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Allanite

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Martin Engi – 1st expert on this subject based on the ideXlab platform

  • Non-matrix-matched standardisation in LA-ICP-MS analysis: general approach, and application to Allanite Th–U–Pb dating
    Journal of Analytical Atomic Spectrometry, 2020
    Co-Authors: Marco Burn, Pierre Lanari, Thomas Pettke, Martin Engi

    Abstract:

    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.

  • non matrix matched standardisation in la icp ms analysis general approach and application to Allanite th u pb dating
    Journal of Analytical Atomic Spectrometry, 2017
    Co-Authors: Marco Burn, Pierre Lanari, Thomas Pettke, Martin Engi

    Abstract:

    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.

  • Allanite behaviour during incipient melting in the southern central alps
    Geochimica et Cosmochimica Acta, 2012
    Co-Authors: Daniela Rubatto, Courtney Gregory, Joerg Hermann, Alfons Berger, Martin Engi

    Abstract:

    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.

Carl Spandler – 2nd expert on this subject based on the ideXlab platform

  • Preservation of Permian Allanite within an Alpine eclogite facies shear zone at Mt Mucrone, Italy: Mechanical and chemical behavior of Allanite during mylonitization
    Lithos, 2020
    Co-Authors: B. Cenki-tok, Emilien Oliot, T B Thomsen, Paola Manzotti, D. Rubatto, A. Berger, M. Engi, E. Janots, D. Regis, Carl Spandler

    Abstract:

    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

  • preservation of permian Allanite within an alpine eclogite facies shear zone at mt mucrone italy mechanical and chemical behavior of Allanite during mylonitization
    Lithos, 2011
    Co-Authors: Benedicte Cenkitok, Martin Engi, Daniela Rubatto, Alfons Berger, Emilie Janots, Daniele Regis, Emilien Oliot, T B Thomsen, Paola Manzotti, Carl Spandler

    Abstract:

    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.

  • 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, 2008
    Co-Authors: Emilie Janots, Martin Engi, Alfons Berger, J Allaz, Jensoliver Schwarz, Carl Spandler

    Abstract:

    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.

Joerg Hermann – 3rd expert on this subject based on the ideXlab platform

  • Allanite behaviour during incipient melting in the southern central alps
    Geochimica et Cosmochimica Acta, 2012
    Co-Authors: Daniela Rubatto, Courtney Gregory, Joerg Hermann, Alfons Berger, Martin Engi

    Abstract:

    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.

  • exploring the potential of Allanite as a geochronometer of high grade crustal processes
    Geochimica et Cosmochimica Acta, 2006
    Co-Authors: Courtney Gregory, Daniela Rubatto, Joerg Hermann

    Abstract:

    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.

  • Allanite thorium and light rare earth element carrier in subducted crust
    Chemical Geology, 2002
    Co-Authors: Joerg Hermann

    Abstract:

    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.