ArticlePDF Available

Using Th-U-Pb geochronology to extract crystallization ages of Paleozoic metamorphic monazite contaminated by initial Pb

Authors:

Abstract and Figures

Geochronology of Th-rich minerals is advantageous as it allows use of three isotopic systems (i.e., ²⁰⁶Pb/²³⁸U, ²⁰⁷Pb/²³⁵U, and ²⁰⁸Pb/²³²Th) for accurate data assessment. The ²⁰⁸Pb/²³²Th system is especially advantageous in cases where the dated mineral includes an initial Pb component, as ²⁰⁸Pb/²³²Th is the least sensitive to the effects of initial Pb amongst the three systems. This benefit is demonstrated with monazite from a white mica schist of the Tsäkkok Lens, Scandinavian Caledonides, where three distinct generations of Paleozoic monazite (MnzI, Mnz-II, Mnz-III) are recognized and dated using laser ablation inductively coupled mass spectrometry. The generations are interpreted to represent monazite crystallization in high-pressure conditions (MnzI), followed by lower-pressure monazite growth (Mnz-II), and likely dissolution-reprecipitation of the pre-existing monazite (Mnz-III). The results are compared in Tera-Wasserburg, Wetherill, and Th-U-Pb concordia space for each monazite generation. In both Tera-Wasserburg and Wetherill space, the data are all discordant and indicate an initial Pb component in the monazite. The trend and magnitude of discordance due to initial Pb in Mnz-I and Mnz-II is generally controlled by UO2 content of the monazite, with higher UO2 equating to greater radiogenic Pb and a dampening of the initial Pb effect, which is most prominent in the ²⁰⁷Pb/²³⁵U system. For the same generations, initial Pb discordance of ²⁰⁶Pb/²³⁸U versus ²⁰⁸Pb/²³²Th is less apparent due to the insensitivity of ²⁰⁸Pb/²³²Th. Mnz-III does not follow the initial Pb trends, likely due to disturbance of the chemical and isotopic systems during recrystallization. Additional discordance in Mnz-I and Mnz-II, which is not related to initial Pb, is recognized and increases with actinide content. The additional discordance may be due to Pb-mobilization in Mnz-I and Mnz-II domains and is revealed when utilizing the ²⁰⁸Pb/²³²Th system due to its insensitivity to initial Pb effects. Consequently, relying only on the UPb systems can lead to significant initial Pb overcorrections in Tera-Wasserburg or Wetherill concordia space and to calculations of erroneously young concordia dates. The Th-U-Pb concordia method, incorporating all three systems, does not require an initial Pb correction and, therefore, can account for the additional discordance. The Th-U-Pb concordia dates are interpretated as accurate crystallization ages for Mnz-I (484.7 ± 1.1 Ma, MSWD: 1.4) and Mnz-II (474.7 ± 1.2 Ma, MSWD: 1.9). The timing for Mnz-III formation is not well-resolved as it formed via result of dissolution-reprecipitation of the pre-existing monazite, likely under lower amphibolite- to greenschist-facies conditions.
Content may be subject to copyright.
A preview of the PDF is not available
... It is divided into several (U)HP terranes > 1000 km along strike of the Scandinavian Caledonides, and are often grouped as the northern, central and west-central SNC terranes. The record of (U)HP metamorphism has often been discussed to be older for the northern terranes (late Cambrian/early Ordovician metamorphism; Root and Corfu 2012; Barnes et al. 2021a;Fassmer et al. 2021) versus the central and west-central SNC (Middle Ordovician metamorphism; Brueckner and Van Roermund 2007;Fassmer et al. 2017). The apparently different temporal (U)HP records have led to tectonic models involving diachronous subduction of the Baltican margin, either progressing from north to the south through time, or reflecting a promontory in the north that was subducted prior to the southern portions of the margin Bukała et al. 2018;Fassmer et al. 2021). ...
... However, the oldest ages of the monazite mantles and rims are older than the monazite cores, indicating Pb re-distribution in the reprecipitated monazite volume, which obfuscates detection of older events in the inclusions. Although multiple partial melting events is suggested by re-melting of possible peritectic garnet, possibly starting at 481.6 ± 2.1 Ma, the timing of the events are Barnes et al. (2021a); 19 Dallmeyer and Gee (1986); 20 Barnes et al. (2021b); 21 Barnes et al. (2020a); 22 Dallmeyer and Stephens (1991) Barnes et al. (2020b); 28 Dallmeyer et al. (1990) equivocal due to Pb behavior within the monazite during dissolution-reprecipitation (Seydoux-Guillaume et al. 2003;Weinberg et al. 2020;Varga et al. 2020). ...
