Al-rich objects in ordinary chondrites: Related origin of carbonaceous and ordinary chondrites and their constituents

Abstract

Ca-Al-rich inclusions (CAIs), common constituents of carbonaceous chondrites, have been studied extensively because they contain refractory, high-temperature minerals, suggesting that they formed at high temperatures during the early history of the solar system. Few CAIs have previously been discovered in ordinary chondrites. Here we describe 169 Al-rich objects (arbitrarily defined as having ≥ 10 wt.% Al 2O 3) from 24 types 3 and 4 ordinary chondrites, five regolith breccias containing unequilibrated material, the unique meteorite Kakangari, and a few types 5 and 6 ordinary chondrites. Based on shape and texture, we divide Al-rich objects into chondrules (round, with igneous textures), irregularly-shaped inclusions (similar to type F and spinel-rich complex CAIs), and fragments (probably fragments of Al-rich chondrules and inclusions). For descriptive purposes, we further subdivide Al-rich chondrules into compositional subgroups, although they are entirely transitional. Conclusions: 1. a) Al-rich chondrules and irregularly-shaped inclusions and fragments are wide-spread, albeit rare, constituents of ordinary chondrites and are similar to CAIs (except the coarse-grained ones) in carbonaceous chondrites, thus providing strong support for a related origin of ordinary and carbonaceous chondrites and their constituents, 2. b) Al-rich chondrules formed as independent, freely-floating, molten droplets and cooled rather rapidly, 3. c) Al-rich chondrules, like the common Mg-Fe-rich chondrules, formed by melting of millimeter- to submillimeter-sized dust-balls. In the case of Al-rich chondrules, the dust-balls consisted largely of mixtures of variable amounts of at least two solid precursor materials, one refractory-rich and the other volatile-rich. Phases which we interpret as unmelted relicts in some of these objects support this model, 4. d) The wide ranges in bulk compositions of Al-rich chondrules from refractory-rich to volatile-rich ones, yet complete transition between them, and the existence of chondrules transitional in composition between the Al-rich and common Mg-Fe-rich chondrules of ordinary chondrites, suggest that all chondrules formed by related processes. Differences in bulk composition result from mixing of different proportions of refractory-, volatile-, and Mg-Fe-rich (and other) solid precursor materials into dust-balls which were then melted, 5. e) Most irregularly-shaped inclusions show no indication of a once molten origin. Secondary alteration has probably changed the original mineral assemblage. Mainly volatile-rich materials entered the refractory objects as a gas, reacted with the constituents, and produced secondary phases. Rims probably formed both by decomposition and reaction with solar gas as well as by accretion, 6. f) Coarse-grained inclusions as found in Allende were not observed in ordinary chondrites.