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Martin Engi - One of the best experts on this subject based on the ideXlab platform.
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Non-matrix-matched standardisation in LA-ICP-MS analysis: general approach, and application to Allanite Th–U–Pb dating
Journal of Analytical Atomic Spectrometry, 2020Co-Authors: Marco Burn, Pierre Lanari, Thomas Pettke, Martin EngiAbstract:To make use of the full geochronological potential of accessory minerals such as zircon, Allanite, monazite, and titanite, high spatial resolution isotopic analysis of the Th–U–Pb system is required. Laser ablation ICP-MS techniques are increasingly applied for this purpose, yet the matrix-dependence of analysis and the paucity of high-quality standards for most of these minerals impose major limitations – as for all in situ microbeam analytical techniques. A novel approach for LA-ICP-MS data reduction is presented here that allows for non-matrix-matched external standardization while yielding highly accurate isotopic ratios and age data. The matrix-dependent downhole fractionation during laser ablation is empirically quantified and corrected; hence, well constrained reference materials (here: Plesovice zircon) can be employed as primary standards for the analysis of rare yet petrologically essential minerals (here: SISS, CAP, and TARA Allanite). Using laser beam sizes of 32 and 24 μm, transient isotope ratio data show systematic differences between zircon and Allanite; these are attributed to matrix effects and are combined to correct for the temporal evolution of the matrix-dependent downhole fractionation. The new data reduction technique was tested on three Allanite standard reference materials demonstrating analytical accuracy at precisions equal to those achieved by ion probe and LA-ICP-MS rastering. The analytical procedures presented here could be applied to any combination of two different matrices (calibration standard and sample), thus greatly mitigating dependence on precisely characterized calibration materials, and contribute to establishing universally applicable LA-ICP-MS dating protocols that can be applied to a much broader range of minerals and chronometers.
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non matrix matched standardisation in la icp ms analysis general approach and application to Allanite th u pb dating
Journal of Analytical Atomic Spectrometry, 2017Co-Authors: Marco Burn, Pierre Lanari, Thomas Pettke, Martin EngiAbstract:To make use of the full geochronological potential of accessory minerals such as zircon, Allanite, monazite, and titanite, high spatial resolution isotopic analysis of the Th–U–Pb system is required. Laser ablation ICP-MS techniques are increasingly applied for this purpose, yet the matrix-dependence of analysis and the paucity of high-quality standards for most of these minerals impose major limitations – as for all in situ microbeam analytical techniques. A novel approach for LA-ICP-MS data reduction is presented here that allows for non-matrix-matched external standardization while yielding highly accurate isotopic ratios and age data. The matrix-dependent downhole fractionation during laser ablation is empirically quantified and corrected; hence, well constrained reference materials (here: Plesovice zircon) can be employed as primary standards for the analysis of rare yet petrologically essential minerals (here: SISS, CAP, and TARA Allanite). Using laser beam sizes of 32 and 24 μm, transient isotope ratio data show systematic differences between zircon and Allanite; these are attributed to matrix effects and are combined to correct for the temporal evolution of the matrix-dependent downhole fractionation. The new data reduction technique was tested on three Allanite standard reference materials demonstrating analytical accuracy at precisions equal to those achieved by ion probe and LA-ICP-MS rastering. The analytical procedures presented here could be applied to any combination of two different matrices (calibration standard and sample), thus greatly mitigating dependence on precisely characterized calibration materials, and contribute to establishing universally applicable LA-ICP-MS dating protocols that can be applied to a much broader range of minerals and chronometers.
