In addition to in-situ trace element compositions, AIR-G is now providing state of the art in situ U-Pb and Sr-Sr of carbonatic rocks for research and oil exploration.

We are currently adapting the old Sr isotope method and U-Pb on carbonates for in situ Laser Ablation ICP MS. We have already tested this method in different materials including very old carbonates of the Quadrilaterro Ferriero and from the Bambui Group. Some staggering results were obtained in a couple of hours particularly with old carbonates of the Gandarela formation in the QF.

The data is acquired using a Thermo Neptune multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS) attached to a Photon Machines 193 Laser ablation, at the Universidade Federal de Ouro Preto, using a method modified after Lana et al. (2017). 

We can use thin sections or polished blocks. The samples are cleaned in an ultrasonic bath and fully imaged with the Photon Machines 193 laser. Samples are ablated in a helium atmosphere (0.15 L min-1) and mixed in the ablation funnel with 0.87 L min-1 16 argon and 0.02 L min-1 17 nitrogen. Signal strength at the ICP-MS was tuned for maximum sensitivity while keeping oxide formation below 0.2% and no fractionation of the Th/U ratio.

Ablation parameters vary from sample to sample. We normally use 85 μm static spot, a repetition rate of 10 Hz, a fluence of 6 Jcm-2, a 15 second washout period between analyses, and a 30 second ablation time.

The U-Pb and Pb/Pb ratios are normalized following the method of Mottram et al. (2014) and Li et al. (2014) where U-Pb ratios are normalized using the 254 Ma  WC-1 calcite (Roberts et al., 2017) and NIST Glass 614. The mean 207Pb/206Pb ratio of each analysis is corrected for mass bias (0.25%) and the 206Pb/238U ratio for interelement fractionation (~5%), including drift over the sequence time, using NIST SRM-614. The Pb-Pb ratios are monitored using BHVO or BCR glasses.

Due to the carbonate matrix, additional offset factors of 1.07 was determined using WC-1 carbonate reference material (Roberts et al., 2017), and has been applied as an external correction to all carbonate analyses.  Data processing used the time-resolved analysis on the Neptune software that was reduced using ours in house data reduction software (Silva et al., 2020) that includes a subtraction for gas blank counts on Pb isotopes, and uncertainty propagation, and Tera-Wasserburg for age determination. All data are plotted at the 2σ confidence interval.

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 basis via U-Pb calcite dating can be crucial for piecing together the possible events of oil reservoir development and for understanding the duration over which hydrothermal fluids have actively circulated in these basins. The obtained ages can then be directly correlated with the diversity of tectonomagmatic events that occurred in synchronization with the crustal extension and basin development. Furthermore, defining the timing and periods of hydrothermal fluid circulation in these carbonate reservoirs may help to establish some key factors to better understand oil production in offshore carbonate basins.