Nebular history of amoeboid olivine aggregates

Abstract

Abstract-Minor element (Ca, Cr, and Mn) concentrations in amoeboid olivine aggregates (AOAs) from primitive chondrites were measured and compared with those predicted by equilibrium condensation in the solar nebula. CaO concentrations in forsterite are low, particularly in porous aggregates. A plausible explanation appears that an equilibrium Ca activity was not maintained during the olivine condensation. CaO and MnO in forsterite are negatively correlated, with CaO being higher in compact aggregates. This suggests that the compact aggregates formed either by a prolonged reheating of the porous aggregates or by condensation and aggregation of forsterite during a very slow cooling in the 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.…”

“…Forsterite grains in Adelaide AOAs that lack low-Ca pyroxene are nearly pure (FeO <1 wt%) with only small amounts of Cr 2 O 3 (<0.3 wt%) and MnO (<0.07 wt%), whereas those in pyroxene-bearing AOAs are richer in FeO (1-4 wt%), Cr 2 O 3 (0.2-0.6 wt%), and MnO (up to 0.8 wt%) (Krot et al, 2004b). Manganese and chromium concentrations in AOAs in a CO3.0 chondrite (Yamato 81020) appear to be higher in fractured or peripheral regions and lowest in AOAs with high CaO concentrations (0.2-0.8 wt%) (Sugiura et al, 2009). This suggests that both manganese and chromium were introduced by gas-solid reactions after Ca-rich forsterite condensed, consistent with thermodynamic models (e.g., Krot et al, 2004a;Sugiura et al, 2009).…”

“…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.…”

“…Manganese and chromium concentrations in AOAs in a CO3.0 chondrite (Yamato 81020) appear to be higher in fractured or peripheral regions and lowest in AOAs with high CaO concentrations (0.2-0.8 wt%) (Sugiura et al, 2009). This suggests that both manganese and chromium were introduced by gas-solid reactions after Ca-rich forsterite condensed, consistent with thermodynamic models (e.g., Krot et al, 2004a;Sugiura et al, 2009).…”

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

“…Most carbonaceous (C) chondrites contain lumpy, olivine-rich inclusions up to a few millimeters across known as amoeboid olivine aggregates (AOAs) (Grossman, 1974;Grossman and Steele, 1976;Grossman et al, 1979;Krot et al, 2004a) or amoeboid olivine inclusions (AOIs) (Rubin, 1998;Chizmadia et al, 2002), which formed primarily as aggregates of fine-grained condensates, with or without the involvement of sintering and small-scale melting (e.g., Grossman and Steele, 1976;Wark, 1979;Kornacki and Wood, 1984;Komatsu et al, 2001;Krot et al, 2004a,b,c;Weisberg et al, 2004;Scott and Krot, 2005;Sugiura et al, 2009). They contain variable proportions of different mineral assemblages that appear to record various formation conditions, including: (a) olivine + metal (e.g., Fig.…”