The Frequency of Compound Chondrules and Implications for Chondrule

Introduction: Among the many properties of chondrules, compound chondrules, two chondrules fused together, have been studied to gain clues as to what the environment that chondrules formed in was like [1,2]. Two methods have been proposed for forming compound chondrules: collisions among individual chondrules while they were plastic [1] and melting of porous aggregates on an already existing primary chondrule [2]. If we can distinguish between these two scenarios, we will be able to gain further insight into how chondrules formed. Previous Studies: Gooding and Keil [1] examined and classified chondrules in thin-section and by removal as whole pieces from meteorites. They concluded that approximately 4% of all chondrules are compounds, with compounds being more common among non-porphyritic chondrules (those that melted completely) than porphyritic. Based on their collisional modeling, these authors concluded that non-porphyritic chondrules were formed in regions of the nebula where the concentration of chondrule precursors was 10 -10 times greater than it was where porphyritic chondrules formed. Wasson et al. [2] examined approximately 10,000 chondrules in thin-section and identified 80 compound chondrules, of which they reported the sizes, contact angles, and textures of the compound components. In addition, these authors divided the types of compound chondrules into three groups: adhering, consorting, and enveloping. These authors acknowledged that they likely were missing some compound chondrules owing to their use of thin-sections, and thus multiplied their statistics by a factor of 3 to conclude that 2.4% of all chondrules are compounds. This factor was derived from the difference in population of compound chondrules observed in thin-section and by removal as whole pieces from meteorites by [1]. Thin-Section Biases: Because thin-section cuts will slice through compound chondrules with random orientations, it is possible that the thin-section will cut through the compound chondrule in such a manner that an observer would not be able to identify it as a compound. We have derived formulae for calculating the probability that a thin-section would cut an ahdering compound chondrule across the area of contact for the two components, allowing it to be identified. Averaged over all possible geometries, roughly 25% of all adhering and consorting chondrules and 50% of all enveloping chondrules would be identified in thin-section studies. However, the probability that we derived for the adhering and consorting compounds depends on the contact angle between the two components. The smaller the contact angle, the less likely the thin-section cut would intersect it. Figure 1 shows the distribution of contact angles for these compounds as measured by [2]. We can correct each bin in the histogram by dividing the number of compounds in that bin by the probability that it would be detected by observation in thin-section. The results of this correction are shown in Figure 2. Summing the number of compounds in each bin in Figure 2 gives 530 compound chondrules, which suggests that approximately 5% of all chondrules are adhering or consorting compounds. The number of enveloping chondrules does not significantly change this total. Figure 2 also strengthens the observation made by [2] that small contact angles are more common among adhering and consorting chondrules. In addition, we calculate, based on the statistics of [2], that the compounds can be broken up into three categories based on the textures of their components: 71% are non-porphyriticnon-porphyritic, 4% are porphyritic-porphyritic, and 25% are mixed. Furthermore, of the secondaries (the most deformed component of the compounds), 92% are non-porphyritic and 8% are porphyritic. Thus, the obsevation that non-porphyritic chondrules are more frequently found as compounds is preserved in this study.