André Navin Paul

During their continuous cooling and differentiation, granitic magmas progressively shift from a medium dominated by crystal-melt interactions to a medium that is dominated by crystal-fluid interactions. We refer to this transition between a purely magmatic to a hydrothermal system as magmatic-hydrothermal transition (MHT), which is often associated with the formation of ore mineralisation. In highly differentiated and volatile-rich magmas (e.g., rare-metal granites and pegmatites), the circulation of hydrothermal fluids often modifies the original rock texture, by dissolving and/or replacing the primary minerals phases by secondary ones. Since most of our petrological interpretations are based on mineral composition (e.g., chemical zoning, geochronology), it is crucial to evaluate and quantify the chemical and mineralogical changes such rocks have undergone during the MHT.To better understand the MHT in highly evolved magmas, especially how these episodes of fluid circulation impacted the texture and composition of the primary mineral phases, we have investigated the internal texture and chemical composition of heterogeneous zircons from the Beauvoir rare-metal granite (Massif Central, France). By combining µ-Raman spectroscopic, mineralogical and geochronological analyses on these grains, we show that primary (magmatic) zircon was partially replaced by secondary (hydrothermal) porous “zircon” through dissolution-reprecipitation mechanisms. The zircon-fluid interactions were notably facilitated by the primary, high trace element content in zircon (especially for U). This newly formed mineral grains (secondary “zircon”) are extremely enriched in non-stoichiometric elements up to few weight percent of P, U, F, Ca, Fe and Mn while they are depleted in Si and Zr compared to pristine zircon. These drastic compositional changes during the MHT of the Beauvoir granite clearly indicate that altered, secondary pseudomorphs after magmatic zircon can be a good tracer for the MHT in evolved silicic systems.As a result of these dissolution-reprecipitation processes, these zircon grains are porous and highly metamict from the high degree of decay damage related to percent levels of Uranium, which considerably limits their use for zircon petrochronology. By comparing the ID-TIMS geochronological analyses performed on these zircon (312 ± 7.2 Ma – discordia upper intercept) with those performed on apatite (313.4 ± 1.3 Ma – 206Pb/238U weighted mean age, 9 analyses), we thus envision that the use of zircon to precisely date the emplacement of highly differentiated magmas is limited, while that of other minerals such as apatite (and potentially columbo-tantalite, cassiterite) may be more appropriate in such systems.

Zircon petrochronology is widely used to quantify the age and duration of magma emplacement and differentiation. However, in highly differentiated magmas, such as those forming rare-metal granites, zircon may form at the magmatic–hydrothermal transition, and its primary crystallisation history, together with its secondary hydrothermal overprint, needs to be resolved and clarified. To resolve zircon formation in such evolved and mineralised granitic systems, we investigated heterogeneous zircons from the Beauvoir rare-metal granite (Massif Central, France). Most of the Beauvoir zircons are characterised by the presence of two distinct domains, designated as Zone 1 and Zone 2. Zone 1 occurs as rounded, Si- and Zr-rich domains, which are embedded in the interconnected Si- and Zr-poor Zone 2 domains that are also extremely P-, U-, F-, Ca-, Fe- and Mn-rich. Both of these zones are strongly damaged (metamict) by radioactive decay, mainly from their high U concentrations. Textures and chemical composition strongly suggest that Zone 1 corresponds to magmatic zircon that has been partly replaced by the Zone 2 material during the magmatic–hydrothermal transition. The crystallisation of Zone 1 zircon is preceded by the crystallisation of U-rich cores (∼6 wt % UO₂) containing UO₂ (uraninite) micro-inclusions, which are then surrounded by a Zone 1 homogeneous rim. These uraninite micro-inclusions resulted from the uranium migration in the metamict and amorphous precursor zircon. U-Pb dating of single zircon grains using chemical abrasion, isotope dilution thermal ionisation mass spectrometry (CA-ID-TIMS) techniques yielded a well-defined discordia line with an upper intercept at 312±2.9 (7.2) Ma (2σ) and a near-zero-age lower intercept. The discordancy reflects the continuous loss of radiogenic lead from a heavily damaged and aperiodic zircon lattice. On the other hand, ID-TIMS data from magmatic apatite of the Beauvoir granite yielded an age of 313.4±0.2 (1.3) Ma (2σ), so far, the most accurate and precise crystallisation age of the Beauvoir granite. Thus, we emphasise that although the study of zircon from highly differentiated systems provides strong insights into the magmatic–hydrothermal transition of these objects, their metamict nature prevents their use to precisely and accurately date the emplacement of rare-metal granite.

