Trace elements in zircon are a promising proxy to quantitatively study long-term Earth’s lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. It specifically provides insight into the water content, magmatic redox conditions, and the extent of pla­gioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomaly from a global compilation of detrital zircons and display it as a timeseries and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Combining these trends with thermodynamic modelling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons are produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

How to cite: Triantafyllou, A., Ducea, M., Jepson, G., Bisch, A., and Ganne, J.: Statistical analysis of Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1954, https://doi.org/10.5194/egusphere-egu23-1954, 2023.

Constraining precise ages for impact events is crucial in establishing Earth’s history, and several geochronometers have been developed to date impacts. We present electron backscatter diffraction (EBSD), sensitive high-resolution ion microprobe (SHRIMP) and atom probe tomography (APT) data from shocked xenotime [(Y,HREE)PO₄] collected from two impact sites to investigate the potential of xenotime as an impact geochronometer. A detrital xenotime grain from the Vredefort dome (South Africa) contains planar fractures, planar deformation bands and {112} twinning, the latter of which are diagnostic shock microstructures. However, APT analysis from the twin domains and also from the host yielded no evidence of Pb mobility at the nanometer scale during the impact. SHRIMP analysis (n=24) on the grain yielded a discordia with an upper intercept of 3136 ± 110 Ma and an imprecise lower intercept of 1793 ± 280 Ma. These correspond, respectively, to the bedrock age and a post-impact, cryptic terrane-wide fluid infiltration event. Three neoblastic grains from the Araguainha dome (Brazil) experienced partial to complete recrystallisation. The least recrystallised grain yields the oldest ²³⁸U/²⁰⁶Pb age of 479 ± 26 Ma, whereas a completely recrystallised neoblastic grain gave an age of 257 ± 11 Ma.  APT analysis on the latter grain showed different nanoscale features that shed light on Pb mobility during shock deformation and recrystallisation.  Based on observations of nanoscale Pb mobility and the correlation between recrystallisation and isotopic resetting, and prior published ages, we interpret 257 ± 11 Ma to date the impact event. These data confirm that recrystallised neoblastic xenotime is a useful impact geochronometer. 

How to cite: Joseph, C., Fougerouse, D., J. Cavosie, A., K. H. Olierook, H., M. Reddy, S., R. Quintero, R., Kennedy, A., W. Saxey, D., and D.A. Rickard, W.: Shocker: xenotime can date impacts, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1082, https://doi.org/10.5194/egusphere-egu23-1082, 2023.

Movement of a large volume of granitic melt is an important factor in the compositional differentiation of the continental crust and the presence of melt in rocks profoundly influences their rheology. Different mechanisms controlling melt migration through crust were proposed. We suggest that pervasive melt flow, analogous to reactive porous melt flow in mantle, could be possibly one of them. It is generally accepted that migration of felsic melts in continental crust starts with short distance pervasive microscopic flow into segregation veins which extract melt. However, we show that pervasive melt flow may be a regional mode of melt migration in continental crust. In such scenario, melt driven by deformation passes pervasively along grain boundaries through the whole rock volume. And the term pervasive melt flow is used for grain-scale, diffuse, porous and reactive flow of felsic silicate melt through rocks. This is effectively an open-system process that thoroughly reworks the resident rock mass. Through-flow of melt destroys pre-existing fabrics and the original chemical and isotopic nature of the protolith. Melt segregation is inefficient and protolith become isotropic granite-like, with partly preserved relics of the original, without ever containing more than a few melt percent at any time. The fabric and geochemical nature of these granites encapsulates the complex history of hybridization.

In order to decipher duration of pervasive melt migration we used precise U-Pb monazite ID-TIMS (isotope dilution thermal ionization mass spectrometry) and U-Pb monazite Laser Ablation Split Stream (LASS) geochronology in combination with monazite chemistry as well as U-Pb zircon SHRIMP geochronology. Monazite reveal continuous chemical equilibration with passing melt. They are getting progressively enriched in HREE and depleted in Eu. Monazites in the least affected rock preserve original magmatic zoning in Th and U, in contrast to more with melt equilibrated rock types, where this zoning is lost. Data for each migmatite type reveal  similar date spread for both cores and the Y-rich well defined rims of single monazite grains, indicating a disconnect between U-Pb dates and chemical zoning. There is also no correlation between U-Pb ages and Yb/Gd ratio. This suggest perturbance of the isotopic system. We interpret these random distribution within-grain date variations as a result of dissolution-reprecipitation reactions between monazite grains and melt. During the coupled dissolution-reprecipitation radiogenic Pb was redistributed within the grain. This is supported by dissolution of apatite into silicate melts that stabilizes monazite during migmatitization, preventing their dissolution but not reaction with passing melt. Redistribution of radiogenic Pb resulted in meaningless individual ages from different migmatite types, but gave overall duration of the thermal event – pervasive melt flow. Duration of pervasive melt flow was dated 8-10myr. This suggest that porous flow of silicate melts in continental crust is a process which can operate over a long time and impacts on the rheology of the crust during orogeny.

