Ca-Al-rich inclusions

The presence of 26Mg excesses correlated with Al/Mg ratios in fifteen Ca-Al-rich inclusions from the Allende and Leoville carbonaceous chrondrites has provided additional strong evidence for the in situ decay of 26A1 (see [9] for a recent review of isotopic anomalies). There are also, however, several examples of minerals whose isotopic compositions depart substantially from a unique Al-Mg isochron, even within a single inclusion [10,11]. Since deviations from the isochron may reflect either differences in the formation age of individual minerals or intrinsic heterogeneities in the initial 26A1/27A1 ratio, the value of the Al-Mg system as a chronometer for early solar system events remains unclear. 

The application of the laser probe to meteorite chronology is illustrated by a study of Ca-Al-rich inclusions from the Allende meteorite [7]. This study was able to show that the K in the inclusions studied mainly concentrated in veins and rims with very little, if any, K in the major minerals. The limit obtained is something of the order of 10 ppm. On the other hand, the major minerals do contain appreciable 40Ar. Individual chondrules and the matrix were also studied in the Allende meteorite from places adjacent to the Ca-Al-rich inclusions. For these samples the ages varied from 3.3 to 4.4 G.y. There appears to be evidence that the Allende meteorite has been subjected to numerous metamorphic events, presumably of a collisional origin. 

In ordinary chondrites, Ca-Al rich inclusions are also present and give the canonical value (Russell et al. 1996), but other object like chondrules or mineral grains give reduced values by a factor of 5 to 100 (Hinton and Bischoff 1984 Hutcheon and Hutchison 1989). Delayed formation relative to CAIs is a probable cause. High precision ICPMS measurements of Mg have been used to address the timing of chondrule formation and show the importance of gas during the formation process (Galy et al. 2000). 

Carbonaceous meteorites. Geochim Cosmochim Acta 36 377-394 Black DC, Pepin RO (1969) Trapped neon in meteorites. II. Earth Planet Sci Lett 6 395-405 Bogdanovski O, Papanastassiou DA, Wasserburg GJ (2002) Cr isotopes in Allende Ca-Al-rich inclusions. Lunar Planet Sci XXXIII 1802... 

Fig. 3.1 vs 0-isotope composition of Ca-Al-rich inclusions (CAl) from various chondrites (Clayton, 1993)... 

Ryerson F.J. and McKeegan K.D. (1994) Determination of oxygen self-diffusion in aker-manite, anorthite, diopside, and spinel implications for oxygen isotopic anomalies and the thermal histories of Ca-Al-rich inclusions. Geochim. Cosmochim. Acta 58, 3713-3734. 

Burkhardt, C., Kleine, T., Bourdon, B. et al. (2008) Hf-W mineral isochron for Ca, Al-rich inclusions age of the solar system and the timing of core formation in planetesimals. Geochimica et Cosmochimica Acta, 72, 6177—6197. 

Sugiura, N. and Krot, A.N. (2007) 26Al-26Mg systematics of Ca-Al-rich inclusions, amoeboid olivine aggregates and chondrules form the ungrouped carbonaceous chondrite Acfer 094. Meteoritics and Planetary Science, 42, 1183-1195. 

Ca-Al-rich inclusions from meteorites Many of these CAIs reveal excess 41K in the most Ca-rich minerals within them. This shows that 41 Ca was alive in the early solar system, qualifying itas an extinct radioactivity. The amountin the CAIs was small but that small amount causes severe restrictions on the timing of the events that placed 41Ca in the solar system. See 41Ca for more on that extinct radioactivity. 

Grossman112 thinks that the Ca, Al rich inclusions found in C 3 chondrites represent these high-temperature condensates (HTC). In Ca, Al-rich inclusions of the Allende meteorite he found Sc, La, Sm, Eu, Yb, and Ir together with Ca and Al to be enriched by a factor of about 20 relative to C 1 chondrites. 

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... 

Aleon J., Krot A. N., and McKeegan K. D. (2002) Ca-Al-rich inclusions and amoeboid olivine aggregates from the CR carbonaceous chondrites. Meteorit. Planet. Sci. 37, 1729-1755. 

Tomeoka K., Nomura K., and Takeda H. (1992) Na-bearing Ca—Al-rich inclusions in the Yamato-791717 CO carbonaceous chondrite. Meteoritics 27, 136—143. 

Fagan T. J., Yurimoto H., Krot A. N., and Keil K. (2002) Constraints on oxygen isotopic evolution from an amoeboid ohvine aggregate and Ca, Al-rich inclusions from the CV3 Efremovka. Lunar Planet. Sci. XXXIII, 1507. The Lunar and Planetary Institute, Houston (CD-ROM). 

Grossman L. (1973) Refractory trace elements in Ca-Al-rich inclusions in the Allende meteorite. Geochim. Cosmochim. Actail, 1119-1140. 

Figure 25 Backscattered electron images showing fine-grained matrix rims on (a) Ca-Al-rich inclusion (CAT), and (b) an amoeboid-olivine aggregate (AOA) in ALHA77307. The light-gray rims contain FeO-rich silicate material and are crossed by dark cracks arrows mark the outer edge of the rims.

Kimura M., El Goresy A., Palme H., and Zinner E. (1993) Ca-, Al-rich inclusions in the unique chondrite ALH85085 petrology, chemistry, and isotopic compositions. Geochim. Cosmochim. Acta 57, 2329-2359. 

Krot A. N., McKeegan K. D., Huss G. R., Liffman K., Sahijpal S., Hutcheon I. D., Srinivasan G., Bischoff A., and Keil K. (2003a) Aluminum-magnesium and oxygen isotope study of rehct Ca-Al-rich inclusions in chondrules. Earth Planet. Sci. Lett, (in preparation). 

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