GMPV1.2 | Quantifying rates of geological processes: techniques and applications of geochronology and thermochronology
EDI
Quantifying rates of geological processes: techniques and applications of geochronology and thermochronology
Co-organized by CL1.1/GM2/SSP2/TS9
Convener: Dawid SzymanowskiECSECS | Co-conveners: Cody CollepsECSECS, Lorenzo TavazzaniECSECS, Marie GengeECSECS, Catherine Mottram, Maxime BernardECSECS, Perach Nuriel
Orals
| Fri, 28 Apr, 14:00–15:45 (CEST), 16:15–18:00 (CEST)
 
Room D1
Posters on site
| Attendance Fri, 28 Apr, 10:45–12:30 (CEST)
 
Hall X2
Orals |
Fri, 14:00
Fri, 10:45
Time is a fundamental variable for the understanding of history and dynamics of Earth and planetary processes. Consequently, precise and accurate determination of crystallisation, deposition, exhumation or exposure ages of geological materials has had, and will continue to have, a key role in the geosciences. In recent years, substantial improvement in spatial and temporal resolution of well-established dating techniques and development of new methods have revealed previously unknown complexity of natural systems and in many cases revolutionised our understanding of rates of fundamental geologic processes.

With this session, we aim to provide a platform to discuss 1) advances in a broad spectrum of geochronological and thermochronological methods (sample preparation, analytical techniques, interpretational and modelling approaches) and 2) applications of such methods to a variety of problems across the Earth sciences, across the geological time and across scales of the process studied. We particularly encourage presentations of novel and unconventional applications or attempts to develop new geo/thermochronometers.

Orals: Fri, 28 Apr | Room D1

Chairpersons: Dawid Szymanowski, Lorenzo Tavazzani, Catherine Mottram
Geochronology
14:00–14:10
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EGU23-1954
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GMPV1.2
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On-site presentation
Antoine Triantafyllou, Mihai Ducea, Gilby Jepson, Alex Bisch, and Jerome Ganne

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.

14:10–14:20
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EGU23-1082
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GMPV1.2
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ECS
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On-site presentation
Cilva Joseph, Denis Fougerouse, Aaron J. Cavosie, Hugo K. H. Olierook, Steven M. Reddy, Raiza R. Quintero, Allen Kennedy, David W. Saxey, and William D.A. Rickard

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.

14:20–14:30
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EGU23-6959
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GMPV1.2
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On-site presentation
Pavlina Hasalová, Karel Schulmann, Urs Schaltegger, Pavla Štípská, Andrew Kylander-Clark, Robert Holder, Roberto Weinberg, and Petra Maierová

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.

14:30–14:40
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EGU23-1147
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GMPV1.2
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ECS
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On-site presentation
Anthony Clarke, Chris Kirkland, and Stijn Glorie

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.

14:40–14:50
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EGU23-8976
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GMPV1.2
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On-site presentation
Etienne Skrzypek, Daniela Gallhofer, Christoph Hauzenberger, and Isabella Haas

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.

14:50–15:00
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EGU23-16555
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GMPV1.2
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On-site presentation
Massimo Coltorti, Nicolò Nardini, Federico Casetta, Lorenzo Tavazzani, Stefano Peres, Theodoros Ntaflos, and Elio Dellantonio

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 (Wo13-51; En2-29; Fs20-85), 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/YbN 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.

15:00–15:10
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EGU23-13440
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GMPV1.2
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ECS
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On-site presentation
Adrianna Virmond, David Selby, Jörn-Frederik Wotzlaw, and Cyril Chelle-Michou

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.

15:10–15:20
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EGU23-1931
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GMPV1.2
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On-site presentation
Chris Clark, Michael Brown, Tim Johnson, Ruairidh Mitchell, and Saibal Gupta

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.

15:20–15:30
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EGU23-5290
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GMPV1.2
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On-site presentation
Dawn Kellett, Kyle Larson, and Diane Skipton

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.

15:30–15:40
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EGU23-7367
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GMPV1.2
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ECS
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On-site presentation
Zeno G. Lugoboni, Gloria Arienti, Valentina Barberini, Andrea Bistacchi, Christian Cannella, Simona Caprarulo, and Igor M. Villa

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.

15:40–15:45
Coffee break
Chairpersons: Cody Colleps, Marie Genge, Maxime Bernard
Thermochronology
16:15–16:35
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EGU23-11870
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GMPV1.2
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ECS
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solicited
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Highlight
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On-site presentation
Using (U-Th)/He dating of iron oxides to constrain past weathering episodes
(withdrawn)
Beatrix Heller, Cécile Gautheron, Guillaume Morin, and Thierry Allard
16:35–16:45
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EGU23-3132
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GMPV1.2
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ECS
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On-site presentation
Alexis Derycke, Kerry Gallagher, and Cécile Gautheron

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.

16:45–16:55
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EGU23-2154
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GMPV1.2
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ECS
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On-site presentation
Riley Rohrer, Chris Clark, Chris Kirkland, and Tim Johnson

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.

