Amoeboid olivine aggregates (AOAs) in the Efremovka, Leoville and Vigarano (CV3) chondrites: A record of condensate evolution in the solar nebula

Chondrites and Their Components

“…AOAs are similar in size to coaccreted chondrules and CAIs being mostly 100-500 mm in size in CO (Chizmadia et al, 2002), up to 5 mm in size in CV (Komatsu et al, 2001), and typically <500 mm in CR chondrites (Aléon et al, 2002). In the least-altered chondrites, they are porous aggregates and their mineralogy matches that expected for high-temperature nebular condensates (Krot et al, 2004bSugiura et al, 2009;Ruzicka et al, 2012a). Unlike CAIs and chondrules, AOAs do not appear to show mineralogical and isotopic differences between groups and thus provide an excellent guide to the alteration history of the meteorite and its constituents.…”

“…AOAs in CO3.0 chondrites contain $8 vol.% metallic Fe,Ni: troilite is rare or absent (Chizmadia et al, 2002). The refractory objects in AOAs are characterized by significant variations in size (1-250 mm), shape (irregular, rounded), distribution (uniform, heterogeneous), modal mineralogy (spinel-rich and spinel-poor), abundance (rare and abundant) (Figure 11), and rare earth element abundances (Ruzicka et al, 2012a). Forsterite grains typically contain numerous pores and tiny inclusions of aluminum-diopside.…”

“…Trace elements concentrations have been measured in AOAs from Allende by INAA (Grossman et al, 1979) and AOAs from Efremovka, Leoville, and Vigarano by secondary ion mass spectrometry (SIMS) (Ruzicka et al, 2012a). Most AOAs have unfractionated abundances of refractory lithophile and siderophile elements ((2-20)ÂCI); some AOAs have group II REE patterns.…”

“…Most AOAs have unfractionated abundances of refractory lithophile and siderophile elements ((2-20)ÂCI); some AOAs have group II REE patterns. Different trace element patterns (group II and non-group II refractory) are sometimes found for Al-rich regions (Ca pyroxene þ anorthite or melilite-bearing) in a given AOA, suggesting that AOAs agglomerated from materials that formed under different conditions and that they did not subsequently homogenize (Ruzicka et al, 2012a).…”

“…The mineralogy, petrology, 16 O-rich compositions, depletion in moderately volatile elements, such as manganese, chromium, and sodium, and the general absence of low-calcium pyroxenes associated with AOAs strongly suggest that AOAs are aggregates of grains of forsterite, aluminum-diopside, spinel, anorthite, and Fe,Ni metal that condensed in the nebula from an 16 Orich gaseous reservoir and aggregated with CAIs that formed earlier (Krot et al, 2004a(Krot et al, ,b, 2005dRuzicka et al, 2012a;Sugiura et al, 2009;Weisberg et al, 2004). Mineralogical and chemical similarities between AOAs and forsterite-rich accretionary rims suggest that AOAs formed contemporaneously with these rims.…”