CAIs isotopic composition

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. 

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. 

When differences in the oxygen isotopic composition of CAIs were first measured in 1973, Robert Clayton and his coworkers attributed these mass-independent variations to mixing of normal solar system gas (plotting on or above the terrestrial mass-fractionation... 

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). 

Srinivasan, G., Sahijpal, S., Ulyanov, A. A. and Goswami, J.N. (1996) Ion microprobe studies of Efremovka CAIs II. Potassium isotope compositions and 41Ca in the early solar system. Geochimica et Cosmochimica Acta, 60, 1823-1835. 

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). 

The observation that bulk chondrites are isotopically homogeneous - with the exception of H, C, N, and O - is evidence for a very thorough mixing in an early phase in the hot nebula (> 2000 K). Chondrules themselves provide a variety of constraints on the dust and gas content of their natal environment. Their chemical and isotopic compositions, size, and shape distributions all suggest dusty gas reservoirs during their formation epoch, which lasted 1-3 Myr after CAI formation, possibly with a peak at 2 Myr. 

With the exception of oxygen, the Solar System has a remarkably uniform isotopic composition, with variations ofless than one partin 1000 for all objects >1 cm in size. Larger isotopic anomalies are seen only in a few submillimeter-sized CAIs, at the level of a few percent for some elements (e.g. MacPherson 2007), and in micrometer-sized presolar, circumstellar grains, which show large isotopic variations in all elements (Zinner 2007). Oxygen is a notable exception to this rule, with non-mass-dependent variations of up to 20% among Solar System materials... 

For volatile-rich carbonaceous chondrites like Cl and CM chondrites, constraints on thermal histories are derived from carbonate ages and oxygen isotopic data. The former indicate that alteration began soon after CAI formation and lasted —20Myr (Endress et al, 1996 Brearley et al, 2001). Oxygen-isotopic compositions of carbonates provide model-dependent temperatures... 

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... 

Oxygen-isotopic compositions of aluminum-rich chondrules from CR and CH chondrites (Krot et al, 2003c) are plotted in Figure 15(b). Three out of six chondrules analyzed exhibit large internal-isotopic heterogeneity, whereas others are iso-topically uniform all chondrules have porphyritic textures. In contrast to aluminum-rich chondrules in ordinary chondrites, oxygen isotopic heterogeneity is due to the presence of relict CAIs (Krot and Keil, 2002). 

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).

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