology can be an efficient reconnaissance tool for evaluating metamorphic and deformational age domains, but more importantly, its in-situ nature and high spatial resolution offer an entirely new level of structurally and texturally specific geochronologic data that can be used to put absolute time constraints on PâTâD paths, constrain the rates of metamorphic and deformational processes, and provide new links between metamorphism and deformation. Microprobe geochronology is particularly applicable to three persistent microstructural/microtextural problem areas: (1) constraining the chronology of metamorphic assemblages; (2) constraining the timing of deformational fabrics; and (3) interpreting other geochronological results. Although some monazite generations can be directly tied to metamorphism or deformation, at present, the most common constraints rely on monazite inclusion relations in porphyroblasts that, in turn, can be tied to the deformation and/or metamorphic history. Microprobe mapping and dating allow geochronology to be incorporated into the routine microstructural analytical process, resulting in a new level of integration of time (t) into PâTâD histories
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In-situ trace element analysis of monazite and other fine-grained accessory minerals by EPMA
Trace element analysis of high Z accessory phases by electron probe micro-analysis offers unique access to data unattainable by other analytical techniques, but also presents great challenges. The intimate relationships between beam conditions (voltage, current), spatial resolution, precision, and spectral complexity require the careful examination of all analytical parameters and their consequences, at a level commensurate with the sensitivity of the desired analysis. The high REE and actinide content, and resulting high Z of monazite, allows high direct analytical resolution (below 500 nm at 10 kV, depending on the realized beam diameter) due to limited electron range, but the high concentration of these elements will also produce significant fluorescence at a distance. The use of high accelerating potential in an effort to improve counting statistics is injudicious in most cases, as direct analytical resolution decreases, and high overvoltage on REEs and Th in monazite results in efficient production of energetic ionizing radiation (L-series) capable of fluorescing elements of interest, or interfering elements at remarkable distances (at least several 10 s of microns from the beam). Boundary fluorescence is a very significant problem in trace element analysis and in geochronologic analysis of monazite, as grains are commonly complex, with micro-domains that differ considerably in composition (especially Th/REE). In such instances, a 15 kV beam can produce fluorescence at a distance, leading to errors of 10 s of ppm within 5 ÎŒm of a compositional boundary. For trace element analysis, effects such as background curvature and minor interferences can result in very large errors, 50% or more for curvature alone for concentrations at or below 100 ppm. Strong background curvature is observed for all makes and varieties of spectrometers in the Pb-M region. In addition, for REE and actinide bearing phases such as monazite, the wavelength regions available for background determination are very limited, requiring routine, high-precision scanning and regression modeling to determine reasonably accurate background intensities. Two-point background interpolation methods are inappropriate for high sensitivity analysis in all cases. Without a direct means to verify the accuracy of low concentration analyses, great attention must be paid to analytical protocol and very detailed spectral analysis. The use of consistency standards to monitor changes is important, but caution must be exercised in the use of secondary âageâ standards in the case of monazite EPMA because systematic error can result in cancellation effects when concentrations of both parent and daughter are proportionally incorrect. In all cases, caution in interpretation and integration of all available information is necessary if the potential of the method is to be realized
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High-resolution compositional mapping of matrix phases: implications for mass transfer during crenulation cleavage development in the Moretown Formation, western Massachusetts
High-resolution compositional maps provide a new tool for investigating mass transfer during cleavage formation. The Moretown Formation of western Massachusetts contains a well-developed crenulation cleavage with alternating mica-rich crenulation limbs and mica-poor crenulation hinges. Compositional mapping shows two generations of plagioclase, the second of which was synchronous with the crenulation cleavage. A significant amount of the syn-crenulation plagioclase (10â20% modally) grew in hinge domains. A small amount of syn-crenulation plagioclase (âŒ1%) and a large amount of phengitic muscovite grew in limb domains. The maps also show that uncrenulated domains experienced mass transfer and reactivation of older cleavages, and thus cannot be used as âundeformedâ reference domains for comparison with crenulated regions. Compositional mapping facilitates a new degree of integration between petrologic and structural analysis. Knowledge of the structural context of compositional domains allows better selection of phases and compositions for interpreting metamorphic reactions and linking metamorphism to deformational stages. Knowledge of syntectonic reactions provides new insights into mass transfer and volume change during deformation. In the Moretown Formation, plagioclase- and phengite-producing reactions play a large role in controlling the nature and magnitude of mass transfer, but microstructures control the location of reactants and products within the evolving fabric
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Age mapping and dating of monazite on the electron microprobe: Deconvoluting multistage tectonic histories
High-resolution X-ray mapping and dating of monazite on the electron microprobe are powerful geochronological tools for structural, metamorphic, and tectonic analysis. X-ray maps commonly show complex Th, U, and Pb zoning that reflects monazite growth and overgrowth events. Age maps constructed from the X-ray maps simplify the zoning and highlight age domains. Microprobe dating offers a rapid, in situ method for estimating ages of mapped domains. Application of these techniques has placed new constraints on the tectonic history of three areas. In western Canada, age mapping has revealed multiphase monazite, with older cores and younger rims, included in syntectonic garnet. Microprobe ages show that tectonism occurred ca. 1.9 Ga, 700 m.y. later than mylonitization in the adjacent Snowbird tectonic zone. In New Mexico, age mapping and dating show that the dominant fabric and triple-point metamorphism occurred during a 1.4 Ga reactivation, not during the 1.7 Ga Yavapai-Mazatzal orogeny. In Norway, monazite inclusions in garnet constrain high-pressure metamorphism to ca. 405 Ma, and older cores indicate a previously unrecognized component of ca. 1.0 Ga monazite. In all three areas, microprobe dating and age mapping have provided a critical textural context for geochronologic data and a better understanding of the complex age spectra of these multistage orogenic belts
Improved Analytical Resolution and Sensitivity in EPMA - Some Initial Results from the Ultrachron Development Project
Format and philosophy for collecting, compiling, and reporting microprobe monazite ages
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