Amoeboid olivine aggregates

The amoeboid descriptor for amoeboid olivine aggregates refers to their irregular shapes. AOAs tend to be fine-grained and porous, and have comparable sizes to CAIs in the same meteorite. They consist mostly of forsterite and lesser amounts of iron-nickel metal, with a refractory component composed of anorthite, spinel, aluminum-rich diopside, and rarely melilite. The refractory component is sometimes recognizable as a CAI embedded within the AOA. The AOAs show no evidence of having been melted, but some contain CAIs that have melted. 

Sugiura, N. and Krot, A.N. (2007) 26Al-26Mg systematics of Ca-Al-rich inclusions, amoeboid olivine aggregates and chondrules form the ungrouped carbonaceous chondrite Acfer 094. Meteoritics and Planetary Science, 42, 1183-1195. 

The most primitive chondrites consist of coarse-grained (mm-sized) mineral assemblages embedded in fine-grained (10 nm-5 pm) matrix material (see Fig. 1.2). The coarse-grained chondritic components are diverse in their composition and mineralogy and include calcium-aluminum-rich inclusions (CAIs), amoeboid olivine aggregates (AOAs), Al-rich chondrules, Fe-Mg chondrules, Fe-rich metals, and iron sulfides. The CAIs are composed largely of calcium, aluminum, and titanium... 

Chondrites consist of four major components chondrules, FeNi-metal, refractory inclusions (Ca-Al-rich inclusions (CAIs) and amoeboid olivine aggregates (AOAs)), and fine-grained matrix material. It is generally accepted that the refractory inclusions, chondrules, and FeNi-metal are formed in the solar nebula by high-temperature processes that included condensation and evaporation. Many CAIs and most chondrules and FeNi-metal were subsequently melted during multiple brief heating episodes. Matrix, some CAIs, and metal in some chondrites (e.g., CH and CB) appear to have escaped these high-temperature nebular... 

Aleon J., Krot A. N., and McKeegan K. D. (2002) Ca-Al-rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites. Meteorit. Planet. Sci. 37, 1729-1755. 

Another argument that the 0-rich end-member was a ubiquitous component of primitive solids is that it is found in many different chemical forms (different minerals) in many classes of meteorites CAls and amoeboid olivine aggregates (AOAs) from Efremovka (CV3) (Aleon et al., 2002 Fagan et al., 2002), AOA from a CO chondrite, Y 81020 (Itoh et al, 2002), and CAl and AOA from CM and CR chondrites (Krot et al., 2002). This isotopic composition can also serve as an end-member for the chondrule mixing line in Figure 2. 

There are two types of refractory inclusions calcium- and aluminum-rich inclusions (this section) and amoeboid olivine aggregates (Section 1.07.5.3). Since the mineralogy, chemistry and isotope chemistry of refractory inclusions were reviewed by MacPherson et al. (1988), many new analyses have been made of CAIs in CV, CM, CO, CR, CH, CB, ordinary and enstatite chondrites that provide important constraints on physicochemical conditions, time, and place of CAI formation. CAIs are addressed in detail in Chapter 1.08, the role of condensation and evaporation in their formation in Chapter 1.15, and their clues to early solar system chronology in Chapter 1.16. 

Amoeboid olivine aggregates (AOAs) are irregularly shaped objects with fine grain sizes... 

Figure 25 Backscattered electron images showing fine-grained matrix rims on (a) Ca-Al-rich inclusion (CAT), and (b) an amoeboid-olivine aggregate (AOA) in ALHA77307. The light-gray rims contain FeO-rich silicate material and are crossed by dark cracks arrows mark the outer edge of the rims.

Fagan T. J., Yurimoto H., Krot A. N., and Keil K. (2002b) Constraints on oxygen isotopic evolution from an amoeboid olivine aggregate and Ca, Al-rich inclusion from the CV3 Efremovka. In Lunar Planet. Sci. XXXIII, 1507. The Lunar and Planetary Institute, Houston (CD-ROM). 

Grossman L. and Steele 1. M. (1976) Amoeboid olivine aggregates in the AUende meteorite. Geochim. Cosmochim. Acta 40, 149-155. 

Grossman L., Ganapathy R., Methot R. L., and Davis A. M. (1979) Trace elements in the AUende meteorite amoeboid olivine aggregates. Geochim. Cosmochim. Acta 43, 817-829. 

Hashimoto A. and Grossman L. (1987) Alteration of Al-rich inclusions inside amoeboid olivine aggregates in the Allende meteorite. Geochim. Cosmochim. Acta 51, 1685-1704. 

Komatsu M., Krot A. N., Petaev M. I., Ulyanov A. A., Keil K., and Miyamoto M. (2001) Mineralogy and petrography of amoeboid olivine aggregates from the reduced CV3 chondrites Efremovka, LeoviUe, and Vigarano products of nebular condensation, accretion, and annealing. Meteorit. Planet Sci. 36, 629-641. 

Hashimoto A. and Grossman L. (1987) Alteration of Al-rich inclusions inside amoeboid olivine aggregates in the AUende meteorite. Geochim. Cosmochim. Acta 51, 1685-1704. Hinton R. W. and Bischoff A. (1984) Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites. Nature 308, 169-172. 

Chondrites, the most primitive of all meteorites, formed in dynamic energetic, dust-rich zones in the solar nebula. In this environment, dust/gas ratios were constantly changing, temperatures fluctuated through 1,000 K, with multiple cycles of melting, evaporation, condensation, and aggregation. In addition there were influxes of matter from the interstellar dust and the periodic removal of batches of chondritic material to small planetesimals. In this section we explore how the most primitive materials of the solar system were formed and what they can tell us about processes during the condensation of the solar nebula. These materials include chondrules, refractory inclusions (CAIs), and amoeboid olivine aggregates (AOAs), the oldest component parts of chondritic meteorites. 

---