16:55–17:05
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EGU23-8741
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GMPV1.2
|
On-site presentation
|
Peter van der Beek and Taylor Schildgen

Interpreting cooling ages from multiple thermochronometric systems and/or from steep elevation transects with the help of a thermal model can provide unique insights into the spatial and temporal patterns of rock exhumation. Several well-established thermal models allow for a detailed exploration of how cooling or exhumation rates evolved in a limited area or along a transect. However, integrating large, regional datasets in such models remains challenging due to the difficulty of extracting exhumation rates from ages that are affected by variable effective cooling temperatures, sampling elevations, and surface temperatures. Here we present a thermal model that can be used to rapidly provide a synoptic overview of exhumation rates from thermochronologic data spread over wide regions. The model incorporates surface temperature based on a defined lapse rate and sample elevation relative to a mean relief value that is dependent on the thermochronometric system of interest. Other inputs include sample age, thermochronometric system, and an initial (unperturbed) geothermal gradient. The model is simplified in that it assumes steady, vertical rock-uplift when calculating exhumation rates. For this reason, it does not replace more powerful and versatile thermal-kinematic models like PECUBE, but it has the advantage of simple implementation and rapidly calculated results. In our example dataset, we show the results of exhumation rates calculated from 1785 thermochronologic ages from the Himalaya associated with five different thermochronometric systems; results were calculated in under a second on a standard laptop. Despite the synoptic nature of the results, we show how they illustrate several fundamental features of the mountain belt, including strong regional differences that reflect known segmentation patterns and changing exhumation rates in areas of newly developed ramp structures. The results can also be correlated with geomorphic metrics to probe potential controls on surface morphology.

How to cite: van der Beek, P. and Schildgen, T.: Age2exhume: A Matlab/Python script to calculate steady-state vertical exhumation rates from thermochronologic ages in regional datasets, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8741, https://doi.org/10.5194/egusphere-egu23-8741, 2023.

17:05–17:15
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EGU23-12472
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GMPV1.2
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On-site presentation
|
David M. Whipp and Dawn A. Kellett

Low temperature thermochronology is a field of research in which the thermally controlled retention of radioactive decay products in geological materials is measured to reconstruct mineral and rock temperature-time histories, especially in regard to their passage through the upper crust (i.e., <350 °C). Such temperature-time histories are most often constructed by inverting low temperature thermochronological data using geological constraints in order to identify envelopes of plausible rock thermal histories. While such inversions are highly informative models of the thermal history of rocks, the ultimate goal of most low temperature thermochronological studies is to relate thermal histories to geological processes in order to reconstruct upper crustal tectonic activity and/or landscape evolution. To do this, the (evolving) depths of thermochronometer effective closure temperatures must be estimated, as both heat transfer processes and crustal rock composition/thermal properties will affect the crustal thermal field. 

Here we present an exploration of the relationships between low temperature thermochronometers, temperature-time histories, and geological processes produced using the software Tc1D (https://doi.org/10.5281/zenodo.7124271). Tc1D is a new, open-source thermal and thermochronometer age prediction model for simulating the competing effects of tectonic and surface processes on thermochronometer ages. The Tc1D software is written in Python and uses the finite difference method to solve the heat transfer equation in 1D including the effects of heat conduction, advection (e.g., erosion, sedimentation), and radiogenic heat production on the thermal profile of the lithosphere. The flexibility of the software means that it can be used to explore the effects of a variety of geological processes, including magmatic intrusion and lithospheric delamination, for example. Thermochronometer ages (U-Th/He and fission track ages for apatite and zircon) are predicted by tracking the thermal history of rock particles in the model as they travel from depth to the surface during their exhumation history, both for samples at the modern-day surface and those reaching the surface at past times. The thermal histories are input to age prediction algorithms, including those that account for the effects of radiation damage in minerals (e.g., Flowers et al., 2009; Guenthner et al., 2013), making the software applicable to thermochronometer age interpretation in a wide variety of geological scenarios.  

In this contribution, we present a selection of results using Tc1D, demonstrating potential applications and providing some examples of unintuitive temperature and age relationships. These examples include cases where sample depth does not correlate with temperature, where variations in predicted effective closure temperatures produce unexpected age relationships, and where the thickness of the layer of exhumed rocks can significantly affect predicted ages. We hope that these illustrative examples demonstrate the role for Tc1D in the thermochronologist’s interpretational toolbox. 

How to cite: Whipp, D. M. and Kellett, D. A.: Exploring the relationships between low-temperature thermochronometers, temperature-time histories, and geological processes using Tc1D, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12472, https://doi.org/10.5194/egusphere-egu23-12472, 2023.

17:15–17:25
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EGU23-1262
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GMPV1.2
|
ECS
|
On-site presentation
Heng Peng, Jianqiang Wang, Chiyang Liu, Jiaoli Li, Xiaoqin Jiao, Liying Zhang, and Massimiliano Zattin

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.

17:25–17:35
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EGU23-14643
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GMPV1.2
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ECS
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On-site presentation
Chloé Bouscary, Georgina King, Jérôme Lavé, Djordje Grujic, György Hetényi, Rafael Almeida, Ananta Gajurel, and Frédéric Herman

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.

