CHONDRULE FORMATION BY RADIATIVE HEATING - A NUMERICAL-MODEL

Based on computer simulations of the thermal behavior of chondrules an d aggregates of dust as well as on existing observational and experime ntal evidence, we propose a model for the formation of chondrules by e lectromagnetic (EM) radiation. Heating occurred primarily by the absor ption of similar to 0.3- to 8-mu m EM radiation, with peak fluxes near similar to 5 x 10(6) W m(-2) and heating durations between 10(3) and 10(5) sec. Chondrules were produced from aggregates of dust having siz e distributions similar to those predicted by models of dust agglomera tion, i.e., an increasing abundance of aggregates with decreasing aggr egate size. Size-dependent heating resulted in an underabundance of sm all (<50 mu m in diameter) chondrules. The paucity of large chondrules (>3 mm in diameter) reflects the low abundance of large precursor dus t aggregates. Dust aggregates rich in metals and sulfides absorbed lig ht more efficiently than those composed purely of silicates, resulting in smaller mean sizes for chondrules having higher densities. Higher radiative fluxes resulted in higher peak chondrule temperatures, small er mean chondrules sizes, and a greater proportion of chondrules havin g nonporphyritic textures. Small unmelted grains and grain aggregates coexisted with molten chondrules; temperature differences between coex isting mu m-size grains and mm-size chondrules may have exceeded sever al hundred K. The continual supply of seed crystals by the incorporati on of solid grains into molten chondrules inhibited the formation of c hondrules having textures characteristic of complete melting. As chond rules cooled and solidified, chondrule seeding graded into formation o f ''dusty'' rims, with the efficiency of rim formation inversely propo rtional to the smallest size of chondrules produced. Small unmelted gr ains and aggregates in chondrule-forming regions ultimately contribute d to the dusty matrix of chondrites. (C) 1995 Academic Press, Inc.