IN-SITU RB-SR DATING AND ELEMENTAL FINGERPRINTING OF SHALES AND DOLERITE BY LA-ICP-MS/MS

Sedimentary rocks make up only 5% of the earth’s crust and yet record the history of the planet’s hydrosphere, biosphere and atmosphere. It is critical to be able to accurately date the time of deposition to read and interpret this record correctly—this is challenging in pre-Cambrian rocks where fossils are of very limited use. Here, we present results from a novel application of in-situ Rb‒Sr dating, using laser ablation-inductively coupled plasma-mass spectrometry, where a reaction gas allows for the separation of ⁸⁷Sr from ⁸⁷Rb. This study targeted samples from the Proterozoic McArthur Basin, northern Australia. The Gibb Member shale was collected from the Urapunga 6 drillhole and yielded a bulk rock Rb-Sr age of 1577 ± 56 Ma with initial ^87/86Sr ratios consistent with ca. 1.5 Ga seawater. We suggest that this age represents a minimum depositional age for the unit. A dolerite sample from the Urapunga 5 drillhole gave a Rb-Sr age of 1295 ± 19 Ma, overlapping with previously reported ages for the Derim Derim magmatic event ca. 1313 Ma. Shales from the Jalboi Formation and the Wooden Duck Member collected close to the dolerite intrusion yielded ages, initial ^87/86Sr ratios and depleted light rare earth element patterns similar to the igneous sample. Therefore, we interpret the shale geochemisty of these shales to reflect subtle hydrothermal alteration due to intrusion of the Derim Derim sill and demonstrate that this has reset the Rb–Sr chronometer. Together, these results indicate the powerful potential of this technique—age and geochemical data are obtained simultaneously to evaluate the nature of the fluids in equilibrium with the Rb–Sr chronometer.