1. Research Focus and Applications

• AIR-G (Applied Isotope Research Group) is developing in situ techniques for U-Pb and Sr-Sr isotope analysis specifically for carbonatic rocks.
• The aim is to support both research and oil exploration by providing precise isotopic compositions directly from carbonate samples.

2. Method Adaptation for Laser Ablation ICP-MS

• The team is adapting existing Sr isotope and U-Pb methods to enable in situ analysis on Laser Ablation ICP-MS.
• This method has already been tested on various carbonate materials, including ancient formations like those from the Quadrilátero Ferrífero (QF) and the Bambuí Group. Check the papers for details.

3. Instrumentation and Setup

• Data acquisition is conducted using a NEOMA multi-collector ICP-MS coupled with an ASI RESOlution 193 Laser Ablation system.
• The method being used is modified from Lana et al. (2017), tailored to fit the specific requirements of carbonate isotope analysis.

4. Sample Preparation and Imaging

• Samples can be either thin sections or polished blocks.
• Preparation includes cleaning the samples in an ultrasonic bath and imaging them fully with the laser before analysis.
• Ablation is performed in a helium atmosphere (0.15 L/min), mixed with argon (0.87 L/min) and nitrogen (0.02 L/min) in the ablation funnel for optimal conditions.

5. Signal Optimization

• The ICP-MS is tuned for maximum sensitivity, maintaining oxide formation below 0.2% and ensuring no fractionation of the Th/U ratio to improve data accuracy.

6. Ablation Parameters

• Ablation parameters are adjusted based on the sample, with typical settings being:
  • Spot size: 85 μm
  • Repetition rate: 10 Hz
  • Fluence: 6 J/cm²
  • Washout period: 15 seconds between analyses
  • Ablation time: 30 seconds per analysis

7. Data Normalization and Correction

Pb-Pb ratios are monitored using BHVO or BCR glasses as standards.

U-Pb and Pb/Pb ratios are normalized using methods by Mottram et al. (2014) and Li et al. (2014).

For U-Pb ratios:

The 254 Ma WC-1 calcite (Roberts et al., 2017) and NIST Glass 614 are used for calibration.

The 207Pb/206Pb ratio is corrected for mass bias (0.25%) and 206Pb/238U for interelement fractionation (~5%), accounting for drift over time.

Sr-Sr isotopes and U_Pb data from aragonite crystals of the Sete Lagoas Formation – from Sambra Quarry

Applications to oil exploration

Direct dating of hydrothermal episodes in carbonate basins via U-Pb calcite dating can play a transformative role in oil exploration by offering insights into the temporal evolution of reservoir development and fluid migration. Carbonate reservoirs, particularly in offshore settings, are often subjected to multiple phases of tectonic activity, fluid circulation, and mineralization. U-Pb calcite dating allows geoscientists to determine the precise timing of these episodes, shedding light on when hydrothermal fluids, which may have played a role in enhancing reservoir porosity and permeability, circulated through the carbonate structures.

The ability to date these hydrothermal events provides critical information on the periods of fluid flow and mineralization, which can be correlated with major tectonomagmatic episodes. Such correlations may reveal the influence of crustal extension, rifting, and subsidence on reservoir evolution. For instance, if U-Pb dating indicates that hydrothermal circulation occurred during a known phase of regional crustal extension, it could suggest that fault systems active at that time facilitated the migration of fluids, possibly contributing to the formation of secondary porosity. These insights can be pivotal in oil exploration, as they help to map the structural and fluid history of a basin, guiding drilling and exploration efforts toward areas with the greatest potential for productive reservoirs.

Additionally, U-Pb calcite dating can help identify the periods of maximum diagenetic alteration, which might affect oil reservoir quality. Understanding when hydrothermal fluids circulated within the carbonate formations allows exploration teams to model the evolution of reservoir porosity and permeability over time, potentially predicting where oil may be trapped or lost due to mineral precipitation that reduces pore space.

The image shows a microphotograph from the laser ablation system depicting calcite growth textures with a series of green laser spots marked across the sample surface. The calcite textures exhibit complex crystalline growth patterns, likely representing multiple generations of calcite deposition, which is common in carbonate rocks subjected to diagenetic and hydrothermal processes.

The growth texture shows banding and variations in crystal size, suggesting episodic calcite formation. Some areas appear to have finer textures, while others have larger, blocky calcite crystals, indicating different stages of mineral growth.

The variations in calcite texture might correlate with changes in fluid chemistry or the timing of growth episodes, essential for understanding the diagenetic history or fluid evolution in this sample.

The image shows a screenshot of the software Saturn U-Th-Pb, used for U-Pb calcite geochronology. The interface contains multiple panels and graphs that indicate the analytical and data interpretation process for isotope ratios in a U-Pb dating system. Here is a breakdown of the key elements visible in the image:

1. Plot Panel (Top Right)

• The main graph in the center is a Tera-Wasserburg diagram, specifically 207Pb/206Pb versus 238U/206Pb often used in geochronology to calculate the ages of Pbc-bearing minerals such as calcite.
• The series of data points is plotted diagonally across the graph, with each point corresponding to different analyses or spots on the calcite minerals. The regression on this plot diagram give use the age of the calcite. The data points are arranged along a trend line that slopes downwards from left to right, indicating that the isotopic ratios are systematically varying, possibly reflecting the progression of lead isotope evolution over time.

