Displays

Improvements to U-Pb geochronology of magmatic zircon have resulted in temporal resolution at the level of <0.1% for individual ²⁰⁶Pb/²³⁸U dates and of 0.02-0.05% for weighted mean ²⁰⁶Pb/²³⁸U ages of a statistically equivalent group of single crystal dates from zircon or baddeleyite (50,000 years for a Mesozoic igneous rock). This talk will give a short overview on the challenges and opportunities such high precision age determination implies in felsic and mafic magmatic systems.

Felsic magmatic systems: Zircon dates from the same hand sample cover a temporal range that integrates their crystallization history in the melt. Since each grain crystalizes over a certain time period, the apparent age range is a minimum estimate of the duration of crystallization or the residence in a magma. A major challenge is the mitigation of decay-related lead loss through refined chemical abrasion procedures (Widmann et al., 2019) to avoid erroneous interpretation of zircon dates that appear too young. Apparent trace element or isotopic trends are typically not coherent with time and therefore reflect fractionation processes at different places and different times in the magmatic system, possibly within compositionally different magma batches.

Mafic magmatic systems: Zircon is not a crystallizing phase in a basaltic melt, but can occur after ~90% fractionation of olivine, pyroxene and amphibole, zircon saturation can then be achieved in low-volume granitic melt pockets (depending on the water content). A zircon date is therefore an age information along the crystallization -cooling path of a mafic intrusion (Zeh et al., 2015). In low-Si and low-Zr melts, baddeleyite may arrive at saturation before zircon and can be used for dating as well. There are two clear problems with zircon/baddeleyite geochronology in mafic systems: (i) since baddeleyite saturates earlier than zircon, it should produce slightly older dates in the same rock; however, these minerals often display the inverse relationship. Since no pre-treatment for the removal of decay-damaged portions exists for baddeleyite, we can demonstrate that this discrepancy is due to lead loss. Mitigating lead loss is also difficult for zircon since it crystallized from residual melt patches of granitoid composition high in uranium, often resulting in metamict crystals; (ii) zircon populations from dolerites may spread over >100,000 years even in cases where simple thermal modeling shows that a dolerite sill has crystallized and cooled at 10³ years timescales. Beside lead loss, we may suspect that certain zircon grains contain minute portions of pre-crystallization radiogenic lead from crustal contamination. We can explore and quantify cryptic inheritance through Hf, O isotopic analysis of the same dated zircon grains. Heterogeneous nucleation on relics of incompletely dissolved zircon is more probably than spontaneous nucleation.

As an overarching challenge, we have no technique or independent approach to quantify lead loss and it remains the biggest uncertainty in U-Pb dating.

References: Davies et al. (2015) Nature Communications, 8, 15596 ; Sell et al. (2014) Earth and Planetary Science Letters, 408, 48-56; Widmann et al. (2019) Chemical Geology, 511, 1-10; Zeh et al. (2015) Earth Planet. Sci. Lett. 418, 103-114

How to cite: Schaltegger, U. and Davies, J. H. F. L.: Changing our ideas about the evolution of magmatic systems with improved temporal resolution: do we get it right?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2178, https://doi.org/10.5194/egusphere-egu2020-2178, 2020.

The pre-Alpine basement of the Adriatic plate in the Southern Alps exposes an exceptionally complete section across the continental crust (Ivrea Verbano: lower crust; Serie dei Laghi: upper crust). The section was weakly reworked during Jurassic extension and Cretaceous to Miocene Alpine shortening. The Insubric Line, an Alpine crustal-scale south-vergent backthrust, separates the Southern Alps from the Alpine nappe stack. The pre-Alpine basement of the Adriatic palaeomargin is intensely reworked in this stack, and is now part of the Sesia-Dent Blanche nappes (Manzotti et al. 2014) and other, smaller, Adria-derived units (e.g. Emilius).

The less deformed part of the Sesia-Dent Blanche nappes are the IIDK and Valpelline Series. Based on lithological similarities, they have been correlated with the Ivrea-Verbano Zone (Carraro et al. 1970). This equivalence has been confirmed by subsequent studies, including detailed U-Pb zircon ages of metamorphic (Kunz et al., 2018) and magmatic events. The other units of the Sesia-Dent Blanche nappes (the Arolla Series, the Gneiss Minuti, and the Eclogitic Micaschists) have been pervasively reworked during the Alpine orogeny, from greenschist to eclogite-facies. Identification of the age and nature of their pre-Alpine protoliths, and of the grade and age of their pre-Alpine metamorphism heavily relies on field and petrological data on key outcrops, supported by U-Pb dating.

