In-situ Rb-Sr dating of detrital mica and its application for provenance studies

Mica, especially white mica, is a common detrital mineral in siliciclastic sediments and sedimentary rocks. It is used in provenance studies to constrain the thermal history and exhumation processes in the source region. It thus records valuable complementary provenance information to e.g. U-Pb geochronology of detrital zircon that date magmatic and high-grade metamorphic events in the provenance area.

However, in contrast to U-Pb geochronology of detrital zircon, there was a major disadvantage of using mica geochronology in provenance investigations until now: the conventional Argon-Argon (Ar-Ar) or Rubidium-Strontium (Rb-Sr) dating techniques are rather time-consuming due to complex sample preparation. Thus, of major interest for mica-based provenance studies is a recent improvement in in-situ Rb-Sr dating by laser ablation (LA) – inductively coupled plasma (ICP) - mass spectrometer (MS/MS) analyses. This analytical setup uses a reaction cell between two mass spectrometers. Induced gases allow the reaction of targeted masses and thus to chemically separating ⁸⁷Rb and its daughter isotope ⁸⁷Sr. This avoids the isobaric overlap during mass-spectrometric analysis. In combination with the novel approach of Rösel and Zack (2022) (GGR 46, 143-168), single-spot Rb-Sr ages can be calculated from individual detrital mica grains. Trace and major elements can be determined contamporaneously from the same laser spots. Consequently, LA-ICP-MS/MS analyses of detrital mica enables collecting of time and cost efficient multi-proxy datasets – a prerequisite for provenance studies.

In this contribution, we focus on (1) the analytical routine, (2) data reduction and age calculation strategy and (3) interpretation of in-situ Rb-Sr age and geochemical data from detral mica for provenance investigations. In-situ Rb-Sr LA-ICP-MS/MS dating was tested on detrital white mica from the Late Miocene Loxton Sand Formation, Murray Bain, South Australia. U-Pb dating from detrital zircon extracted from the same formation was performed for comparison.