A 3-D tomographic survey of compound chondrules in two carbonaceous chondrites : Acfer 139 and Renazzo

Abstract

Compound chondrules (CCs) are two or more chondrules fused or mechanically joined together. Studies of compound chondrules have been conducted in order to understand the chondrule formation process, including the environment and precursor materials. Chondrule formation is still not currently understood and much doubt still remains as to whether the chondrule formation process is nebular and/or planetary However, our evidence suggests that a primary and secondary process are responsible for CCs: (1) semi-molten collisions among individual (I) chondrules (Gooding and Keil, 1981) and (2) Parent-body impacts that cause jostling, fracturing and compaction (Wasson et al., 1995). By understanding chondrule collisions and impacts we can gain greater knowledge into what was happening during early solar system formation, including mean interparticle distances and velocities (Gooding and Keil, 1981). This thesis introduces x-ray tomography as a new method of measuring and surveying CCs; and explores the advantages and limitations of this method. A 3-D tomographic study of two CR carbonaceous meteorites, Acfer139 and Renazzo, reveals a higher frequency of compound chondrules in CR chondrites than in ordinary chondrites. Previous two-dimensional studies of compound chondrules are reexamined and two new categories, touching and aggregate compounds, are also introduced. Our reexamination suggests a 9.6% frequency of compound chondrules (excluding the new categories), twice more than the value of 4% found by Gooding and Keil (1981) and almost 4 times the value of 2.4% found by Wasson et al. (1995) in their studies of ordinary chondrites. The implications of a higher compound chondrule frequency on chondrule formation are also examined.