How to cite: Hasalová, P., Schulmann, K., Schaltegger, U., Štípská, P., Kylander-Clark, A., Holder, R., Weinberg, R., and Maierová, P.: Timescale of pervasive melt migration in the continental crust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6959, https://doi.org/10.5194/egusphere-egu23-6959, 2023.

Determining the crystallization of S-type granitic material can be challenging due to a lack of neoblastic zircon growth (e.g. thin overgrowths) and the potential of large inherited zircon cargos. Coupled apatite–zircon geochronology can help address such complexities and also clarify post-magmatic thermal history, given the disparate Pb closure temperatures in these minerals. Here we present a case study on the Johnston Complex, a rare outcrop of the Precambrian basement in southern Britain, representing a window into the tectonic regime of Avalonia. Zircon and apatite yield identical U-Pb ages, within uncertainty, of 569 ± 2 Ma and 576 ± 11 Ma, respectively. A minor antecrystic zircon core component is identified at 615 ± 11 Ma. Given the previously reported zircon U-Pb age of 643 Ma, these results demonstrate that the Complex represents a composite suite of plutons along its ca. 20 km length. Zircon Lu-Hf data imply a broadly chondritic source, with model ages consistent with crustal extraction during Rodinia formation. Zircon trace elements are consistent with a calc-alkaline continental magmatic arc setting. Whilst, apatite trace elements demonstrate a sedimentary component within the melt. Combined, these results support arc granite production within the peri-Gondwanan realm during amalgamation of Eastern Avalonia and associates the Johnston Complex to the Cymru subterrane. Importantly, congruent zircon–apatite ages imply rapid cooling after crystallisation, and that subsequent thermal heating did not exceed the apatite Pb closure temperature.

How to cite: Clarke, A., Kirkland, C., and Glorie, S.: A detective duo; an apatite–zircon case study of the Johnston Complex, Wales, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1147, https://doi.org/10.5194/egusphere-egu23-1147, 2023.

Allanite-group minerals are known to incorporate not only U and Th but also initial, non-radiogenic Pb. Allanite can therefore be analyzed in order to assess its crystallization age as well as the ambient Pb composition at the time of crystallization. Whereas allanite age dating has been the focus of many studies, constraining its initial Pb composition has received much less attention. We collected a series of Phanerozoic, allanite-bearing magmatic rock samples (volcanic, plutonic, pegmatite) and measured both the age and initial Pb composition of allanite by laser ablation-multi collector-inductively coupled plasma-mass spectrometry. We show that allanite data can be corrected for mass bias and fractionation using zircon (for U/Pb and Th/Pb ratios) and glass (for Pb/Pb ratios) as reference material as long as allanite is not metamict. A lower intercept age and y-axis intercept Pb composition can be determined by linearly regressing U-Pb data in a Tera-Wasserburg diagram, and a ²³⁰Th disequilibrium correction is highly recommended. We find a good agreement between our allanite U-Pb dates and published U-Pb zircon ages for the same localities. Our initial Pb compositions are validated by a fair agreement with Pb isotopic data measured on co-genetic feldspars from the same samples. The initial Pb composition of samples ranging from ca. 530 to 18 Ma reveals fluctuations in initial ²⁰⁷Pb/²⁰⁶Pb ratio, which points to different degrees of crustal (elevated μ=²³⁸U/²⁰⁴Pb) contribution. These variations could be due to post-magmatic deformation, weathering or metamorphism, but we believe that they rather reflect differences in initial magma composition. We thus emphasize the usefulness of allanite initial Pb compositions to discuss the source of igneous rocks.

How to cite: Skrzypek, E., Gallhofer, D., Hauzenberger, C., and Haas, I.: How useful is the initial Pb composition of magmatic allanite ?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8976, https://doi.org/10.5194/egusphere-egu23-8976, 2023.