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Allanite behaviour during incipient melting in the southern central alps
Geochimica et Cosmochimica Acta, 2012Co-Authors: Daniela Rubatto, Courtney Gregory, Joerg Hermann, Alfons Berger, Martin EngiAbstract:Abstract The response of Allanite to incipient melting was investigated in migmatites from the Tertiary Barrovian-type sequence of the Central Alps (southern Switzerland, northern Italy). Inheritance and new mineral growth were recorded in composite Allanite grains sampled from meta-granitoids and leucosomes. Ion microprobe (SHRIMP) dating of high Th/U Allanite cores in meta-granitoids yield Permian ages consistent with magmatic crystallisation dating protolith intrusion. In contrast, low Th/U Allanite overgrowths and weakly-zoned Allanite in meta-granitoids and leucosomes yield Alpine U–Pb intercept ages between 30 ± 4 and 20 ± 5 Ma; these date Allanite formation during the Barrovian cycle. Major and accessory mineral REE compositions suggest that Alpine Allanite crystallised in the presence of a low-temperature melt. Whereas new zircon growth is rare in the migmatites, Allanite readily recorded growth during the Alpine cycle. Allanite U–Th–Pb isotopes may therefore present a complementary approach to zircon for dating low-temperature partial melting, where the preservation of Allanite is aided by low LREE solubility in hydrous granitic melt. The Th–Pb age is preferred to date high-Th magmatic Allanite, however the U–Pb and Th–Pb ages of Allanite overgrowths may differ (by up to 25%), and this demands a comparison of both U–Pb and Th–Pb isotopic systems to obtain a best estimate for the timing of low-Th Allanite crystallisation. Protolith Allanite preserves a substantial memory of its initial age in spite of upper amphibolite facies re-working during migmatisation ( T = 620–700 °C), which places strong constraints on Pb closure temperature. Magmatic Allanite contains 60% of initial Pb. Therefore, the initial Pb may be useful as a sensor for the amount of melt present during Allanite formation. The Pb isotope composition of Allanite overgrowths indicates ⩽5% inherited radiogenic Pb from precursor Allanite, which suggests efficient redistribution and homogenisation of Pb isotopes during the Alpine partial melting period.
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redistribution of ree y th and u at high pressure Allanite forming reactions in impure meta quartzites sesia zone western italian alps
American Mineralogist, 2012Co-Authors: Daniele Regis, James Darling, Benedicte Cenkitok, Martin EngiAbstract:Accessory phases are important hosts of trace elements; Allanite may contain >90% of the REE in a bulk rock. The mobility and redistribution of several trace elements, notably HREE, Th, U, and Y is thus controlled by reactions involving Allanite and other REE phases, as well as several rock-forming minerals. As these elements are commonly concentrated in mature clastic sediments, a suite of impure quartzite was studied. Two eclogite facies samples from the Monometamorphic Cover Complex of the Sesia Zone (Western Italian Alps) are presented in some detail, as they reveal a remarkably rich spectrum of reaction relationships that involve REE phases. Two Allanite-forming reactions were inferred from textures and phase compositions monazite + Ca - silicate ( ? ) + fluid → Allanite + apatite + thorite ; (1) monazite + thorite + Ca - silicate ( ? ) + fluid → Th - rich Allanite + auerlite ± apatite . (2) Petrographic observations and thermodynamic models suggest that Allanite entered the HP assemblage at ~530 °C and 17–18 kbar during prograde metamorphism. In one sample, Allanite is rimmed by epidote rich in Y and HREE that grew at the expense of xenotime. Two net transfer reactions were derived xenotime + Allanite + fluid → Y - rich epidote + apatite + thorite ; (3) xenotime + Allanite + fluid → Y - rich epidote + aeschynite + thorite + ( phosphate ? ) . (4) Textural relationships and trace element analyses of coexisting Allanite/monazite and xenotime/Y-rich epidote reveal systematic partitioning of the REE. Partition coefficients for the HREE are compatible with equilibrium fractionation, whereas those for the LREE show patterns that seem to be inherited from the precursor phases, in this case zircon with variable LREE composition.
