Constraints on the thermal history of the Allende parent body as derived from olivine-spinel thermometry and Fe/Mg interdiffusion in olivine

Abstract

Attempts are made to determine the extent of thermal metamorphism on the Allende parent body. The chemical composition of olivine and coexisting spinel in type II chondrules and Fe-rich isolated olivine grains from the Allende meteorite were analysed with an electron microprobe. The Fe/Mg exchange reaction between olivine and spinel yields upper limits for peak metamorphic temperatures on the Allende parent body of approximately 800 K. This may represent the closure temperature for Fe/Mg interdiffusion in olivine and spinel. Steep Fe/Mg concentration gradients in Allende olivine of various textural settings provide an independent method for estimating peak metamorphic temperatures. Modelling of Fe/Mg interdiffusion in olivine leads to an upper limit of peak metamorphic temperatures between 600 and 750 K, assuming a duration of the thermal event of 106 years. The extrapolation of experimentally determined diffusion data to low temperatures and low oxygen fugacities is the major cause for the uncertainties in the calculated equilibration temperatures. A critical evaluation of existing Fe/Mg interdiffusion data and equations proposed for low temperature extrapolation suggests that the lower value of 600 K is more realistic.