COMPOUND CHONDRULES

Abstract

The properties of compound chondrules offer information about (1) the local density of solid matter at the moment of chondrule formation, (2) the mean number of flash heating events experienced by chondrules, and (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 combined surface area of 79 cm2; compositions of the mafic minerals were determined in fifty-six compound-chondrule sets. With rare exceptions, textural evidence shows that, at the time compound chondrules fused together, one chondrule (the primary) was a spheroid rigid enough to retain its shape, and the other (the secondary) was molten. On the basis of textural and compositional criteria we classify compound chondrules either as siblings (57%), which have closely similar textures and compositions, or independents (43%), which have textures or compositions inconsistent 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 pyroxene (RP), with most of the remainder being barred olivine (BO) or cryptocrystalline (C). Among the independent compound chondrules, BO accounts for 38% and the three porphyritic types 38% of the primaries; the remaining 24% are RP. The independent secondaries are mainly BO, RP, and C.We interpret the evidence to indicate that sibling compound chondrules formed in the same flash heating event and collided as a result of turbulent motions before their secondaries solidified. This seems to require simultaneous formation as small (centimeter-size) clouds of chondrules. Although a small fraction of independent chondrules were produced by random collisions while molten, the mean time between particle collisions is much too large (hours or longer) for this model to have 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 independent primaries and secondaries are much more similar than would be 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 combined with the drift in the composition of nebular solids towards higher FeO contents in silicates, enhanced by the lower liquidus temperatures of FeO-rich silicates. In addition, a tendency for low-FeO olivine-normative independent primaries to have low-FeO secondaries suggests that some chondrules became isolated and did not participate in the final epoch of chondrule-forming flash-heating events.