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Allanite u th pb geochronology by laser ablation icpms
Chemical Geology, 2012Co-Authors: James Darling, Craig Storey, Martin EngiAbstract:Abstract The rare earth element (REE) rich epidote-group mineral Allanite has great potential as a geochronometer for a wide range of geological processes. Its utilization has been hampered by several analytical challenges: often-high common-Pb contents (208Pbcomon/208Pbtotal from Using a 213 nm Nd:YAG laser, coupled with an Agilent 7500cs (quadrupole) ICPMS, analytical protocols focused upon minimizing the causes of matrix sensitivity in the laser ablation ICPMS process. This has primarily been achieved via dynamic (raster) ablation, which greatly reduces time-dependent laser induced elemental fractionation. Accordingly, a non-matrix matched external standardization approach is adopted, utilizing the zircon standards Plesovice, 91500 and GJ1. Accurate common-Pb corrections are critical to Allanite geochronology, and here we advocate an approach based upon the measured intensity of 204Pb, as it minimizes assumptions and allows for simple and robust error propagation. For each of the Allanite reference materials, mean common-Pb corrected 208Pb/232Th ages and Tera–Wasserburg regression ages are within uncertainty of reference values. Three of the samples (SISS, BONA and AVC) have significantly higher 206Pb/238U ages, which reflects incorporation of 230Th during crystallization, and hence 206Pb excess. Such isotopic disequilibrium, together with the high Th/U ratios of Allanites (up to 1000), makes the Th–Pb dating system preferable, particularly for relatively young Allanites (i.e. Phanerozoic). That accurate age information has been generated from Allanites of wide ranging composition, suggests that matrix effects are not significant in our analyses, on the scale of uncertainties generated: 0.5 to 1.5% (2σ) on mean 208Pb/232Th ages. The final propagated uncertainty in ages is a function of the common-Pb content, uncertainty in the measurement of 202Hg/204Pb, and uncertainty in the isotopic composition of the common-Pb inherited into the mineral. Our results show that accurate U–Th–Pb geochronological information, at geologically useful levels of precision, can be determined from Allanite with relatively simple analytical and data reduction protocols, and without the requirement for matrix-matched standardization.
Carl Spandler - One of the best experts on this subject based on the ideXlab platform.
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Preservation of Permian Allanite within an Alpine eclogite facies shear zone at Mt Mucrone, Italy: Mechanical and chemical behavior of Allanite during mylonitization
Lithos, 2020Co-Authors: B. Cenki-tok, Emilien Oliot, T B Thomsen, Paola Manzotti, D. Rubatto, A. Berger, M. Engi, E. Janots, D. Regis, Carl SpandlerAbstract:International audienceThis study addresses the mechanical and chemical behavior of Allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in Allanite during geological processes is essential for robust petrochronology. Dating of Allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet Allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287 ± 7 Ma). These mm-sized Allanite grains are rimmed by an aggregate of coarse-grained garnet + phengite, thought to derive from former epidote. These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291 ± 5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian Allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning. These results indicate that Allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of Allanite and not by mere Pb diffusion alone under the HP conditions
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preservation of permian Allanite within an alpine eclogite facies shear zone at mt mucrone italy mechanical and chemical behavior of Allanite during mylonitization
Lithos, 2011Co-Authors: Benedicte Cenkitok, Martin Engi, Daniela Rubatto, Alfons Berger, Emilie Janots, Daniele Regis, Emilien Oliot, T B Thomsen, Paola Manzotti, Carl SpandlerAbstract:This study addresses the mechanical and chemical behavior of Allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in Allanite during geological processes is essential for robust petrochronology. Dating of Allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet Allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287 ± 7 Ma). These mm-sized Allanite grains are rimmed by an aggregate of coarse-grained garnet + phengite, thought to derive from former epidote. These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291 ± 5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian Allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning. These results indicate that Allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of Allanite and not by mere Pb diffusion alone under the HP conditions.
