39AR-40AR AGES AND THERMAL HISTORY OF MESOSIDERITES

Abstract

Determinations of 39Ar-40Ar ages for plagioclase separates of three clasts from the Vaca Muerta and Mount Padbury mesosiderites give similar results. Ages increase monotonically by #0.2-0.3 Ga when plotted against increasing 39Ar release from the samples and give average values of #3.95 Ga. When combined with revised ages for mesosiderite analyses previously reported, nineteen analyses of thirteen mesosiderites give an average Ar-Ar age of 3.94 ± 0.10 Ga. The average age at 90% 39Ar release is 4.13 ± 0.14 Ga, and the ages at 10% 39Ar release range over 3.32-3.97 Ga. It seems unlikely that the 39Ar-40Ar ages were produced by direct impact heating, but they probably are independent evidence for slow cooling deep within the mesosiderite parent body. We conclude that the sloped age plateaus and the significant range in Ar-Ar closure temperatures calculated for these mesosiderites (#75-340°C) are due to significant differences in Ar diffusion properties among samples. From the Ar diffusion data, we estimate an approximate mesosiderite cooling rate of #0.2°C/Ma, which is similar to the preferred cooling rate of 0.1-0.5°C/Ma derived by others from Ni diffusion and metal textures. The 39Ar/40Ar ages seem incompatible with very slow cooling rates of <0.2°C/Ma, unless the closure temperatures for Ar diffusion and formation of metal cooling textures on the parent body substantially overlap. Any possible role of fragmentation of the mesosiderite parent body in producing the younger Ar-Ar ages and the very slow cooling rates is unclear.