Due to the complex geodynamic framework and the excellent state of preservation of the stratigraphic relationships towards the host metamorphic and sedimentary rocks, the Permo-Triassic magmatic sequences of the Southern Alps (Italy) are intensely studied. Throughout the Southalpine domain, the main peaks of the volcano-plutonic activity are both pre- and post-dated by the emplacement of small volume of magmas with variable chemical affinity. These magmas, preserved as dykes and veins intruded into the plutonic bodies and/or the overlying volcanites, are powerful tools for tracing the evolution of the magma source and reconstructing the temporal evolution of the magmatic episode. Here, we present a detailed geochemical and geochronological study of phonolitic dykes (SiO₂ from 56.8 to 57.8 wt.%; Na₂O + K₂O from 11.1 to 15.3 wt.%) cropping out near Predazzo (Southern Alps; Italy) and intruded into the basaltic to trachyandesitic Middle Triassic lavas. The phonolites are mostly aphyric with a porphyricity index <10%. The main mineral phases are concentric-zoned clinopyroxene, ranging in composition from diopside-hedenbergite, to aegirine (Wo_13-51; En_2-29; Fs_20-85), K-feldspar and rare sodalite. Accessory phases are titanite, apatite and magnetite embedded in a aphyric matrix. Titanite has a highly variable U-Th concentration (U from 24 to 478 ppm and Th from 170 to 4328 ppm) and is characterized by a chondrite-normalized REE pattern with a convex-upward shape (La/Yb_N from 18.9 to 41.5) with enrichment in LREE and depletion in HREE. Thermometry through Zr-in-Titanite calculations (Hayden et al., 2008) indicate crystallisation temperatures between 860.3 and 942.8 ± 57 °C. In-situ, U-Pb dating on titanite phenocrysts performed by laser ablation-inductively coupled-mass spectrometry (LA-ICP-MS) shows that the age of phonolite dykes is comprised between 240.4 ± 3.2 Ma and 242.0 ± 3.6 Ma, partially overlapping with the emplacement of the Middle-Triassic plutonic bodies of the Dolomites (238.190 ± 0.050 - 238.075 ± 0.087; Storck et al. 2019).

These results provide new insights into the timing of the Middle Triassic magmatic event in the Southern Alps, fostering the debates about the temporal and chemical evolution of the magmatism in between the Variscan orogeny and the opening of the Alpine Tethys.

References:

Hayden, L. A., Watson, E. B., & Wark, D. A. (2008). A thermobarometer for sphene (titanite). Contributions to Mineralogy and Petrology, 155(4), 529-540.

Storck, J. C., Brack, P., Wotzlaw, J. F., & Ulmer, P. (2019). Timing and evolution of Middle Triassic magmatism in the Southern Alps (northern Italy). Journal of the Geological Society, 176(2), 253-268.

How to cite: Coltorti, M., Nardini, N., Casetta, F., Tavazzani, L., Peres, S., Ntaflos, T., and Dellantonio, E.: In-situ, U-Pb dating of titanite in phonolitic dykes from the Dolomites area (Southern Alps, Italy): new insights on the timing of the Middle Triassic magmatism, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16555, https://doi.org/10.5194/egusphere-egu23-16555, 2023.

Porphyry Copper Systems (PCS) represent a significant source of metals, and will continue to play a key role in future with the development of green technology. Despite being one the most studied mineral systems, the primary controls on the ore tonnage of deposits (that varies up to 5 orders of magnitude in nature) remain poorly constrained. The Eocene Chuquicamata Intrusive Complex (CIC) in northern Chile hosts one of the world’s largest porphyry copper deposits and represent a perfect natural laboratory to explore the influence of timescales in controlling the formation and size of PCS.

Here we investigate the tempo of multiple magmatic-hydrothermal events in the CIC applying molybdenite geochronology (Re-Os ID-NTIMS) and high precision zircon petrochronology (U-Pb CA-ID-TIMS geochronology in tandem with LA-ICPMS trace element composition). Preliminary geochronological results may suggest a partial decoupling of the magmatic and hydrothermal events. Zircon U-Pb geochronology results point to a multi-million-year protracted magmatic history with at least two discrete pulses separated by 500 kyrs. The hydrothermal event appears slightly younger than the youngest magmatic pulse and lasted for ca. 1 Myrs.

The extensive duration of the mineralization scales with the behemothian size of the Chuquicamata deposit (more than 110 Mt of contained copper) and corresponds to predictions from numerical modelling of magma degassing. Interestingly, the apparent temporal decoupling between magmatism and hydrothermal activity at Chuquicamata suggests that syn-mineralization ore-forming magmas might not always intrude as dyke or stock at mineralization depth and can remain hidden at upper to mid-crustal depth. In the absence of high-precision geochronological data, this may bear consequences when assuming a direct genetic link between spatially associated porphyritic rocks and the mineralization.

