Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb geochronology for carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. In this study, we review the latest technical progress in LA-ICP-MS carbonate geochronology, including the pre-screening strategies (on-line spot selection with a threshold, image-guided approach, and image-based approach), preferred instrumentation (Q-ICP-MS, SF-ICP-MS and MC-ICP-MS), calibration methods, common Pb corrections and the development of reference materials, with the aim of further improving the precision and accuracy of this technique. We emphasized the characterization of two calcite reference materials (TLM and LSJ07) for micro-beam U-Pb geochronology and C, O isotope ratio measurements. The latest geological applications of LA-ICP-MS U-Pb carbonate geochronology in dating of diagenesis and hydrothermal activity were reviewed.

How to cite: Wu, S., Roberts, N., and Lan, Z.: LA-ICP-MS U-Pb carbonate geochronology and its geological applications, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-15974, https://doi.org/10.5194/egusphere-egu23-15974, 2023.

Great Holocene civilizations on Pacific islands were created by Homo sapiens. However, most of the construction histories remain uncertain due to the lack of developed writing system and the limitation of dating techniques. Nan Madol (0.7 km in width and 1.5 km in length), an abandoned city called the “Venice of the Pacific” with over 100 artificial islets, is located on the southeastern coast of island Pohnpei in Micronesia. This world-unique ruin, inscribed onto UNESCO’s World Heritage List in 2016, was built with basalt megaliths and scleractinian coral cobbles. Oral histories and previous charcoal ¹⁴C ages suggested that the main construction of Nan Madol of Pohnpei could begin in the 13th or 14th century and ceased at the 16th or 17th century, associated with the rise and fall of the Saudeleur Dynasty. However, after 150 years or more of studies, the timing of construction and the dynasty, and the probable influence of environmental factors, remain unresolved. High-precision U-Th dating techniques, developed at the High-Precision Mass Spectrometry and Environment Change Laboratory (HISPEC), Department of Geosciences, National Taiwan University, were used to date the selected pristine coral infills and reveal the construction time of the two ruins. With over 150 coral ages determined, results show a peak of construction activity during the middle 11th century could be related to the rise of the Saudeleur Dynasty. In the early 15th century, construction activities ceased, associated with the dynasty’s downfall. Our study shows that Nan Madol construction and the rise and fall of the dynasty occurred 2-3 centuries earlier than previously estimated. Moreover, the entire development was dominated by El Niño-Southern Oscillation variability and tectonic-related sea level rise.

How to cite: Shen, C.-C. (., Beardsley, F., Gong, S.-Y., Kataoka, O., Yoneda, M., Yokoyama, Y., Jiang, L., Lin, A. Y.-M., Fox, J., Barnabas, J., Kohler, G., Richards, Z. T., and Hobbs, J.-P. A.: Dating a world-unique Pacific ruin: Nan Madol, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-123, https://doi.org/10.5194/egusphere-egu23-123, 2023.

The application of U-Pb dating method performed on calcite has exponentially increased over the last years, since constraining the age of the crystallization for such syn-kinematic minerals, would provide a specific timing of faults movement. The potential gain of this approach is evident but the robustness of the U-Pb method performed on calcites has been not yet systematically tested.  Here, we firstly demonstrated that a mineral/fluid interaction indeed affects the regression of the ²³⁸U/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb data-points and therefore the age; and secondly, we propose an innovative application of U-Pb dating method and a new strategy to identify and reject analytically robust isochrons.

We explore 36 samples, combining U-Pb dating performed with different methods along with carbon and oxygen stable isotopes compositions measured on the same fibres of calcite. The extremely high precision ²⁰⁷Pb/²⁰⁶Pb measured by Thermal Ionisation Mass Spectrometry  revealed that every sample experienced a certain degree of fluid interaction. We find no correlation between ²³⁸U/²⁰⁶Pb and the spread in δ¹⁸O. The higher spread in δ¹⁸O is instead coupled with a remarkable scattered data-points that yield U-Pb ages calculated with the different methods on the same samples with a large variability. In conclusion, our study demonstrates that great care must be taken when considering radiometric ages made on calcite since LA-ICP-MS large uncertainties might obscure the isotopic reorganization.

