CAIs mineral composition

CAI s that were once molten (type B and compact type A) apparently crystallized under conditions where both partial pressures and total pressures were low because they exhibit marked fractionation of Mg isotopes relative to chondritic isotope ratios. But much remains to be learned from the distribution of this fractionation. Models and laboratory experiments indicate that Mg, O, and Si should fractionate to different degrees in a CAI (Davis et al. 1990 Richter et al. 2002) commensurate with the different equilibrium vapor pressures of Mg, SiO and other O-bearing species. Only now, with the advent of more precise mass spectrometry and sampling techniques, is it possible to search for these differences. Also, models prediet that there should be variations in isotope ratios with growth direction and Mg/Al content in minerals like melilite. Identification of such trends would verify the validity of the theory. Conversely, if no correlations between position, mineral composition, and Mg, Si, and O isotopic composition are found in once molten CAIs, it implies that the objects acquired their isotopic signals prior to final crystallization. Evidence of this nature could be used to determine which objects were melted more than once. 

Many more high-precision whole-rock analyses of both Mg and O isotope ratios for chondrite components would help to establish whether or not the mixing trends in Figure 15 are valid, or even if the concept of mixing is useful. Bulk objects are desirable because their isotopic compositions are not affected by secondary inter-mineral exchange reactions that are know to be important for both the Mg and O isotopic systems in CAIs and chondrules. 

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). 

Consider first the CAIs. In general, their bulk compositions are consistent with those calculated for the first 5% of condensable matter (Davis and Richter, 2004). Moreover, many of the minerals that comprise CAIs (hibonite, perovskite, spinel, melilite, diopside, anor-thite) are predicted to have been the earliest condensed phases. However, not all CAIs are... 

Oxygen isotopic compositions of minerals in CAIs on an oxygen three-isotope diagram. Axes are given in delta notation 81S0 =[((180/160)sampie/(180/160)s,andard)-1] x 1000, and similarly for S170. After Clayton et al. (1977). 

Oxygen isotopic compositions (relative to standard mean ocean water, SMOW) of several mineral grains in three Stardust particles, each grain denoted by a separate symbol. Open circles are from a CAI comet particle. After McKeegan et al. (2006). 

Bulk chemical compositions of CAIs, AOAs, and chondrules, compared to trends for calculated total condensed solids formed by equilibrium condensation of a hot solar gas at two different pressures. Condensed minerals are corundum (Cor), hibonite (Hib), end members of melilite series gehlenite (Geh) and akermanite (Ak), anorthite (An), diopside (Di), and forsterite (Fo). Modified from MacPherson etal. (2005). 

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... 

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. 

Aluminum-rich chondrules are a broad class of objects with compositions intermediate between those of CAIs and the more common ferromagnesian chondrules. Their bulk compositions are generally Mg-, Si-rich and Ca-, Al-poor relative to most CAIs. The most abundant minerals are usually olivine, orthopyroxene, and... 

Figure 6 Oxygen-isotopic compositions of individual minerals in CAIs from the CO chondrites Y-81020, Colony, Kainsaz and Ornans. Primary minerals in CAIs from the least metamoprhosed CO chondrites Y-81020 (type 3.0) and Colony (3.0) are uniformly 0-enriched, whereas CAIs from Kainsaz (3.2) and Omans (3.3) tend to show oxygen isotopic heterogeneity with spinel and high-calcium pyroxene enriched in 0 and melilite and secondary nepheline depleted in 0. Based on these observations, Wasson et al (2001) inferred that oxygen isotope exchange took place during thermal metamorphism and alteration in an asteroid (data from Itoh et al, 2000 Wasson et al, 2001).

Figure 7 (a,b) Oxygen-isotopic compositions of individual minerals in CAIs from CR chondrites (data from Aleon et al, 2002a). Most CAIs are °0-rich and isotopically... 

Figure 15 Oxygen-isotopic compositions of individual minerals in (a) aluminum-rich chondrules from ordinary chondrites (Russell et at, 2000) and (h) CR carhonaceous chondrites (Krot et ah, 2002h). (c) Oxygen-isotopic compositions of individual minerals in the CAI-hearing chondmle 17 from Acfer 094 (data from Krot et al, 2003a) (Ahhreviations chd, chondmle cpx, clinopyroxene gl, glass hih, hihonite nph, nepheline ol, olivine opx, orthopyroxene pi, plagioclase px, pyroxene sp, spinel).

Matrix material is best defined as the optically opaque mixture of mineral grains 10 nm to 5 p.m in size that rims chondrules, CAIs, and other components and fills in the interstices between them (Scott et al, 1988). Matrix grains are generally distinguished from fragments of chondrules, CAIs and other components by their distinctive sizes, shapes, and textures. Minerals found in matrices include silicates, oxides, sulfides, metallic Fe,Ni, and especially in type 2 chondrites, phyllosilicates and carbonates (Table 5). Matrices are broadly chondritic in composition though richer in FeO than chondrules and have refractory abundances that deviate more from bulk chondrite values (McSween and Richardson, 1977 Brearley, 1996). Matrix typically accounts for 5 -50 vol.% of the chondrite (Table 1). 

Figure 6 Oxygen-isotopic compositions of CV chondrites and their dark inclusions plotted on a three-isotope diagram. Bulk CVS chondrites plot close to the CCAM line defined by the isotopic composition of anhydrous minerals from CAIs in Allende. The reduced CV chondrites are isotopically lighter than the oxidized CV chondrites such as Allende. Dark inclusions from Allende also plot on the CCAM line, whereas inclusions from the reduced CV chondrites, Vigarano, Leoville, and Efremovka have oxygen-isotopic compositions that show significant heavy isotope enrichments, indicative of aqueous alteration (source Krot et al., 1999).

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