APPENDIX

Presolar refractory phases (oxides and silicates) with high vaporization temperatures in excess of ~1350 °K (the approximate condensation temperature of forsterite) began to condense near the midplane of the hot primordial nebula outward to ~3 AU during the rapid infall stage which began over 4,568 m.y. ago. Mineral condensation progressed in the following sequence: corundum => hibonite => perovskite => grossite => melilite => spinel => diopside => forsterite => anorthite. The condensation of these refractory minerals and the formation of CAIs persisted for ~1 m.y. During this time interval, radiogenic 26Al was introduced into the solar nebula by one or more supernovae (Ciesla and Yang, 2010). Many of these refractory minerals, specifically those which formed within the initial 100,000 years, survived (perhaps within planetesimals) as primary condensates of a dust-enhanced (10 × solar gas composition which was 16O-rich: Δ17O ≈ Δ18O ~–60‰ to –40‰) nebular gas, having properties of variable but low pressure, variable but high temperature, and a reduced environment (conditions attested to by volatility fractionated REEs). Episodic melting of these CAIs occurred over the next 300,000 years, with some experiencing remelting in the chondrule-forming region at least 900,000 years after initial formation (MacPherson et al., 2010).

Some of the earliest refractory phases (hibonite-bearing) formed prior to the incorporation and mixing of 26Al into the solar nebula and are present in CM chondrites (no resolvable 26Mg-excesses; e.g., platy-crystals [PLACs] and blue aggregates [BAGs]), while others do show evidence of in situ 26Al decay (e.g., spinel–hibonite spherules [SHIBs] and calcium–aluminum-rich inclusions [CAIs]). CAIs may be direct condensates or evaporative residues formed from one or more episodes of rapid heating and slow cooling of precursor dust. This process continued over a time span as short as 20–100 t.y. (consistent with an FU-Orionis outburst, see below) (Krot et al., 2009; Wurm and Haack, 2009, and references therein), which is datable by the Hf–W and U–Pb systems to 4,568.3 (±0.7) m.y. ago relative to the angrites D'Orbigny, NWA 4590, and NWA 4801 (Burkhardt et al., 2008; Nyquist et al., 2009). This age is in agreement with that obtained from Allende CAIs. It was calculated that initial nebular condensation processes account for 80% of the refractory element enrichment (e.g. Ca, Al) in type A and type B CAIs, while 20% is due to subsequent evaporation of more volatile elements (e.g. Mg, Si) (Grossman et al., 2008). Evidence for an early, instantaneous, impact-generated origin for CAIs has been presented by Sanders (2008).

Following formation near the proto-Sun, many of these refractory minerals were transported radially outward by turbulent diffusion mechanisms to chondritic accretionary regions. Others may have been confined to the protoplanetary disk embedded in the center of spiral arms (Haghighipour and Boss, 2003), or possibly transported to cooler heliocentric regions (2–5 AU) by bipolar outflows (x-wind) or by photophoresis, a force created in response to an ~100-fold increase in the Sun's luminosity during an FU-Orionis outburst—an event resulting from an enhanced accretion rate within ~1 AU of the central star (Wurm and Haack, 2009). When cm-sized CAIs are fully illuminated in an optically thin region of the disk, they are transported vertically and radially outward along the surface of the (flared) protoplanetary disk following a temperature gradient from hot to cold. Calculations show that CAIs measuring up to 1 cm in size, representing a total mass of 0.005 Earth masses, could have been easily transported to the asteroid belt at a distance of ~3 AU, and some may have exceeded 10 AU. Here the condensation sequence was arrested and these minerals remained stable against gas drag and the accretionary influence of the Sun for at least 1 m.y. Finally, they rained down to the nebular midplane where they eventually coalesced with newly forming chondrules to form the nascent chondritic planetesimals. Some km-sized planetesimals, such as the parent bodies of some iron meteorites and that of the basaltic meteorite Asuka 881394, have been determined to have very ancient ages indicating they had accreted contemporaneously with CAI formation (Wadhwa et al., 2009).

CAIs are particularly abundant in the CV-group of carbonaceous chondrites, but they also occur in the other carbonaceous chondrite groups, in K chondrites, in ordinary and enstatite chondrites, and have been identified in comet samples from NASA's STARDUST mission (81P/Wild-2). Wark–Lovering monomineralic rims commonly occur on most all CAI types, reflecting episodic flash heating events in the solar nebula. These energetic events are often cited as being associated with magnetic reconnectiion flares. These events resulted in the volatilization of Mg, Si, and Ca from the outermost layer of the CAI, followed by the diffusion of elements, possibly derived from accetionary forsterite dust (which is texturally and mineralogically similar to AOA forsterite), back onto the surface of the CAI. Alternatively, the formation of Wark–Lovering rims may be attributed to relatively slow evaporation from solid CAIs. Additional information on CAI formation can be found on the Allende page.

CAIs were originally grouped as 'coarse-grained' and 'fine-grained' inclusions (Grossman, 1975). However, continued studies have led to further refinement in their classification, with an emphasis on those from the CV group. Coarse-grained CAIs have been classified into three main groups (A, B, and C) based primarily on the proportions of fassaite and melilite, the latter corresponding to the series with Ca-rich end member åkermanite and Al-rich end member gehlenite. Other characteristic phases include spinel, hibonite, perovskite, and anorthite.

NWA 2086, CV3, 46 g end section with large CAI
Photo courtesy of Dr. Martin Horejsi

COARSE-GRAINED CAIs