17:35–17:45
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EGU23-3705
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GMPV1.2
|
ECS
|
On-site presentation
Erick Prince, Tsukamoto Sumiko, Grützner Christoph, Vrabec Marko, and Ustaszewski Kamil

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.

17:45–17:55
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EGU23-9864
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GMPV1.2
|
Virtual presentation
Joel Spencer and David Sanderson

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.

17:55–18:00

Posters on site: Fri, 28 Apr, 10:45–12:30 | Hall X2

Chairpersons: Cody Colleps, Marie Genge, Dawid Szymanowski
X2.57
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EGU23-581
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GMPV1.2
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ECS
Dru-Ann Harris, Karl Lang, Duna Roda Boluda, and Marcus Kurth

The erosion of mountain landscapes is the greatest source of terrestrial sediment to global ocean basins and a critical part of the global carbon cycle regulating Earth’s climate over geologically relevant timescales. In particular, the expansion of mountain glaciers may accelerate bedrock erosion and rapidly increase the flux of terrestrial sediment from source areas. However, the mechanisms by which glaciation augments sediment flux are complex, and understanding them requires further research. Our research adopts a novel approach to determine the source of sediment in rivers exiting a glaciated landscape, combining detrital zircon fission-track “tracer” thermochronology and Raman spectroscopy of carbonaceous material (RSCM). Our research focuses on the Southern Alps of New Zealand as a model landscape with well-constrained lithology and a predictable exhumation gradient. In 5 west-draining transverse river catchments, we test the hypothesis that modern sediment is preferentially derived from glaciated, high-elevation areas of the catchment. Our 5 rivers span a range of glacial coverage, allowing us to further test the hypothesis that glacially-sourced sediment increases with the degree of glaciation in the catchment. Our preliminary results suggest that sediment is not exclusively derived from glaciated areas of the catchment, but may instead reflect additional deglaciated source areas affected by landsliding, possibly induced by seismicity along the Alpine Fault. Our research demonstrates a powerful and novel approach to tracing sediment sources within an individual catchment area and highlights complex interrelationships between mountain glaciation and changes in the magnitude and sources of sediment fluxes.

How to cite: Harris, D.-A., Lang, K., Roda Boluda, D., and Kurth, M.: Tracing sediment source within a glaciated landscape: new observations from detrital thermochronology and Raman spectroscopy in the Southern Alps of New Zealand, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-581, https://doi.org/10.5194/egusphere-egu23-581, 2023.

X2.58
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EGU23-1407
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GMPV1.2
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|
Ralf Hetzel, Reinhard Wolff, Kyra Hölzer, István Dunkl, Qiang Xu, Aneta Anczkiewicz, and Zhenyu Li

Active graben systems in south Tibet and the Himalaya are the expression of ongoing E-W extension, however, the cause and spatio-temporal evolution of normal faulting remain debated. We reconstruct the history of normal faulting at the southern Tangra Yumco graben by using new thermochronological data and thermo-kinematic modelling (Wolff et al., 2022). The Miocene cooling history of the footwall of the main graben-bounding fault is constrained by zircon (U-Th)/He ages between 16.7±1.0 and 13.3±0.6 Ma, apatite fission track ages (15.9±2.1 to 13.0±2.1 Ma), and apatite (U-Th)/He ages (7.9±0.4 to 5.3±0.3 Ma). Thermo-kinematic modelling of the data indicates that normal faulting began 19.0±1.1 Ma ago at a rate of ~0.2 km/Myr and accelerated to ~0.4 km/Myr at ~5 Ma. In the northern Tangra Yumco rift, re-modelling of published thermochronological data (Wolff et al., 2019) shows that faulting started ~5 Ma later at 13.9±0.8 Ma. The age difference and the distance of 130 km between the two sites indicates that rifting and normal faulting propagated northward at an average rate of ~25 km/Myr. As this rate is similar to the Miocene convergence rate between India and south Tibet, we argue that the under-thrusting of India beneath Tibet has exerted an important control on the propagation of rifts in south Tibet.

References

Wolff, R., Hetzel, R., Hölzer, K., Dunkl, I., Xu, Q., Anczkiewicz, A.A., Li, Z. (2022). Rift propagation in south Tibet controlled by underthrusting of India: A case study at the Tangra Yumco graben (south Tibet). J. Geol. Soc. Lond., https://doi.org/10.1144/jgs2022-090.

Wolff, R., Hetzel, R., Dunkl, I., Xu, Q., Bröcker, M. & Anczkiewicz, A.A. (2019). High-angle normal faulting at the Tangra Yumco graben (southern Tibet) since ~15 Ma. J. Geology, 127, 15–36, http://doi.org/10.1086/700406.

 

How to cite: Hetzel, R., Wolff, R., Hölzer, K., Dunkl, I., Xu, Q., Anczkiewicz, A., and Li, Z.: Rift propagation in south Tibet controlled by under-thrusting of India: A case study at the Tangra Yumco graben (south Tibet), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1407, https://doi.org/10.5194/egusphere-egu23-1407, 2023.

