THERMAL AND SHOCK HISTORY OF MESOSIDERITES AND THEIR LARGE PARENT ASTEROID

To elucidate the geological evolution of the mesosiderite stony-iron m eteorites and their 3.3-3.8 Gy Ar-40-Ar-39 ages, we have investigated their shock and thermal histories. We have studied shock metamorphism in sixteen mesosiderites and find that none have been shocked to more than 10 GPa (shock stage S1-S2). Three mesosiderites contain a tiny fr action (approximate to 0.1% overall) of mineral fragments that were sh ocked to shock stage S3-S6 levels. The uniformity of shock features wi thin all fragments shows that these fragments were not shocked in situ . Our shock data for mesosiderites, the absence of evidence of 3.6-3.9 Gy impact melt, the shock history of impact-heated ordinary chondrite s, and the difficulty in quantitatively removing Ar in an impact event , all suggest that the mesosiderite parent body did not suffer a major impact event 3.6-3.9 Gy ago. Metallographic cooling rates of approxim ate to 0.03 degrees C/My at 400 degrees C were estimated from taenite lamellae in four mesosiderites using the latest diffusion coefficients and Fe-Ni-P phase diagram. Cooling rates of 0.01 degrees C/My at 425- 325 degrees C were estimated from published compositional data for kam acite grains in four mesosiderites. These two techniques and four othe r semiquantitative, metallographic cooling rate indicators show that t he mesosiderites cooled slower than any iron meteorite. We infer that cooling rates at 400 degrees C were similar to 0.02-0.03 degrees C/My and certainly less than 0.5 degrees C/My. The inferred cooling rate is too slow to allow Ar closure before 4 Gy. All of the shock, thermal, and age data for mesosiderites are consistent with slow cooling at dep th <1 My after metal and silicate were mixed around 4.4 Gy ago. Therma l models indicate that the mesosiderites probably cooled in an asteroi d some 200-400 km in radius.