If the IIDK and Valpelline Series represent the lower Adriatic crust, the other units may derive from the upper Adriatic crust, i.e. may be similar to the Serie dei Laghi in the Southern Alps. Alternatively, they may also represent pieces of the Adriatic lower crust that were pervasively re-hydrated during the Jurassic extension and/or the Alpine subduction (Engi et al., 2018), thus allowing re-equilibration at HP conditions during Alpine deformation.

This contribution will summarize a range of field, petrological, and geochronological data (obtained by LA-ICP MS on zircon, combined with in situ-oxygen isotope data measured by SIMS). This data set reveals significant differences in the timing of crustal melting, as well as magma emplacement at different depths. It can be concluded that the history of the Adriatic crust in the Alpine stack is comparable with that of the Southern Alps, with implications for the mechanical behaviour of the crust during the Alpine orogeny.

Manzotti et al. (2014). Swiss Journal of Geosciences, 107, 309-336

Carraro et al. (1970). Memorie della Società Geologica Italiana, 9, 19-224

Kunz et al. (2018). International Journal of Earth Sciences, 107, 203-229

Engi et al. (2018). Geochemistry, Geophysics, Geosystems, 19, 865-881

How to cite: Manzotti, P., Bégué, F., Kunz, B., Rubatto, D., and Ulianov, A.: Timing of crustal melting and magma emplacement at different depths: insights from the Permian in the Western Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9175, https://doi.org/10.5194/egusphere-egu2020-9175, 2020.

In situ age and trace element determinations of monazite and rutile grains from an ultrahigh temperature (UHT) metapelite hosted leucosome from the Napier Complex using laser split-stream analysis reveals highly variable behavior in both the U–Pb, REE and trace element systematics that are directly linked to the petrographic setting of individual grains.

Monazite grains armored by garnet and quartz retain a concordant 2.48 Ga age that is the same as the age for peak UHT metamorphism in the Napier Complex. Yttrium in the armored grains are unzoned with contents around 700 ppm in the garnet-hosted monazite and range between 400-1600 ppm in the monazite enclosed within quartz. A monazite grain hosted within a mesoperthite grain records a spread of concordant ages from 2.42 to 2.20 Ga and Y contents ranging between 400 to 1700 ppm. This grain exhibits core to rim zoning in both Y and age with the cores enriched in Y relative to the rim and younger ages in the core relative to the rim. A monazite grain that sits on a grain boundary between mesoperthite and garnet records the largest spread in ages­– from 2.42 to 2.05 Ga. The youngest ages in this grain are within a linear feature that reaches the core and is connected to the grain boundary between the garnet and mesoperthite, the oldest ages are observed where monazite is in contact with garnet. Yttrium in the grain is enriched in the core and depleted at the rim with the strongest depletions where monazite in adjacent to grain boundaries between the silicate minerals or in contact with garnet.

By contrast, rutile which is petrologically part of the peak-UHT assemblage and therefore inferred to have grown at c. 2.48 Ga records a complex discordant array of ages with the oldest concordant ages at 1.90 Ga with a spread down concordia to 1.70 Ga and a lower, imprecisely defined intercept at 0.55 Ga. The most discordant rutile grains sit within the residual garnet-sillimanite-spinel domains and record Zr-in rutile temperature of <800 °C. The least discordant and oldest grains sit within the leucosome and record Zr-in-rutile temperatures of >1000 °C. There is no correlation between grain size and age/degree of discordance.

The age and chemical relationships outlined above illustrate decoupling between the geochemical and geochronological systems in monazite and rutile. Individual grains are suggestive of a range of processes that modify these systems, including volume diffusion, flux-limited diffusion and recrystallisation, all operating at the scale of a single thin section and primarily controlled by the host minerals and their microstructural setting. These relationships, while complicated, can be interpreted in terms of the thermal history of this rock allowing the potential identification of a previously cryptic thermal event. This would not be possible without the petrographic information for the location of individual grains enabled through analysis of the different accessory minerals in thin section.

How to cite: Clark, C. and Taylor, R.: Petrographically-controlled elemental mobility in monazite and rutile in ultrahigh temperature granulites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6407, https://doi.org/10.5194/egusphere-egu2020-6407, 2020.