X2.59
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EGU23-2925
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GMPV1.2
Andreas Wölfler, Andrea Hampel, Reinhard Wolff, Ralf Hetzel, and István Dunkl

The Austroalpine nappes in the Eastern European Alps have preserved the record of two orogenic phases in the Cretaceous and Tertiary but their cooling and exhumation history remains poorly constrained. Here we use new low-temperature thermochronological data and thermokinematic modeling to unravel the exhumation history of the Austroalpine nappes in the Nock Mountains east of the Tauern Window (Wölfler et al., submitted). Our data show that the central Nock Mountains (Ötztal-Bundschuh and Drauzug-Gurktal nappes) cooled through the zircon fission track closure temperature (~240 °C) already in the Late Cretaceous. Apatite fission track ages cluster around 35-30 Ma, indicating that the rocks have been at depths of ≤5-6 km since the Eocene-Oligocene boundary. In contrast, the Radenthein and Millstatt Complexes, which are located south of the Hochstuhl Fault, cooled below 240 °C during the Eocene and show apatite fission track ages of ~15 Ma. Thermokinematic modeling of an age-elevation profile in the central Nock Mountains (near Innerkrems) revealed a phase of enhanced exhumation (~0.62 km/Ma) between ~100 and ~85 Ma, which we relate to syn- to late-orogenic Late Cretaceous extension. After a period of slow exhumation (~0.03 km/Ma), the exhumation rate increased to ~0.16 km/Ma at ~32 Ma. In contrast, thermokinematic modeling of an age-elevation profile near Millstatt shows that rocks of the Radenthein and Millstatt Complexes were rapidly exhumed (~0.78 km/Ma) from ~44 Ma to ~38 Ma during the initial Europe-Adria collision. After a phase of slow exhumation (~0.07 km/Ma) between ~38 and ~19 Ma, the exhumation rate increased to ~0.3 km/Ma with the onset of Miocene lateral extrusion in the Eastern Alps. Altogether, ~16 km of rock have been removed since ~100 Ma in the Innerkrems region, whereas ~11 km of rock have been removed in the last ~44 Ma in the Millstatt area. These findings are consistent with pressure-temperature estimates for the Ötztal-Bundschuh nappe and the Radenthein/Millstatt Complexes, respectively (Koroknai et al., 1999; Schuster, 2003; Krenn et al., 2003, 2011). The distinct differences in the cooling histories north and south of the Hochstuhl Fault further suggest that this fault, which has hitherto been considered as a dextral strike-slip fault during Miocene lateral extrusion (Polinski & Eisbacher, 1992; Linzer et al., 2002), also accommodated a considerable amount of thrust movement. The difference between the amount of exhumation north and south of the Hochstuhl Fault indicates ca. 5 km of vertical offset between ~44 and ~38 Ma.

How to cite: Wölfler, A., Hampel, A., Wolff, R., Hetzel, R., and Dunkl, I.: Phases of enhanced exhumation during the Cretaceous and Tertiary orogenies in the Eastern European Alps: new insights from thermochronological data and thermokinematic modeling, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2925, https://doi.org/10.5194/egusphere-egu23-2925, 2023.

X2.60
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EGU23-10060
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GMPV1.2
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ECS
Thermal/exhumation histories of the late Neogene plutons in the Tanigawa-dake area, central Japan, based on multi-thermochronometries
(withdrawn)
Saki Minami, Shigeru Sueoka, Shoma Fukuda, Mitsuhiro Nagata, Barry P. Kohn, Tatsunori Yokoyama, Saya Kagami, and Takahiro Tagami
X2.61
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EGU23-11638
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GMPV1.2
Benjamin Guillaume, Nathan Cogné, Kerry Gallagher, Pierre G. Valla, and Christian Crouzet

This study tests the application of combined detrital apatite fission track (AFT) and U-Pb dating to infer both glacial erosion spatial patterns and long-term rock cooling histories in Alpine mountainous settings. We have dated 716 detrital apatite grains from glacial sediments collected in the Maurienne and Arve valleys (Western European Alps, France) from moraine deposits corresponding to different stages of glacial retreat since the Last Glacial Maximum (LGM, ca. 24-21 ka).
The Maurienne valley crosses the internal and external Alps, which exhibit contrasting in-situ AFT and U-Pb ages. Here, we present the measured distribution of both detrital AFT and U-Pb ages at 6 locations along the valley, with catchment elevations ranging from 390 to 1740 m. We show that during glacial retreat, erosion is mainly concentrated in the downstream part of the glacier, near the sampled moraine deposits. This inference suggests that during glacial retreat, glacial erosion is more effective below the ELA (Equilibrium Line Altitude) and specifically close to the glacier front, in areas where ice flow velocity is high and subglacial water is abundant, as predicted by ice-dynamics reconstructions in the European Alps over the last 20 ka.
In the Arve valley, previous studies showed that in situ AFT ages are systematically younger than 7 Ma for the Mont-Blanc massif. We compare the thermal history obtained from these literature bedrock-derived data to that derived from the new detrital AFT data collected in the Little Ice Age (LIA) moraine, just at the front of the Bossons glacier (~1300 m elevation). We also compare our results with 5 other samples down the valley at catchment elevations between 460 and 1050 m to evaluate potential changes in the detrital AFT signal as well as the consistency in the retrieved long-term cooling history.
Based on these first results, we plan to extend our study to other areas (e.g., Patagonia) to investigate both (1) spatial patterns of glacial erosion for older glacial periods (pre-LGM), and (2) long-term rock cooling histories from moraine deposits where modern bedrock is inaccessible (e.g. under modern glaciers or ice fields).

