The effect of chemical and isotopic exchange on mica Rb-Sr closure temperatures in the Lepontine Alps
thesisposted on 15.12.2014, 10:39 by Hannah. Townley
The aim of this study is to provide a greater understanding of the resetting processes, which produce the Rb-Sr closure temperature, allowing the production of more relevant models of closure temperature. This study provides information on the scale, mechanism and pathways of transport of species within rocks to understand how chemical exchange during cooling affects the closure temperatures of isotopic systems.;To complete these objectives three areas are addressed: a literature review of the processes thought to be involved in defining a closure temperature, microanalytical work, and modelling of results. The samples studied are closed system marbles, implying that any exchange effects can be quantified, as any species lost from one mineral must be balanced by gain in another mineral or minerals.;TIMS Rb-Sr suggests the phlogopite closure temperature is higher than that of biotite. This study and previous modelling work of chemical exchange suggest that closure temperatures are partly controlled by the mineral mode and the elemental concentration of species within each mineral. LA-ICP-MS studies of calcite reveal gradients of 87Sr/86Sr, which decrease away from the mica bands present in the samples over a few centimetres. Ion and electron microprobe work suggest that the movement of elements may be in the opposite sense to that of isotopic exchange, but the results from this part of the study are inconclusive.;During cooling from peak metamorphism, isotopic and chemical gradients are "frozen" in different minerals. Modelling suggests that exchange occurs by a combination of volume and grain boundary diffusion. 87Sr/86Sr gradients in calcite are used to produce a new method of producing the age of peak metamorphism and the profile shape describes the cooling history. The results of this study imply that exchange within the Rb-Sr system is very complex and the closure temperature process is more complicated than previously thought.