Experimental zircon/melt and zircon/garnet trace element partitioning and implications for the geochronology of crustal rocks (original) (raw)
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Zircon ages in granulite facies rocks: decoupling from geochemistry above 850 °C?
Contributions to Mineralogy and Petrology
Granulite facies rocks frequently show a large spread in their zircon ages, the interpretation of which raises questions: Has the isotopic system been disturbed? By what process(es) and conditions did the alteration occur? Can the dates be regarded as real ages, reflecting several growth episodes? Furthermore, under some circumstances of (ultra-)high-temperature metamorphism, decoupling of zircon U-Pb dates from their trace element geochemistry has been reported. Understanding these processes is crucial to help interpret such dates in the context of the P-T history. Our study presents evidence for decoupling in zircon from the highest grade metapelites (> 850 °C) taken along a continuous high-temperature metamorphic field gradient in the Ivrea Zone (NW Italy). These rocks represent a well-characterised segment of Permian lower continental crust with a protracted high-temperature history. Cathodoluminescence images reveal that zircons in the mid-amphibolite facies preserve mainly detrital cores with narrow overgrowths. In the upper amphibolite and granulite facies, preserved detrital cores decrease and metamorphic zircon increases in quantity. Across all samples we document a sequence of four rim generations based on textures. U-Pb dates, Th/U ratios and Ti-in-zircon concentrations show an essentially continuous evolution with increasing metamorphic grade, except in the samples from the granulite facies, which display significant scatter in age and chemistry. We associate the observed decoupling of zircon systematics in high-grade non-metamict zircon with disturbance processes related to differences in behaviour of non-formula elements (i.e. Pb, Th, U, Ti) at high-temperature conditions, notably differences in compatibility within the crystal structure. Keywords Zircon • High-temperature metamorphism • Decoupling zircon characteristics • U-Pb dating • Th/U ratios • Ti-in-zircon thermometry Communicated by Timothy L. Grove.
REE partition among zircon, orthopyroxene, amphibole and garnet in a high-grade metabasic system
Geological Magazine, 2017
A mafic amphibole-bearing granulite with porphyroblastic garnet was investigated to evaluate: (1) the rare earth element (REE) partition among garnet, zircon, orthopyroxene and amphibole during the metamorphic evolution; (2) the significance of the REE distribution along lobes and bights of reabsorbed garnet rim; and (3) REE distribution coefficient values (DREE) suggestive of chemical equilibrium, assuming garnet as a reference. The results have been compared with those deriving from an intermediate granulite containing porphyroblastic garnet, without amphibole. Porphyroblastic garnet from both samples is rimmed by a continuous corona formed during post-peak decompression characterized by REE-enriched lobes and REE-poor bights. The amphiboles from corona have various REE abundances, reflecting a different dissolution rate of original garnet rim. The initial slow rate of garnet dissolution caused high REE concentration in the new garnet rim due to intra-crystalline diffusion, leading to the formation of REE-poorer amphiboles in corona. Subsequently, under an increasing geothermal gradient and fluid-present conditions, the faster dissolution of garnet determined the formation of bights and the transfer of REEs towards the corona. The timing of garnet growth and its dissolution were checked by U-Pb zircon ages. The zircons dated from 339 Ma to 303 Ma in two rock types combined with the garnet domains (core, outer core, rim) show similar distribution of patterns relative to heavy rare earth elements for zircon and garnet (DHREE zrn/grt), suggesting chemical equilibrium. Zircons dated at c. 300 Ma do not appear in equilibrium with REE-rich garnet lobes, and younger zircons (278 Ma) show a new equilibrium with REE-poor garnet bights. On this basis, the DHREE amph/grt values obtained in specific textural sites might be interpreted as suggestive of equilibrium under granulite conditions.