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prograde metamorphic sequence of ree minerals in pelitic rocks of the central alps implications for Allanite monazite xenotime phase relations from 250 to 610 c
Journal of Metamorphic Geology, 2008Co-Authors: Emilie Janots, Martin Engi, Alfons Berger, J Allaz, Jensoliver Schwarz, Carl SpandlerAbstract:The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of Allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well-established metamorphic field gradient that record conditions from very low grade metamorphism (250 C) to the lower amphibolite facies (600 C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and Allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low-grade metamorphism (<440 C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid (440–450 C, thermometry based on chlorite–choritoid and carbonaceous material), monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of Allanite distinguishable by EMPA and X-ray mapping. Prior to garnet growth, Allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid–out zone boundary (556–580 C, based on phase equilibrium calculations), Allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience reequilibration following their prograde formation. Hence, the partial breakdown of Allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of Allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade.
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Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for Allanite-monazite-xenotime phase relations from 250 to 610 C
Journal of Metamorphic Geology, 2008Co-Authors: Emilie Janots, Martin Engi, Alfons Berger, J Allaz, Jensoliver Schwarz, Carl SpandlerAbstract:The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of Allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well-established metamorphic field gradient that record conditions from very low grade metamorphism (250 C) to the lower amphibolite facies (600 C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and Allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low-grade metamorphism (
Joerg Hermann - One of the best experts on this subject based on the ideXlab platform.
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Allanite behaviour during incipient melting in the southern central alps
Geochimica et Cosmochimica Acta, 2012Co-Authors: Daniela Rubatto, Courtney Gregory, Joerg Hermann, Alfons Berger, Martin EngiAbstract:Abstract The response of Allanite to incipient melting was investigated in migmatites from the Tertiary Barrovian-type sequence of the Central Alps (southern Switzerland, northern Italy). Inheritance and new mineral growth were recorded in composite Allanite grains sampled from meta-granitoids and leucosomes. Ion microprobe (SHRIMP) dating of high Th/U Allanite cores in meta-granitoids yield Permian ages consistent with magmatic crystallisation dating protolith intrusion. In contrast, low Th/U Allanite overgrowths and weakly-zoned Allanite in meta-granitoids and leucosomes yield Alpine U–Pb intercept ages between 30 ± 4 and 20 ± 5 Ma; these date Allanite formation during the Barrovian cycle. Major and accessory mineral REE compositions suggest that Alpine Allanite crystallised in the presence of a low-temperature melt. Whereas new zircon growth is rare in the migmatites, Allanite readily recorded growth during the Alpine cycle. Allanite U–Th–Pb isotopes may therefore present a complementary approach to zircon for dating low-temperature partial melting, where the preservation of Allanite is aided by low LREE solubility in hydrous granitic melt. The Th–Pb age is preferred to date high-Th magmatic Allanite, however the U–Pb and Th–Pb ages of Allanite overgrowths may differ (by up to 25%), and this demands a comparison of both U–Pb and Th–Pb isotopic systems to obtain a best estimate for the timing of low-Th Allanite crystallisation. Protolith Allanite preserves a substantial memory of its initial age in spite of upper amphibolite facies re-working during migmatisation ( T = 620–700 °C), which places strong constraints on Pb closure temperature. Magmatic Allanite contains 60% of initial Pb. Therefore, the initial Pb may be useful as a sensor for the amount of melt present during Allanite formation. The Pb isotope composition of Allanite overgrowths indicates ⩽5% inherited radiogenic Pb from precursor Allanite, which suggests efficient redistribution and homogenisation of Pb isotopes during the Alpine partial melting period.