Re and Os (rhenium and osmium) are chalcophile-siderophile elements (transition metals) with a unique position in isotope geochemistry. Unlike other commonly used decay schemes for radiometric dating, these metals take residency in resource-related media, for example, sulfide minerals, the organic component in black shales, coals, and bitumens and oils. In short, the reducing environment is their haven whereas under oxidizing conditions, Re and Os become unmoored and the radiometric clock becomes compromised. The clock is not temperature sensitive, and its applicability spans Early Archean to Pleistocene.

This Bunsen Medal lecture will explore and review the challenges in bringing Re-Os from the meteorite-mantle community into the crustal environment. At the center of it all is our ability to turn geologic observation into a thoughtful sampling strategy. The possibility to date ore deposits was an obvious application, and molybdenite [Mo(Re)S₂], rarely with significant common Os and lacking overgrowths, became an overnight superstar, yielding highly precise, accurate, and reproducible ages. Yet, molybdenite presented our first sampling challenge with recognition of a puzzling parent-daughter (¹⁸⁷Re-¹⁸⁷Os) decoupling in certain occurrences. A strategic sampling procedure was employed. From there, the diversity of applications spread, as molybdenite is also an accessory mineral in many granitoids, and can be a common trace sulfide in metamorphic rocks conformable with and/or cross-cutting foliation linking timing and deformation. Pyrite and arsenopyrite can also be readily dated.

Applications jumped from sulfides to organic matter. Extracting and dating the organic (hydrogeneous) component in black shales gives us Re-Os ages for sedimentary units in the Geologic Time Scale. This led to construction of an Os isotope seawater curve – an ongoing process. Unlike the well-known Sr seawater curve, the short residence time of Os in the oceans creates a high-definition time record with unambiguous high-amplitude swings in ¹⁸⁷Os/¹⁸⁸Os. Re-Os puts time pins into the biostratigraphic record, and we have even directly dated fossils. Re-Os opened the door for a new generation of paleoclimate studies to evaluate seawater conditions at the time of organic blooms and organic sequestration in bottom mud. Uplift and continental erosion can be balanced with hydrothermal input into oceans based on changes in the Os isotope composition of seawater. The timing and connectivity of opening seaways can be determined, and the timing of glaciation and deglaciation events can be globally correlated. The timing and instigators of mass extinctions are carried in the Re-Os record. A meteorite impact places an enormous scar in the Os isotope record as seawater drops toward mantle values and recovers in a few thousand years. Most recently, Re-Os has transformed our understanding of the events and fluids involved in the construction of whole petroleum systems.

Looking to the future, what kinds of data sets will be explored and what are the interdisciplinary skill sets needed to interpret those data? Re-Os will continue to provide us with new ways to dismantle geologic media for new scientific understanding of processes that have shaped our lithosphere, biosphere and hydrosphere and their intersection and exchange.

How to cite: Stein, H.: How Two Unassuming Elements, Re and Os, Assumed Acclaim in the Geosciences, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21442, https://doi.org/10.5194/egusphere-egu2020-21442, 2020.

The Barberton Greenstone Belt (BGB) is a well-preserved remnant of Paleo- to Mesoarchean crust. The oldest supracrustal rocks of the BGB consist of the 3.5-3.3 Ga Onverwacht Group. These rocks form a NE-SW trending belt deformed and metamorphosed largely under lower greenschist-facies conditions. In the southern BGB, the Komati Fault separates the structurally uppermost, lower greenschist-facies Onverwacht Group from its stratigraphically lowest components – the Sandspruit and Theespruit Formations (hereafter referred to as Lower Onverwacht Group), which occur south of the Komati Fault and have been metamorphosed under high-pressure amphibolite-facies conditions. The Lower Onverwacht Group rocks occur as a band along the southern edge of the greenstone belt and as septa between several ca. 3.55, 3.45 and 3.23 Ga Tonalite-Trondhjemite-Granodiorite plutons. The Lower Onverwacht Group rocks record a complex history of metamorphism and retrogression. An early phase of amphibolite-facies metamorphism is recorded at ca. 3.44 Ga by monazite in metasediments, whilst the main phase of the regional metamorphism occurred at ca. 3.23 Ga (e.g. Cutts et al., 2014).