How to cite: Guillaume, B., Cogné, N., Gallagher, K., Valla, P. G., and Crouzet, C.: Using detrital thermochronology on moraine deposits to infer glacial erosion patterns and rock thermal history : insights from the Arve and Maurienne valleys (Western European Alps), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11638, https://doi.org/10.5194/egusphere-egu23-11638, 2023.

X2.62
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EGU23-12637
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GMPV1.2
Christoph Glotzbach and Sarah Falkowski

Applications of tracer thermochronology exploit a known or assumed surface thermochronometric age map (based on either interpolated observed or modelled bedrock ages) to determine the provenance of detrital grains within fluvial or glacial catchments. The goal is to interpret the erosion pattern and processes within the sampled catchment. So far, most studies focused on modern sediments and glacial deposits.

We extend this approach to several time slices (between 28 and 12 Ma) of well-dated stratigraphic sections of pro- and retro-foreland basins of the European Alps. Foreland basin deposits represent a rich archive of erosional processes that were controlled by tectonics, climate, and lithology. However, importantly, before we reconstruct and interpret past erosion patterns and exhumation from detrital zircon fission-track (ZFT) age distributions and modelled bedrock ZFT ages back in time, we produce a frame of reference of today's situation. We do this by investigating signals from modern river samples and the present-day erosion pattern and mineral fertility in the Alps.

Here, we focus on 26 modern river samples (21 previous samples from the Western and Central Alps, and 5 new samples from the Eastern Alps) and discuss observed and predicted (based on possible erosion scenarios) ZFT age distributions, as well as potential pitfalls of the method (such as poor bedrock control in some areas of the Alps). We also show preliminary results from stratigraphic sections.

How to cite: Glotzbach, C. and Falkowski, S.: Erosion patterns in the European Alps from zircon fission-track tracer thermochronology, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12637, https://doi.org/10.5194/egusphere-egu23-12637, 2023.

X2.63
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EGU23-6766
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GMPV1.2
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ECS
Julien Amalberti, Peter van der Beek, Cody Colleps, Maxime Bernard, and Isabel Wapenhans

Step-heating experiments constitute a key technique to study the release of volatile elements from geological materials as a function of temperature. In the case of noble gases (He, Ne, Ar, Kr, and Xe), step-heating is particularly useful to determine diffusion kinetics, structural defects, or spatial homogeneity within the material. These parameters are critical in the application of diffusion-based thermochronology such as the apatite (U-Th)/He system, where mapping out the spatial distribution of natural 4He provides crucial information on the thermal history of apatite crystals. Characterizing the diffusion and distribution of 4He via step-heating additionally has the potential to detect anomalously behaved grains and to directly constrain grain-to-grain variability in diffusivities within samples with significant radiation damage-induced age dispersion.

Within the ERC-funded COOLER project, we aim to further the development of high-resolution, ultra-low temperature 4He/3He thermochronology. To this end, we developed a new technique for precise step-heating experiments coupled with a diode laser including an inline single-wavelength pyrometer. The new protocol uses an all-alumina ceramic crucible fitted with a K-thermocouple ~0.1 mm below the center of the crucible pit. The head of the thermocouple is located directly below the sample within the ceramic matrix, allowing precise temperature measurements of the sample. The crucible is mounted on an alumina rod connected to a noble-gas preparation line. Gas released from the sample is purified and analyzed by a Thermo Scientific Helix SFT™ multi-collector mass spectrometer. The sample is wrapped in Pt foil and indirectly illuminated with a diode laser. Laser and PID temperature controls are carried out by a custom LabVIEW program. Temperature calibration is performed by comparing measured and theoretical melting points of well-known materials loaded in the alumina crucible pit.

Our initial results show very short response times for the thermocouple (a few seconds) and excellent agreement with the melting point of Indium (Tmelt = 157°C). Although the current design is limited to hold only a single sample, it enables precise calibration of the emissivity value for a specific capsule assembly, which is a key parameter for pyrometer control of the temperature. Consequently, by calibrating the Pt capsule emissivity prior to the step-heating experiment, they can then be mounted in a multiple laser sample holder (up to 36 samples per chamber). The single-wavelength pyrometer of our system enables temperature measurements for large sample batches. Temperature is also cross-calibrated between the pyrometer and the thermocouple to ensure its correct reading.  This new approach, coupled with analytical automation, will lead to significant improvement in the accessibility and efficiency of routine 4He/3He analyses for geologic applications.

How to cite: Amalberti, J., van der Beek, P., Colleps, C., Bernard, M., and Wapenhans, I.: New high-resolution step heating experiments using a coupled Diode laser and thermocouple for thermochronology applications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6766, https://doi.org/10.5194/egusphere-egu23-6766, 2023.

X2.64
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EGU23-1426
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GMPV1.2
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ECS
Lorenzo Tavazzani, Dawid Szymanowski, Francesca Forni, Giuseppe Cadel, and Peter Brack

Silicic magma flare-up episodes are characterized by the addition of large volumes of evolved magma (>65 wt% SiO2) to the continental crust in geologically short time intervals (106-107 years). Flare-up events are often associated with (trans-)extensional tectonics and contribute to crustal differentiation and critical metal mineralization. Related volcanic aerosol dispersion in the atmosphere can also trigger global environmental changes. During flare-up episodes, long-lived caldera complexes are thought to be primary eruptive sources at the Earth’s surface. However, a substantial proportion of the overall mobile magma can be trapped in extra-caldera dikes, fissures and monogenetic edifices controlled by the extensional stress regime.

In the Southern Alps of Northern Italy, a post-Variscan magmatic flare-up is recorded in a ca. 400 km long array of largely undeformed magmatic bodies of Early Permian age (285-275 Ma; [1]), then located along the northern margin of Gondwana. In the Southern Alps this flare-up produced more than 5*104 km3 of rhyolitic volcanic and cogenetic intrusive rocks. Two major caldera complexes (Sesia Caldera; Ora Caldera) were capable of ejecting volumes >103 km3 of magma during individual catastrophic eruptive events. However, magmatic activity also resulted in numerous scattered volcanic centers with relatively small eruptions (0.1 – 1 km3 each) and punctuated by quiescent intervals.

In this study we focus on two Early Permian fault-bounded basins, ca. 40 km apart, in the central Southern Alps: the Orobic Basin (Bergamo) and the Collio Basin (Brescia). The stratigraphic records of both basins preserve proximal and distal volcanic products and both successions terminate with erosional unconformities of Middle- to Late Permian age. New zircon LA-ICP-MS U-Pb ages indicate that the onset of explosive, rhyolitic magmatism was essentially coeval at ~284 Ma. The Collio Basin contains just a few ignimbrite sheets dispersed in an essentially (fluvio)-lacustrine sedimentary fill and recording a pulsated volcanic activity of nearly 5 Myr (youngest ignimbrite ~280 Ma). After an initial phase (1-2 Myr) of a similar pulsed nature, the Orobic Basin became the locus of extrusion of much larger volumes of rhyolitic magma (probably in excess of 100 km3) in less than 1 Myr (283-282 Ma). This was followed by a depositional style similar to the Collio but with a scarcer pyroclastic contribution.

The contrasting volcanic record in these two basins, which share size and tectonic environment but not magmatic evolution, provides a striking example of magmatic architecture diversity in the midst of a silicic flare-up event. Further investigation into the timing (CA-ID-TIMS U-Pb geochronology) and compositional evolution (e.g., zircon d18O, eHf) of volcanic products in the Collio and Orobic basins is expected to provide a much better resolved comparison and open a window into the combined tectono-magmatic processes that ultimately regulate the size and frequency of catastrophic, caldera-forming eruptions in silicic flare-up provinces.

[1] Schaltegger, U., & Brack, P. (2007). International Journal of Earth Sciences, 96(6), 1131-1151.

How to cite: Tavazzani, L., Szymanowski, D., Forni, F., Cadel, G., and Brack, P.: Magmatic architecture and basin evolution in the midst of a silicic flare-up: U-Pb zircon geochronology of volcanic deposits from two Early Permian, Collio-type basins of Southern Alps (Italy), EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1426, https://doi.org/10.5194/egusphere-egu23-1426, 2023.

X2.65
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EGU23-8495
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GMPV1.2
Sebastian Stumpf, Etienne Skrzypek, Kurt Stüwe, and Christoph Iglseder

The affiliation of the Ennstal Phyllite Zone (EPZ) to either the micaschist units of the Koralpe-Wölz nappe system (KW-NS) to its south or to nappes of the “Greywacke Zone” to its north and east is still debated. Due to similarities with phyllites of the “Greywacke Zone” in the north and phyllonitic micaschists in the south, no clear lithological boundary between these units is observable. Petrographic observations suggest a continuous eoalpine metamorphic gradient with no metamorphic gap between the KW-NS and the EPZ. In order to clear this debate and further constrain the tectonic and temporal evolution of these units, we present new LA-MC-ICP-MS U/Pb age dating results for metapelite samples from the EPZ as well as for the adjacent units of the KW-NS.

Two samples (EA09 and SP02) from the central EPZ and one sample (SP62) from the northernmost part of the Wölz-Complex of the KW-NS were selected for detrital zircon age dating. The distribution of approximately 150 dates per sample reveals major peaks at the Ediacaran-Cryogenian boundary (624 – 646 Ma), a smaller peak at the Neoproterozoic-Mesoproterozoic boundary (~1000 Ma) followed by a hiatus and a smaller peak in the mid-Paleoproterozoic (~2000 Ma). All samples show similar mid-Paleoproterozoic and Neoproterozoic-Mesoproterozoic peaks. Sample SP62 contains one grain of Cambrian age (523 Ma) and one grain of mid-Ordovician age (460 Ma) whereas the youngest zircons from the EPZ samples yield Ediacaran ages of 629 Ma and 625 Ma. The lack of zircons of Ordovician age in samples EA09 and SP02 indicate an affiliation of the EPZ with the basal units of the “Greywacke Zone”.

