Time and temperature variation of the intracrystalline Fe2+,Mg fractionation in Johnstown meteoritic orthopyroxene

Abstract

The partitioning of Fe 2+ and Mg on the M1 and M2 sites of orthopyroxenes from the Johnstown meteoritic diogenite has been equilibrated between 1000°C and 700°C in ordering and disordering runs. The method of bivariate high order truncation analysis (Kroll et al., 1997) has been employed to refine the site occupancies from conventional X-ray intensity data. The Fe 2+ ,Mg distribution coefficient varies according to \[\mathrm{In K_{D} = 0.417 (121) {-} 2540(136)/T[K].}\] From isothermal kinetic ordering and disordering experiments an exceptionally large activation energy was derived. Combining our data with those of Zema et al. (1997a) results in the Arrhenius equation for the rate constant: \[\mathrm{In k_{dis}[min^{{-}1}] = 41.4({\pm}0.9) {-} 97.8({\pm}1.9)[kcal/mol] / RT.}\] For the first time, non-linear continuous cooling experiments were performed in which the crystals were cooled from 850°C to 250°C at an average rate of 10°C/day. The Fe 2+ ,Mg distributions were determined after the crystals had reached 650°C, 550°C, 450°C, 350°C, and 250°C, respectively. Using the Mueller rate equation (Ganguly, 1982) and employing the temperature dependencies of K D and k dis as given above, the experimentally delineated ordering path is closely reproduced by the calculated path. However, due to the large activation energy, cooling rates calculated for the untreated crystals turn out to be physically unreasonable, i.e. some 10 −5 K/My. By contrast, Arrhenius parameters determined in the literature on orthopyroxenes with compositions similar to the Johnstown crystals produce physically reasonable rates of some hundred K/My. TEM studies do not show a significant difference between the microtextures of untreated and annealed samples. All orthopyroxenes studied contain clinopyroxene exsolution lamellae and abundant Guinier-Preston zones. At present, we can neither prove nor disprove the concept that the large activation energy of the Johnstown orthopyroxenes is related to their intricate exsolution microtexture.