Argon 40-argon 39 chronology of four calcium-rich achondrites

Abstract

Results are presented from thermal-release argon 40-argon 39 dating experiments on four calcium-rich achondrites. Pasamonte shows an apparent age of 4.1 AE over 70 per cent of its gas release indicating a degassing event at this age. At higher temperatures, the apparent age for Pasamonte rises to 4.51 AE, defining a lower limit gas retention age which is identical to that of the chondrites St. Severin and Guareña. Petersburg has a high-temperature gas retention age of 4.40 ± 0.03AE, 100 million years younger than St. Severin, in agreement with previously reported Pu-Xe and K-Ar gas retention ages and the absence of Xe 129from the decay of I 129. Stannern has a complicated pattern of apparent age with gas release which cannot be accounted for in detail by simple diffusive loss following a well-defined initiation of gas retention. In broad scale, however, the Stannern pattern is relatively flat and confined to the range 3.5–3.9 AE, indicating that this meteorite was strongly degassed in this interval. The nakhlite Lafayette has an exceedingly young gas-retention age in the range 1.4–1.7 AE. These results indicate that as a group the calcium-rich achondrites have experienced a more diverse and extensive thermal history than have the chondritic meteorites.