Thermal history of Acapulco and ALHA81261 acapulcoites constrained by Fe2+-Mg ordering in orthopyroxene

The quenched Fe2+-Mg ordering states of four orthopyroxene crystals from the Acapulco meteorite and two from the Antarctic ALHA81261 acapulcoite were determined from X-ray single-crystal diffraction and electron-microprobe analysis. The closure temperatures of cation ordering were 49028 and 46543°C for Acapulco and ALHA81261, respectively. Kinetic analysis of the Fe2+-Mg intracrystalline reaction, on the basis of Mueller's theory [1,2], was carried out using the method developed by Ganguly [3] to measure the cooling rates of the host rock, assuming both asymptotic and exponential models. The determined ordering states yield cooling rates, near their closure temperatures, of ∼ 0.17 and ∼ 0.05°C/day for Acapulco and ALHA81261 respectively, slightly slower than that measured for the FRO90011 lodranite [4]. A break-up of the A-L parent body, with subsequent radiative heat loss of small ejected fragments, may explain such rapid cooling. The closure temperatures obtained for the two acapulcoites were then compared with those calculated, using the same method, for other achondrites and their cooling rates with those measured for the FRO90011 lodranite [4] and the Estherville mesosiderite [5]. The sequence of decreasing closure temperatures from the FRO90011 lodranite to the Landes IAB iron meteorite reflects decreasing cooling rates. Silicates enclosed in iron meteorites, and thus involved in core formation processes, deeper in the interior of a parent asteroid, seem to have cooled more slowly than meteorites that were affected by partial melting events.