IRON-MAGNESIUM ORDER-DISORDER IN AN ORTHO-PYROXENE CRYSTAL FROM THE JOHNSTOWN METEORITE

Equilibrium reversals of Fe2+-Mg distribution between the M1 and M2 sites of an orthopyroxene from the Johnstown meteorite were achieved at several temperatures between 700 and 1000-degrees-C. One single crystal was used for the whole thermal treatment and for collecting all the X-ray data after quenching. The intracrystalline ion exchange for the bulk chemical composition: (Mg1.453Fe0.441Cr0.024Ca0.054Mn0.015Fe0.005Ti0.003Al0.005)(Si1.960Al0.040)O6 is given by: ln K(D) = -3027(+/- 39)/T(K) + 0.872(+/- 0.013) where K(D) is the distribution coefficient for the reaction: Fe(M2)2+ + Mg(M1) = Mg(M2) + Fe(M1)2+. A transmission electron microscopy (TEM) study of part of the crystal showed the presence of very thin augite lamellae and Guinier-Preston zones indicating a relatively rapid cooling of the host rock at temperatures close to 1000-degrees-C. The new temperature scale yields a relatively high quenching temperature of 379 (+/- 8)-degrees-C for the pyroxene which appears consistent with a rapid cooling (estimated few degrees per hundred years) of a magmatic cumulate excavated by an impact on its parental body.