Cation ordering in orthopyroxenes from two stony-iron meteorites: Implications for cooling rates and metal-silicate mixing

We have determined the cooling rates of orthopyroxene crystals from two gro up IVA stony iron meteorites-Steinbach (ST) and Sao Joao Nepomuceno (SJN)-o n the basis of their Fe-Mg ordering states. The rate constant was calibrate d as a function of temperature by controlled cooling experiments using orth opyroxene crystals separated from ST. These data were used along with earli er calibrations of the equilibrium intracrystalline fractionation of Fe and Mg as a function of temperature for crystals separated from both meteorite s to calculate their cooling rates. The site occupancies of the orthopyroxe ne crystals were determined by single-crystal X-ray diffraction subject to the bulk compositional constraints. The closure temperatures (T-c) of catio n ordering for the untreated crystals from SJN are similar to 400 degrees C , whereas those from ST vary between similar to 430 and 470 degrees C. Reco nciliation of the metallographic and orthopyroxene cooling rate data, withi n the framework of the metal-silicate mixing model of Haack et al. (1995), suggests that these two stony irons had cooled at a similar rate of similar to 400 degrees C/Ma through the closure temperatures for cation ordering i n the orthopyroxenes. This was followed by slow cooling for ST at similar t o 50 degrees C/Ma at T < 425 degrees C Similar slow cooling was not recorde d by the metals in SJN, which implies that if this stony iron were subjecte d to slow cooling, it must have been below 350 degrees C. The similar cooli ng rates above 425 degrees C for both ST and SJN, as required to reconcile the metal and orthopyroxene cooling rate data, is at variance with the earl ier notion (Rasmussen et al., 1995) of distinctly different cooling rates f or the high and low Ni IVA irons and stony irons. The cation ordering and m etallographic cooling rate data are also amenable to an alternative interpr etation, which requires two different parent bodies for the two stony irons , and mixing of the metal and silicate components of ST after the metals ha d cooled below the closure temperature of Fe-Ni interdiffusion. However, th e available textural data for ST seems to argue against such metal-silicate mixing model. Copyright (C) 2000 Elsevier Science Ltd.