AR-39-AR-40 AGES AND THERMAL HISTORY OF MESOSIDERITES

Determinations of Ar-39-Ar-40 ages for plagioclase separates of three clasts from the Vaca Muerta and Mount Padbury mesosiderites give simil ar results. Ages increase monotonically by similar to 0.2-0.3 Ga when plotted against increasing Ar-39 release from the samples and give ave rage values of similar to 3.95 Ga. When combined with revised ages for mesosiderite analyses previously reported, nineteen analyses of thirt een mesosiderites give an average Ar-Ar age of 3.94 +/- 0.10 Ga. The a verage age at 90% Ar-39 release is 4.13 +/-0.14 Ga, and the ages at 10 % Ar-39 release range over 3.32-3.97 Ga. It seems unlikely that the Ar -39-Ar-40 ages were produced by direct impact heating, but they probab ly are independent evidence for slow cooling deep within the mesosider ite parent body. We conclude that the sloped age plateaus and the sign ificant range in Ar-Ar closure temperatures calculated for these mesos iderites (similar to 75-340 degrees C) are due to significant differen ces in Ar diffusion properties among samples. From the Ar diffusion da ta, we estimate an approximate mesosiderite cooling rate of similar to 0.2 degrees C/Ma, which is similar to the preferred cooling rate of 0 .l-0.5 degrees C/Ma derived by others from Ni diffusion and metal text ures. The Ar-39/Ar-49 ages seem incompatible with very slow cooling ra tes of <0.2 degrees C/Ma, unless the closure temperatures for Ar diffu sion and formation of metal cooling textures on the parent body substa ntially overlap. Any possible role of fragmentation of the mesosiderit e parent body in producing the younger Ar Ar ages and the very slow co oling rates is unclear. Copyright (C) 1998 Elsevier Science Ltd.