Full-text available
Article
The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U–Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjället gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 ± 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 ± 2.0 Ma. In the Marsfjället Gneiss exposed near Kittelfjäll, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 ± 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 ± 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjället gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision.
... Fig. 9. Results of LA-ICPMS isotopic analyses of monazite from the felsic granulite SUD43 on the diagrams presenting 207 Pb/ 235 U, 206 Pb/ 238 U and 208 Pb/ 232 Th mean dates. Jastrzębski et al., 2020Jastrzębski et al., , 2021Barnes et al., 2021;Budzyń et al., 2021a), low to moderate degree of metamictization related to radiation damage (Seydoux-Guillaume et al., 2018b;Nasdala et al., 2020;Budzyń et al., 2021b), fluid-mediated compositional alteration via coupled dissolution-reprecipitation reactions resulting in removal of Pb in altered monazite domains Williams et al., 2011;Budzyń et al., 2015Budzyń et al., , 2021bGrand'Homme et al., 2016 or the presence of Pb-bearing nanoinclusions (Seydoux-Guillaume et al., 2018a;Turuani et al., 2022). Below we discuss the influence of these on the U-Th-Pb age record and the meaning of the Pb/U and Pb/Th dates in monazite that experienced multiple metamorphic episodes. ...
Full-text available
Article
Eleven monazite grains, two from a migmatitic gneiss and nine from two felsic granulites from the Góry Sowie Block (SW Poland) were studied with transmission electron microscopy (TEM), electron probe microanalysis (EPMA), Raman microspectroscopy and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) U-Th-Pb analysis in order to assess processes affecting U-Th-Pb age record. Two monazite grains from the migmatitic gneiss are patchy zoned in BSE imaging and overgrown by allanite, whereas Raman results indicate moderate radiation damage. Monazite in the corresponding TEM foils shows twins and nanoinclusions of fluorapatite, thorianite, goethite, titanite, chlorite and CaSO4. Furthermore, monazite is partially replaced by secondary monazite forming ca. 100 nm-thick layers and calcite along grain boundaries. The submicron alterations had little or no effect on the Pb/U and Pb/Th dates, when compared to earlier age constraints on the metamorphism in the studied region. In contrast, monazite from both granulites is homogeneous in eight investigated TEM foils, contains no solid or fluid nanoinclusions or any signs of fluid-induced alterations, with only one exception of a ca. 140 nm-thick crack filled with monazite. The 206Pb/238U and marginally older 208Pb/232Th mean dates pulled for all data show good coherence. However, the 207Pb/235U isotopic record is disturbed due the presence of common Pb within the entire monazite grain in one granulite and in the cores of two monazite grains in the second granulite, where the Usingle bondPb data of the rims are not compromised and concordant. Due to lack of TEM evidence for fluid-mediated alterations, the age discordance has to be related to addition of common Pb in the monazite lattice or in the micro-cracks. To summarize, the 208Pb/232Th data reveal the most accurate ages, which are consistent with previous geochronological studies in the region. Therefore, the Pb/Th chronometer, which is less compromised by age disturbance compared to Pb/U ages, is recommended for monazite geochronology. Application of the submicron scale investigations using TEM is recommended to evaluate potential presence of the submicron inclusions of Pb-bearing phases or compositional alterations of monazite that can remain unnoticed by using standard microanalytical instruments.
... Despite of the high closure temperature of Pb diffusion, the isotopic U-Th-Pb system in monazite may be disturbed under conditions corresponding to middle-to upper crustal depths due to metasomatic alteration (Cherniak et al., 2004). In addition, recent studies have shown that monazite can accumulate substantial amounts of common or initial Pb (Seydoux-Guillaume et al., 2012; Skrzypek et al., 2017;Jastrzębski et al., 2021;Barnes et al., 2021;Budzyń et al., 2021). Previous experimental studies demonstrated that alteration induced by alkali-rich fluids via coupled dissolutionreprecipitation reactions result in incomplete removal of Pb in the temperature range from 250 • C to 600 • C (Budzyń et al., , 2017Grand'Homme et al., 2016) or nearly complete removal of Pb at 450 • C . ...