The rocks targeted in this study have felsic metavolcanic protoliths and occur as a greenstone remnant within deformed and undeformed phases of 3.45 Ga Trondhjemites. They contain cm-sized garnets and the mineralogy of the samples indicate amphibolite-facies peak metamorphism. The garnets show major element growth zonation from core to rim (Alm_0.63-0.80 Grs_0.15-0.08Pyr_0.0.05-0.09Sps_0.17-0.0.03). U-Pb rutile geochronology gives an age at 3.15 Ga and Zr-in-rutile thermometry yields a temperature of ca. 640 °C (at 5 kbar). The rutile grains contain small, pristine zircon inclusions and the rutile is assumed to have grown in equilibrium with both zircon and quartz as buffer phases. The amphibolite-facies assemblage and the Zr-in-rutile temperature indicate that the rutile dates are cooling ages, which are difficult to interpret without information on the age of peak metamorphism of the samples. The objective of this study is to attempt to elucidate the early metamorphic record of these samples by directly dating the large garnet grains using in situ U-Pb laser-ablation inductively-coupled-plasma mass-spectrometry geochronology. Ongoing research shows that low-U garnet is datable by this method (Albert Roper et al., 2018). Preliminary results have been obtained from a different Lower Onverwacht Group sample, yielding a 3.45 Ga age for the garnet core and a 3.22 Ga age for the garnet rim (Cutts et al, unpublished data). The results indicate that U-Pb in rutile and in garnet from Archaean greenstones can be used in order to date metamorphic events. This is especially relevant when other potential datable accessory minerals, such as zircon or monazite, are not present.

References

Cutts et al., 2014. Geological Society of America Bulletin 126, 251–270.

Albert Roper et al., 2018. Goldschmidt Abstracts, 2018, 32.

How to cite: van Schijndel, V., Cutts, K., Stevens, G., Lana, C., and Zack, T.: In situ U-Pb geochronology on garnet and rutile: New age data from the Palaeoarchaean Onverwacht Group, Barberton Greenstone Belt, South Africa., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20167, https://doi.org/10.5194/egusphere-egu2020-20167, 2020.

Ultra-high temperature (UHT) metamorphism is a thermal regime that can be attained by the lower continental crust in exceptional contexts and that is usually accompanied by fluid-absent dehydration melting. Such conditions are observed in the Gruf Complex, a 12 x 10 km migmatitic body located in the Central Alps, which is characterized by the presence of UHT granulitic schlieren and enclaves within migmatitic orthogneisses and charnockites. Two types of granulites, both with a massive and melanocratic texture, were investigated. The first granulite contains sapphirine, garnet, orthopyroxene, K-feldspar and biotite in the peak mineral assemblage, whereas the second type displays garnet, orthopyroxene, sillimanite and biotite. In both granulites, garnets are porphyroblastic and can reach up to 2 cm in size. These garnets are almost pure almandine-pyrope solid solutions and are zoned, showing pyrope-richer rims (Alm_43-54Prp_43-55Sps_0-2Grs_1-6) compared to cores (Alm_47-62Prp_32-48Sps_0-3Grs_2-9). A clear zoning is also observed in the rare earth elements (REE), with garnet cores showing the highest REE concentrations. Moreover, the porphyroblastic garnets are characterized by the presence of numerous melt inclusions (MI), which can be noticed both in garnet cores and rims. The MI occur as polycrystalline (nanogranitoids) and glassy inclusions, and dominantly display a peraluminous, rhyolitic composition, suggesting that they were originated, along with the host garnet, by incongruent, fluid-absent melting reactions during crustal anatexis. Lu-Hf ages obtained for the MI-bearing garnet cores of both granulites indicate that they formed at about 41 ± 4 Ma, which therefore can be interpreted as the time that crustal anatexis generated the UHT granulites. Considering the granulites in the context of the alpine framework, it is also inferred that UHT conditions in the lower crust were achieved as a consequence of asthenospheric upwelling, probably related to slab steepening or slab breakoff.

How to cite: Gianola, O., Cesare, B., Bartoli, O., Ferri, F., and Anczkiewicz, R.: Dating crustal anatexis in UHT granulites with Lu-Hf, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8729, https://doi.org/10.5194/egusphere-egu2020-8729, 2020.