How to cite: Virmond, A., Selby, D., Wotzlaw, J.-F., and Chelle-Michou, C.: Partially decoupled magmatic and hydrothermal events in porphyry copper systems?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13440, https://doi.org/10.5194/egusphere-egu23-13440, 2023.

The rate of cooling of metamorphic rocks provides a first-order constraint on the tectonic processes controlling heat flow and exhumation. For example, for small crustal terranes that were subducted to ultrahigh pressure conditions during the early stages of collisional orogenesis, exhumation is generally fast with rates similar to plate velocities, such that cooling is also rapid. Similarly, rates of cooling are commonly fast (generally ~20–30°C/Myr) during exhumation of metamorphic core complexes or due to transpression. By contrast, cooling in some granulite terranes can be slow and close-to-isobaric, leading to time-integrated cooling rates of <5°C/Myr. The implication of such slow rates of cooling is that these granulite terranes were close to isostatic equilibrium as a result of sustained high mantle heat flow that limited exhumation by erosion. However, constraining initial cooling rates in granulite terranes can be difficult, particularly where the rocks reached ultrahigh temperatures (>900 °C) that exceed the closure temperature of many geochronometers. In order to overcome this difficulty, we combine U–Pb zircon geochronology with Ti-in-zircon thermometry to investigate the thermal history of metapelitic rocks from the Eastern Ghats Province of eastern India. For the combined dataset of metamorphic zircon from the samples, concordant dates decrease continuously within 2σ uncertainty from around 950 Ma to 800 Ma, consistent with c. 150 Ma of zircon crystallization. Ti-in-zircon temperatures for each dated spot during this period decrease with age, corresponding to linear cooling rates ranging from 0.26 to 0.90°C/Myr. We propose that retention of heat producing elements in the lower crust of the Eastern Ghats Province and a low net erosion rate were responsible for c. 150 Myr of ultra-slow cooling. The location of the Eastern Ghats Province on the margin of the supercontinent Rodinia may have been a contributing factor enabling the region to remain relatively undisturbed until it was exhumed during the formation of Gondwana.

How to cite: Clark, C., Brown, M., Johnson, T., Mitchell, R., and Gupta, S.: Ultra-slow cooling of ultra-hot orogens, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1931, https://doi.org/10.5194/egusphere-egu23-1931, 2023.

⁴⁰Ar/³⁹Ar dating has been a valuable and widely used method for dating orogenic processes such as prograde and retrograde metamorphism and brittle and ductile deformation, through the analysis of K-bearing rock-forming minerals such as white mica. The in situ ⁴⁰Ar/³⁹Ar method, in which a short wavelength laser is used to ablate an analyte and deliver the liberated Ar to a noble gas spectrometer, is particularly valuable as an approach to dating deformation or metamorphism because it allows for targeting of specific chemical and structural domains, and the mapping of intragrain age distributions. Rb-Sr dating can also be applied to K-bearing minerals because of Rb’s propensity to substitute for K. The Rb-Sr method has been under-used in recent decades because the isobaric interference between parent ⁸⁷Rb and daughter ⁸⁷Sr has necessitated the chemical separation of Rb from Sr via ion exchange chromatography prior to mass spectrometric analysis, and hence bulk sampling of the target analyte. New tandem mass spectrometers, in which two quadrupoles are separated by an intervening reaction chamber into which a reactive gas can be introduced, have opened up the opportunity of applying laser-based in situ sampling approaches to beta decay geochronometers, including Rb-Sr (Zack and Hogmalm, 2016).

We have collected new in situ Rb/Sr data for white mica from three different tectono-metamorphic settings previously dated using the in situ ⁴⁰Ar/³⁹Ar method: recrystallization of white mica in a Paleozoic low-temperature ductile shear zone; development of multiple cleavage domains in low-temperature metamorphic rocks deformed in the Paleozoic, and; slow cooling of rocks following regional amphibolite-facies metamorphism in a Paleoproterozoic orogeny. , This allows a direct comparison between these two approaches, with the goal of exploring the functionality and utility of in situ Rb-Sr data, and testing geological interpretations based upon the in situ ⁴⁰Ar/³⁹Ar method. Our results show that the in situ Rb-Sr method is a highly complementary approach to the ⁴⁰Ar/³⁹Ar method for white mica, particularly in cases for which the target mica population has a large internal spread in Rb/Sr. allowing for the rigorous testing of assumptions and hypotheses about timing and conditions of rock cooling, deformation, and fluid events developed using ⁴⁰Ar/³⁹Ar datasets.