Slow cooling of deep crustal granulites and Pb-loss in zircon
Geochimica et Cosmochimica Acta, 1999
We attempted to obtain the magmatic crystallization age of a metamorphosed (granulite facies, T ϭ 800 Ϯ 90°C; P ϭ 8 Ϯ 1 kbar) massif-type anorthosite from the Ankafotia body of southwest Madagascar. The sample studied is a coarse-grained leuconorite with good preservation of igneous texture and mineralogy, although plagioclase, which contains abundant rutile and zircon inclusions, has been slightly recrystallized. Thirty three isotope dilution U-Pb analyses of zircons representing single-grain fragments (29 analyses) and multi-grain fractions (4 analyses) yield a spectrum of concordant ages from 631 to 549 Ma, a time span of more than 80 myr. Back-scattered electron and cathodoluminescence images show that most grains are either homogeneous, structureless fragments (35%), or are permeated to a variable degree by anastamosing cracks occupied by relatively U-and/or Th-enriched zircon (45%); a smaller percentage of grains show relict magmatic zoning (20%). Thin, U-and/or Th-rich overgrowths occur on about 25% of grains. Raman spot analyses demonstrate that all fragments are highly crystalline and non-metamict. There are marked correlations between zircon grain size and internal features, such that the oldest grains are larger, and show relict magmatic zoning; the youngest grains are small fragments containing high-U crack networks. Ion microprobe spot analyses show that each zircon grain preserves a distinct trace element signature; rare earth element patterns show heavy REE-enrichment, with negative Eu anomalies and positive Ce anomalies. We suggest that the ca. 80 myr spread in concordant U-Pb ages in this sample is indicative of high-temperature Pb-loss during one or more protracted periods of granulite facies metamorphism, with only minor episodic or continuous metamorphic zircon growth. Volume diffusion and/or fracture-assisted diffusion seems to be the dominant mechanism of Pb-loss. Cooling curves, calculated using recently-measured Pb diffusion parameters, conform to the age-size relationship, and imply very slow cooling rates (1-2°C/myr or less), as might be expected for a terrane in which granulite conditions were maintained for an extended period of time. Our results, therefore, suggest a note of caution for interpretation of concordant zircon ages in meta-igneous rocks affected by high-grade metamorphism of long duration.
Zircon equilibria in metamorphic rocks
2002
This study defines a number of fundamental principles of zircon equilibria in metamorphic rocks u ing a combination of traditional metamorphic petrology. trace element geochemistry. experimental petrology. thermobarometry and geochronology. It i demonstrated that solid-state zircon formation in metamorphic rocks is more likely to occur during retrograde rather than prograde processes. Zircon equilibria in metamorphic rocks is also found to be highly temperature dependent; phases such as garnet and rutile incorporate more Zr with increasing temperature. thereby resorbing coexisting zircon. These findings have profound implications for the interpretation of ages derived from metamorphic zircons. Experimental investigation of the reaction Zr-rutile + quartz -. rutile + zircon reveals a strong temperature dependence on the Zr content of rutile. From experiments. a new geothermometcr is defined based on Zr02 in rutile. in the presence of zircon and qua11z: T(OC) = 89297.49 + 0.63(P l) 27...
Geological Society, London, Special Publications, 2003
In-situ U-Th-Pb analyses by ion-microprobe on zircon in intact textural relationships are combined with backscatter and cathodoluminescence imaging and trace element analyses to provide evidence for growth episodes of zircon. This approach helps: (a) to unravel the polymetamorphic history of aluminous migmatitic and granitoid gneisses of the regional contact aureole around the Rogaland anorthosite-norite intrusive complex; and (b) to constrain the age of M2 ultrahigh-temperature (UHT) metamorphism and the subsequent retrograde M3 event. All samples yield magmatic inherited zircon of c. 1035 Ma, some an additional group at c. 1050 Ma. This suggests that loss of Pb by volume diffusion in non-metamict zircon is not an important factor even under extreme crustal conditions. Furthermore, the identical inheritance patterns in aluminous (garnet, cordierite _+ osumilite-bearing) migmatites and orthogneisses indicate a metasomatic igneous instead of a sedimentary protolith for the migmatite. Results for the M1 metamorphic event at c. 1000 Ma BP are consistent in all samples, including those from outside the orthopyroxene-in isograd. The latter do not show evidence for zircon growth during the M2 metamorphic episode.
