Al-rich inclusions from

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. 

Lin, Y. T., Guan, Y. B., Leshin, L.A., Ouyang, Z. Y. and Wang, D. (2005) Short-lived chlorine-36 in a Ca- and Al-rich inclusion from the Ningqiang carbonaceous chondrite. Proceedings of the National Academy of Sciences of the United States of America, 102, 1306-1311. 

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. 

McKeegan K. D., Sahijpal S., Krot A. N., Weber D., and Ulyanov A. A. (2003) Preservation of primary oxygen isotopic compositions in Ca, Al-rich inclusions from CH chondrites. Earth Planet. Sci. Lett, (submitted). 

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). 

Ivanova M. A., Petaev M. I., MacPherson G. J., Nazarov M. A., Taylor L. A., and Wood J. A. (2002) The first known natural occurrence of CaAl204, in a Ca-Al-rich inclusion from the CH chondrite NWA470. Meteorit. Planet. Sci. 37, 1337-1344. 

Stolper E. and Paque J. (1986) CrystaUization sequences of Ca-Al-rich inclusions from AUende the effects of coohng rate and maximum temperature. Geochim. Cosmochim. Acta 50, 1785-1806. 

Wark D. A. and Lovering J. F. (1977) Marker events in the early evolution of the solar system evidence from rims on Ca-Al-rich inclusions from carbonaceous chondrites. Proc. 8th Lunar Planet. Sci. Cortf. 95-112. 

Palme H. and Wlotzka F. (1976) A metal particle from a Ca, Al-rich inclusion from the meteorite Allende, and the condensation of refractory siderophile elements. Earth... 

Stolper E, Paque JM (1986) Crystalhzation sequences of Ca-Al-rich inclusions from Allende The effects of coohng rate and maximum termperatrrre. Geochim Cosmochim Acta 50 1785-1806 Sugiitra N, Hoshino H (2000) Hydrogen-isotopic compositions in Allan Hills 84001 and the evolution of the martian atmosphere. Meteoritics Planet Sci 35 373-380 Taylor HP, Jr., Epstein S (1970) 18-0/16-0 ratios of Apollo 11 lunar rocks and soils. Proc Apollo 11 Lunar Sci Conf, p 1613-1626... 

Laser ablation combined with LA-MC-ICPMS provides a new dimension to the analysis of Mg isotopes in calcium aluminum-rich inclusions from primitive meteorites. Dispersion in 26Mg - Al/ Mg evolution lines can be correlated with mass-dependent variahons in 5 Mg that distinguish open-system from closed-system processes. The ultimate product of such studies will be a better understanding of the chronological significance of variations in Mg in these objects. 

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

Hashimoto A. and Grossman L. (1987) Alteration of Al-rich inclusions inside amoeboid olivine aggregates in the AUende meteorite. Geochim. Cosmochim. Acta 51, 1685-1704. Hinton R. W. and Bischoff A. (1984) Ion microprobe magnesium isotope analysis of plagioclase and hibonite from ordinary chondrites. Nature 308, 169-172. 

Swindle T. D., Davis A. M., Hohenberg C. M., MacPherson G. J., and Nyqtiist L. E. (1996) Formation times of chondrules and Ca-Al-rich inclusions constraints from short-lived radionuclides. In Chondrules and the Proto-planetary Disk (eds. R. H. Hewins, R. H. Jones, and E. R. D. Scott). Cambridge University Press, New York, pp. 77-86. 

The solar system accreted from a dust cloud, formed after a supernova explosion. From this primitive solar nebula condensed the Sun and the planets. Some of the oldest objects in the solar system yet found are Ca-Al-rich inclusions in meteorites, —4.566 Ga old (Allegre et al, 1995). It is possible that these grains predate the solar nebula and may have been formed in the expanding envelope of the supernova explosion (Cameron, 2002). 

Most meteorites are chondrites, so-called because almost all contain spherical mm- to cm-sized chondrules or their fragments early in the Solar System s history. Cooling rates for some were 1000 C/hr, and 10-100°C/hr for others. Rapid heating and cooling are easily done in the laboratory but difficult on a Solar System scale. Yet, large volumes of chondrules must have existed in the Solar System because chondrites are numerous (Table I). Chondrites (and many achondrites) date to the Solar System s formation - indeed provide chronometers for it and represent accumulated primary nebular condensate and accretionary products. Some condensate formed from the hot nebula as mm-sized Ca- and Al-rich inclusions (CAI), mineral aggregates predicted as vapor-deposition products by thermodynamic calculations. These CAI, found mainly in chondrites rich in carbonaceous (organic) material exhibit many... 

Figure 4 shows the isotopic anomalies of the iron peak elements predicted by the multi-zone mixing model as compared with the average excesses as observed in Ca-Al-rich inclusions. The match between the two data sets is impressive, except for Fe and Zn. In the case of Fe no significant anomalies have been measured, but the multi-zone mixing model only predicts a Fe excess of approximately 1 part in 10", which is at the limit of present mass spectrometric capability. In the case of Zn, the excess in Zn is approximately an order of magnitude less than that expected. This can be explained in terms of the volatility of Zn with respect to the other iron peak elements, as it would be the last of these elements to condense from the expelled stellar material. The correlation between anomalies in neutron-rich isotopes in the iron peak elements can therefore be explained in terms of the nuclear statistical equilibrium processes, which took place at a late stage in the evolution of massive stars. 

Figure 5-9 Al- Mg isochron for a calcium-aluminum-rich inclusion (CAI) E60. The initial isotopic ratio of (26ai/27ai)o 4,52 X 10 . Adapted from Amelin et al. (2002).

We have included here, for comparison, the results of a study of zirconolite-rich Synroc nominally composed of 80 wt% Ce- or Pu-doped zirconolite plus 10 wt% hollandite and 10 wt% rutile (Hart et al. 1998). Inclusion of this study in this section is significant because the two additional phases are both highly durable in their own right and the experiments were conducted at two different temperatures (90 and 200 °C) and in three different aqueous solutions (pure water, silicate, and brine). The authors found no major differences in the release rates of Ca, Ce, Hf, Ti, Zr, Pu, and Gd apart from those for Ce and Ti, which appeared to be somewhat higher in the brine. On average, for all elements, the increase in temperature caused the release rates to increase by a factor of approximately seven. Release rates were generally below 10 2 g/m2/d for Ca, 10 3 g/m2/d for Ce and Gd, and 10 4 g/m2/d for Ti, Zr, Hf, and Pu (except for the brine at 200 °C, which gave a Ti release rate of 2 x 10 3g/m2/d). Hart et al. (2000) also determined the release rate of Pu in an LLNL-type zirconolite ceramic. After nearly one year in pure water at 90 °C the release rate of Pu decreased from 2 x 10-3 g/m2/d to less than 10-5 g/m2/d (Fig. 7). 

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