Full-text available
Article
Experimentally metasomatised monazite was studied in terms of preservation of U-Pb and Th-Pb ages during alkali-bearing fluid-induced alteration over a broad range of temperature conditions 250–750 °C. Starting materials for experiments included Burnet monazite (Concordia age 1100.5 ± 11.6 Ma, 2σ), albite, K-feldspar, biotite, muscovite, SiO2, CaF2, Na2Si2O5 and H2O. Monazite from experiments at 250–550 °C is partially replaced by secondary REE-rich fluorapatite [(Ca,LREE,Si,Na)5(PO4)3F], fluorcalciobritholite [(Ca,REE)5(SiO4,PO4)3F] and REE-rich steacyite [K1-x(Na,Ca)2(Th,U)Si8O20], and developed patchy zoning, whereas partial replacement by fluorcalciobritholite and cheralite [CaTh(PO4)2] occurred at 650 and 750 °C, with no signs of compositional alteration based on EPMA data and BSE imaging. Raman microspectroscopy results show narrowing of the ν1(PO4) stretching band in unaltered domains, which indicates advancing annealing of the monazite structure with increasing temperature, and narrow ν1(PO4) band with low FWHM values in altered domains. TEM investigations revealed that unaltered domains of monazite from experiments at 250–550 °C have mottled diffraction contrast, similar to the starting Burnet monazite, which indicates low to moderate degree of metamictization. On the contrary, the altered domains of monazite (patchy zones) show no mottled contrast, suggesting an ordered crystalline structure. TEM imaging demonstrated low degree of metamictization in monazite from the experiment at 650 °C; fluid-aided alteration along the cleavage planes resulted in the development of nanoporosity or partial replacement by fluorcalciobritholite and cheralite. Monazite from the experiment at 750 °C has crystalline structure with no signs of metamictization and shows significant development of nanoporosity and formation of secondary cheralite nanocrystals across the grain. For comparison, TEM and Raman evaluation of xenotime from similar experiments at 350 and 650 °C revealed that both starting xenotime and xenotime from experimental products are crystalline with no signs of radiation damage or fluid-induced alteration affecting internal domains on submicron scale, which could result in compositional alteration of the xenotime. The unaltered domains of monazite from runs at 250–550 °C yielded U-Pb and Th-Pb dates similar to the age of Burnet monazite, whereas altered domains yielded discordant dates due to various degree of Pb-loss (up to 99.4%). Linear regressions on the Concordia diagrams show lower intercept ages from −266 ± 160 Ma (run 350 °C, 200 MPa) to −1 ± 48 Ma (450 °C, 800 MPa), which reflect the “true age” of experimental alteration. The monazite from runs at 650 and 750 °C yielded data indicating initial disturbance of the U-Th-Pb system, ranging from 8.4% Pb-gain to 18.6% Pb-loss. Linear regressions with lower intercepts of −53 ± 420 Ma and −55 ± 610 Ma roughly correspond to the timing of the experiments. Furthermore, LA-ICPMS results demonstrate discrepancy between Th-Pb and U-Pb dates suggesting higher mobility of 208Pb than that of 207Pb and 206Pb. To summarize, TEM and Raman data indicate increasing annealing of the radiation damaged monazite with increasing temperature. Alteration processes induced by alkali-bearing fluid can result in recrystallization of monazite and various degrees of the age disturbance at temperatures 250–550 °C, whereas isotopic U-Th-Pb microanalysis provide an opportunity to constrain the age of the metasomatic processes as the lower intercept in the Concordia diagram. The particular importance of this study lies in submicron alteration of monazite at 650–750 °C induced by alkali-bearing fluid and/or melt, which remains unnoticed using common electron microscopy BSE imaging. Such alteration, however, induces substantial disturbance of U-Pb and Th-Pb ages, which can cause misinterpretations in reconstructions of geological processes.
