COMPOUND CHONDRULES

The properties of compound chondrules offer information about (I) the local density of solid matter at the moment of chondrule formation, (2 ) the mean number of flash heating events experienced by chondrules, a nd (3) the physical and chemical state of solid matter when chondrule formation was occurring. We examined eighty-three compound chondrules in seventy-nine ordinary chondrite (OC) thin sections having a combine d surface area of 79 cm(2); compositions of the mafic minerals were de termined in fifty-six compound-chondrule sets. With rare exceptions, t extural evidence shows that, at the time compound chondrules fused tog ether, one chondrule (the primary) was a spheroid rigid enough to reta in its shape, and the other (the secondary) was molten. On the basis o f textural and compositional criteria we classify compound chondrules either as siblings (57%), which have closely similar textures and comp ositions, or independents (43%), which have textures or compositions i nconsistent with the individual chondrules having formed from the same batch of melt. We estimate that 1.4% of all OC chondrules are sibling and 1.0% are independent compound chondrules. Among siblings the most common textural class of both primary and secondary is radial pyroxen e (RP), with most of the remainder being barred olivine (BO) or crypto crystalline (C). Among the independent compound chondrules, BO account s for 38% and the three porphyritic types 38% of the primaries; the re maining 24% are RP. The independent secondaries are mainly BO, RP, and C. We interpret the evidence to indicate that sibling compound chondr ules formed in the same flash heating event: and collided as a result of turbulent motions before their secondaries solidified. This seems t o require simultaneous formation as small (centimeter-size) clouds of chondrules. Although a small fraction of independent chondrules were p roduced by random collisions while molten, the mean time between parti cle collisions is much too large (hours or longer) for this model to h ave general validity. Much more plausible is that independents formed by the mechanism commonly accepted for the formation of relict grains: flash heating of a porous aggregate of small particles containing an embedded primary chondrule. Mafic mineral compositions in conjugate in dependent primaries and secondaries are much more similar than would b e expected on the basis of random associations. Simulations show that much of this similarity can be attributed to the small fraction of low -FeO secondaries. The latter reflects later formation of secondaries c ombined with the drift in the composition of nebular solids towards hi gher FeO contents in silicates, enhanced by the lower liquidus tempera tures of FeO-rich silicates. In addition, a tendency for low-FeO olivi ne-normative independent primaries to have low-FeO secondaries suggest s that some chondrules became isolated and did not participate in the final epoch of chondrule-forming hash-heating events.