The latest Cretaceous(?)–Paleocene Barra Honda Formation represents one of the largest carbonate shoals (>900 km², 350 m thick) of the convergent margin of Costa Rica. Although the mode of formation of the carbonate shoal is well understood, how environmental and tectonic factors interacted to cause its demise near the Paleocene‐Eocene boundary remains poorly constrained. Stable isotopic, biostratigraphic, mineralogical, and geochronological analyses from the Barra Honda Formation and overlying siliceous Buenavista Formation provide new constraints on the timing and causes of the demise of the carbonate shoal. We report one new U–Pb zircon chemical abrasion, isotope dilution, and thermal ionization mass spectrometry date (56.30 ± 0.13 Ma, 2σ) obtained from an ash‐rich layer at the boundary between the two formations. The sharp transition from Barra Honda massive limestones to Buenavista marl‐chert alternations coincides with a negative shift in carbon isotope (δ¹³C_carb) values of about 3–5 ‰ and a 50% decrease in carbonate contents. The timing of the combined lithological‐mineralogical‐isotopic change is coeval with the Paleocene‐Eocene Thermal Maximum (PETM, 56 Ma). The onset of clay‐rich sedimentation is consistent with a PETM‐related increase in the terrestrial influx of nutrients and detrital particles, which promoted eutrophication and decreased light availability in the photic zone. Combined with seawater acidification and warming, these environmental parameters were fatal to the carbonate‐producing benthic communities of Barra Honda. High subsidence rates of the forearc basin and renewed arc volcanic activity must have closely followed the cessation of shallow carbonate production, preventing further formation of the carbonate shoal.

The highest precision and accuracy in U–Pb geochronology is achieved using isotope dilution thermal ionisation mass spectrometry (ID-TIMS), a technique which owes its reliability to precise Pb and U isotope ratio analysis, a largely unified framework of lab protocols, and common isotopic tracers with accurately determined compositions. However, while hardware and protocol developments have steadily improved the analytical precision, the level to which ID-TIMS U–Pb dates from different laboratories agree remains largely unquantified. To better assess both internal repeatability and interlaboratory reproducibility of this method, we have conducted an experiment in which a large batch of natural zircon was dissolved, mixed with a newly prepared ²⁰⁵Pb–²³³U–²³⁵U tracer, and distributed as solution to participating laboratories. Thus prepared, pre-spiked, homogeneous PLES535 solution underwent the full sample preparation and analysis process separately in each lab, allowing a maximally unbiased comparison of the entire analytical procedure on a sample of unknown age. The results from 14 instruments at 11 institutions demonstrate internal repeatability of individual labs at 5 to 10 U–Pb analyses, with MSWD values generally indicative of single age populations. Lab weighted-mean ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ages for the 337 Ma zircon solution agree within 0.05 % and 0.09 % (two standard deviations), respectively. This underscores the reliability of the participating laboratories for precise and accurate zircon U–Pb analyses, while highlighting the need for continued exchange on lab protocols and method improvement. We identify likely reasons for the remaining interlaboratory bias and discuss ways forward toward the goal of 0.01 % reproducibility.

The highest precision and accuracy in U–Pb geochronology is achieved using isotope dilution thermal ionisation mass spectrometry (ID-TIMS), a technique which owes its reliability to precise Pb and U isotope ratio analysis, a largely unified framework of lab protocols, and common isotopic tracers with accurately determined compositions. However, while hardware and protocol developments have steadily improved the analytical precision, the level to which ID-TIMS U–Pb dates from different laboratories agree remains largely unquantified. To better assess both internal repeatability and interlaboratory reproducibility of this method, we have conducted an experiment in which a large batch of natural zircon was dissolved, mixed with a newly prepared ²⁰⁵Pb–²³³U–²³⁵U tracer, and distributed as solution to participating laboratories. Thus prepared, pre-spiked, homogeneous PLES535 solution underwent the full sample preparation and analysis process separately in each lab, allowing a maximally unbiased comparison of the entire analytical procedure on a sample of unknown age. The results from 14 instruments at 11 institutions demonstrate internal repeatability of individual labs at 5 to 10 U–Pb analyses, with MSWD values generally indicative of single age populations. Lab weighted-mean ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²³⁵U ages for the 337 Ma zircon solution agree within 0.05 % and 0.09 % (two standard deviations), respectively. This underscores the reliability of the participating laboratories for precise and accurate zircon U–Pb analyses, while highlighting the need for continued exchange on lab protocols and method improvement. We identify likely reasons for the remaining interlaboratory bias and discuss ways forward toward the goal of 0.01 % reproducibility.

