An amoeboid olivine inclusion (AOI) in CK3 NWA 1559, comparison to AOIs in CV3 Allende,and the origin of AOIs in CK and CV chondrites

An amoeboid olivine inclusion in CK3 NWA 1559 (0.54 × 1.3 mm) consists of a diopside‐rich interior (approximately 35 vol%) and an olivine‐rich rim (approximately 65 vol%). It is the first AOI to be described in CK chondrites; the apparent paucity of these inclusions is due to extensive parent‐body recrystallization. The AOI interior contains irregular 3–15 μm‐sized Al‐bearing diopside grains (approximately 70 vol%), 2–20 μm‐sized pores (approximately 30 vol%), and traces of approximately 2 μm plagioclase grains. The 75–160 μm‐thick rim contains 20–130 μm‐sized ferroan olivine grains, some with 120º triple junctions. A few coarse (25–50 μm‐sized) patches of plagioclase with 2–18 μm‐thick diopside rinds occur in several places just beneath the rim. The occurrence of olivine rims around AOI‐1 and around many AOIs in CV3 Allende suggests that CK and CV AOIs formed by the acquisition of porous forsteritic rims around fine‐grained, rimless CAIs that consisted of diopside, anorthite, melilite, and spinel. Individual AOIs in carbonaceous chondrites may have formed after transient heating events melted their olivine rims as well as portions of the underlying interiors. In AOI‐1, coarse plagioclase grains with diopside rinds crystallized immediately below the olivine rim. Secondary parent‐body alteration transformed forsterite in the rims of CV and CK AOIs into more‐ferroan olivine. Some of the abundant pores in the interior of AOI‐1 may have formed during aqueous alteration after fine‐grained melilite and anorthite were leached out. Chondrite groups with large chondrules tend to have large AOIs. AOIs that formed in dust‐rich nebular regions (where CV and CK chondrites later accreted) tend to be larger than AOIs from less‐dusty regions.