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exploring the potential of Allanite as a geochronometer of high grade crustal processes
Geochimica et Cosmochimica Acta, 2006Co-Authors: Courtney Gregory, Daniela Rubatto, Joerg HermannAbstract:The REE-rich accessory mineral Allanite plays a key role in the storage and mobility of geochemically important trace elements (LREE, Th) in magmatic and high-grade rocks. Allanite occurs in a wide range of rock types, but of particular interest is its common presence in mafic, migmatitic and high-pressure rocks. We report on the response of Allanite trace element chemistry and its U-Th-Pb isotopic system to magmatism, partial melting and eclogite-facies metamorphism. In situ U, Th–Pb geochronology of Allanite has been carried out using SHRIMP ion microprobe in conjunction with LA ICP-MS analysis. We analyse Allanite with a variety of FeO and trace element compositions from anatectic rocks, granodioritic to tonalitic plutons and high-pressure mafic rocks. Allanite Th– Pb ages are capable of reliably addressing geochronological problems with a precision of 1–2% and 2–5% (2r) for magmatic and metamorphic rocks, respectively. This study allows preliminary conclusions to be made for the closure temperature of Allanite. In the case studies presented, no indication of inheritance has been observed for Allanite. The trace element composition of Allanite records changes in paragenesis. Allanite HREE content relative to LREE provides an indicator for the co-crystallization of garnet. Where Allanite formed in migmatites display a small negative Eu anomaly, eclogitic Allanite lacks a Eu anomaly, which is related to crystallization in the absence of plagioclase. In addition, the Sr content in Allanite can be used as an indicator of crystallization above the stability field of plagioclase. This study demonstrates that Allanite can be correlated to major metamorphic and rock-forming minerals and therefore the P-T conditions of crystallization.
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Allanite thorium and light rare earth element carrier in subducted crust
Chemical Geology, 2002Co-Authors: Joerg HermannAbstract:Abstract The investigation of deeply subducted eclogites from the Dora-Maira massif, Western Alps reveals that accessory minerals are important hosts for trace elements. Rutile contains most of the bulk rock Ti, Nb and Ta while zircon hosts nearly all Zr and Hf. More than 90% of the bulk rock light rare earth elements (LREE) and Th and about 75% of U are incorporated in accessory Allanite. Phengite is the most interesting major mineral because it hosts more than 95% of Rb, Ba and Cs. Synthesis piston cylinder experiments in a model crustal composition in the range 2.0–4.5 GPa, 680–1150 °C doped with trace elements demonstrate that accessory Allanite forms at the expense of the major mineral zoisite at temperatures above 700 °C and 2.0 GPa. Allanite is stabilised by the incorporation of light rare earth elements and was found up to temperatures of 1050 °C and to pressures of at least 4.5 GPa. Disappearance of Allanite is caused by dissolution in a coexisting hydrous granitic melt and is not related to the breakdown of any major phase. Experimentally determined element partitioning between Allanite and a hydrous granitic melt at 900 °C, 2.0 GPa yield DLREEall/melt of about 200, DThall/melt of 60 and DUall/melt of 20. These results, combined with literature data, are used to estimate light rare earth element saturation levels of subduction zone liquids as a function of temperature. Allanite is a residual phase up to temperatures of at least 900 and 1000 °C for metabasalts and metasediments, respectively, at ≤10% partial melting. Allanite is therefore capable of controlling LREE and to a lesser extent Th contents for the critical temperature range of subduction zone liquid extraction. Low temperature fluids coexisting with Allanite are not capable of transporting significant amounts of light rare earth elements and therefore arc lavas showing enrichment of these elements probably originate from a mantle source that experienced metasomatism by a hydrous granitic melt.
Emilie Janots - One of the best experts on this subject based on the ideXlab platform.
