Geological Survey of Canada

The use of clumped isotopes (Δ₄₇) as a geothermometer in carbonates has become widespread in geosciences because carbonates are ubiquitous in the environment and this technique also constrains the precipitating fluid δ¹⁸O compositionHowever, in some contexts, Δ₄₇ values cannot be used as a geothermometer.Instead, they provide insights into processes affecting the isotopic composition of CO₂ and dissolved inorganic carbon (DIC) prior to and during precipitation, as observed in speleothems, corals, and cold seep-associated authigenic carbonates.Among all these precipitates, authigenic carbonates associated with cold seeps stand out, as the measurement of their clumped isotopic abundances has revealed an unprecedented range of isotopic ordering, and several explanations were put forward to explain the various signals observedTo get a firmer understanding of the cause of natural variability in modern cold seeps, here we report δ¹³C, δ¹⁸O and Δ₄₇ results performed on carbonates from two deep-sea cold seeps offshore Nova Scotia, Canada, i.e., >2300 m water depth and within the gas hydrate stability zone.We report a broad Δ₄₇ range (i.e., 0.502-0.663 ‰ I-CDES) that differs from values expected at isotopic equilibrium at seafloor temperatures (i.e., 0.671-0.672 ‰ I-CDES).Using micro computed X-ray tomog., micro-X-ray fluorescence, and micro sampling, we demonstrate that methane-derived authigenic carbonates at these deep-water sites are closely associated with colocalized methane-hydrate dissolutionThe isotopic composition of these authigenic carbonates are influenced by the complex mixing of different dissolved inorganic carbon (DIC) pools at various stages of isotopic re-equilibration, resulting in fine-scale isotopic variability.We propose several mixing models and stages of DIC re-equilibration to explain this isotopic variability and apply them to evaluate the contributions of the proposed sources of DIC.The isotopic domains resulting from this exercise encompass the entire range of dual isotopic values (δ¹⁸O and Δ₄₇) ever observed in cold seeps and suggests that the isotopic diversity observed in these environments may be broader than that observed to date.

This study examines the microstructural and geochronological record in specimens collected along transects away from the Shannon fault and d'Abbadie fault, in the northern Canadian Cordillera. Microstructural analysis for both shear zones indicate dextral shear and an increase in paleostress and inferred strain magnitude toward each fault. In situ mica Rb–Sr geochronology from across a strain gradient developed by a single phase of deformation within the Shannon pluton, adjacent to the Shannon fault, dominantly records syn-emplacement deformation and/or rapid cooling at ca. 100 Ma with sparse analyses ranging to as young as ca. 63 Ma. U–Pb geochronology in apatite from the same specimens returns three ca. 100 Ma dates and two ca. 85 Ma dates. In contrast, lithologically heterogenous specimens that have experienced multiple deformational events, collected in proximity to the d'Abbadie fault, exhibit more variation in Rb–Sr dates, ranging from presumed regional cooling at ca. 195 Ma, a possible thermo-tectonic event at ca. 150 Ma, syn-pluton emplacement deformation and/or rapid cooling at ca. 100 Ma, and local partial resetting of Rb–Sr in mica most likely by brittle fault movement at ca. 80 Ma. U–Pb in apatite geochronology in the same specimens also records the ca. 100 Ma event. The results of this study demonstrate that lithological differences and multi-phase deformational histories create complexity in patterns of in situ Rb–Sr geochronology, and require interpretation within thermal and structural contexts, to quantify the timing of deformation in ductile high-strain shear zones.

Exploration for graphite in Canada is of economic, strategic and governance priority. In this study, we aimed to develop a reliable prospectivity map for graphite in Canada. Our approach mitigated multiple sources of workflow-induced uncertainty by propagating uncertainty due to the selection of negative labels, machine learning algorithms, feature space dimensionality, and hyperparameter tuning metrics. By averaging an ensemble of de-correlated models, we produced a single-merged model that clearly represents propagated uncertainty through a consensus map and an uncertainty map. These maps adhere to the metrological convention of "result plus/minus associated uncertainty" and are intuitive to use. Our ensemble demonstrated robustness, quickly converging to the consensus model, suggesting that new mineral prospectivity mapping (MPM) products using the same data would unlikely perturb our consensus model’s coverage. We conducted a maximally double-blind study, avoiding geoscientific knowledge during model generation to ensure impartial post-hoc analysis and interpretation. Therefore, our MPM products complement geoscientific knowledge-based exploration, because the targeting information provided in our MPM products constitute a maximally independent source. Our MPM products showed excellent spatial variability, aligning with existing knowledge of graphite deposits in Canada, indicating that combining data-driven rigor with independent interpretation enhances the robustness of our MPM products. Consequently, we believe our MPM products could effectively guide regional exploration of natural graphite in Canada.