Article
The massive exploitation and application of heavy metals and rare earth elements (REEs) lead to their exceeding the standard in soil. Herein, a new type of biochar supported phosphorus doped ferrihydrite ([email protected]) has been designed and enhance passivation of Pb and Ce in soil. SEM images of [email protected] showed P-FH nanoparticles adhered to the natural cavity and large pore diameter on the surface of biochar, which greatly avoided the agglomeration of nanoparticles. The residual state of lead or cerium increased 161.4% or 43.9% by adding 3% [email protected] after 90 days of incubation in 500 mg/kg lead or cerium simulated contaminated soil. The passivation of cerium by [email protected] is obviously inhibited with the coexistence of lead. The results of [email protected] magnetically separated from the soil characterization indicate that complexation, co-precipitation and the formation of secondary minerals mainly contribute to the high efficiency passivation ability of [email protected] for lead and cerium. By changing the addition of [email protected], the soil pH can be adjusted and the soil organic matter and P contents can be improved. Moreover, [email protected] is an environmentally friendly material without ecotoxicity. And bacterial richness and diversity in soil were improved after passivation of Pb and Ce by adding [email protected]
Full-text available
Article
In this study, data from garnet‐kyanite metapelites in ultrahigh‐pressure (UHP) domains of the Western Gneiss Region (WGR), Norway is presented. U–Pb geochronology and trace element compositions in zircon, monazite, apatite, rutile and garnet were acquired, and pressure–temperature (P–T) conditions were calculated using mineral equilibria forward modelling and Zr‐in‐rutile thermometry. Garnet‐kyanite gneiss from Ulsteinvik record a prograde evolution passing through ~690–710 °C and ~9–11 kbar. Zircon and rutile age and thermometry data indicate these prograde conditions significantly pre‐date Silurian UHP subduction in the WGR and are interpreted to record early Caledonian subduction of continental‐derived allochthons. Minimum peak conditions in the Ulsteinvik metapelite occur at ~28 kbar, constrained by an inferred garnet + kyanite + omphacite + muscovite + rutile + coesite + H2O assemblage. The retrograde evolution passed through ~740 °C and ~7 kbar, firstly recorded by the destruction of omphacite and followed by the partial replacement of kyanite and garnet by cordierite and spinel. Garnet‐kyanite metapelite from the diamond‐bearing Fjørtoft outcrop document a polymetamorphic history, with garnet forming during the late Mesoproterozoic and limited preservation of high‐pressure Caledonian assemblages. Similar to the Ulsteinvik metapelite, zircon and rutile age data from the Fjørtoft metapelite also record pre‐Scandian Caledonian ages. Two potential tectonic scenarios are possible: (1) the samples were exhumed at different times during pre‐Scandian subduction of the Blåhø nappe, or (2) the samples do not share a history in the same nappe complex, instead the Ulsteinvik metapelite is a constituent of the Seve‐Blåhø Nappe, whilst the Fjørtoft metapelite shares its history within a separate nappe complex.
Full-text available
Article
The interaction of trace elements, fluids and crystal defects plays a vital role in a crystalline material’s response to an applied stress. Fluid inclusions are typically known to facilitate crystal-plastic deformation in minerals. Herein we discuss a model of fluid hardening whereby dislocations are pinned at fluid inclusions during crystal-plastic deformation, initiating pipe diffusion of trace elements from the fluid inclusions into crystal defects that leads to their stabilization and local hardening. We derive this hypothesis from atom probe tomography data of naturally deformed pyrite, combined with electron backscatter diffraction mapping, electron channelling contrast imaging and scanning transmission electron microscopy. The 2D and 3D micro- to nanoscale structural and chemical data reveal nanoscale fluid inclusions enriched in As, O, Na and K that are linked by As-enriched dislocations. Our efforts advance the understanding of the interplay between nanostructures and impurities during relatively low temperature deformation, which yields insight into the larger scale mass transfer processes on Earth.