Journal of Petrology, 2017
This work aims to show the importance of Zr-in-rutile thermometry for evaluating the P-T history of granulite-facies rocks, where higher diffusion rates in the main constituent minerals impede the use of geothermometers based on element distributions. We apply Zr-in-rutile thermometry to a garnet-clinopyroxene (Grt-Cpx) granulite from the Moldanubian Zone of the Bohemian Massif. Three major metamorphic evolutionary stages are identified from the Grt-Cpx granulite. The early high-pressure (HP) stage is represented by an inclusion assemblage in garnet: a high-Ca garnet core (32-38% grossular, 30-32% pyrope and 32-35% almandine) þ omphacite (36-39% jadeite and 3-5% Ca-Tschermak) þ plagioclase (18% anorthite) þ pargasitic amphibole þ rutile þ zircon þ quartz. The subsequent medium-pressure (MP) stage is represented by matrix minerals composed of augitic clinopyroxene (2-6% jadeite and 2-6% Ca-Tschermak) þ orthopyroxene þ ternary feldspar (17-23% anorthite, 41-44% albite, 33-43% orthoclase; reintegrated compositions from antiperthite grains in the matrix) þ rutile þ ilmenite þ quartz. The final low-pressure (LP) stage is represented by a symplectic corona composed of calcic plagioclase ($90% anorthite) þ orthopyroxene þ magnetite. Application of Grt-Cpx and/or jadeite-quartz-albite geobarometers gives pressures of 1Aˊ8GPafortheearlyHPstageand1Aˊ3−1Aˊ4GPafortheMPstage.ThefinalLPstageisconstrainedtolowerthan1Á8 GPa for the early HP stage and 1Á3-1Á4 GPa for the MP stage. The final LP stage is constrained to lower than 1Aˊ8GPafortheearlyHPstageand1Aˊ3−1Aˊ4GPafortheMPstage.ThefinalLPstageisconstrainedtolowerthan0Á7 GPa using conventional geothermobarometers. Rutile inclusions in high-grossular garnet have a rather low and limited range of Zr contents (mostly 1100-1500 ppm), regardless of inclusion size. This suggests that rutile inclusions preserved the initial Zr compositions without much modification by later re-equilibration. Application of Zr-in-rutile thermometry yields a temperature of 830Cat830 C at 830Cat1Á8 GPa for the early HP stage of granulite evolution. Rutile grains in undeformed clinopyroxene-rich domains of the matrix generally occur as small euhedral crystals and have higher Zr contents (mostly 8000-10000 ppm), corresponding to 980-1066 C at 1Á35 GPa using Zr-in-rutile thermometry. In contrast, those in strongly deformed quartz-rich domains of the matrix occur as coarser and more elongated grains with lower Zr contents (3000-5000 ppm), yielding slightly lower temperatures owing to retrogressive re-equilibration. Based on these results, we reveal that the studied Grt-Cpx granulite underwent a significant heating by about 200 C during the early stage of decompression from the peak pressure. Sensitive high-resolution ion microprobe U-Pb dating for the zircon inclusions in high-grossular garnet indicates that the HP stage of the studied granulite occurred at c. 340 Ma, which is indistinguishable from reported LP
Journal of Petrology, 2010
In situ mineral (garnet, zircon) trace element and garnet^rock REE distribution data obtained on leucogranulites from the Bohemian Massif are consistent with an origin of these now strongly deformed rocks as migmatitic leucogranites formed through dehydration-melting of muscovite-bearing protoliths at high pressures during the Variscan orogeny. Partial melting at P^T conditions of 900^9408C and 1·6^1·8 GPa consumed mica and feldspar to produce peritectic kyanite and high-Ca (20^24 mol % grossular), high-Eu garnet along with leucogranitic melt. D REE (Grt/rock) values calculated from this garnet and its host rock are consistent with garnet^melt equilibrium at these P^T conditions. Ternary feldspar and rutile (2000^3200 ppm Zr) crystallized from the leucogranite melts in the migmatites at minimum temperatures of 4880^9208C. These formed along with apatite prior to and accompanying the crystallization of garnet rims and the majority of the garnet that occurs in the leucogranulites. This garnet is typified by strong depletion in Eu and D REE (Grt/rock) values consistent with garnet^melt equilibrium only after the fractionation of apatite (50·3 wt %) and ternary feldspar (5^15 wt %). Zircons in one leucogranulite are dominated by oscillatory-zoned grains that yield pre-Variscan U^Pb ages (495 and 433 Ma) and crystallized, from Ti-in-zircon thermometry, at 700^8108C in their original host magmas. These xenocrystic grains are potentially preserved because of the short timespan of the high P^T Variscan event at 340 Ma and the moderate amounts of leucogranitic partial melt present in the rocks, estimated as at least 30 wt % melt from Zr mass balance. The production of new zircon associated with the Variscan event in this leucogranulite is limited to weakly zoned zircon rims characterized by flat heavy REE patterns that approach REE equilibrium with garnet rims. This post-peak zircon was formed at minimum P^T conditions of 8408C and 0·8^1·0 GPa along the decompression-cooling path defined by previous work for the Bohemian Massif granulites. The garnetŵ hole-rock REE relationships, observation of Eu depletion between early and later garnet growth phases, and Zr^Ti thermometric estimates indicating temperatures of 48408C for this evolution cannot be explained by the alternative interpretation of the leucogranulites as reworked and recrystallized pre-Variscan low-P granitoids, but support a model in which the Bohemian leucogranulites were high-P Variscan-age migmatites containing significant amounts of melt.