Zack, T. and Hogmalm, K.J., 2016. Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell. Chemical Geology, 437, pp.120-133.

How to cite: Kellett, D., Larson, K., and Skipton, D.: Integration of in situ Rb-Sr and in situ 40Ar/39Ar dates under diverse tectono-metamorphic scenarios, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5290, https://doi.org/10.5194/egusphere-egu23-5290, 2023.

Pseudotachylytes are solidified frictional melts produced by seismic fault slip. Being melts that solidified in seconds or minutes after the seismic slip event, they have always been considered a very favourable tool to date brittle deformation. However, since all pseudotachylytes are composed of inherited clasts, melt-derived matrix and (quite often) also alteration products, it is necessary to discriminate the Ar contribution of these three reservoirs to obtain meaningful ages. This can be done by analyzing Ca/K and Cl/K signatures provided by Ar systematics. Furthermore, microstructural analysis and microCT allow quantifying the clast-to-pseudotachylyte matrix ratios for each sample, and XRPD allows detecting potential alteration phases. Here we present the results of step-heating ⁴⁰Ar/³⁹Ar analyses performed on pseudotachylytes of the Trois Villes Fault and the Quart Fault, which crop out in a region of the Western Alps (Aosta Valley) affected by three different post-metamorphic brittle deformation phases: D1 characterized by NW-SW extension, D2 with NE-SW extension, and D3 showing N-S extension. The relative chronology of these deformation phases is based on consistent cross-cutting relationships. D1 ages of 29–32 Ma have been inferred from syn-kinematic magmatic dikes and hydrothermal veins. However, no absolute ages were so far available for D2 and D3, as direct radiometric dating of fault rocks has never been performed before in the area. Our results are consistent with the relative chronology and greatly improve our understanding of the tectonics of this area.

How to cite: Lugoboni, Z. G., Arienti, G., Barberini, V., Bistacchi, A., Cannella, C., Caprarulo, S., and Villa, I. M.: 40Ar/39Ar dating of pseudotachylytes: a case study on post-metamorphic brittle fault in the NW Alps, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7367, https://doi.org/10.5194/egusphere-egu23-7367, 2023.

In low temperature thermochronology, reliable interpretation of (U-Th)/He data is controlled by our understanding of helium diffusion in a crystal. The diffusion kinetics can be simulated through the classic Arrhenius-type equation, with parameters frequency factor Do and activation energy Ea (Farley, 2000). For apatite, it has been demonstrated that accumulated radiation damage perturbed the Arrhenius-type equation and exerts a strong control on He diffusion. Two models have been developed to parameterise the evolution of diffusion kinetics in apatite in terms of accumulated radiation damage: one based on the physical phenomenon (Gautheron et al., 2009) and the other calibrated on empirical observations (Flowers et al., 2009). As the amount of radiation damage depends on both time (U and Th decay producing damage) and temperature (annealing of radiation damage), both of these models are routinely used to interpret apatite (U-Th)/He data in terms of thermal histories. However, results obtained from inverse thermal history modelling with these two models can differ and be inconsistent with other low thermochronological data (e.g., apatite fission tracks). In this contribution we present a new radiation damage-based diffusion model that combines the approaches of both the Gautheron et al. and Flowers et al. models.

Our new model is based on the theoretical diffusion model proposed by Gerin et al. (2017) but incorporates a new calibration from the available He diffusion experiment results. The Gerin et al. model is built on a theoretical understanding of the fundamental physical processes and predicts diffusion parameters for different levels of crystal lattice damage, using quantum calculus. We recalibrated this model through an empirical law based on real crystal mesh damage calculated from available experimental data. To test the reliability of the revised model and to compare it to the existing models, it was implemented in the modelling software, QTQt (Gallagher, 2012). Here we present results of both forward and inverse modelling to highlight the benefits of the new model. The results are assessed in terms of the impact for “deep time” (>500 Ma) thermochronology, in which accumulated radiation damage can have a significant control on the inferred thermal history models.

