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

The partitioning of Fe2+ and Mg on the M1 and M2 sites of orthopyroxenes fr om the Johnstown meteoritic diogenite has been equilibrated between 1000 de grees C and 700 degrees C in ordering and disordering runs. The method of b ivariate high order truncation analysis (Kroll et al., 1997) has been emplo yed to refine the site occupancies from conventional X-ray intensity data. The Fe2+,Mg distribution coefficient varies according to ln K-D = 0.417(121) - 2540(136) / T[K]. From isothermal kinetic ordering and disordering experiments an exceptional ly large activation energy was derived. Combining our data with those of Ze ma et al. (1997a) results in the Arrhenius equation for the rate constant: In k(dis)[min(-1)] = 41.4(+/-0.9) - 97.8(+/-1.9)[kcal/mol] / RT. For the first time, non linear continuous cooling experiments were performe d in which the crystals were cooled from 850 degrees C to 250 degrees C at an average rate of 10 degrees C/day. The Fe2+,Mg distributions were determi ned after the crystals had reached 650 degrees C, 550 degrees C, 450 degree s C, 350 degrees C, and 250 degrees 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 t he 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 anneale d samples. All orthopyroxenes studied contain clinopyroxene exsolution lame llae and abundant Guinier-Preston zones. At present, we can neither prove nor disprove the concept that the large ac tivation energy of the Johnstown orthopyroxenes is related to their intrica te exsolution microtexture.