Journal of Metamorphic Geology, 2002
SHRIMP U-Pb ages have been obtained for zircon in granitic gneisses from the aureole of the Rogaland anorthosite-norite intrusive complex, both from the ultrahigh temperature (UHT; >900°C pigeonite-in) zone and from outside the hypersthene-in isograd. Magmatic and metamorphic segments of composite zircon were characterised on the basis of electron backscattered electron and cathodoluminescence images plus trace element analysis. A sample from outside the UHT zone has magmatic cores with an age of 1034 ± 7 Ma (2r, n ¼ 8) and 1052 ± 5 Ma (1r, n ¼ 1) overgrown by M1 metamorphic rims giving ages between 1020 ± 7 and 1007 ± 5 Ma. In contrast, samples from the UHT zone exhibit four major age groups: (1) magmatic cores yielding ages over 1500 Ma (2) magmatic cores giving ages of 1034 ± 13 Ma (2r, n ¼ 4) and 1056 ± 10 Ma (1r, n ¼ 1) (3) metamorphic overgrowths ranging in age between 1017 ± 6 Ma and 992 ± 7 Ma (1r) corresponding to the regional M1 Sveconorwegian granulite facies metamorphism, and (4) overgrowths corresponding to M2 UHT contact metamorphism giving values of 922 ± 14 Ma (2r, n ¼ 6). Recrystallized areas in zircon from both areas define a further age group at 974 ± 13 Ma (2r, n ¼ 4). This study presents the first evidence from Rogaland for new growth of zircon resulting from UHT contact metamorphism. More importantly, it shows the survival of magmatic and regional metamorphic zircon relics in rocks that experienced a thermal overprint of c. 950°C for at least 1 Myr. Magmatic and different metamorphic zones in the same zircon are sharply bounded and preserve original crystallization age information, a result inconsistent with some experimental data on Pb diffusion in zircon which predict measurable Pb diffusion under such conditions. The implication is that resetting of zircon ages by diffusion during M2 was negligible in these dry granulite facies rocks. Imaging and Th ⁄ U-Y systematics indicate that the main processes affecting zircon were dissolution-reprecipitation in a closed system and solid-state recrystallization during and soon after M1.
European Journal of Mineralogy, 2002
Zircon from two high-pressure, low-temperature orthogneisses (1-1.5 GPa and 550°C) from the Sesia Zone, Western Alps were examined by means of scanning electron, back-scattered and cathodoluminescence imaging and isotope dilution thermal ionization mass spectrometry to investigate U-Pb systematics of zircon in high-pressure, low-temperature terranes. The results of the U-Pb geochronology show a simple, two-stage mixing trend between crystallization of the leucogranitic protolith and eo-Alpine high-pressure metamorphism. The upper intercept ages of 435 ± 8 and 396 ± 21 Ma constrain the age of the protoliths and can be correlated to the zircon interiors that display strong cathodoluminescence. The lower intercept age of 68 ± 6.6 Ma dates the eo-Alpine high-pressure metamorphism and can be correlated with weakly luminescent, slightly zoned overgrowth seen in CL-and BSE-images, respectively. Two air-abraded single-grain analyses of apparently inherited zircons give a concordant age of 3.7 Ga and a discordant age of 2.4 Ga. Eo-Alpine zircon dissolution and overgrowth are characterized by internal and external embayed surfaces and multi-faceted crystal forms, respectively. A notable lack of evidence for pervasive fluid-rock interactions supports the view that dissolution of zircon was aided by local pore fluids of low salinity produced during metamorphic dehydration reactions of biotite and zoisite to form garnet. Precipitation of zircon overgrowths may have accompanied the H 2 O-consuming reaction of phengite formation from garnet.