We also dated metamorphic allanite and REE-bearing epidote rims which are interpreted to form at low pressure and temperature conditions in metapelites. Allanites from the EPZ yield metamorphic ages of 105 ± 3.5 Ma in the northern part of the unit and 279 ± 6 Ma in the southern part. Allanite cores from two micaschist samples from the northern and central Wölz-Complex yield ages of 316 ± 21 Ma and 286 ± 11 Ma. Their respective epidote rims yield eoalpine ages of 98 ± 2 Ma and 96 ± 2 Ma. One micaschist sample from the Rappold-Complex yields ages of 326 ± 9 Ma for the allanite cores and 101 ± 1 Ma for the epidote rims. These ages are interpreted as prograde crystallization of allanite and epidote and give us petrochronological information about three distinct metamorphic events: Variscan, Permian and Eoalpine. By gathering three distinct eoalpine ages within the EPZ and the KW-NS, we can further constrain the metamorphic evolution of the eoalpine lower plate.

How to cite: Stumpf, S., Skrzypek, E., Stüwe, K., and Iglseder, C.: From sedimentation to multiple tectono-thermal events: U/Pb zircon and allanite dating in the Eastern Alps, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8495, https://doi.org/10.5194/egusphere-egu23-8495, 2023.

X2.66
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EGU23-7959
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GMPV1.2
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ECS
Dawid Szymanowski, Lorenzo Tavazzani, Yannick Buret, Marcel Guillong, Alejandro Cortes Calderon, and Cyril Chelle-Michou

Tin-tungsten magmatic-hydrothermal deposits are sources of critical raw materials (Sn, W, Nb, Ta, Li), key to the development of technologies involved in the green transition. However, the current and projected supply of many of these mineral commodities is often dominated by entities whose practices or geopolitical setting may raise issues from a social, political, or environmental standpoint. To meet a steadily increasing demand, new responsible mineral extraction projects must therefore be developed. Successful exploration and economic appraisal of newly identified mineral deposits require (1) an understanding of the ore-forming processes to build an exploration model, and (2) an early estimate of the deposit size to facilitate well-targeted investments. One key parameter that helps to achieve both goals is the knowledge of absolute timing and duration of the mineralisation process.

We present new analytical developments in U-Pb dating of strategic Sn-W ore minerals (cassiterite, wolframite, scheelite) using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). We used a suite of Sn-W mineral specimens to characterise U/Pb downhole fractionation behaviour and polyatomic interference patterns for these three matrices, allowing the optimisation of ablation and ICP-MS settings. In parallel with technical developments, we compiled a large library of potential primary and secondary cassiterite, wolframite, and scheelite reference materials (RMs) which we characterised for major and trace elements. To further our understanding of geochemistry of Sn-W phases, we also performed high-resolution compositional mapping of key trace elements (e.g. U, Pb, REE) with an ultra-fast washout laser ablation system.

Promising RM candidates will be developed into primary RMs with a careful characterisation of compositional homogeneity and precise age determination by isotope dilution-thermal ionisation mass spectrometry (ID-TIMS). Thus characterised RMs and a set of analytical best practices will be made available to laboratories wishing to test and further develop such methods. The ultimate goal of this effort is to build a set of community shared materials and techniques that will allow precise and accurate temporal characterisation of Sn-W mineralisation.

How to cite: Szymanowski, D., Tavazzani, L., Buret, Y., Guillong, M., Cortes Calderon, A., and Chelle-Michou, C.: Developing techniques and reference materials for LA-ICP-MS U-Pb geochronology of Sn-W minerals, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7959, https://doi.org/10.5194/egusphere-egu23-7959, 2023.

X2.67
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EGU23-11918
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GMPV1.2
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ECS
Learning from Failure? Carbonate U-Pb and Quartz Fluid Inclusion 40Ar/39Ar Geochronology
(withdrawn)
Daniel Rutte, Frank Tomaschek, Klaudia Kuiper, Jan Wijbrans, and Nikolaus Froitzheim
X2.68
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EGU23-4567
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GMPV1.2
Servando De la Cruz-Reyna