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Allanite petrochronology in fresh and retrogressed garnet biotite metapelites from the longmen shan eastern tibet
Journal of Petrology, 2019Co-Authors: Laura Airaghi, Emilie Janots, Pierre Lanari, Julia De Sigoyer, Valérie MagninAbstract:Linking the timing of Allanite growth to metamorphic conditions in metapelites is particularly challenging because of the large variety of Allanite textures and chemical compositions. This study focuses on five garnet–biotite metapelites retrogressed to different extents, from the internal domain of the Longmen Shan (eastern Tibet) from which few petrochronological data documenting the metamorphic ages are available. Microstructural observations were combined with whole-rock compositions, detailed mineral characterization, phase equilibria modelling, and in situ U–Th/Pb Allanite dating to gain insights into the Allanite reactivity relative to other rock-forming minerals. All samples experienced similar peak temperatures of 560–600°C. Allanite is the main rare earth element (REE)-bearing accessory mineral but it exhibits different textures, such as epidote rims with distinct REE contents as well as late inclusion-like dissolution features. Garnet is, along with Allanite, critical to reconstruct the REE budget of these rocks. In the two samples where Allanite is observed as inclusions in garnet, garnet shows no textural zoning and a low Y content ( 520°C). This relative chronology is confirmed by in situ U–Th/Pb Allanite dating: pre-garnet Allanite shows ages of c. 200 Ma, whereas syn- to post-garnet Allanite has ages of c. 180 Ma. The timing of Allanite appearance strongly correlates with the biotite-in reaction predicted by the models and observed in microstructures. In the two samples collected close to the major Wenchuan Shear Zone—deformed up to mylonitization and retrogressed under greenschist-facies conditions (3–4 ± 1 kbar, 350–400°C)—Allanite still preserves the peak metamorphic ages (c. 200 and c. 180 Ma). Allanite and its epidote rims, however, are fragmented and partly replaced by a retrograde assemblage of Qz + Pb-depleted (<100 ppm) monazite, providing an age <90–100 Ma. Our results allow different metamorphic stages for the studied transect in the central Longmen Shan to be refined: a prograde path at c. 200 Ma, a thermal relaxation and exhumation from c. 180 Ma, and a late greenschist overprint. This study also shows that when Allanite is the principal phase in a mineral assemblage datable by U–Th/Pb, detailed observations of Allanite textures may be highly informative in constraining the timing of its growth relative to other major mineral phases.
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Allanite Petrochronology in Fresh and Retrogressed Garnet–Biotite Metapelites from the Longmen Shan (Eastern Tibet)
Journal of Petrology, 2018Co-Authors: Laura Airaghi, Emilie Janots, Pierre Lanari, Julia De Sigoyer, Valérie MagninAbstract:Linking the timing of Allanite growth to metamorphic conditions in metapelites is particularly challenging because of the large variety of Allanite textures and chemical compositions. This study focuses on five garnet–biotite metapelites retrogressed to different extents, from the internal domain of the Longmen Shan (eastern Tibet) from which few petrochronological data documenting the metamorphic ages are available. Microstructural observations were combined with whole-rock compositions, detailed mineral characterization, phase equilibria modelling, and in situ U–Th/Pb Allanite dating to gain insights into the Allanite reactivity relative to other rock-forming minerals. All samples experienced similar peak temperatures of 560–600°C. Allanite is the main rare earth element (REE)-bearing accessory mineral but it exhibits different textures, such as epidote rims with distinct REE contents as well as late inclusion-like dissolution features. Garnet is, along with Allanite, critical to reconstruct the REE budget of these rocks. In the two samples where Allanite is observed as inclusions in garnet, garnet shows no textural zoning and a low Y content ( 520°C). This relative chronology is confirmed by in situ U–Th/Pb Allanite dating: pre-garnet Allanite shows ages of c. 200 Ma, whereas syn- to post-garnet Allanite has ages of c. 180 Ma. The timing of Allanite appearance strongly correlates with the biotite-in reaction predicted by the models and observed in microstructures. In the two samples collected close to the major Wenchuan Shear Zone—deformed up to mylonitization and retrogressed under greenschist-facies conditions (3–4 ± 1 kbar, 350–400°C)—Allanite still preserves the peak metamorphic ages (c. 200 and c. 180 Ma). Allanite and its epidote rims, however, are fragmented and partly replaced by a retrograde assemblage of Qz + Pb-depleted (
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preservation of permian Allanite within an alpine eclogite facies shear zone at mt mucrone italy mechanical and chemical behavior of Allanite during mylonitization
Lithos, 2011Co-Authors: Benedicte Cenkitok, Martin Engi, Daniela Rubatto, Alfons Berger, Emilie Janots, Daniele Regis, Emilien Oliot, T B Thomsen, Paola Manzotti, Carl SpandlerAbstract:This study addresses the mechanical and chemical behavior of Allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in Allanite during geological processes is essential for robust petrochronology. Dating of Allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet Allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287 ± 7 Ma). These mm-sized Allanite grains are rimmed by an aggregate of coarse-grained garnet + phengite, thought to derive from former epidote. These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291 ± 5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian Allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning. These results indicate that Allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of Allanite and not by mere Pb diffusion alone under the HP conditions.
