RB-SR CLOSURE TEMPERATURES IN BI-MINERALIC ROCKS: A MODE EFFECT AND TEST FOR DIFFERENT DIFFUSION MODELS

Abstract

Various diffusion models can be applied to Rb-Sr systems in cooling rocks. It is argued that a closed-system diffusion-controlled model, involving local equilibrium along grain boundaries, is most appropriate for describing the exchange of Sr isotopes between minerals in a rock sample during cooling. A finite-difference numerical method is used to solve the diffusion and mass-balance equations. Closure temperatures of mineral-pair Rb-Sr isochrons are predicted to depend on the factors involved in Dodson's infinite reservoir closure temperature formulation (cooling and diffusion rate, grain size and shape) ofboth the minerals. In addition, the closure temperature for mineral pairs is also dependent on the proportion of Sr in each mineral, which is dominantly a function of rock mode. This implies that Rb-Sr cooling ages from interbedded rocks having the same cooling history, but distinct modes, should differ: at slow cooling rates age differences could be > 100 Ma. Such effects, if unrecognised could result in erroneous cooling curves, but if recognised could be utilised in estimating true cooling rates. Our closed-system model may also apply to other isotope decay schemes, such as the Sm-Nd system. A simple test is proposed which would allow the relevance of different diffusion models to be assessed.