Xin-Min Hua

Fayalite-rich rims, veins, and halos around and in forsteritic olivines in CAIs and chondrules in carbonaceous chondrites: Types, compositional profiles and constraints of their formation

Abstract Fayalite-rich rims, veins, and halos around and in forsteritic olivines are a wide-spread phenomenon in chondrules, Ca, Al-rich inclusions (CAIs), and single grains in carbonaceous chondrites. The presence of fayalite rod-like crystals and laths in rims, veins, in wall of pores, and as fluffy network bridging neighbouring olivines, pyroxenes, feldspars, etc. is strongly suggestive that the fayalitic olivine was formed by condensation presumably from the solar nebula gas. The formation of the fayalitic olivine was probably caused by an increase in the H 2 O H 2 ratio (to a ratio between 0.1–1) subsequent to condensation of forsterite. At that stage, FeNi inclusions in olivine were also oxidized and fayalitic halos around the metal were then formed by Fe diffusion along with addition of SiO 2 from the solar gas or loss of MgO to the solar gas. The Fa-rich olivine rims and veins display a narrow compositional variation between Fa 34 and Fa 46 . Subsequent to condensation of Fa-rich olivine and oxidation of FeNi metal, Fe diffused in forsterite. This diffusion was probably enhanced due to the presence of point defects in olivine or the formation of a nonstoichiometric phase analogous to laihunite enriched in Al 2 O 3 and Cr 2 O 3 . However, the presence of Al 2 O 3 - and Cr 2 O 3 - rich discrete domains cannot be excluded. Cooling rates calculated by modelling of the diffusion profiles are indicative of rapid cooling subsequent to the condensation of fayalitic olivines. We obtain cooling rates ranging from 2000°C/ day and 10°C/day at an initial temperature of 1200°C and 900°C, respectively.