THERMAL HISTORY OF ACAPULCO AND ALHA81261 ACAPULCOITES CONSTRAINED BYFE2-MG ORDERING IN ORTHO-PYROXENE()

The quenched Fe2+-Mg ordering states of four orthopyroxene crystals fr om the Acapulco meteorite and two from the Antarctic ALHA81261 acapulc oite were determined from X-ray single-crystal diffraction and electro n-microprobe analysis. The closure temperatures of cation ordering wer e 49028 and 46543 degrees C for Acapulco and ALHA81261, respectively. Kinetic analysis of the Fe2+-Mg intracrystalline reaction, on the basi s of Mueller's theory [1,2], was carried out using the method develope d by Ganguly [3] to measure the cooling rates of the host rock, assumi ng both asymptotic and exponential models. The determined ordering sta tes yield cooling rates, near their closure temperatures, of approxima tely 0.17 and 0.05 degrees C per day for Acapulco and ALHA81261 respec tively, 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 compa red with those calculated, using the same method, for other achondrite s and their cooling rates with those measured for the FRO90011 lodrani te [4] and the Estherville mesosiderite [5]. The sequence of decreasin g closure temperatures from the FRO90011 lodranite to the Landes IAB i ron meteorite reflects decreasing cooling rates. Silicates enclosed in iron meteorites, and thus involved in core formation processes, deepe r in the interior of a parent asteroid, seem to have cooled more slowl y than meteorites that were affected by partial melting events.