NOBLE GASES AND THE CLASSIFICATION OF BRACHINA

Abstract

Noble‐gas systematics show that Brachino is not a member of the SNC‐group of meteorites. The whole‐rock K‐Ar gas retention age is (3.11 ± 0.07) AE as compared to the 1.3 AE solidification ages of SNCs; the content of radiogenic129 Xe* of (3.47 ±. 15) × 10−10 cm3 STP/g is about two orders of magnitude higher, and the129 Xe/132 Xe ratio (11.0), the ratio of radiogenic129 Xe* to fissiogenic136 Xef (300), and the ratio36 Ar/132 Xe in the trapped gases are about one order of magnitude higher than observed for SNCs. The same evidence argues strongly against any simple genetic relationship with eucrites. The noble‐gas abundance pattern resembles closely that in silicate inclusions from the iron meteorites Campo del Cielo and Udei Station. Abundances of cosmic‐ray produced3 He and21 Ne (5.7 and. 99 × 10−8 cm3 STP/g, resp.) indicate an exposure age of ∼ 2.4 Ma. Irradiation conditions appear to have been perfectly normal except for an unaccountably low content of spallogenic 38Ar. Losses by diffusion of radiogenic4 He are severe; they must have occurred at or before the onset of the exposure of the meteoroid to the cosmic radiation.