Dating fossils

Direct dating of diagenetic episodes in fossil-bearing carbonate deposits via U-Pb calcite dating can offer groundbreaking insights into the timing of fossilization, diagenesis, and the environmental conditions surrounding fossil formation. Many fossils, especially those preserved in carbonate matrices, are subjected to multiple phases of mineral growth, fluid circulation, and recrystallization after their initial burial. U-Pb calcite dating allows geoscientists to determine the precise timing of these post-depositional processes, shedding light on when fluids circulated through the carbonate matrix, which may have played a role in the preservation or alteration of fossil material.

The ability to date these diagenetic events provides critical information on the timing of fossilization and related geochemical alterations, which can be correlated with major environmental or tectonic events. Such correlations may reveal how factors like subsidence, burial, or regional fluid flow influenced the fossilization process. For example, if U-Pb dating indicates that fluid circulation occurred during a known period of tectonic activity or climatic change, it could suggest that these events facilitated the mineralization or preservation of the fossilized organisms. This temporal data is essential for reconstructing the paleoenvironments in which fossils formed, offering a direct link between fossil preservation and broader geological events.

Additionally, U-Pb calcite dating can help identify periods of intense diagenetic alteration, which may impact the preservation quality of the fossils. Understanding when fluids circulated within the fossil-bearing carbonate formations allows paleontologists and geologists to model the diagenetic history of the fossil material over time. This can provide clues about the conditions that promoted fossil preservation or caused partial dissolution and replacement, offering a more complete understanding of the fossilization process and its relation to the paleoenvironment.

By establishing the timing of these diagenetic episodes, U-Pb dating helps refine the chronology of fossil-bearing sequences and offers insights into the environmental changes that influenced fossilization in both marine and terrestrial settings.

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Tracing Silurian Seascapes via LA-ICP-MS: U-Pb Chronology, Sr Isotopes, and REY Patterns in Orthoconic Nautiloids.

Orthoconic nautiloids (orthocerids, endocerids, and actincocerids, among others) hold a significant place in the field of palaeontology, not only for their widespread presence in Paleozoic fossil records but also for the insights they provide into the evolution of early marine ecosystems (Errben, 1966; Kröger et al., 2009; Monnet et al., 2015). As one of the earliest known cephalopods, these orthocones played a pivotal role in Paleozoic evolutionary history, surviving the late Ordovician extinction (e.g., Bambach et al., 2004; Bond and Grasby, 2020). Their global distribution offers crucial insights into the evolutionary developments that gave rise to modern squids, octopuses, and cuttlefish. Orthocones worldwide provide valuable information about the morphology, behaviour, and adaptive strategies of marine life from the Ordovician to Devonian periods (e.g., Evans, 1992, King 1999, King and Evans 2019). Furthermore, the distribution of such fossils across ancient seafloors assists palaeontologists in mapping the shifting continents and marine environments of early Earth, revealing patterns of biodiversity, ocean circulation, and climatic conditions (Servais et al., 2008).

These slender, conical creatures thrived in Palaeozoic seas hundreds of millions of years ago, offering a unique window into Earth’s deep past. While their elegant fossils have fascinated palaeontologists for decades, a critical piece of the puzzle has remained elusive: the precise age of the environments in which they lived. Two independent U-Pb LA-ICP-MS analyses of nautiloids from the Anti-Atlas region of Morocco have yielded identical ages of 423.5 ± 2.2 Ma and 422.5 ± 1.2 Ma, consistent with the Late Ludlow/Early Pridoli epoch of the Silurian. Multiple specimens returned identical ages within uncertainty, affirming the primary shell material as a reliable geochronological marker. In contrast, calcite infills associated with these fossils produced younger ages between 390 and 400 Ma, suggesting post-depositional diagenetic processes. These findings highlight the necessity of distinguishing between original and altered material in paleontological U-Pb studies, as diagenetic calcite can lead to significantly younger ages. Sr isotope analysis of the orthocone shells provided an ⁸⁷Sr/⁸⁶Sr ratio of 0.70846±0.00021, matching the Silurian seawater signature and reinforcing the integrity of the fossil material in recording ancient seawater chemistry. Geochemical analysis revealed an enrichment of MREEs and HREEs, pointing to deposition in low-oxygen, reducing conditions. These characteristics, coupled with high uranium concentrations, indicate that the nautiloids thrived in anoxic, upwelling-driven environments with limited terrestrial influence. These findings link the mass occurrence of the nauitloids in the Anti-Atlas region to global sea-level fluctuations and nutrient upwelling, correlating with the glacio-eustatic lowstands of the Late Silurian.

This image presents a detailed U-Pb geochronology analysis of an Orthocone Nautiloid Fossil. The dating results, shown on the concordia plot, indicate that the fossil is from the Ludfordian stage of the Silurian, approximately 423.2 million years old. The accompanying timescale helps place the fossil’s age within the broader geological context.

Fossils of orthoconic nautiloids, are characterized by their elongated, conical shells segmented into chambers. These fossils show internal chamber structures and suture patterns, commonly observed in Orthoceras or related genera. The specimens are likely preserved in a matrix, possibly from the Orthoceras Limestone, given the distinctive appearance and preservation style.

The image above depicts a phylogenetic tree of cephalopods, showing the evolutionary relationships among various groups, including Nautiloids, Orthocone cephalopods, Ammonoids, Belemnites, and modern cephalopod groups like Decabrachia (e.g., squids, cuttlefish) and Octopodiformes (e.g., octopuses). Extant taxa are in bold, and some lineages, such as Endoceratoidea, Actinoceratoidea, and Bactritoidea, are marked with an asterisk to indicate uncertainty in their placement in the evolutionary tree.