AOAs melting

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. 

Other components of chondrites such as AOAs (>90+ vol% forsterites), their cousins, and aggolmeratic chondrules have all experienced some level of melting, albeit very minor in most cases (Weisberg et al. 2003 Zanda et al. 2005). The reasons why AOAs were not melted extensively like chondrules and Type-B CAIs is unknown and no quantitative constraints on their thermal histories exist. 

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... 

Although AOAs show no clear evidence of being melted, some appear to contain CAIs that have been melted (Figure 10). AOAs in CO3.0 chondrites contain 8 vol.% metallic Fe,Ni troilite is rare or absent (Chizmadia et al, 2002). 

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. 

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