„Wer hat an der Uhr gedreht ?“ Mechanisms of Rb-Sr age resetting revealed by high-resolution LA-ICP-MS/MS isotope mappings

The Rb-Sr isotope system is sensitive to secondary processes that disturb or even completely reset the age signal in datable minerals like mica or feldspars. To interpret age signatures correctly, the factors governing isotope mobility in real-world minerals have to be identified. Three main processes are known to influence isotopic clocks: (1) diffusional re-equilibration of minerals with their surroundings, which is expected to be strongly temperature-sensitive provided that the matrix of a given mineral facilitates rapid isotope transport (2) fluid-mediated dissolution-reprecipitation and (3) deformation-induced dynamic recrystallization.

Distinction between these processes is notoriously difficult. However, different types of Sr isotope mobility should leave characteristic microscale isotope distribution patterns, with bell-shaped Sr diffusion patterns, preferential rejuvenation close to high-strain zones in the case of ductile deformation, and probably element zoning patterns resulting from fluid-driven reprecipitation. To recognize and to distinguish such patterns, two-dimensional images of the Rb/Sr elemental and Sr-isotope distribution would be required, in analogy to what has been done using Ar-Ar in K-feldspar [1],[2].

Visualization of such patterns has long been analytically impossible due to the absence of an in-situ method that provides sufficient Rb Sr-sensitivity, isotope selectivity and high spatial resolution. By combining state-of the art ICP-MS/MS spectrometry and a fast aerosol transfer lasing system, we developed a routine for rapid Rb-Sr age mappings of mica that offers resolution at the µm-scale. We present first age maps for muscovite crystals from metamorphosed granitic pegmatites.

[1] Wartho, J. A. et al.(1999) https://doi.org/10.1016/S0012-821X(99)00088-6

[2] Popov, D. V. et al. (2020) https://doi.org/10.1016/j.chemgeo.2020.119841