UA GEOS 474A - Diffusion versus recrystallization processes

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Diffusion versus recrystallization processes in Rb-Sr geochronology: Isotopic relics in eclogite facies rocks, Western Gneiss Region, NorwayIntroductionGeological settingThe Western Gneiss RegionThe Dalsfjord areaSamples and sample petrographyMineral chemistryPyroxeneGarnetSheet silicatesAmphibolePlagioclaseEpidoteApatiteThermobarometryGranulite faciesEclogite faciesSummary of PT dataAnalytical procedures for isotope analysesResultsDiscussionAge of granulite facies metamorphismAge of eclogite facies metamorphismThe role of fluids, deformation and diffusion for reaction progressSr redistribution in a dry systemIsotope redistribution: the role of apatiteConclusionsAcknowledgmentsReferencesDiffusion versus recrystallization processes inRb–Sr geochronology: Isotopic relics in eclogite faciesrocks, Western Gneiss Region, NorwayJohannes Glodnya,*, Alexander Ku¨hnb,1,Ha˚kon AustrheimcaGeoForschungsZentrum Potsdam, Telegrafenberg C2, 14473 Potsdam, GermanybInstitut fu¨r Geowissenschaften, Johannes Gutenberg-Universita¨t, Becherweg 21, 55099 Mainz, GermanycPGP and Department of Earth Sciences, Postboks 1047, University of Oslo, N-0316 Oslo, NorwayReceived 23 January 2007; accepted in revised form 29 October 2007; available online 4 November 2007AbstractRb–Sr and U–Pb isotopic data for granulite facies rocks, forming textural relics with respect to eclogite facies metamor-phism in the Western Gneiss Region (WGR) of Norway, highlight the importance of textures and mineral reaction kinetics forthe interpretation of geochronological data. Studied rocks from Ba˚rdsholmen, southern WGR, were subjected to granulitefacies metamorphism at 955 ± 3 Ma (U–Pb, zircon). Later on, they experienced a subduction-related, kinetically strandedeclogitization (T > 650 Cat20 kbar) at 404 ± 2 Ma (Rb–Sr multimineral internal isochron data), followed by exhumationthrough amphibolite facies conditions. Full conversion of granulite to eclogite was restricted to zones of fluid infiltration anddeformation. Despite the fact that metamorphic temperatures vastly exceeded the commonly assumed ‘closure temperature’for Rb–Sr in submillimeter-sized biotite for several Ma during eclogite facies overprint, Sr-isotopic signatures of relic biotitehave not been fully reset. Large biotite crystals nearly record their Sveconorwegian (Grenvillian) crystallization age. Sr sig-natures of other granulite facies phases (feldspar, pyroxenes, amphibole) remained unchanged, with the exception of apatite.The results imply that isotopic signatures much closer correspond to the P, T conditions of formation recorded by a datedphase and its paragenesis, than to a temperature history. In texturally well-equilibrated high-grade rocks which experiencedno mineral reactions and remained devoid of free fluids during exhumation, like granulites or eclogites, isotopic resetting dur-ing cooling is either kinetically locked, or restricted to sluggish intermineral diffusion which demonstrably does not lead to fullisotopic homogenization. In texturally unequilibrated rocks, textural relics are likely to represent isotopic relics. It is shownthat for both high-grade rocks and for rocks with textural disequilibria, geologically meaningful isotopic ages based on iso-chron methods can only be derived from sub-assemblages in isotopic equilibrium, which have to be defined by analysis of allrock-forming minerals. Conventional two-point ‘mica ages’ for such rocks are a priori geochronologically uninterpretable,and valid multimineral isochron ages a priori do not record cooling but instead date recrystallization-inducing processes likefluid–rock interaction. 2007 Elsevier Ltd. All rights reserved.1. INTRODUCTIONA persistent challenge in radiogenic isotope geochronol-ogy is to accurately link isotopic age data to specific pro-cesses in a rock, like crystallization and recrystallization,fluid–rock interaction, metamorphic mineral reactions,deformation, and cooling. This step in the interpretationof isotopic data is critical if data are to be converted to geo-dynamic information on nature and velocity of orogenicprocesses at depth. Two fundamental factors have beenidentified capable of setting and modifying element distri-bution signatures as well as isotopic signatures of the widely0016-7037/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.gca.2007.10.021*Corresponding author. Fax: +49 331 2881370.E-mail address: [email protected] (J. Glodny).1Gexco AS, Postboks 500, 8601 Mo i Rana, Norway.www.elsevier.com/locate/gcaAvailable online at www.sciencedirect.comGeochimica et Cosmochimica Acta 72 (2008) 506–525used Rb–Sr, K–Ar, U–Pb and Sm–Nd systems in rocks andminerals, namely (a) recrystallization-inducing processes,like deformation and fluid–rock interaction with dissolu-tion/reprecipitation, and (b) temperature-driven diffusion(see Villa, 2006 for review). The potential of temperature-driven diffusion to modify isotopic mineral signatures inrocks has been discussed for long. It has been used to devel-op elaborate concepts, essentially translating diffusivities ofradiogenic nuclides within specific minerals into a narrowtemperature range separating high-temperature nuclidemobility from low-temperature immobility, the so-called‘blocking temperature’ or ‘closure temperature’ (e.g., Ja¨ger,1967; Dodson, 1973; Lovera et al., 1989). A system of dif-ferent closure temperatures for different decay systems ina number of minerals has been established, which is oftenused to infer thermal histories of geologic units. Isotope dif-fusivities in this context have either been deduced from nat-ural examples, i.e., from isotopic signatures in rocks withwell-constrained thermal histories, or from experimentaldiffusion data. While extrapolation of experimental diffu-sion or other kinetic data to geological timescales and tem-peratures is fraught with difficulties (see Baxter andDePaolo, 2002; Villa, 2006), some of the early geologicalstudies of isotopic closure were biased towards a searchfor temperature effects, and did not take adequately into ac-count other factors than temperature influencing isotopesignatures.Geochronological studies involving independent tex-tural information or observational data from microanalyt-ical techniques, like electron microprobe analysis orcathodoluminescence imaging, generally reveal close link-age between isotopic signatures, texture, and


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