A novel in-situ method for rubidium-strontium (Rb-Sr) dating of igneous rocks and minerals

Abstract

The Rb-Sr isotopic system represents a popular geochronological tool for dating igneous rocks and minerals. This study presents recent advances in the Rb-Sr radiogenic dating via a direct ‘in-situ’ age determination of biotites based on laser ablation system (LA) coupled with a new generation of triple quadrupole (QQQ) ICP-MS instrument (Agilent 8900). In addition this study also tests (i) the sensitivity and performance of online chemical separation of Rb and Sr in QQQ, using O2 and N2O reaction gases, and (ii) the precision and reproducibility of elemental concentrations and 87Sr/86Sr data on selected standards with respect to their certified values. Results performed by Agilent 8900 on standard and sample solutions confirmed the reliability and robustness of the chemical separation of Rb and Sr via QQQ where yields of SrO+ reaction products reach ~12% for O2 gas, and up to ~99.85% for N2O gas. The latter was thus applied for the in-situ Rb-Sr dating as well as further tests of Agilent 8900 using rocs standards, which showed that the certified Rb and Sr concentrations in G-2 and GSP-2 granitic standards were reproduced within 2%, and their 87Sr/86Sr rations measured by QQQ with N2O gas agree with the first 4 decimal places with respect to certified values and/or our TIMS measurements.

The acquired in-situ Rb-Sr isochrons ages of biotites from the studied rocks (including granite, gabbro, gneiss) and mineral separates (LP-6 standard) showed a good agreement with the published crystallisation ages for these samples. Specifically LP-6 yielded an in-situ Rb-Sr age of 127±12 Ma (published whole rock Rb-Sr and K-Ar age of 128.2±2.2 Ma); Padthaway Granite yielded 479.4±9.4 Ma (published U-Pb age of 487.1±1.2 Ma); Black Hill Gabbro yielded 481.6±6 Ma (published whole rock Rb-Sr and K-Ar age of 487±5 Ma); and finally Rathjen Gneiss was dated at 502.0±9.9 Ma (published U-Pb and Rb-Sr age of 514±5 Ma). Overall, this study and the above in-situ Rb-Sr ages of biotites confirmed the potential and robustness of the LA-QQQ system (i.e. Agilent 8900 with N2O gas) for geochronology applications, such as direct dating of biotite-bearing igneous and metamorphic rocks.