Full-text available
Article
The collision of Baltica and Laurentia during the Caledonian Orogeny has happened around 400 – 420 Ma. However, subduction and collision processes also took place before this main collisional phase and the tectonic history of these is still not fully resolved. The Seve Nappe Complex in Sweden has recorded these earlier phases. The Seve Nappe Complex in Norrbotten (North Swedish Caledonides) comprises four superimposed nappes emplaced by eastward thrusting (from base to top according to the conventional structural interpretation): Lower Seve Nappe, Vaimok, Sarek, and Tsäkkok Lenses. Eclogites occur in the Vaimok and Tsäkkok Lenses. The Vaimok Lens represents rocks of the Baltican continental margin intruded by Neoproterozoic dolerite dikes which were later eclogitized and boudinaged. In contrast, eclogites of the Tsäkkok Lens are former oceanic basalts associated with calcschists, possibly representing the ocean‐continent transition between Baltica and Iapetus. Previous age determinations for eclogitization yielded various ages between c. 500 and 480 Ma, in contrast to younger (460‐450 Ma) ages of (ultra‐) high‐pressure metamorphism in the Seve Nappe Complex further south in Jämtland. Eclogites from the Vaimok (one sample) and Tsäkkok (three samples) lenses were dated using Lu‐Hf garnet geochronology. Garnet from all samples shows prograde zoning of major element and Lu contents and yielded well‐defined isochrons of the following ages: 480.4 ± 1.2 Ma (Vaimok); 487.7 ± 4.6 Ma, 486.2 ± 3.2, 484.6 ± 4.5 Ma (Tsäkkok). The ages from Tsäkkok are interpreted to date the burial of the Iapetus‐Baltica ocean‐continent transition in a west‐dipping subduction zone around ~485 Ma, and the age from the structurally deeper Vaimok Nappe the following subduction of the continental margin. Previously reported ages of 500 Ma and older are not supported by this study. The age difference between eclogites in the Seve Nappe Complex in Jämtland (c. 460‐450 Ma) and Norrbotten (c. 488‐480 Ma) may reflect the collision of an island arc with an irregularly shaped passive continental margin of Baltica or alternatively the collision of a straight margin with a microcontinent (Sarek Lens) accreted to the upper plate.
Full-text available
Article
Mechanical twins form by the simple shear of the crystal lattice during deformation. In order to test the potential of narrow twins in monazite to record the timing of their formation, we investigated a ca. 1700 Ma monazite grain (from the Sandmata Complex, Rajasthan, India) deformed at ca. 980 Ma, by electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and atom probe tomography (APT). APT 208Pb/232Th ages indicate that the twin was entirely reset by radiogenic Pb loss during its formation at conditions far below the monazite closure temperature. The results are consistent with a model where Pb is liberated during rupture of rare earth element–oxygen (REE-O) bonds in the large [REE]O9 polyhedra during twinning. Liberated Pb likely migrated along fast diffusion pathways such as crystal defects. The combination of a quantitative microstructural investigation and nano-geochronology provides a new approach for understanding the history of accessory phases.
Full-text available
Article
The Tsäkkok Lens of the Scandinavian Caledonides represents the outermost Baltican margin that was subducted in late Cambrian/Early Ordovician time during closure of the Iapetus Ocean. The lens predominantly consists of metasedimentary rocks hosting eclogite bodies that preserve brittle deformation on the μm-to-m scale. Here, we present a multidisciplinary approach that reveals fracturing related to dehydration and eclogitization of blueschists. Evidence for dehydration is provided by relic glaucophane and polyphase inclusions in garnet consisting of clinozoisite + quartz ± kyanite ± paragonite that are interpreted as lawsonite pseudomorphs. X-Ray chemical mapping of garnet shows a network of microchannels that propagate outward from polyphase inclusions. These microchannels are healed by garnet with elevated Mg relative to the surrounding garnet. Electron backscatter diffraction (EBSD) mapping revealed that Mg-rich microchannels are also delimited by very-low angle (<3°) subgrain boundaries. X-ray computed microtomography demonstrates that some garnet is transected by up to 300 μm wide microfractures that are sealed by omphacite ± quartz ± phengite. Locally, mesofractures sealed either by garnet- or omphacite-dominated veins transect through the eclogites. The interstices within the garnet veins are filled with omphacite + quartz + rutile + glaucophane ± phengite. In contrast, omphacite veins are predominantly composed of omphacite with minor apatite + quartz. Omphacite grains are elongated along [001] crystal axis and are preferably oriented orthogonal to the vein walls, indicating crystallization during fracture dilation. Conventional geothermobarometry using omphacite, phengite and garnet adjacent to fractures, provides pressure-temperature conditions of 2.47±0.32 GPa and 620±60°C for eclogites. The same method applied to a mesoscale garnet vein yields 2.42±0.32 GPa at 635±60°C. Zirconium-in-rutile thermometry applied to the same garnet vein provides a temperature of ~620°C. Altogether, the microchannels, microfractures and mesofractures represent migration pathways for fluids that were produced during glaucophane and lawsonite breakdown. The microfractures are likely precursors of the mesoscale fractures. These dehydration reactions indicate that high pore-fluid pressure was a crucial factor for fracturing. Brittle failure of the eclogites thus represents a mechanism for fluid-escape in high-pressure conditions. These features may be directly associated with seismic events in a cold subduction regime.
