A COMPARATIVE-STUDY OF DIFFUSION PROFILES IN HIMALAYAN AND DALRADIAN GARNETS - CONSTRAINTS ON DIFFUSION DATA AND THE RELATIVE DURATION OF THE METAMORPHIC EVENTS

Cation diffusion rates at 690 +/- 30 degrees C have been calculated by inverse modelling of observed manganese (Mn) zonation profiles in 40 garnets from two kyanite-bearing metapelite samples from the High Hima layan Crystalline Series, Zanskar. northwest India. Knowledge of the i nitial growth profile of Mn in garnet is a pre-requisite for this tech nique. Following previous workers we model Mn partitioning into growin g garnet in terms of a Rayleigh fractionation process, and demonstrate that the Mn-garnet:whole rock partition coefficient is 60-100. Three- dimensional zonation profiles were obtained by successively grinding a nd polishing similar to 1 cm slabs of each sample at 0.1-0.2 mm interv als and analysing the garnets at each stage, thus ensuring that core s ections were measured. The diffusion model assumes that garnet has a s pherical geometry and behaves as a closed system, and simulates diffus ive modification of the hypothetical Mn Rayleigh growth profile for ea ch garnet. The derived measure of the time-integrated diffusion histor y for each garnet is then combined with radiometric and field-relation constraints for the duration of the Himalayan metamorphic event to ca lculate cation diffusion rates. The average cation interdiffusion rate calculated for garnets in the two samples examined is (6 +/- 3.2) x 1 0(-23) s(-1). This interdiffusion rate pertains to a temperature of 69 0 +/- 30 degrees C, which is 0.97 x T-PEAK, the peak temperature condi tions experienced by the samples estimated using standard thermobarome tric techniques. Garnet compositions are P(y2-17)Alm(65-77)Gro(6-16)Sp (1-17). These new diffusion data are consistent with, and more precise than, existing high-temperature (> 1000 degrees C) experimentally det ermined diffusion data, although some uncertainties remain difficult t o constrain, Qualitative comparison between diffusively modified Mn gr owth profiles in garnets from the Scottish Dalradian and the Himalayan garnets suggests that the duration of metamorphism affecting the Dalr adian garnets was 10-20 times longer than that endured by the Himalaya n garnets.