M. W. Caffee

Xenon and other noble gases in shergottites

We have determined the isotopic composition of the xenon component trapped in EETA 79001 lithologies B and C, which we refer to as SPB-xenon. SPB-xenon is isotopically distinct from known xenon reservoirs, but differs in regular fashion. Normalized to 132Xe, the light isotope ratios are indistinguishable from air, the 129Xe/132Xe ratio is about 2.4, and 134Xe and 136Xe are enhanced relative to the terrestrial atmosphere or AVCC. The apparent heavy-isotope enrichments are not generated by in situ fission and there is no spectral evidence for the presence of 244Pu fission xenon. However, the xenon composition does match that of fractionated AVCC except at 129Xe, and consequently may be derived from or related to that component. ALHA 77005, Shergotty and EETA 79001 lithology A also have enhanced 129Xe/132Xe ratios in most temperature steps, and are seemingly consistent with varying mixtures of SPB-xenon and terrestrial xenon. Our results for neon and argon in EETA 79001 confirm earlier results on the exposure ages. We have also verified that the trapped 38Ar/36Ar ratio in lithology C is apparently substantially different from the terrestrial or meteoritic value. Krypton in EETA 79001,C is more fractionated with respect to AVCC than is terrestrial krypton and in the opposite direction as xenon. EETA 79001,C contains excess 80Kr (and perhaps 82Kr and 128Xe), presumably from neutron capture on bromine and iodine, but these neutron captures do not appear to have occurred by in situ processes.

Iodine-xenon studies of petrographically and chemically characterized Chainpur chondrules

Abstract We have performed petrographie, instrumental neutron activation analysis (INAA) and noble gas studies on samples of 18 chondrules and matrix from the Chainpur (LL3) meteorite to study variations in R0, the ratio of 129I to 129I at the time of xenon isotopic closure. R0 varies by more than a factor of 10 among the chondrules, corresponding to a span of more than 50 Ma in apparent I-Xe ages, including the latest apparent I-Xe ages ever observed for a chondritic sample. Variations are not closely related to any petrographie properties, although low values of R0 (late apparent ages) may be associated with high sulfide abundances and/or non-porphyritic textures. Similarly, R0 is not closely related to any chemical components, but does seem to correlate with abundance of refractory lithophile elements. Also, R0is correlated with 244 Pu 238 U and anticorrelated with trapped 129 Xe 132 Xe , as might be expected if the variations in iodine isotopic composition are dominated by decay of 129I. We have not found a completely satisfying explanation of the variations in R0. Models involving gas-dust mixing or nebular heterogeneity cannot satisfactorily explain the Chainpur data. However, there are also difficulties with explanations attributing the variations to differences in formation age, metamorphic age, or time of aqueous alteration. We believe the most plausible explanation is that the variations represent times of low-grade shock events.

Iodine-xenon studies of Allende inclusions: Eggs and the Pink Angel

The iodine-xenon systems of six Allende inclusions (five Eggs and the Pink Angel) appear to have been altered by non-nebular secondary processes. Evidence for this includes temperature-ordered variations in the initial I isotopic composition within several objects (with older apparent I-Xe ages associated with higher extraction temperatures) and the absence of primitive I-Xe ages. The span of apparent ages seen in Allende objects (10 Ma or more) is probably too long to reflect any nebular process, so at least some alteration probably occurred on the parent body. The range in initial 244Pu238U ratios for the Eggs (3–14 × 10−3) includes the current best estimates of the bulk solar system value (4–7 × 10−3). For Egg 3, the PuU ratio varies by a factor of two between extractions, probably the result of fractionation of Pu from U among different phases.

I-Xe studies of individual Allende chondrules

Abstract 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 129 I 127 I 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 244 Pu 238 U 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.