Integration of in situ Rb-Sr and in situ 40Ar/39Ar dates under diverse tectono-metamorphic scenarios

⁴⁰Ar/³⁹Ar dating has been a valuable and widely used method for dating orogenic processes such as prograde and retrograde metamorphism and brittle and ductile deformation, through the analysis of K-bearing rock-forming minerals such as white mica. The in situ ⁴⁰Ar/³⁹Ar method, in which a short wavelength laser is used to ablate an analyte and deliver the liberated Ar to a noble gas spectrometer, is particularly valuable as an approach to dating deformation or metamorphism because it allows for targeting of specific chemical and structural domains, and the mapping of intragrain age distributions. Rb-Sr dating can also be applied to K-bearing minerals because of Rb’s propensity to substitute for K. The Rb-Sr method has been under-used in recent decades because the isobaric interference between parent ⁸⁷Rb and daughter ⁸⁷Sr has necessitated the chemical separation of Rb from Sr via ion exchange chromatography prior to mass spectrometric analysis, and hence bulk sampling of the target analyte. New tandem mass spectrometers, in which two quadrupoles are separated by an intervening reaction chamber into which a reactive gas can be introduced, have opened up the opportunity of applying laser-based in situ sampling approaches to beta decay geochronometers, including Rb-Sr (Zack and Hogmalm, 2016).

We have collected new in situ Rb/Sr data for white mica from three different tectono-metamorphic settings previously dated using the in situ ⁴⁰Ar/³⁹Ar method: recrystallization of white mica in a Paleozoic low-temperature ductile shear zone; development of multiple cleavage domains in low-temperature metamorphic rocks deformed in the Paleozoic, and; slow cooling of rocks following regional amphibolite-facies metamorphism in a Paleoproterozoic orogeny. , This allows a direct comparison between these two approaches, with the goal of exploring the functionality and utility of in situ Rb-Sr data, and testing geological interpretations based upon the in situ ⁴⁰Ar/³⁹Ar method. Our results show that the in situ Rb-Sr method is a highly complementary approach to the ⁴⁰Ar/³⁹Ar method for white mica, particularly in cases for which the target mica population has a large internal spread in Rb/Sr. allowing for the rigorous testing of assumptions and hypotheses about timing and conditions of rock cooling, deformation, and fluid events developed using ⁴⁰Ar/³⁹Ar datasets.

 

Zack, T. and Hogmalm, K.J., 2016. Laser ablation Rb/Sr dating by online chemical separation of Rb and Sr in an oxygen-filled reaction cell. Chemical Geology, 437, pp.120-133.