Mathematics – Logic
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27r.232h&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 232
Mathematics
Logic
1
Scientific paper
Where, how, and from what are chondrules formed? Assuming isotopic data require a nebular "where," the "how" defines heating events in the accretion disk we otherwise would have no knowledge of. Experiments reproducing chondrule textures and phase compositions then are potentially important in revealing the physical conditions in these heating events. The well-known conclusion (e.g. 1) that chondrules were heated to close to their liquidus temperatures, though based on dynamic crystallization experiments that succeeded well in matching chondrules (except perhaps for severe Na loss), was premature. These experiments followed petrologic tradition in achieving reproducible initial melt conditions by isothermal heating for at least 30 minutes before cooling and crystallization and hence appeared to rule out a flash heating mechanism. However, very different thermal histories have recently been shown to reproduce chondrule features equally well. For example, totally melted spherules can grow chondrule-like textures (instead of becoming glass) if they collide with crystalline dust during cooling (2). Then, if chondrule formation involved very localized heating in a very dusty environment, there might be no limit to the highest temperature reached. Similarly, with very short heating times (a few minutes), chondrule textures are reproduced with temperatures 100-300 degrees C above the liquidus, depending on grain size (3). Na loss problems, though expected to become more severe with high temperatures, are actually less as very short heating times are used: a five-second laser heating experiment in Ar-H2 resulted in 99% melting but only 10% Na loss. Oxygen isotope exchange experiments may also provide information about chondrule heating, but initial work shows strong kinetic effects in no way duplicating the Allende chondrule trend. Recent experiments then have modified constraints on chondrule formation conditions: temperatures were near-liquidus or (much) higher; heating times could have been short, perhaps only a second or so. However, to reproduce the Fe-Mg zonation in olivine crystals, cooling rates of 100-1000 degrees C/hr (1) are still required and indicate that chondrules were formed en masse. Given a number density of chondrules of 10/m**3, the size of chondrule- forming regions providing the appropriate bulk cooling rates has been calculated (4). The most plausible configuration involves a volume at most 300 km across embedded within a thicker region with an identical particle number density. The borders of this volume are warmed enough to sinter dust onto chondrules as rims, by comparison with rim-forming experiments (5). This model is compatible with very many transient heating events after solids are concentrated in the mid-plane of the disk. The range of bulk compositions of chondrules has the potential to tell us "from what" chondrules were formed. Unfortunately, precursor interstellar dust would yield a composition spectrum, while it was partially evaporated during chondrule melting, quite similar to that for sequences of nebular condensates aggregated at different temperatures. The existence of a group of chondrules with a Na/Al ratio of one (higher than CI), plus the observation that short-duration melting can involve very little Na loss, suggests that precursors were condensates, some albite-bearing, rather than interstellar dust (6). References: (1) Hewins R.H. and Radomsky P.M. (1990) Meteoritics 25, 309-318. (2) Connolly H.C. Jr. and Hewins R.H. (1990) Meteoritics 25, 354-355. (3) Connolly H.C. Jr. et al. (1991) Meteoritics 26, 329. (4) Sahagian D.L. and Hewins R.H. (1992) Lunar Planet. Sci. XXIII, 1197-1198. (5) Connolly H.C. Jr. and Hewins R.H. (1991) Lunar Planet. Sci. XXII, 233-234. (6) Hewins R.H. (1991) Geochim. Cosmochim. Acta 55, 935-942.
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