Full-text available
Article
The actinide-containing mineral monazite-(Ce) is a common accessory rock component that bears petrogenetic information, is widely used in geochronology and thermochronology, and is considered as potential host material for immobilisation of radioactive waste. Natural samples of this mineral show merely moderate degrees of radiation damage, despite having sustained high self-irradiation induced by the decay of Th and U (for the sample studied herein 8.9 ± 0.3 × 1019 α/g). This is assigned to low damage-annealing temperature of monazite-(Ce) and "alpha-particle-assisted reconstitution". Here we show that the response of monazite-(Ce) to alpha radiation changes dramatically, depending on the damage state. Only in radiation-damaged monazite-(Ce), 4He ions cause gradual structural restoration. In contrast, its high-temperature annealed (i.e. well crystalline) analogue and synthetic CePO4 experience He-irradiation damage. Alpha-assisted annealing contributes to preventing irradiation-induced amorphisation ("metamictisation") of monazite-(Ce); however, this process is only significant above a certain damage level.
Full-text available
Article
The Kamieniec Ząbkowicki Metamorphic Belt (KZMB) is a narrow zone of mainly mica schists, subordinate acid metavolcanics and scarce eclogites, sandwiched between Brunovistulia and the northern tip of the Teplá-Barrandia microplates. Locally occurring high-pressure relics indicate subduction of the metasedimentary succession of the KZMB, the origin and provenance of which remain unclear. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) investigations of detrital zircons show that the metapelites represent an Ediacaran-Cambrian sedimentary basin, with a maximum depositional age of 561±9 Ma. This basin was filled with detritus from a source or sources, composed of rocks containing zircons that are mainly Cryogenian-Ediacaran and Palaeoproterozoic in age. No younger component was found in the zircon population studied. The isotopic U-Pb LA-ICP-MS and chemical U-Th-total Pb electron probe microanalysis (EPMA) monazite geochronology data indicate an important regional tectono-metamorphic event at ca. 330 Ma. Though these data do not permit determination of the peak pressure from the peak temperature stages, the event was part of a complex collision of the Saxothuringian plate with Brunovistulia. [pdf available at https://geojournals.pgi.gov.pl/asgp/article/view/28567 ]
Full-text available
Article
Integrated structural, geochemical, and geochronological investigations were conducted on metasedimentary rocks in the eclogite‐bearing Tsäkkok Lens of the Seve Nappe Complex (Scandinavian Caledonides) to resolve its exhumation history. Three deformation events are defined. D1 is likely related to the prograde to peak‐metamorphic stages, represented by a locally preserved S1. D2 resulted in vertical shortening and is defined by a pervasive S2 and cm‐/m‐scale F2 closed folds. D2 terminated with Scandian thrusting, which emplaced the overlying Köli Nappe Complex. D3 records NE‐SW shortening and constitutes m‐/km‐scale F3 open folds that deformed the Tsäkkok Lens and Köli Nappe Complex together. In‐situ white mica 40Ar/39Ar geochronology was conducted on select metasedimentary samples possessing S1 or S2 to resolve the timing of exhumation. Post‐decompression cooling of the Tsäkkok Lens is best recorded by samples containing S1 or S2 that yield homogeneous white mica chemistry and 40Ar/39Ar dates. The timing of cooling is resolved to 477.2 ± 4.1 Ma (S1) and 475.3 ± 3.5 Ma (S2). Vertical shortening of the lens during exhumation may have proceeded until 458.1 ± 9.0 Ma. Later‐stage deformation during Scandian thrusting penetrated the Tsäkkok Lens at 429.9 ± 9.0 Ma, or younger. This resulted in non‐coaxial deformation of the metasedimentary rocks, producing heterogeneous white mica chemistry and partially reset the older 40Ar/39Ar cooling record. Temperatures for deformation are resolved to the upper greenschist‐ lower amphibolite‐facies. Altogether, the Tsäkkok Lens records rapid exhumation from eclogite‐facies conditions to mid‐crustal depths or shallower, followed by emplacement of the overlying Köli Nappe Complex.