The Paricutin-Tancítaro region (PTR), located in the SW sector of the Michoacán-Guanajuato monogenetic field, in central Mexico, is characterized by a high spatial density of monogenetic scoria cones around Tancítaro, a stratovolcano active in the middle Pleistocene. The PTR area has been active for around one million years, and the latest eruption, beginning in 1943, formed the Paricutin volcano. We use the Average Erosion Index (AEI) to estimate the relative ages of 170 PTR scoria cones located within latitudes 19°N and 20°N and longitudes -102.0° E and -102.7° E. The AEI quantifies the erosional state of scoria cones from a morphological analysis of their level contours extracted from a high-resolution DEM (the 12-m TanDEM-X in this case). The analysis provides a metric for the undulations along the level contour curves at different altitudes, reflecting the width and amplitude of erosional rills and gullies on the cone’s surface. We compute a functional relationship between AEI and age by correlating 10 published radiometric ages with the measured AEIs of those cones. Then, using that function, we assign an age to each of the 170 cones, assuming that all the monogenetic volcanoes in the analysis have been exposed to similar erosive conditions. Finally, we tessellate the study area with a 0.1° x 0.1° grid and identify the number of events per grid module to compute the probability of at least one eruption occurring in the module in a specific time, using a Poisson process distribution obtained from the count of the number of events per 20 ky time intervals. Our results suggest that the dispersed volcanic activity in the PTR started to increase after the last eruption of Tancitaro (~237 ka), with a further activity increase during the Holocene, mainly concentrated on the NE sector of Tancítaro, where Paricutin is located. Holocene vents align to the NE, parallel to the Tepalcatepec-Tangancícuaro normal fault system. Furthermore, our results suggest a spatial coincidence between the regions with a higher probability of an eruption, based on the obtained eruption history, and the location of the recent seismic swarms in the PTR, the last two in 2020-2021, suggesting an increase in volcanic and seismic hazards in that area. To what extent? It is the subject of our forthcoming research.

How to cite: De la Cruz-Reyna, S.: Temporal and Spatial distribution of scoria cones in the Paricutin-Tancítaro volcanic region, Mexico: A morpho-chronometric approach to monogenetic hazard evaluation, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4567, https://doi.org/10.5194/egusphere-egu23-4567, 2023.

X2.69
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EGU23-12969
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GMPV1.2
Daniela Gallhofer, Etienne Skrzypek, Christoph Hauzenberger, Andreas Möller, Joseph Andrew, Luis A. Parra-Avila, Laure Martin, Anthony Kemp, Rohan Fernando, and He Dengfeng

Zircon megacrysts are unusually large crystals (> 5 mm) that are commonly associated with mantle-derived kimberlites, carbonatites, alkali basalts and syenitic pegmatites (e.g., Hoskin and Schaltegger 2003). Such zircons form during relatively short timespans and therefore, are often used as reference material for U-Pb geochronology. Here, we determine the geochemical and isotopic (U-Pb, Hf, O) characteristics of a little-known occurrence of zircon megacrysts at Kawisigamuwa, Sri Lanka.

The dark brown megacrysts are euhedral, commonly elongate crystals with double pyramidal terminations and have faintly corroded crystal surfaces. The zircons consist of oscillatory zoned and nearly featureless cathodoluminescence-bright patches, some of which appear to follow sealed cracks. All zircon domains show a low to moderate FWHM of the ν3 (SiO4) Raman band (2.5 to 7.3 cm-1), have a low to moderate radiation damage (total α-dose mainly <0.5 x 1018 events/g) and therefore are intermediate to well crystalline. Contents of most trace element (U, Th, REE, P) are elevated in the oscillatory zoned domains, while Hf content is elevated in the CL-bright domains and seems to be grain-dependant. The oscillatory zoned domains yielded a TIMS weighted mean 206Pb/238U age of 532.39 ± 0.66 Ma (2sd). The206Pb/238U dates within the CL-bright domains are partially reset by a single event of recrystallisation at ~518 Ma. The mean Hafnium isotopic compositions of the tested grains show a narrow range of 176Hf/177Hf from 0.281969 to 0.282003. Oxygen isotopes determined on two oscillatory zoned zircon megacrysts are homogeneous (mean δ18O of 12.1 and 12.2).    

While some of the trace and major element characteristics (Th/U, Zr/Hf, Hf content) of the Kawisigamuwa megacrysts resemble those of carbonatite zircons, their hafnium and oxygen isotope ratios are clearly different from mantle values. The isotopic values indicate that a significant amount of a crustal component must be involved in the formation of the zircons. Recently, several studies have found evidence for melting of carbonate rocks under high grade metamorphic conditions in Sri Lanka (e.g., Wang et al. 2021). It might be feasible that zircons grow from interaction of crustal derived carbonate melts and silicate melts or wall rocks under high grade metamorphic conditions.

Hoskin P.W.O. and Schaltegger U. (2003). The Composition of Zircon and Igneous and Metamorphic Petrogenesis. Reviews in Mineralogy and Geochemistry, 53 (1), 27–62.

Wang J., Su B.-X., Chen C., Ferrero S., Malaviarachchi S.P.K., Sakyi P.A., Yang Y.-H. and Dharmapriya P.L. (2021). Crustal derivation of the ca. 475-Ma Eppawala carbonatites in Sri Lanka. Journal of Petrology, 62 (11), 1-18.

How to cite: Gallhofer, D., Skrzypek, E., Hauzenberger, C., Möller, A., Andrew, J., Parra-Avila, L. A., Martin, L., Kemp, A., Fernando, R., and Dengfeng, H.: Characterization of zircon megacrysts from an atypical occurrence of carbonatite at Kawisigamuwa, Sri Lanka, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12969, https://doi.org/10.5194/egusphere-egu23-12969, 2023.