How to cite: Lanari, R., Buzenchi, A., Bragagni, A., Dhuime, B., Brilli, M., Del Ventisette, C., Mattei, M., Conticelli, S., and Avanzinelli, R.: Mineral/Fluid interaction as a potential bias in calcite U-Pb dating, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-265, https://doi.org/10.5194/egusphere-egu23-265, 2023.

Carbonate veins are ubiquitous in many ore deposits and are often interpreted as a late stage or cross cutting fluid flow events in the ore deposit history. Faults accommodate rock displacement and the resulting zones of weakness act as conduits for magma and localised magmatic-hydrothermal fluid flow, leading to the formation of ore deposits. Dating of both low temperature veins and brittle fault material has been notoriously difficult because of a lack of ‘datable’ material. Using innovative techniques, it is now possible to date carbonate with the U-Pb isotopic system.

Here we use in-situ U-Pb carbonate geochronology to date a variety of fault material and mineralised and unmineralised veins within a major fault-controlled Cu-Au-Mo porphyry system in the central Yukon, Canadian Cordillera. Over 50 samples have been dated, revealing a long history of faulting and fluid flow in the deposit spreading over 10s of millions of years between ~75 Ma and <20 Ma. We combine petrography, U-Pb carbonate geochronology, trace element geochemistry, and clumped isotope analysis to interpret the full temperature-time evolution of the fluids within the deposit. Our results show the carbonate veins crystallised during the main ore-forming event at ~75 Ma. Subsequently, there was a prolonged period of fault-controlled fluid pulsing that likely concentrated metallic minerals in the deposit. The findings show that carbonate veins are not always late features within ore deposits and are an underutilised resource for understanding the full temporal and fluid evolution of a system. Carbonate U-Pb geochronology is therefore potentially incredibly useful for telling the previously untold and long history of fluid flow in a variety of deposit types.

How to cite: Mottram, C., Kellett, D., Dennis, P., and Clog, M.: Longevity of fault-controlled fluid flow within a Cu-Au-Mo porphyry (Yukon, Canada) revealed by coupled U-Pb carbonate geochronology and clumped isotope analysis, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13236, https://doi.org/10.5194/egusphere-egu23-13236, 2023.

The presence of magnesite (MgCO₃) in the Earth’s mantle plays a fundamental role in reducing the melting point of the mantle [1] and forming carbonate‑rich melts such as kimberlites and carbonatites [2]. The melting curve of (dry) magnesite is well constrained [3, 4], but melting of magnesite in the presence of H₂O, providing the basis for more complex (natural) systems, is poorly understood from some quenched experiments [5]. Also, the distribution of trace elements such as Li, Sr, Pb, and rare earth elements during melting of magnesite is poorly considered in models that evaluate the trace element budget of carbonate‑rich melts parental to kimberlites [6].

Here we report, first, the H₂O‑present melting curve of magnesite between 2 and 12 GPa. The melting curve of magnesite mixed with 16 wt% brucite was established by in ‑ situ X‑ray diffraction measurements using the large volume press at P61B at PETRA III (DESY). Second, we report trace element partitioning data for congruent melting of calcite and incongruent melting of magnesite producing carbonate melt and periclase between 6 and 9 GPa. Those results were obtained from quenched experiments using a rocking multi‑anvil press at the GFZ overcoming equilibrium and quenching problems in previous studies [7].

1          Dasgupta and Hirschmann, The deep carbon cycle and melting in Earth's interior. Earth and Planetary Science Letters, 2010. 298(1-2): p. 1-13.

2          Jones, Genge, and Carmody, Carbonate Melts and Carbonatites. Reviews in Mineralogy and Geochemistry, 2013. 75(1): p. 289-322.

3          Solopova, Dubrovinsky, Spivak, Litvin, and Dubrovinskaia, Melting and decomposition of MgCO3 at pressures up to 84 GPa. Physics and Chemistry of Minerals, 2014. 42(1): p. 73-81.