Farley, K.A., 2000. Helium diffusion from apatite: General behavior as illustrated by Durango fluorapatite. J. Geophys. Res. 105, 2903–2914. https://doi.org/10.1029/1999JB900348

Flowers, R.M., Ketcham, R.A., Shuster, D.L., Farley, K.A., 2009. Apatite (U–Th)/He thermochronometry using a radiation damage accumulation and annealing model. Geochimica et Cosmochimica Acta 73, 2347–2365. https://doi.org/10.1016/j.gca.2009.01.015

Gallagher, K., 2012. Transdimensional inverse thermal history modeling for quantitative thermochronology. Journal of Geophysical Research: Solid Earth 117, n/a-n/a. https://doi.org/10.1029/2011JB008825

Gautheron, C., Tassan-Got, L., Barbarand, J., Pagel, M., 2009. Effect of alpha-damage annealing on apatite (U–Th)/He thermochronology. Chemical Geology 266, 157–170. https://doi.org/10.1016/j.chemgeo.2009.06.001

Gerin, C., Gautheron, C., Oliviero, E., Bachelet, C., Mbongo Djimbi, D., Seydoux-Guillaume, A.-M., Tassan-Got, L., Sarda, P., Roques, J., Garrido, F., 2017. Influence of vacancy damage on He diffusion in apatite, investigated at atomic to mineralogical scales. Geochimica et Cosmochimica Acta 197, 87–103. https://doi.org/10.1016/j.gca.2016.10.018

How to cite: Derycke, A., Gallagher, K., and Gautheron, C.: A new calibration of radiation damage control on He diffusivity in apatite: implications for (U-Th)/He thermochronology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3132, https://doi.org/10.5194/egusphere-egu23-3132, 2023.

In situ analysis of the Rb–Sr isotopic composition of biotite via triple quadropole LA–ICPMS is an increasingly popular method for constraining the time through the Sr closure temperature in rocks. Although interpreting the radiogenic product can be complicated by various factors that can affect diffusion of Rb and Sr, the role of the different minerals that may be in contact with biotite in regard to local diffusion gradients is poorly understood. In this study, we show the importance of analysing Rb–Sr isotopic data in the context of detailed petrographic observations, which reveals that the ratios obtained are affected by various diffusion pathways between like material and minerals that preferentially incorporate Sr. The studied samples are metapelites from the Fraser Zone (Western Australia) that have peak metamorphic conditions of about 850 °C and 9 kbar and a history of cryptic biotite Ar-Ar ages of ~1205 Ma, which on face value could imply exhumation rates that are some of the fastest recorded in Earth’s history. However, new biotite data from in-situ Rb-Sr analysis highlights differences in Sr retentivity. While calculated isochrons may at first yield large errors, sorting based on the location of the grains in terms of surrounding minerals yields a possible solution for varying Sr values skewing the ages in the sample. This results in an average age of 1205 Ma for biotite and sillimanite surrounded grains and 1107 Ma, from biotite and sillimanite surrounded grains and quartz and K-feldspar surrounded grains. This shows that the diffusive properties of Sr between biotite and the surrounding minerals creating variable re-equilibration between the different domains surrounding biotite. The complexities of Sr diffusion within between the various phases are still unknown, but the apparent effect between the surrounding material on the biotite and the measured initial Sr values does play a key factor in the final calculated ages and the interpretations they represent.

How to cite: Rohrer, R., Clark, C., Kirkland, C., and Johnson, T.: Multiple dates in millimetres; diffusion as an explanation for Rb-Sr age discrepancies in biotite, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2154, https://doi.org/10.5194/egusphere-egu23-2154, 2023.

Qinling Orogenic Belt with its Meso-Cenozoic intracontinental orogeny and uplift, is a key physiographic element that characterized the differential evolution of the geology, geography and climate in continental China (Dong et al., 2022). However, numerous thermochronological dates of the Qinling bedrocks (Dong et al., 2011; Yang et al., 2017) show that there is a wide cooling gap between Triassic and Early Cretaceous. In this study, we studied this gap by multiple geochronology and geochemistry on Lower Cretaceous molasse granitic gravel samples, with the aim to recover the hidden Mesozoic exhumation history. We report the first detailed zircon U-Pb ages, whole-rock major and trace elements and Sr-Nd-Pb isotopic data, which suggest that these clasts derive from Late Triassic I-type granites which were emplaced in a syn-collisional setting during a subduction phase. Their provenances were also determined by comparison with the geochemical fingerprint of Qinling granitic bedrocks. New zircon and apatite U-Pb, (U-Th)/He and fission-track data, as well as biotite ⁴⁰Ar-³⁹Ar, were performed on the granitic gravels dated between ca. 222 Ma to 110 Ma. Thermal history modeling, based on the multiple geochronological data, shows rapid cooling from ca. 700 °C to 200 °C during Late Triassic-Early Jurassic, then followed by a period of slow cooling from Middle Jurassic to Early Cretaceous.