The Early Triassic is a key interval to understand biotic and environmental recovery after the major disturbance of the planetary system at the Permo-Triassic Boundary (PTB). The collapse of biodiversity and the dramatic change of the global carbon cycle are thought to be the result of intrusions of mafic melts of the Siberian Large Igneous Province (S-LIP) into sediments rich in organic matter and evaporites. However, the Early Triassic is marked by a long record of global carbon cycle perturbations, suggesting that severe climatic instability and delayed biotic recovery may go beyond S-LIP volcanism.To further explore this question, we compiled and constructed a continuous stratigraphic record in the platform-slope sequences of the Nanpanjiang Basin, South China, where bio- and chemostratigraphic control is supported by precise geochronology. We present here a U-Pb zircon-based Bayesian age model integrated with δ¹³C isotope data, providing precise temporal calibration of the carbon cycle fluctuations and of lithostratigraphic and biostratigraphic boundaries over 5.5 million years. This record is calibrated to absolute age via the Bayesian age model relying on 20 published and 6 new high-precision ID-TIMS U-Pb ages of volcanic zircon in interbedded ashes.Our compilation covers six major δ¹³C excursions, three negative (termed N1 to N3) and three positive ones (termed P1 to P3): The initial sharp negative excursion N1 at the PTB (~252.02 Ma) reflects rapid input of volcanogenic CO₂ from the Siberian Large Igneous Province (S-LIP) activity. An early Griesbachian positive peak P1 (~251.88 Ma) is associated with transient recovery of organic carbon burial in microbial-rich environments. A negative δ¹³C shift (N2) in the Dienerian (~251.40 Ma) coincides with ecological stress and enhanced weathering. A sharp positive excursion P2 at the Dienerian-Smithian transition (~250.39 Ma) reflects carbon sequestration in a dynamic climate regime. The largest negative excursion N3 is found during the middle Smithian thermal maximum (~249.56 Ma) and correlates with the climax of global warming and marine anoxia. The Smithian-Spathian boundary positive excursion P3 (~249.33 Ma) marks a cooling phase and coincides with significant biotic turnover, particularly affecting nektonic faunas. The Spathian interval features a stepwise decrease in δ¹³C (N4; ~248.14 Ma), followed by a broad positive excursion near the Spathian-Anisian boundary (P4; ~246.88 Ma), reflecting stabilization of carbon cycling as ecosystems began recovering. The positive excursions P1 to P4 coincide with 100ky-long gaps in the sedimentary record, related to low sea-level and possibly to presence of continental ice.Published and new U-Pb zircon and baddeleyite dates put the minimum estimate for intrusive as well as extrusive S-LIP activity into the Dienerian (ca. 251.0-250.0 Ma), pointing to a non-volcanic trigger of the P2, P3, P4, N3, N4 isotope excursions. In summary, the robust U-Pb calibrated early Triassic carbon isotope record is interpreted to reflect the interplay of volcanic influence, intrinsic carbon cycle feedback, and other extrinsic factors, which appear to have paced the alternation between extreme warming and transient cooling phases during the Early Triassic post-extinction recovery, and post-recovery period.

Marine carbonates are archives of geochemical proxies, such as e.g. Sr and U-Pb isotopes, which potentially can be utilized in the reconstruction of past climate conditions, ancient seawater composition and/or their alteration during burial and fluid interaction, if interpreted accurately. The ability to confidently reconstruct environmental conditions in the past times is of great importance since they can be linked with changes in the biosphere. For example, the Ediacaran-Cambrian transition was a period where significant evolutionary change modified the biosphere towards appearance of extant animal clades and the establishment of metazoan-dominated ecosystems. Any contribution towards an intact, continuous record of environmental conditions through multiple proxies will help to understand better the timing, nature and sequence of events that preceded or accompanied such changes in biodiversity. However, carbonate rocks are susceptible to numerous post-depositional processes (such as: oxidative weathering, diagenesis, burial, lithification, deformation, dissolution and reprecipitation), which may alter the geochemical record. Additionally, detrital components may increase the complexity of the geochemical signature and the carbonate composition, particularly in shallow marine settings with variable continental run-off.Thus, we have to understand and identify the presence or absence of such processes, before extracting meaningful geological information from these archives. Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS), is a tool that offers spatial resolution when performing geochemical analyses, which may help to interpret the geochemical data more confidently. In this work, we combine observations from Sr and U-Pb isotopic systematics supported by trace element abundances to identify domains that are indicative of post-depositional processes, over protracted time and variable in their extent. The sample material is taken from drill cores in Namibia, obtained within the frame of the GRIND project, which aims to investigate the late Edicaran period. Situated in the south of Namibia, the Gariep belt was geologically active during the Pan African orogenesis and possibly affected by the Atlantic Ocean opening.