The availability of high-temperature thermochronometers suitable for generation of continuous thermal histories at mid- to lower-crustal temperatures (i.e., ≥ 400 °C) is limited. Available thermochronometers include the recently developed apatite and rutile U-Pb thermochronometers ( ≤ 550 and 640 °C; Kooijman et al., 2010; Cochrane et al., 2014) and arguably the K-Ar system in white mica (sensitive to temperatures ≤ 500 °C. Recent work has focussed on micro-beam U-Pb analysis of apatite and rutile by sector-field and multi-collector LA-ICPMS to generate single-crystal U-Pb age profiles. Such profiles can be inverted to yield continuous thermal histories for high-temperature processes (e.g., Smye et al., 2018). However, both apatite and rutile can exhibit crystal growth and dissolution-reprecipitation reactions in the same temperature ranges at which measurable Pb diffusion occurs: neither behaves as a pure thermochronometer in all circumstances (e.g., Chambers and Kohn, 2012; Harlov et al., 2005). Thus, it is critical to develop protocols which unequivocally identify age profiles arising from volume diffusion.

Here, we present case studies from greenschist- to granulite-facies-grade metamorphic systems from the Eastern Alps and the Western Gneiss Region of Norway. We demonstrate the utility of trace-element analysis (Sr-Y-REE-Th-U) and isotopic forward modelling to discriminate age resetting arising from (re)crystallisation from diffusion. Both rutile and especially apatite routinely incorporate non-trivial amounts of common-Pb during crystallisation (as opposed to radiogenic Pb generated by in-situ radionuclide decay), rendering them discordant in U-Pb isotope space. This common-Pb must be corrected for during age calculation. However, common-Pb is isotopically distinct from radiogenic Pb but exhibits the same diffusion behaviour, so the predicted U-Pb isotopic distribution for a given crystal arising from a proposed thermal history can be estimated by isotopic forward modelling. Thus, common-Pb can be exploited to validate both the assumption of Pb-loss by volume diffusion, and the thermal history predicted by age profile inversion.

Chambers, J.A., & Kohn, M.J., Am. Mineral., 97, 543–555 (2012); Cochrane, R., et al., Geochim. Cosmochim. Acta, 127, 39–56, (2014); Harlov, D.E., et al., Contrib. Mineral. Petrol, 150, 268–286 (2005); Kooijman, E., et al., Earth Planet. Sci. Lett, 293, 321–330, (2010); Smye, A.J., et al., Chem. Geol., 494, 1–18 (2018).

How to cite: Mark, C., Daly, J. S., Chew, D., and Cogné, N.: What are we dating, volume diffusion or recrystallisation? Isotopic modelling and trace-element analysis as tools to interpret the high-temperature U-Pb thermochronometer in apatite and rutile, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9405, https://doi.org/10.5194/egusphere-egu2020-9405, 2020.

The Ryoke plutono-metamorphic belt exposed in SW Japan is the type locality for low-Pressure/high-Temperature (LP/HT) metamorphism. The Ryoke metamorphic field gradient is, however, a complex object shaped by several deformation phases, multiple magmatic pulses and protracted metamorphism. In the western part of the Ryoke belt (Iwakuni-Yanai area), a petrological and geochronological study of two plutons emplaced before metamorphism is used to explore the behaviour of magmatic monazite along the LP/HT gradient and constrain the thermal history of the belt. We compare a massive granite adjoining schistose rocks affected by greenschist facies metamorphism with a gneissose granite adjoining migmatitic gneiss affected by upper-amphibolite facies conditions. Despite contrasting textures, the granite samples have similar mineral modes and compositions. Monazite in the massive granite is dominated by primary domains with limited secondary recrystallization, and is variably replaced by allanite+apatite±xenotime±Th−U-rich phases. Primary domains yield an average 206Pb/238U date of 102 ± 2 Ma while Th−U phases show Th−U−Pb dates of ca. 58 and 15−14 Ma. Monazite in the gneissose granite preserves sector- or oscillatory-zoned primary domains cross-cut by inclusion-rich secondary domains enriched in Ca, Y, U, P. Primary domain analyses are commonly discordant (116−101 Ma) while secondary domains preserve concordant 206Pb/238U dates spreading from 102 ± 3 to 91 ± 2 Ma.