Article
A single megacryst of Neoproterozoic monazite (“TS-Mnz”) was investigated by electron probe microanalysis (EPMA), Raman microspectroscopy, U-Pb dating by isotope dilution-thermal ionization mass spectrometry (ID-TIMS), and U-Th-Pb dating and trace element analysis by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). The monazite contains both distinct unaltered domains that are either homogeneous or zoned, and altered domains with secondary inclusions of ThSiO4 along fractures. Altered domains can be easily recognized in backscattered electron images and avoided during microanalysis. The U-Pb ID-TIMS measurements in two laboratories revealed slight reverse discordance of U-Pb data and provided 207Pb/235U mean dates of 908.2 ± 1.5 Ma (95% conf., MSWD = 2.9) and 910.42 ± 0.34 Ma (2σ, MSWD = 1.0). Three individual LA-ICPMS sessions yielded 207Pb/235U mean dates of 899.8 ± 6.9 Ma (95% conf., MSWD = 0.10), 896.3 ± 6.5 Ma (95% conf., MSWD = 0.18) and 901.9 ± 2.0 Ma (95% conf., MSWD = 0.57), and 208Pb/232Th mean dates of 909.2 ± 5.8 Ma (95% conf., MSWD = 0.36), 915.1 ± 11 Ma (95% conf., MSWD = 0.31) and 909.9 ± 1.5 Ma (95% conf., MSWD = 0.56). The lower precision of the LA-ICPMS with respect to the ID-TIMS technique hides discrepancies between ID-TIMS 207Pb/235U dates as well as the reverse discordance, which is common for monazite. The discrepancies between ID-TIMS and LA-ICPMS data are interpreted as related to standardization and individual setup of analytical techniques; however, minor variance of discordance between individual analyzed spots also cannot be excluded. Further electron microprobe U-Th-total Pb dating provided dates of 914.6 ± 2.0 Ma (95% conf., MSWD = 0.99) and 922.9 ± 3.1 Ma (95% conf., MSWD = 4.3) for two fragments of TS-Mnz. The LA-ICPMS U-Th-Pb round-robin test employing TS-Mnz as a primary reference material and various monazite samples of known age as unknowns provided accurate U-Pb and Th-Pb dates within errors of the reference ages. To conclude, the TS-Mnz despite compositional variations reveals relatively reproducible U-Pb, Th-Pb and U-Th-total Pb dates and is proposed as a new age reference material for in-situ U-Th-Pb microanalysis with recommended reference mean 207Pb/235U and 208Pb/232Th ages of 910.42 ± 0.34 Ma (2σ) and 910.7 ± 1.3 Ma (95% conf.), respectively.
Article
The zirconium-in-rutile thermometer enjoys widespread use, but confidence in its accuracy is limited because experiments were conducted at higher temperatures than many rutile-bearing rocks and calibration uncertainties have not been quantitatively assessed. Refined calibrations were developed using bootstrap regression to minimize residuals in the natural logarithm of the equilibrium constant, based on experiments only (n = 32) and on a combined compilation of experiments and natural data (n = 94, total). Rearranging the regression to solve for T, and expressing Zr concentration (C) in parts per million (μg/g), the calibrations in the α-quartz stability field are: Experimental data set: T ( C ∘ ) = 68740 + 0 . 441 · P ( bars ) - 0 . 114 · C ( ppm ) 129 . 76 - R · ln [ C ( ppm ) ] - 273 . 15 . Combined data set: T ( C ∘ ) = 71360 + 0 . 378 · P ( bars ) - 0 . 130 · C ( ppm ) 130 . 66 - R · ln [ C ( ppm ) ] - 273 . 1 . Thermodynamics of the quartz-coesite transition as applied to the calibration for α-quartz yields calibrations for the coesite stability field: Experimental data set T ( C ∘ ) = 71290 + 0 . 310 · P ( bars ) - 0 . 114 · C ( ppm ) 128 . 76 - R · ln [ C ( ppm ) ] - 273 . 15 . Combined data set: T ( C ∘ ) = 73910 + 0 . 247 · P ( bars ) - 0 . 130 · C ( ppm ) 129 . 65 - R · ln [ C ( ppm ) ] - 273 . 15. Propagated temperature uncertainties are ±20–30 °C (2σ) for the experimental data set calibration, and ±10–15 °C (2σ) for the combined data set. Compared to previous experimental calibrations, the refined thermometer predicts temperatures up to 40 °C lower for T ≤ 550 °C, and systematically higher temperatures for T > 800 °C. With careful attention to distributions of Zr in rutile grains, precisions of ±5 °C and accuracies ~±15 °C may be possible, although a poor understanding of how to select compositions for thermometry will typically lead to larger uncertainties. The ZiR calibration promises continued high-precision and accurate thermometry, and possibly improved thermodynamic properties, but the sources of compositional variability in rutile warrant further scrutiny.