Astronomy and Astrophysics – Astronomy
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.527s&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 527
Astronomy and Astrophysics
Astronomy
2
Chondrites, Chondrule, Melting, Metamorphism (Geology), Meteoritic Composition, Astronomical Models, Heat Sources, Petrology, Protoplanets, Radioactive Isotopes
Scientific paper
Chondrules evidently formed by the flash melting of precursor solids in the dusty midplane of the solar nebula. Since chondrule production probably overlapped in time with accretion of the earliest planetesimals, possible genetic links between chondrules and accretion have been explored. Primitive bodies that accreted less than 1 m.y. after the solar system began were rapidly heated to melting by the decay of short-lived radioactive isotopes. Subsequent collisions yielded copious volumes of incandescent spray that stuck to, irradiated, sintered, and even melted dust and coarser fragments in the local toroidal 'nebula' already orbiting each planetesimal. In this way various kinds of additional chondrules were produced. Because impact velocity was low, much of the collisional debris fell back to the growing planetesimal surface. The planetesimal interior was still molten, so a steep thermal gradient quickly developed through the insulating blanket of chondritic debris, and metamorphism ensued. This scenario seems reconcilable with most, possibly all, petrographic, chemical, and isotopic constraints imposed by observations in chondrites. The collision scenario is consistent with three important constraints. It yields chondrules with high efficiency, in sufficient quantities to retard radiative heat loss, and with a restricted range of subliquidus temperatures. Total disruption of the smaller molten body would tend to re-mix segregated metal and silicate and restore primitive chemistry on a small scale. The metal core of the larger body would tend not to re-mix. An attractive feature of the model is that a single heat source, namely radiogenic heat stored as thermal energy inside planetesimals, accounts both for chondrule formation and for later metamorphism. The model simplifies the metamorphic regime; it circumvents the narrow time window during which in-situ radioactive decay would be effective, and it can accommodate either hot or cold accretion.
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