4          Müller, Koch-Müller, Rhede, Wilke, and Wirth, Melting relations in the system CaCO3-MgCO3 at 6 GPa. American Mineralogist, 2017. 102(12): p. 2440-2449.

5          Ellis and Wyllie, Carbonation, hydration, and melting relations in the system MgO-H2O-CO2 at pressures up to 100 kbar. American Mineralogist, 1979. 64(1-2): p. 32-40.

6          Girnis, Bulatov, Brey, Gerdes, and Höfer, Trace element partitioning between mantle minerals and silico-carbonate melts at 6–12GPa and applications to mantle metasomatism and kimberlite genesis. Lithos, 2013. 160-161: p. 183-200.

7          Buob, Experiments on CaCO3-MgCO3 solid solutions at high pressure and temperature. American Mineralogist, 2006. 91(2-3): p. 435-440.

How to cite: Sieber, M. J., Reichmann, H., Farla, R., Appelt, O., Oelze, M., Lathe, C., and Koch-Müller, M.: H2O-present melting curve of magnesite and trace element distribution during melting of (dry) magnesite and calcite in the upper mantle, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-7797, https://doi.org/10.5194/egusphere-egu23-7797, 2023.

The global-scale glacial events recorded by diamictite and cap carbonate couplets occurred in the late Neoproterozoic and has been recognized on at least 15 paleo-continents. Diamictite and cap carbonate couplets play a pivotal role in establishing regional stratigraphic correlations and understanding the extreme climatic conditions and glacial-interglacial cycles of the Neoproterozoic glaciation. Here we report newly discovered Neoproterozoic glaciogenic diamictite and cap carbonate couplet in the Longshoushan area at the southwestern margin of the Alxa Block, NW China. Based on detailed stratigraphic and sedimentologic studies, we identified massive and stratified diamictites at the bottom of the Hanmushan Group, both with poorly sorted and rounded gravels. The presence of glacial striations and ice-rafted dropstones in stratified diamictites supports a glaciogenic origin. The upward transition from massive diamictites to stratified diamictites indicates the process of glacier retreat. The occurrence of thin-bedded phyllites in the stratified diamictites suggests a short-term deglaciation during the glaciation. The 2- to 2.6-m-thick cap carbonates cover the stratified diamictites and consist of thinly laminated microcrystalline dolomites. The basal cap carbonates contain closely linked sheet cracks, cemented breccias, tepee-like structures and cavities. The cap carbonates show high-resolution ¹³C_PDB chemostratigraphy and negative δ¹³C values (ca. −2.9 to −4.1‰), typical of the Marinoan cap carbonates. Regional sedimentary characteristics and the C-O isotope values suggest that the diamictites and cap carbonate couplet in the Alxa Block likely correspond to the Marinoan glaciation and subsequent deglaciation (ca. 635 Ma), not the previously assumed Ediacaran glaciation. Thus, the diamictite and cap carbonate sequence marks the Cryogenian-Ediacaran boundary in the Alxa Block and provides evidence for further stratigraphic correlation and investigation. This work was financially supported by NSFC projects (grants 42072264, 41730213, 41902229, 41972237) and Hong Kong RGC GRF (17307918).

How to cite: Shao, D., Han, Y., Li, M., Lu, L., Cao, X., and Ju, P.: Discovery of Neoproterozoic glaciogenic diamictites and cap carbonate couplet in the Alxa Block, NW China: Evidence from stratigraphic, sedimentologic and geochemical studies, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4758, https://doi.org/10.5194/egusphere-egu23-4758, 2023.

