Conference Agenda

The Rb-Sr geochronometer is among the earliest isotopic dating methods and became widely adopted due to the abundance of datable minerals such as micas and alkali feldspars, facilitated by their contrasting Rb-Sr geochemistry. However, any meaningful interpretation of Rb-Sr ages requires a thorough understanding of the processes responsible for isotopic resetting. Although Rb-Sr isotope redistribution has often been regarded as diffusion-controlled i.e. primarily temperature-activated, natural examples demonstrate that deformation, recrystallization and fluid interaction affect (and sometimes dominate) Rb-Sr age resetting. The interplay of these processes may produce complex intracrystalline isotope patterns, which make it often difficult to relate Rb-Sr ages obtained from bulk analyses to specific events in a rock’s history, such as cooling, hydration or deformation.

Since the introduction of in situ Rb-Sr analysis by reaction-cell tandem mass spectrometry ~10 years ago, many of these complexities can be assessed more directly. The technique relies on the online chemical separation of the isobaric ⁸⁷Rb/⁸⁷Sr interference, which otherwise would require a currently unachievable mass resolution of ~300,000. Combined with laser-ablation sampling (typically using 193 nm excimer lasers) this now enables the determination of Rb-Sr ages at a spatial resolution of a few tens of micrometers and has become a routine method in an ever-growing number of applications.

In this talk we will demonstrate both the strengths and current limitations of in situ Rb-Sr geochronology. We will highlight recent methodological advances, including Rb-Sr age mapping at a spatial resolution below 10 µm and the use of ultrashort 157 nm laser ablation systems.

The development of in-situ radiometric dating techniques using triple quadrupole and MC-ICP-MS/MS instruments have spurred new interest in the Rb–Sr isotope system [e.g., 1,2]. Here, we compare measurement protocols and data reduction strategies for in-situ Rb–Sr dating using our 193 nm Excimer laser coupled to a Neoma MC-ICP-MS/MS that is equipped with a pre-filter and collision/reaction cell (CRC). For the analysis of Rb-poor phases (carbonate, apatite, and plagioclase), we apply a weak pre-filter B-field of 20% with an open pre-filter slit and no gas in the CRC. When applying Kr⁺, Rb⁺, and improved REE⁺⁺ interference corrections, this measurement configuration yields an individual spot precision and accuracy better than 0.1 ‰ (2 s.e.) for ⁸⁷Sr/⁸⁶Sr. Using SF₆ reaction gas, we analyse mass-shifted SrF species free from Kr⁺, Rb⁺, and REE⁺⁺ interferences in Rb-rich phases (micas, K-feldspars, glasses) with the pre-filter B-field increased to 50 % and the slit closed to 65 %. This limits but does not entirely prevent masses >88 amu from entering the CRC, complicating measurements using SF₆ in materials with low Sr/Zr and Sr/Y ratios (e.g., phlogopite, phosphates, glasses). This is because Zr- and Y-oxides, formed inside the CRC, interfere on SrF masses. Taking into account these findings, we present an in-situ dating protocol for K-feldspars, which long remained difficult to date with triple quadrupole instruments.

[1] Zack, T., Hogmalm, K.J., 2016. Chem. Geol. 437, 120–133.

[2] Cruz-Uribe, A.M., Craig, G., Garber, J.M., Paul, B., Arkula, C., Bouman, C., 2023. GGR. 47, 795–805.

White mica is a common mineral in metamorphic, igneous, and sedimentary rocks, and its Rb-Sr and K-Ar systematics enable tracking of cooling and recrystallization events below ~500 °C. In-situ Rb-Sr geochronology of detrital white mica/muscovite thus offers intermediate- to low-temperature provenance information absent from high-closure-temperature systems such as zircon.

We present new in-situ Rb-Sr LA-ICP-MS/MS analyses in white mica and integrate it with K-Ar dating of tuffs and fault rocks, detrital apatite U-Pb geochronology, and illite textural and thermometric characterization to constrain the metamorphic and deformational history of the Neoproterozoic sub-metamorphic Itajaí foreland basin northern Dom Feliciano Belt , Brazil) and provide a framework for evaluating detrital mica ages. Comparison of detrital muscovite and zircon spectra reveals a general provenance signal similarity, with detrital mica only displaying ages younger than 1.2 Ga. A significant proportion of single-spot muscovite ages are younger than cross-cutting rhyolites, indicating post-depositional isotopic resetting. Our findings show that reset age clusters might be correlated with faulting events, suggesting that Rb-Sr isotopic resetting to be related to deformation under temperatures lower than expected. These results indicate that detrital muscovite datasets should be interpreted cautiously in light of available deformation, metamorphic, and stratigraphic constraints.

Over the past 30 years, Ruthenium along with the other elements from the Highly Siderophile Elements (HSE) and their isotopic systems (Re-Pt-Os) have been recognized as tracers of planetary processes (core-mantle differentiation, late accretion), mantle partial melting and metasomatism/refertilisation as well as mantle-crust/fluid interactions.

High-quality analyses of HSE in mantle samples has brought his load of challenges (incomplete digestion of refractory HSE metal nuggets, HFSE interferences in HSE during ICP-MS measurements), which have been solved through analytical development investigations. Still, although mentioned in few studies in the 2000s, the effect of Cr-based interferences on the Ru isotope masses has never been explored in depth. In this presentation, we will present our analytical investigations based on pure Cr and pure Ru as well as mixed Cr-Ru solution analyses using both the Agilent 8900 QQQ-MS (Bochum University) and the Thermo-Fisher Element XR HR-ICP-MS (University of Bonn). This will provide insights into (1) whether such Cr-based interferences are affecting the accuracy of Ru concentrations determination and in which extend, (2) how such interferences could be eliminated during measurement, and (3) discuss thus our understanding of the Ru concentrations so far determined for mantle samples (e.g. peridotites).