Time and temperature variation of the intracrystalline Fe (super 2+) , Mg fractionation in Johnstown meteoritic orthopyroxene
Time and temperature variation of the intracrystalline Fe (super 2+) , Mg fractionation in Johnstown meteoritic orthopyroxene
European Journal of Mineralogy (February 2000) 12 (1): 163-176
The partitioning of Fe (super 2+) and Mg on the M1 and M2 sites of orthopyroxenes from the Johnstown diogenite has been equilibrated between 1000 and 700 degrees C in ordering and disordering runs. The Fe (super 2+) ,Mg distribution coefficient varies according to ln K (sub D) = 0.417(121)-2540(136)/TK. From isothermal kinetic ordering and disordering experiments, an exceptionally large activation energy was derived. Non-linear continuous cooling experiments were performed in which crystals were cooled from 850 to 250 degrees C at an average rate of 10 degrees C/day; the Fe (super 2+) ,Mg distributions were determined after the crystals had reached 650, 550, 450, 350 and 250 degrees C and were found to match closely 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 (super -5) K/m.y. By contrast, Arrhenius parameters from the literature on compositions similar to the Johnstown crystals produce physically reasonable rates of some hundred K/m.y. At present the concept that the large activation energy of the Johnstown orthopyroxenes is related to their intricate exsolution microtexture can be neither proved nor disproved.