Isotopic analysis using high resolution Isotope Ratio Laser Spectroscopy (IRLS) has been shown to be advantageous to multiple geochemical applications during the last decade.  These advances include isotopic analysis of the bulk isotopic compositions of water, carbon dioxide, methane and nitrous oxide.  More recently, laser spectroscopy has been used by several groups to examine the isotopic compositions of methane, carbon dioxide and nitrous oxide when carrying two rare isotopes (so called clumped isotopes).   Our recent work using Tunable Infrared Laser Direct Absorption Spectroscopy (TILDAS) has demonstrated highly accurate (~0.01 ‰) measurements of the clumped (¹⁶O¹³C¹⁸O or 638 in HITRAN isotope notation) isotopic composition of carbon dioxide derived from carbonate samples with spectroscopic measurement times of ~30 minutes using a dual laser spectrometer.  That spectrometer is optimized for the measurement of the four isotopologues required to calculate Δ₆₃₈ (or Δ₄₇).  We present here our parallel project to develop a novel dual laser isotope analyzer capable of measuring multiple carbon dioxide isotopic signatures simultaneously.   Specifically, we simultaneously measure the isotopic abundances of the three most abundant clumped isotopologues (Δ₆₃₈, Δ₆₃₇ and Δ₈₂₈) as well as ¹⁷O oxygen excess or Δ¹⁷O.  Δ₆₃₈ and Δ₈₂₈ correspond to the quantities Δ₄₇ and Δ₄₈ when measured by isotope ratio mass spectroscopy (IRMS).   Δ¹⁷O is very difficult to measure with IRMS and Δ₆₃₇ has not been previously measured with any technique to the best of our knowledge.  The new instrument utilizes carefully chosen spectral windows, operates at low sample pressure and exploits automated laser frequency hopping.  This prototype instrument simultaneously measures seven isotopologues of carbon dioxide: 626, 636, 628, 627, 638, 637 and 828.  Our preliminary results for Δ₈₂₈ (or Δ₄₈) are displayed as an Allan-Werle plot which shows that the precision in the measurement of Δ₈₂₈ is ~0.09‰ for a single 3 minute sample measurement referenced to a working reference gas.  The plot shows that instrumental drift is very small over periods of several hours and that the precision can be improved to 0.03‰ by processing 10 sub-samples or to 0.01‰ by processing 100 sub-samples.  These measurements are preliminary and somewhat idealized but show promise for this new technique.

How to cite: Nelson, D., Herndon, S., Wang, Z., Quade, J., and Dettman, D.: TILDAS measurement of multiple clumped isotope ratios in carbonates: progress and new horizons, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-9499, https://doi.org/10.5194/egusphere-egu23-9499, 2023.

For fundamental thermodynamic reasons, ¹³C-¹⁸O bonds in carbonate minerals formed under isotopic equilibrium conditions are more abundant than predicted for a random distribution of isotopes, yielding positive Δ₄₇ clumped-isotope signatures which decrease as a function of formation temperature [1]. Although most Earth-surface carbonates are unlikely to achieve complete isotopic equilibrium, Δ₄₇ values of many biogenic and abiotic calcites formed under very different crystallization conditions (and with irreconcilable water-calcite oxygen-18 fractionation laws) appear to follow indistinguishable temperature calibrations, as independently documented by various groups over the years [e.g., 2-4]. That is not to say that all groups agree on a single calibration linking Δ₄₇ and temperature, and a recent comparison of 14 reprocessed calibration studies still found evidence for statistically significant inter-laboratory discrepancies [3]. Rigorous statistical tests aiming to prove or disprove consistency between Δ₄₇ calibrations are particularly challenging because of potentially large and non-independent analytical errors associated with standardization procedures [5], and even in some cases by large correlations in the uncertainties of estimated formation temperatures, making classical least-squares regression approaches ill suited to model these calibration data sets.

Here I propose a new formulation for least-squares regression of data with an arbitrarily complex covariance structure linking all predictor and response observations, generally applicable to all sorts of geochemical data. I use this “Omnivariant Generalized Least-Squares” (OGLS) approach to compare 7 published Δ₄₇ calibration data sets which have been (re)processed according to the newly established InterCarb Carbon Dioxide Equilibrium Scale (I-CDES), supposedly allowing robust comparisons between Δ₄₇ measurements across laboratories [6]. None of these reprocessed calibration data sets are found to deviate significantly from a single, unified regression line, with an overall reduced chi-squared statistic (adjusted for data covariance according to OGLS) of 0.8 consistent with slightly overestimated uncertainties on temperature constraints. This finding marks another milestone in the 17-year-long progress of Δ₄₇ thermometry, which has now solved most of the methodological challenges standing in the way of its widespread application to many scientific issues. In short: carbonate clumped-isotope thermometry is all grown up now.

