Calcium-aluminum inclusions compositions

Oxygen 3-isotope plot showing excesses of 160 in minerals from calcium-aluminum inclusions (CAIs). All samples from Earth rocks plot along the terrestrial fractionation line. Mass-dependent fractionation processes cannot move a composition off of this line, so the excesses of 160 were clearly isotopic anomalies. After Clayton et al. (1977). 

Clayton et al. (1973) observed that the high-temperature calcium-aluminum inclusions (CAIs) present in the carbonaceous chondrite Allende possess an oxygen isotope composition of rather than the expected mass-dependent = 0.56 O. It was suggested that this anomalous isotopic composition must derive from a nuclear, rather than chemical process. In general, an equality may arise in... 

Richter FM, Davis AM, Ehel DS, Hashimoto A (2002) Elemental and isotopic fractionation of Type B calcium-, aluminum-rich inclusions Experiments, theoretical considerations, and constraints on their thermal evolution. Geochim Cosmochim Acta 66 521-540 Richter FM, Davis AM, DePaolo DJ, Watson EB (2003) Isotope fractionation by chemical diffusion between molten basalt and rhyolite. Geochim Cosmochim Acta 67 3905-3923 Rudnick RL, Fountain DM (1995) Nature and composition of the continental crust—a lower crustal perspective. Rev Geophys 33 267-309... 

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... 

Ma (Wadhwa et al. 2007 and references therein), which is actually the age of a group of inclusions within chondrites known as calcium-aluminum-rich inclusions (CAIs). The word primitive refers to the fact that the bulk compositions of all chondrites, within a factor of two, are solar in composition for all but the most volatile elements (Weisberg et al. 2006). This fact indicates that chondrites have not been through a planetary melting or differentiation process in their parent body, indicating that they have recorded the materials that were present and the processes that operated within the disk before or during planet formation. 

Our description of meteorite mineralogy starts with the minerals characteristic of the calcium-aluminum-rich inclusions (CAIs). The mineralogy of CAIs varies systematically with their composition. The most Al-rich CAIs contain spinel, hibonite, and/or grossite. More rarely, corundum or calcium mono-aluminate is present. As the bulk composition becomes more Si-rich, the melilite solid solution becomes important. With additional Mg and Si in the bulk composition, fassaite and anorthite are present. 

Scott E. R. D. and Krot A. N. (2001) Oxygen isotopic compositions and origins of calcium-aluminum-rich inclusions and chondmles. Meteorit. Planet. Sci. 36, 1307-1319. 

The refractory condensate model has fallen out of favor, including with Lewis (1988). Nevertheless, it is a useful end-member case. Goettel (1988) calculated the composition of the silicate portion of an ultrarefractory Mercury (Table 2, column 2). This model composition contains no FeO or volatiles, and has large concentrations of the refractory elements—aluminum, calcium, and magnesium. We calculated the thorium and uranium contents of such refractory condensates by assuming chondritic Al/Th and Al/U ratios. A surface of this composition will contain many of the phases in calcium-aluminum-rich inclusions (CAls), such as forsterite, anorthite, spinel, perovskite, hibonite, and melilite. 

Probably the most informative objects in meteorites are the refractory, calcium-aluminum-rich inclusions (CAIs). They are sub-millimeter- to centimeter-sized objects found in all types of primitive (chondritic) meteorites. On the basis of their uranium/lead radiometric ages, they are believed to be the first-formed rocks in the Solar System 4). Their chemical compositions are consistent with equilibrium condensation as solids from a gas of solar composition at high temperatures 1700 K). The major mineral phases are spinel (MgAl204), pyroxene (Mg, Ca, Al, Ti silicate), melilite (another Mg, Ca, A1 silicate), and anorthite (CaAl2Si20s). They are enriched in refractory (less volatile) trace elements, such as the rare-earth elements, by a factor of 15-20 (5), reflecting their high temperature of condensation. The abundances of the three stable isotopes of oxygen exhibit a pattern not seen in any terrestrial rocks (6). On earth, ratios of abundances of isotopes, such as and vary by... 

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