Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka, Leoville and Vigarano: Products of nebular condensation, accretion and annealing

Abstract

Amoeboid olivine aggregates (AOAs) from the reduced CV chondrites Efremovka, Leoville and Vigarano are irregularly-shaped objects, up to 5 mm in size, composed of forsteritic olivine (Fa<10) and a refractory, Ca, Al-rich component. The AOAs are depleted in moderately volatile elements (Mn, Cr, Na, K), Fe, Ni-metal and sulfides and contain no low-Ca pyroxene. The refractory component consists of fine-grained calcium-aluminum-rich inclusions (CAIs) composed of Al-diopside, anorthite (An100), and magnesium-rich spinel (ε11 wt% FeO) or fine-grained intergrowths of these minerals; secondary nepheline and sodalite are very minor. This indicates that AOAs from the reduced CV chondrites are more pristine than those from the oxidized CV chondrites Allende and Mokoia. Although AOAs from the reduced CV chondrites show evidence for high-temperature nebular annealing (e.g., forsterite grain boundaries form 120° Triple junctions) and possibly a minor degree of melting of Al-diopside-anorthite materials, none of the AOAs studied appear to have experienced extensive (.50%) melting. We infer that AOAs are aggregates of high-temperature nebular condensates, which formed in CAI-forming regions, and that they were absent from chondrule-forming regions at the time of chondrule formation. The absence of low-Ca pyroxene and depletion in moderately volatile elements (Mn, Cr, Na, K) suggest that AOAs were either removed from CAI-forming regions prior to condensation of these elements and low-Ca pyroxene or gas-solid condensation of low-Ca-pyroxene was kinetically inhibited.