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metamorphic rates in collisional orogeny from in situ Allanite and monazite dating
Geology, 2009Co-Authors: Emilie Janots, Martin Engi, Courtney Gregory, Daniela Rubatto, Alfons Berger, Meinert RahnAbstract:The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), Allanite is partly replaced by monazite at 560–580 °C. Relic Allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and Allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of Allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fission track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.
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prograde metamorphic sequence of ree minerals in pelitic rocks of the central alps implications for Allanite monazite xenotime phase relations from 250 to 610 c
Journal of Metamorphic Geology, 2008Co-Authors: Emilie Janots, Martin Engi, Alfons Berger, J Allaz, Jensoliver Schwarz, Carl SpandlerAbstract:The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of Allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well-established metamorphic field gradient that record conditions from very low grade metamorphism (250 C) to the lower amphibolite facies (600 C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and Allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low-grade metamorphism (<440 C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid (440–450 C, thermometry based on chlorite–choritoid and carbonaceous material), monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of Allanite distinguishable by EMPA and X-ray mapping. Prior to garnet growth, Allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid–out zone boundary (556–580 C, based on phase equilibrium calculations), Allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience reequilibration following their prograde formation. Hence, the partial breakdown of Allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of Allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade.
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Allanite behaviour during incipient melting in the southern central alps
Geochimica et Cosmochimica Acta, 2012Co-Authors: Daniela Rubatto, Courtney Gregory, Joerg Hermann, Alfons Berger, Martin EngiAbstract:Abstract The response of Allanite to incipient melting was investigated in migmatites from the Tertiary Barrovian-type sequence of the Central Alps (southern Switzerland, northern Italy). Inheritance and new mineral growth were recorded in composite Allanite grains sampled from meta-granitoids and leucosomes. Ion microprobe (SHRIMP) dating of high Th/U Allanite cores in meta-granitoids yield Permian ages consistent with magmatic crystallisation dating protolith intrusion. In contrast, low Th/U Allanite overgrowths and weakly-zoned Allanite in meta-granitoids and leucosomes yield Alpine U–Pb intercept ages between 30 ± 4 and 20 ± 5 Ma; these date Allanite formation during the Barrovian cycle. Major and accessory mineral REE compositions suggest that Alpine Allanite crystallised in the presence of a low-temperature melt. Whereas new zircon growth is rare in the migmatites, Allanite readily recorded growth during the Alpine cycle. Allanite U–Th–Pb isotopes may therefore present a complementary approach to zircon for dating low-temperature partial melting, where the preservation of Allanite is aided by low LREE solubility in hydrous granitic melt. The Th–Pb age is preferred to date high-Th magmatic Allanite, however the U–Pb and Th–Pb ages of Allanite overgrowths may differ (by up to 25%), and this demands a comparison of both U–Pb and Th–Pb isotopic systems to obtain a best estimate for the timing of low-Th Allanite crystallisation. Protolith Allanite preserves a substantial memory of its initial age in spite of upper amphibolite facies re-working during migmatisation ( T = 620–700 °C), which places strong constraints on Pb closure temperature. Magmatic Allanite contains 60% of initial Pb. Therefore, the initial Pb may be useful as a sensor for the amount of melt present during Allanite formation. The Pb isotope composition of Allanite overgrowths indicates ⩽5% inherited radiogenic Pb from precursor Allanite, which suggests efficient redistribution and homogenisation of Pb isotopes during the Alpine partial melting period.
