Origin of low-Ca pyroxene in amoeboid olivine aggregates: Evidence from oxygen isotopic compositions

Amoeboid olivine aggregates (AOAs) in primitive carbonaceous chondrites consist of forsterite (Fa(< 2)), Fe,Ni-metal, spinel, Al-diopside, anorthite, and rare gehlenitic melilite (angstrom k(< 15)) similar to 10% of AOAs contain low-Ca pyroxene (Fs(1-3)Wo(1-5)) that is in corrosion relationship with forsterite, and is found in three major textural occurrences: (i) thin (< 15 mu m) discontinuous layers around forsterite grains or along forsterite grain boundaries in AOA peripheries; (ii) 5-10-mu m-thick haloes and subhedral grains around Fe,Ni-metal nodules in AOA peripheries, and (iii) shells of variable thickness (up to 70 mu m), commonly with abundant tiny (3-5 mu m) inclusions of Fe,Ni-metal grains, around AOAs. AOAs with the low-Ca pyroxene shells are compact and contain euhedral grains of Al-diopside surrounded by anorthite, suggesting small (10%-20%) degree of melting. AOAs with other textural occurrences of low-Ca pyroxene are rather porous. Forsterite grains in AOAs with low-Ca pyroxene have generally O-16-rich isotopic compositions (Delta(17)O < -20%). Low-Ca pyroxenes of the textural occurrences (i) and (ii) are O-16-enriched (Delta(17)O < -20%), whereas those of (iii) are O-16-depleted (Delta(17)O = -6% to -4%). One of the extensively melted (> 50%) objects is texturally and mineralogically intermediate between AOAs and Al-rich chondrules. It consists of euhedral forsterite grains, pigeonite, augite, anorthitic mesostasis, abundant anhedral spinel grains, and minor Fe,Ni-metal; it is surrounded by a coarse-grained igneous rim largely composed of low-Ca pyroxene with abundant Fe,Ni-metal-sulfide nodules. The mineralogical observations suggest that only spinel grains in this igneous object were not melted. The spinel is O-16-rich (Delta(17)O similar to -22%), whereas the neighboring plagioclase mesostasis is O-16-depleted (Delta(17)O similar to -11%). We conclude that AOAs are aggregates of solar nebular condensates (forsterite, Fe,Ni-metal, and CAls composed of Al-diopside, anorthite, spinel, and +/- melilite) formed in an O-16-rich gaseous reservoir, probably CAI-forming region(s). Solid or incipiently melted forsterite, in some AOAs reacted with gaseous SiO in the same nebular region to form low-Ca pyroxene. Some other AOAs appear to have accreted O-16-poor pyroxene-normative dust and experienced varying degrees of melting, most likely in chondrule-forming region(s). The most extensively melted AOAs experienced oxygen isotope exchange with O-16-poor nebular gas and may have been transformed into chondrules. The original O-16-rich signature of the precursor materials of such chondrules is preserved only in incompletely melted grains. Copyright (c) 2005 Elsevier Ltd.