As a whole, our new multiple geochronological and geochemical data and the related thermal history modeling results provide new insights on the prolonged pre-Cenozoic cooling history as well as the intracontinental deformation of the Qinling, which were mostly related to Paleo-Tethyan subduction and Late Triassic North China-South China Block collision.

Reference:

Dong, Y., Genser, J., Neubauer, F., Zhang, G., Liu, X., Yang, Z. and Heberer, B., 2011. U-Pb and 40Ar/39Ar geochronological constraints on the exhumation history of the North Qinling terrane, China. Gondwana Research, 19(4): 881-893.

Dong, Y., Sun, S., Santosh, M., Hui, B., Sun, J., Zhang, F., Cheng, B., Yang, Z., Shi, X., He, D., Yang, L., Cheng, C., Liu, X., Zhou, X., Wang, W. and Qi, N., 2022. Cross Orogenic Belts in Central China: Implications for the tectonic and paleogeographic evolution of the East Asian continental collage. Gondwana Research, 109: 18-88.

Yang, Z., Shen, C., Ratschbacher, L., Enkelmann, E., Jonckheere, R., Wauschkuhn, B. and Dong, Y., 2017. Sichuan Basin and beyond: Eastward foreland growth of the Tibetan Plateau from an integration of Late Cretaceous-Cenozoic fission track and (U-Th)/He ages of the eastern Tibetan Plateau, Qinling, and Daba Shan. Journal of Geophysical Research: Solid Earth, 122(6): 4712-4740.

How to cite: Peng, H., Wang, J., Liu, C., Li, J., Jiao, X., Zhang, L., and Zattin, M.: Revealing the hidden Mesozoic exhumation history of the Qinling orogenic belt, Central China: insights from multiple geochronological and geochemical data of the molasse granitic gravels, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1262, https://doi.org/10.5194/egusphere-egu23-1262, 2023.

Two end-member competing models have been proposed to describe the kinematics of the central Nepal Himalayas in the last few Myr. They differ in their interpretations of which surface breaking faults accommodate current shortening and the kinematics responsible for driving rapid exhumation in the topographic transition zone around the Main Central Thrust (MCT). These locally higher uplift and erosion rates in the High Himalaya could reflect (1) thrusting over a midcrustal ramp with the growth of a Lesser Himalaya duplex at midcrustal depth causing underplating along the Main Himalayan Thrust ramp, or (2) out‐of-sequence thrusting along the front of the High Himalaya, possibly driven by climatically controlled localized exhumation.

To decipher between the two tectonic models, we compare existing low and medium-temperature thermochronometric data (⁴⁰Ar/³⁹Ar on muscovite, apatite (U-Th)/He - AHe, zircon (U-Th)/He - ZHe, apatite fission track - AFT, and zircon fission track - ZFT), extracted from the world thermochronometric data file of Herman et al. (2013), to luminescence thermochronometry data from 61 newly collected rock samples along transhimalayan rivers between the Kali Gandaki and the Trisuli. The luminescence thermochronometry data provide a new perspective on Late Pleistocene exhumation rates (timescales of 10⁴ to 10⁵ years) of the Nepalese Himalayas, by offering quantitative high-resolution constraints of rock cooling histories within the upper kilometres of the Earth’s crust.

All of the thermochronometric data show younger ages and higher exhumation rates around the topographic transition and the MCT zone through central Nepal. For the higher temperature thermochronometers, there is a continuous trend towards younger ages from the Lesser Himalaya through the topographic transition and the MCT zone. These data suggest that the in-sequence model, with exhumation rates linked to increased erosion and the formation of a duplex below the Higher Himalayas, coincident with the MCT location in some areas, is the model that best describes the thermochronometric ages of this study area on Myr timescales. However, the luminescence thermochronometry data reveal a spatial and temporal variability of the higher exhumation rates at different timescales, suggesting an intermittency of exhumation signal due to geomorphological processes. The luminescence thermochronometry data also highlight a systematic sharp transition at the MCT, pointing to out-of-sequence activity at this tectonic boundary on 100-kyr timescales. Whether this difference in tectonic model between the two timescales is due to low resolution of the higher temperature thermochronometers, shallow isotherms deflected by fluid circulation and hot spring activity near the MCT, or to a change in tectonic regime during the last 200 kyr, out-of-sequence activity of the MCT needs to be considered in seismic hazard models as it could put the local population at risk.

How to cite: Bouscary, C., King, G., Lavé, J., Grujic, D., Hetényi, G., Almeida, R., Gajurel, A., and Herman, F.: Out-of-sequence fault activity in the High Himalaya revealed by luminescence thermochronometry, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14643, https://doi.org/10.5194/egusphere-egu23-14643, 2023.

The Periadriatic Fault System (PAF) is among the largest and most important post-collisional structures of the Alps; it accommodated between 150-300 km of right-lateral strike-slip motion between the European and Adriatic plates from about 35 until 15 Ma. Recent GPS data suggest that Adria-Europe convergence is still being accommodated in the Eastern Alps. However, according to instrumental and historical seismicity records, seismotectonic deformation is mostly concentrated in the adjacent Southern Alps. In this contribution, we present our first results for dating earthquakes along the PAF during the Quaternary by applying two trapped charge dating methods. Both Electron Spin Resonance (ESR) and Optically Stimulated Luminescence (OSL) are especially useful as ultra-low temperature thermochronometers due to their dating range (a few decades to ~1 Ma) and low closing temperature (below 100°C). We aim to show which segments of the PAF system accommodated seismotectonic deformation by directly dating quartz and feldspar from fault gouges. For ESR, we measure the signals from the Al center in quartz following the single aliquot additive (SAAD) and single aliquot regenerative (SAR) protocols, focusing on the 100-150 µm grain size fraction. For OSL, we measure the IRSL signal at 50°C (IR₅₀) and the post-IR IRSL signal at 225°C (pIRIR₂₂₅) on potassium feldspar aliquots of the 100-150 µm grain size fraction. Our ESR results indicate the PAF system accommodated seismotectonic deformation during the last 1 Ma, while the OSL signals for all samples were in saturation. The minimum ages obtained from OSL suggest that the events are likely not younger than 0.4 Ma. We also studied a segment of the nearby Lavanttal Fault, for which our ESR results suggest that the last earthquakes strong enough to produce sufficient shear heating to produce a partial reset on the geochronometer probably happened before 4 Ma.

How to cite: Prince, E., Sumiko, T., Christoph, G., Marko, V., and Kamil, U.: Finding Quaternary Seismogenic Activity Along the Eastern Periadriatic Fault System: Dating of Fault Gouges via Trapped Charge Methods, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-3705, https://doi.org/10.5194/egusphere-egu23-3705, 2023.

In this work we have been investigating the luminescence properties of plant opal phytoliths to assess their suitability for determination of age and/or thermometric information from soil and sediment sequences. Opal phytoliths, or bio-opal, form when monosilicic acid from soil-waters is taken up by plants and chemically altered to silica, producing intra- or extra- cellular structures that give grasses and stems their strength. Opal phytoliths are usually considered to be non-crystalline and referred to as silica mineraloid structures, with ~4-9% water, <5% other elements, and specific gravity ranging from ~1.5-2.3. They are known to be resistant to degradation and hence preserved in soil or sediment even after decomposition of organic matter. Our earlier work examined a <2.37 g/cm³ density fraction in parallel with quartz grains from samples collected from fluvial terraces and soil pits on Konza Prairie Biological Station native tall grass prairie a few km from Kansas State University. We observed generally similar luminescence characteristics from the phytolith fractions to quartz, with bright blue optically stimulated luminescence (OSL) signals and good single-aliquot regenerative-dose characteristics. In two hours the OSL signal is ~90% bleached by white light, whereas red fluorescence lab lighting has a negligible effect over the same exposure time. Thermoluminescence (TL) data suggested the presence of feldspatic-like minerals or perhaps thermal degradation of the phytoliths during TL measurement; the phytolith fractions were also stimulated by low-temperature infrared stimulated luminescence (IRSL₅₀) perhaps also indicating presence of contaminant minerals. Initial SEM analyses identify what appear to be weathered silica grains, but also highly weathered, pitted concretions with silicate-like structures according to element mapping but actual mineral identification is presently unclear.

Most recently we have begun analyzing samples collected from a suite of stratified paleosols from the mid-continent stream type-site of Claussen, Mill Creek, Wabaunsee County, Kansas. This site has documented phytolith examples and a radiocarbon framework. We are continuing luminescence characterization studies, incorporating screening of prepared fractions with SEM and IRSL₅₀ evaluation, and pulsed time domain analysis measurements are being explored.

We think luminescence from opal phytoliths shows great promise as an alternative target to quartz or feldspar, but moreover as a sensitive recorder of climatic change or fire exposure on plant communities. This presentation will review our earlier work on phytoliths and discuss most recent findings from the Claussen site.

How to cite: Spencer, J. and Sanderson, D.: Luminescence chronology and thermometry studies of plant opal phytoliths, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9864, https://doi.org/10.5194/egusphere-egu23-9864, 2023.