Despite alteration, primary monazite domains preserve the age of magmatic crystallization for both plutons (102 ± 2 Ma and 106 ± 5 Ma). In the massive granite, monazite replacement is ascribed to the influx of aqueous fluid enriched in Ca+Al+Si±F during hydrothermal alteration below 500 °C. The oldest date (58 ± 5 Ma) obtained from the Th−U-rich alteration products is regarded as a minimum age for chloritization during final exhumation of the granite. In the gneissose granite a small amount of anatectic melt was responsible for a pseudomorphic recrystallization of monazite by dissolution-reprecipitation above 600 °C. The spread in 206Pb/238U dates for the secondary domains is attributed to incomplete isotopic resetting during dissolution-reprecipitation, and the youngest date of 91 ± 2 Ma is considered as the age of monazite recrystallization during a suprasolidus metamorphic event. These results reveal a diachronous, ca. 10 Ma-long HT history and an overall duration of about 15 Ma for the metamorphic evolution of the western part of the Ryoke belt.

How to cite: Skrzypek, E., Shuhei, S., and Dominik, S.: Using the alteration of magmatic monazite to constrain the thermal history of the Ryoke metamorphic belt (SW Japan), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2877, https://doi.org/10.5194/egusphere-egu2020-2877, 2020.

The Pontiac subprovince consists of metaturbidites, plutons and thin ultramafic rock layers of Archean age and lies south of the Cadillac-Larder Lake (C-LL) fault zone which is the boundary between the Pontiac and the extensively mineralized Abitibi Greenstone Belt. The sediments show a Barrovian metamorphic gradient which increases southwards, away from the C-LL fault. The most likely tectonic provenance for the Pontiac sedimentary rocks is that they represent a relic accretionary prism with material derived from both the Abitibi and an older terrane. Zircon U-Pb dating shows that deposition occurred not later than 2685±3 Ma ago and recent, robust Lu-Hf dating of garnets bracketed Pontiac's peak metamorphic conditions at 2658±4 Ma. For this study we used a recently developed LA-ICP-MS/MS method for in-situ Rb-Sr dating of biotite and plagioclase in samples ranging in metamorphic grade (biotite to sillimanite zones) from the Pontiac subprovince. Calibration of the instrument was achieved by repeated ablations on several reference materials (see Hogmalm et al. 2017) which also provided the monitoring of accuracy and precision throughout the analyses. Results show a range in dates between 2550 Ma and 2200 Ma with an average of 2440±50 Ma (2σ). Samples from the staurolite and kyanite zones have a larger range with respect to the other zones, but no significant differences are observed in the data with any method of data handing. These dates are ≈300Ma younger than the peak metamorphism in the area and this is attributed to either overgrowth and re-setting of the Rb-Sr system by a second metamorphic/hydrothermal event, or diffusional resetting with core-rim age variations. Possible influence from the adjacent late syntectonic to post-tectonic monzodiorite-monzonite-granodiorite-syenite (MMGS) plutons dated 2671±4 Ma and the garnet-muscovite-granite series (GMG) dated ≈2650 Ma cannot be ruled out. This study provides insights about the metamorphic history of the sequence and supports previous findings regarding resetting of some isotopic systems with relatively low closure temperatures (≈350-400°C) by later thermal events.

How to cite: Leventis, N., Zack, T., Pitcairn, I., and Högmalm, J.: First in-situ Rb-Sr dating of metasedimentary rocks from the Pontiac subprovince, Superior Craton, Canada. Implications towards the regional metamorphic evolution of the sequence., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2699, https://doi.org/10.5194/egusphere-egu2020-2699, 2020.

