Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.483a&link_type=abstract
Meteoritics, vol. 30, no. 5, page 483
Mathematics
Logic
4
Chondrites, Ll, Chondrules, Iodine-Xenon Ages, Meteorites, Parnallee, Radionuclides, Extinct
Scientific paper
We have attempted to unravel the evolutionary complexities of the Parnallee (LL3) ordinary chondrite by the chronometric analysis of distinct, mineralogically well characterized, structural components (chondrules, matrix, igneous and exotic clasts). The problems associated with variable I-Xe closure ages, as encountered in the analysis of achondrites, would not occur in the dating of individual chondrules from unmetamorphosed meteorites as the cooling rates of chondrules is on the order of minutes or hours. Hence the Xe closure of all minerals would be essentially contemporaneous. Parnallee is a relatively unaltered meteorite, chondrules from which have already yielded useful I-Xe isochrons [1]. The age differences between the chondrules suggest that the metamorphism was insufficient to reset the I-Xe clock. Chondrules and 'matrix' were hand picked from Parnallee and irradiated for Xe analysis using the RELAX resonance ionization mass spectrometer [2]. Chondrules. The age range for most of the chondrules is ca 8Myr, similar to that determined for chondrules from other meteorites [3]. However two chondrules (macrochondrule and P9) had no detectable live l29I present upon Xe closure. This implies that either; there was no I present (but they contain l28Xe from 127I), that they have undergone a highly efficient degassing process (but still contain their usual planetary Xe complement) or that they formed very late, after the decay of 129I (which would extend the chondrule forming event to >80Myr). Whilst the latter may be understood for the macrochondrule, which has been described as an impact melt [4], the chondrule P9 is more difficult to explain. Matrix. The matrix samples show a greater variation in age than do the chondrules, generally offering poorly constrained isochrons. This is due to the difficulty of excluding non-matrix fragments and the smaller grain size of the matrix increasing the ease with which the chronometer can be reset. The minimum reliable age of matrix material is 32.6+/-1.45) Myr after Bjurbole chondrule closure. This contrasts with previous studies of Allende, Chainpur and Allegan which have given I-Xe ages the same as, or older than, their chondrule ages. In contrast Ar-Ar and Rb-Sr dating of matrix have given ages younger than the chondrule ages [5]. Formation. As noted previously [1] Parnallee has had a complex thermal history. Whilst the majority of the chondrule formation took place at least 30Myr before the Xe closure of the matrix the presence of chondrule-like objects with no live l29I implies that the closure of the matrix did not mark the end of the geological history of the Parnallee meteorite parent and that accretion continued after the metamorphism of the meteorite. References: [1] Ash et al. (1994) Meteoritics, 29, 439. [2l Gilmour et al. (1994) Rev. Sci. Instrum., 65, 617. [3] Swindle and Podosek (1987) Meteorites and the Early Solar System, 1127, Univ. of Arizona, Tucson. [4] Dodd (1969) Mineral. Mag., 37, 231. [5] Swindle et al. (1983) in Chondrules and Their Origins (E. A. King, ed.), 246. [6] Bridges et al. (1995) Proc NIPR Symp. Antarc. Meteorites.
Ash Richard D.
Bridges John C.
Gilmour James D.
Hutchison Robert
Turner Gary
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