[1] Schauble et al. (2006) 10.1016/j.gca.2006.02.011
[2] Kele et al. (2015) 10.1016/j.gca.2015.06.032
[3] Petersen et al. (2019) 10.1029/2018GC008127
[4] Anderson et al. (2021) 10.1029/2020GL092069
[5] Daëron (2021) 10.1029/2020GC009592
[6] Bernasconi et al. (2021) 10.1029/2020GC009588

How to cite: Daëron, M.: Making the Case for Reconciled Δ47 Calibrations Using Omnivariant Generalized Least-Squares Regression, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10066, https://doi.org/10.5194/egusphere-egu23-10066, 2023.

Oxygen-17 excess (Δ17O) in carbonate minerals can provide valuable insights into past continental and marine environments, long-term trends in the temperature and oxygen-isotope composition of ancient oceans, isotopic disequilibrium effects in biogenic and abiotic carbonates, and cryptic diagenesis. Triple oxygen isotope analyses of carbonates and/or CO2 using isotope-ratio mass spectrometers (IRMS) remain challenging, however, because of isobaric interference between 16O13C16O and 16O12C17O. Using spectroscopic methods, the abundance of each CO2 isotopologue may be directly quantified, potentially providing simple, non-destructive measurements of δ13C, δ18O and Δ17O on small samples of CO2.

Here we report new data characterizing the application of VCOF-CRDS (V-shaped Cavity Optical Feedback - Cavity Ring Down Spectroscopy [1]) to the analysis of small samples (<40 μmol) of pure CO2, as typically produced by phosphoric acid digestion of carbonate minerals.

Instrumental drifts from various sources are observed to bias apparent isotopic abundances by a few tens of ppm, but these drifts are slow enough that they may be precisely monitored and corrected for by repeated analyses of a working gas interspersed between other analyses, requiring only ~8 mn per aliquot and 30 mn between consecutive “unkown” analyses. This approach was tested by analyzing repeated aliquots of another CO2 tank with a different isotopic composition, yielding instrumental repeatabilities of 12 ppm, 13 ppm and 7.4 ppm for δ13C, δ18O and Δ17O, respectively (95 % CL, Nf = 66).

The accuracy of our measurements was tested over a wide range of Δ17O values spanning 130 ppm, by analyzing CO2 equilibrated at 25 °C with different waters whose triple oxygen compositions were independently constrained in the SMOW-SLAP scale by IRMS measurements and by simple nonlinear mixing predictions. We find that our Δ17O measurements are well within analytical uncertainties of predicted values (RMSE = 1.2 ppm), with analytical repeatabilities (including isotopic equilibration and gas manipulation) of 8.6 ppm (95 % CL, Nf = 27).

We will also report the results of our ongoing investigation regarding the isotopic fractionation and analytical noise associated with different acid digestion protocols at different reaction temperatures, and the triple oxygen composition of various international reference materials already used for δ13C, δ18O, and clumped-isotope measurements.

Based on these results, we conclude that VCOF-CRDS offers excellent accuracy, along with state-of-the-art levels of analytical precision/linearity, for straightforward analyses of 17O excess in CO2, water, and carbonate minerals.

[1] Stoltmann et al. (2017) 10.1021/acs.analchem.7b02853

How to cite: Chaillot, J., Daeron, M., Casado, M., Landais, A., Pesnin, M., and Kassi, S.: Triple oxygen isotope analyses of carbon dioxide, water and carbonates by VCOF-CRDS technique, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12216, https://doi.org/10.5194/egusphere-egu23-12216, 2023.