Sulfur is an important geochemical element that is part of many natural compounds. Sulfur takes part in most natural processes. In this case, a change in the valency of this element can occur. The change in sulfur valency is accompanied by isotope fractionation, which gives rise to noticeable shifts in the isotopic composition of this element. Thus, sulfur is one of the important geochemical indicators that can be used to reconstruct the redox conditions of various geological processes, including sedimentation processes in marine basins. For many decades, the traditional method of analyzing the isotopic composition of sulfur in sulfides remains the method involving the oxidation of sulfur to SO2 and the subsequent measurement of the 34S/32S ratio in gas using a mass spectrometer (Giesemann et al., 1994; Grassineau et al., 2001; Studley et al., 2002, etc.). However, with the advent of the inductively coupled plasma method of multi-collector mass spectrometry, new in isotope geochemistry, the corresponding methodological direction began to develop in which this method is used to analyze the sulfur isotopic composition (Krupp et al., 2004; You and Li, 2005; Clough et al ., 2006; Mason et al., 2006; Craddock et al., 2008 and others). The results presented in these works suggest that the MC-ICP-MS method can be used to solve certain problems of studying the natural variations in the sulfur isotopic composition. Therefore, we created optimized methodic for measuring isotope ratio of sulfur. The experimental work was carried out on the instrument base of the laboratory of SEC "Geothermohronology" of the Institute of Geology and Oil and Gas Technologies at Kazan Federal University (IGiNGT KFU, Kazan) and was aimed at creating a set of analytical procedures for chemical sample preparation and mass spectrometric analysis methods. The chemical preparation of pyrite samples included the stage of their acid decomposition using concentrated inorganic acids, which ensured the quantitative transfer of the sulfur of the sample into solution, as well as the stage of obtaining pure sulfur preparations based on the use of ion exchange chromatography. The optimal methodological scheme of mass spectrometric measurement of the 34S / 32S ratio was determined on a Neptune Plus multi-collector mass spectrometer. It takes into account the influence of factors unfavorable for analysis of the isotopic composition of sulfur, such as interference overlays of ions of oxygen, carbon, and nitrogen compounds, as well as the effect of instrumental mass discrimination. The work was supported by the Ministry of Science and High Education of the Russian Federation contract No. 14.Y26.31.0029 in the framework of the Resolution No.220 of the Government of the Russian Federation.

How to cite: Gareev, B., Batalin, G., and Chugaev, A.: Determination of sulfur isotopic ratio by HR-ICP-MS method, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17744, https://doi.org/10.5194/egusphere-egu2020-17744, 2020.

The thermal history of the Siberian platform has not been studied and only single thermochronological study is available now [Rosen et al., 2009]. According to high-precision U-Pb dating the main phase of magmatic activity of the Siberian Traps Large Igneous Province took place ~252.0-251.3 Ma [Kamo et al., 2003] and its duration didn’t exceed ~1 Myr. But according to Ar/Ar dating (~240 Ma) [Ivanov et al., 2013] the total duration of the Siberian Traps formation may be estimated as long as ~10 Myr. In addition, single apatite fission track (AFT) ages are approximately 222-185 Ma [Rosen et al., 2009].

We present the first results of AFT dating from the Guli pluton and computer modeling of its post-magmatic cooling, as well as some new AFT ages from other magmatic bodies within the Siberian platform. Based on these data we present the first model of the tectonothermal evolution of the Siberian platform in Mesozoic and Cenozoic.

The Guli massif is located within the Maymecha-Kotuy region of the Siberian Permian-Triassic Traps and is the world's largest alkaline-ultrabasic complex. Results of U-Pb dating of baddeleyite from the carbonatites – the latest intrusion phase – 250.2±0.3 Ma [Kamo et al., 2003] correspond to the time of massif’s crystallization.

AFT dating was conducted by an external detector method at the University of Arizona (Tucson). The fission track ages of the Guli are in the range of ~250-231 Ma with the mean standard error (1σ) ±34 Myr. In addition, we obtain five new AFT ages as well as U-Pb age obtained from different intrusive bodies within the Siberian platform: Kontayskaya intrusion, Odikhincha massif and Padunsky sill. All obtained AFT ages are in the range of 195-173 ±13 (1σ) Ma, which corresponds to the Early-Middle Jurassic. At the same time, the U-Pb LA-ICPMS age of apatite from Padunsky sill is 242±7 Ma.

Thermal history modeling using fission track age data and track lengths distribution was performed in HeFTy v.1.8.3. Based on the obtained results we consider the following model of tectonic-thermal evolution of the studied intrusive massifs: (1) the emplacement of intrusions ca. 250 Ma; (2) their burial under a thick sedimentary (volcanic?) cover; (3) regional exhumation and cooling below 110°C about 220-190 Ma.

The research was carried out with the support of RFBR (grants 18-35-20058 and 18-05-00590) and Programs of development of Lomonosov Moscow State University.

How to cite: Bagdasaryan, T., Veselovskiy, R., Myshenkova, M., Zaitsev, V., Thomson, S., Latyshev, A., and Zakharov, V.: New apatite fission track thermochronology data from the Siberian Permian-Triassic Traps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-692, https://doi.org/10.5194/egusphere-egu2020-692, 2020.