Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

Abstract

Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CBb chondrite Hammadah al Hamra 237 and none were observed in the CBa chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa<2), Fe, Ni-metal (5–12wt% Ni), and Ca, Al-rich inclusions (CAIs) composed of Al-diopside, spinel, anorthite, and very rare melilite. Melilite is typically replaced by a fine-grained mixture of spinel, Al-diopside, and ±anorthite; spinel is replaced by anorthite. About 10% of AOAs contain low-Ca pyroxene replacing forsterite. Forsterite and spinel are always 16O-rich (δ17,18O∼−40‰ to −50‰), whereas melilite, anorthite, and diopside could be either similarly 16O-rich or 16O-depleted to varying degrees; the latter is common in AOAs from altered and metamorphosed carbonaceous chondrites such as some CVs and COs. Low-Ca pyroxene is either 16O-rich (δ17,18O∼−40‰) or 16O-poor (δ17,18O∼0‰). Most AOAs in CV chondrites have unfractionated (∼2–10×CI) rare-earth element patterns. AOAs have similar textures, mineralogy and oxygen isotopic compositions to those of forsterite-rich accretionary rims surrounding different types of CAIs (compact and fluffy Type A, Type B, and fine-grained, spinel-rich) in CV and CR chondrites. AOAs in primitive carbonaceous chondrites show no evidence for alteration and thermal metamorphism. Secondary minerals in AOAs from CR, CM, and CO, and CV chondrites are similar to those in chondrules, CAIs, and matrices of their host meteorites and include phyllosilicates, magnetite, carbonates, nepheline, sodalite, grossular, wollastonite, hedenbergite, andradite, and ferrous olivine.Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in 16O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in 16O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with 16O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in 16O-poor gaseous reservoirs or coated by 16O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with 16O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids.