TIMS Techniques for Measuring Strontium Isotope Ratios in Apatite Grains and Application to Igneous Rock Petrology

This study evaluates the potential of Sr isotopic compositions in apatite grains from magmatic rocks as a proxy for initial ⁸⁷Sr/⁸⁶Sr ratios, addressing the need for alternative approaches to conventional whole-rock analyses in isotopic studies of granitoid systems.

Chemical and spectrometric procedures were optimized and validated for the determination of Sr isotopic ratios in apatite by TIMS. Three representative samples from the Ribeira Orogen, Brazil, were selected, and apatite Sr isotopic compositions were compared with initial whole-rock ⁸⁷Sr/⁸⁶Sr ratios calculated from U-Pb zircon crystallization ages, whole-rock isotopic ratios, and Rb and Sr concentrations. The reliability of the analytical approach was evaluated using the EN-1 and Durango apatite reference materials.

Apatite grains yielded Sr concentrations ranging from 80 to 950 μg g^-1, with total Sr masses between 0.5 and 50 ng. Comparisons between apatite-derived and whole-rock initial ⁸⁷Sr/⁸⁶Sr ratios show isotopic variability below 0.05%, indicating high analytical precision. Validation results demonstrate consistent agreement between methods, with uncertainties within 0.5%.

The proposed TIMS-based method for Sr isotope analysis in apatite grains provides a potential approach for investigating the initial Sr isotopic compositions of granitoid magmatic systems. This approach represents a cost-effective and time-efficient alternative to traditional whole-rock analyses, with strong potential for large-scale isotopic applications in orogenic terrains.