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preservation of permian Allanite within an alpine eclogite facies shear zone at mt mucrone italy mechanical and chemical behavior of Allanite during mylonitization
Lithos, 2011Co-Authors: Benedicte Cenkitok, Martin Engi, Daniela Rubatto, Alfons Berger, Emilie Janots, Daniele Regis, Emilien Oliot, T B Thomsen, Paola Manzotti, Carl SpandlerAbstract:This study addresses the mechanical and chemical behavior of Allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in Allanite during geological processes is essential for robust petrochronology. Dating of Allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet Allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287 ± 7 Ma). These mm-sized Allanite grains are rimmed by an aggregate of coarse-grained garnet + phengite, thought to derive from former epidote. These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291 ± 5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian Allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning. These results indicate that Allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of Allanite and not by mere Pb diffusion alone under the HP conditions.
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metamorphic rates in collisional orogeny from in situ Allanite and monazite dating
Geology, 2009Co-Authors: Emilie Janots, Martin Engi, Courtney Gregory, Daniela Rubatto, Alfons Berger, Meinert RahnAbstract:The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), Allanite is partly replaced by monazite at 560–580 °C. Relic Allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and Allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of Allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fission track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.
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prograde metamorphic sequence of ree minerals in pelitic rocks of the central alps implications for Allanite monazite xenotime phase relations from 250 to 610 c
Journal of Metamorphic Geology, 2008Co-Authors: Emilie Janots, Martin Engi, Alfons Berger, J Allaz, Jensoliver Schwarz, Carl SpandlerAbstract:The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of Allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well-established metamorphic field gradient that record conditions from very low grade metamorphism (250 C) to the lower amphibolite facies (600 C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and Allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low-grade metamorphism (<440 C); these are enriched in La, but depleted in Th and Y, compared with inherited grains. Upon the appearance of chloritoid (440–450 C, thermometry based on chlorite–choritoid and carbonaceous material), monazite is consumed, and MREE and LREE are taken up preferentially in two distinct zones of Allanite distinguishable by EMPA and X-ray mapping. Prior to garnet growth, Allanite acquires two growth zones of clinozoisite: a first one rich in HREE + Y and a second one containing low REE contents. Following garnet growth, close to the chloritoid–out zone boundary (556–580 C, based on phase equilibrium calculations), Allanite and its rims are partially to totally replaced by monazite and xenotime, both associated with plagioclase (± biotite ± staurolite ± kyanite ± quartz). In these samples, epidote relics are located in the matrix or as inclusions in garnet, and these preserve their characteristic chemical and textural growth zoning, indicating that they did not experience reequilibration following their prograde formation. Hence, the partial breakdown of Allanite to monazite offers the attractive possibility to obtain in situ ages, representing two distinct crystallization stages. In addition, the complex REE + Y and Th zoning pattern of Allanite and monazite are essential monitors of crystallization conditions at relatively low metamorphic grade.
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Prograde metamorphic sequence of REE minerals in pelitic rocks of the Central Alps: implications for Allanite-monazite-xenotime phase relations from 250 to 610 C
Journal of Metamorphic Geology, 2008Co-Authors: Emilie Janots, Martin Engi, Alfons Berger, J Allaz, Jensoliver Schwarz, Carl SpandlerAbstract:The distribution of REE minerals in metasedimentary rocks was investigated to gain insight into the stability of Allanite, monazite and xenotime in metapelites. Samples were collected in the central Swiss Alps, along a well-established metamorphic field gradient that record conditions from very low grade metamorphism (250 C) to the lower amphibolite facies (600 C). In the Alpine metapelites investigated, mass balance calculations show that LREE are mainly transferred between monazite and Allanite during the course of prograde metamorphism. At very low grade metamorphism, detrital monazite grains (mostly Variscan in age) have two distinct populations in terms of LREE and MREE compositions. Newly formed monazite crystallized during low-grade metamorphism (
