Iodine-xenon studies have been performed on nine Allende chondrules and a sample of oxidized Allende matrix material. The chondrules are all very rich in radiogenic xenon relative to trapped xenon, making it possible to determine a relatively precise model initial iodine composition for each temperature extraction. These model compositions show a total range in variation of about 20 percent, spanning the compositions seen in Bjurbole chondrules. One of the chondrules (chondrule 6) gives a well-defined isochron, with an apparent age .53 ±. 15 m.y. later than Bjurbole whole rock. The rest of the chondrules show a pattern of increasing apparent antiquity with increasing extraction temperature, which could be interpreted as relatively slow cooling (100-200°C/m.y.). Alternatively, poorly-defined plateaus in composition can be seen, perhaps indicative of a few phases with distinct initial iodine compositions (but also temperature-ordered), as has been previously suggested for Allende inclusions. Possible consequences of these interpretations are discussed. Elemental abundances were determined for some elements several months after the irradiation by INAA, and suggest that all the chondrules except chondrule 6 might be pyroxene- or mesostasis-rich. The oxidized matrix sample gives a well-defined isochron with an initial 129I 127I ratio higher than any plateaus seen in the chondrules, suggesting that, if this sample is representative of matrix, the matrix pre-dates the chondrules. The initial 244Pu 238U ratios of the Allende chondrules and 10 Bjurbole chondrules irradiated earlier appear to be consistent with. 004-.007 values quoted for unfractionated material in the early solar nebula.
ASJC Scopus subject areas
- Geochemistry and Petrology