Brøgger Seminar Series – Rb-Sr geochronology: New concepts, and applications in geodynamics 

When? 10 May. 3 p.m.
Where? Nordenskiöldsalen, Geovetenskapens hus

 

Abstract:
Isotopic ages, namely Rb-Sr mineral ages, have long been considered as particularly sensitive to temperature and have been used to infer thermal histories. However, this approach is problematic because of the common observation of microscale spatial correlations between isotopic ages, mineral growth zonation, and recrystallization fronts. These correlations indicate that factors like availability of fluids, mineral recrystallization, and the modal rock composition play crucial roles in determining the behaviour of isotopic systems of mineral grains. It has been observed that Rb-Sr mica-based isotopic ages are almost insensitive to fluid-absent, nonreactive static tempering. This observation opens the doors for new applications of Rb-Sr geochronology in geodynamics.
Field examples illustrate that the Rb-Sr system of white mica in eclogite may persist unchanged up to nearly 700°C, which allows insights into subduction-related metamorphic processes at high temperatures. A valid Rb-Sr multimineral isochron comprising all phases of an eclogitic assemblage can be interpreted as dating eclogitization, and linked to PT data to constrain PTt conditions of eclogitization. The same concept applies to assemblages formed later in a high-pressure rocks’ history. Veins, precipitated from fluids during exhumation, may be used to determine age and PT conditions of amphibolite- or greenschist facies overprints. Mylonitic deformation in shear zones leads to complete Sr-isotopic reequilibration among all simultaneously recrystallizing phases. Rb-Sr multimineral isochrons from deformation zones may thus constrain the timing of deformation up to the ductile-brittle transition. Examples show how petrology-guided Rb-Sr multimineral geochronology can be applied to establish PTt paths of tectonic units, to determine exhumation and erosion rates, and to guide geodynamic modelling of accretionary wedge growth.

 

 

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