Cosmic chemical memory

Clayton DD (1982) Cosmic chemical memory anew astronomy. Quart J Roy Astron Soc 23 174-212 Clayton DD (1983) Principles of stellar evolution and nucleosynthesis. University of Chicago Press, Chicago Clayton DD (1986) Interstellar fossil Mg and its possible relationship to excess meteoritic Mg. Astrophys 1310 490-498... 

The discovery in 1973 that refractory minerals found in meteorites are enriched in the l60 isotope launched a new era of cosmic chemical memory of nucleosynthesis. Though only 5%, the excess showed that not all rocks in the solar system were assembled from well mixed gas of the solar system. It suggested that presolar materials also can be found. The study of isotopes in natural history was accelerated by this pivotal discovery. 

The isotopes of Ti provide a rich spectrum ofisotopic deviations from the solar ratios in presolar materials and in solar materials prepared retaining a cosmic chemical memory (see Glossary) of presolar materials. 

Chromium exhibits a rich display of isotopic anomalies. The clearly documented cases include an unknown presolar component rich in 54Cr, and variability in the 53Cr/52Cr ratio owing to various effects from radioactive 53Mn. Cosmochemists rejoice at the richness of the Cr isotopic data but remain perplexed about the detailed causes, which derive from cosmic chemical memory (see Glossary) rather than from a supernova admixture into the forming solar system. 

Calcium-aluminum-rich inclusions (CAIs) After removing the progressive mass-dependent fractionation that occurs in the measuring process and in the formation process for the samples, isotopic anomalies for 7°Zn are observed in certain types of CAIs ( FUN inclusions). Only one detection exists to date, a deficit of 2 parts per thousand for 7°Zn in one FUN inclusion. An excess of 1.7 parts per thousand for 66Zn exists in that same CAI. Some form of cosmic chemical memory (see Glossary) is probably involved. 

The cosmic chemical memory interpretation was advanced by the writer as a superior way to think of these isotopic anomalies. This picture argued that the early solar system was not hot enough to vaporize the entirety of most solids but only their volatile parts and portions of their refractory Ca-and-Al-rich minerals. The refractory parts had survived to that time from their earlier condensation as stardust and were fused into the CAI assemblages found today in the meteorites. That fusion occurred while the gas that was vaporized from a dust-rich presolar mixture recondensed as the main minerals of the CAIs. The refractory cores, being stardust that had condensed even earlier within individual stars and supernovae, contain the isotope ratios from those distinct sources. When these cores were fused into the CAIs found today, the chemistry remembered the isotope ratios of the source presolar grains, so thatsolar-system rocks (CAIs) remembered their isotopic parentage. Hence the name cosmic chemical memory. See l60 for a fuller account of the historical role played by the experimental discovery of l60-rich minerals within the CAIs, and of how the memory of l60-richness was saved. 

Some of these presolar grains did survive. When incontestably presolar grains were discovered in 1987 the cosmic-chemical-memory picture was suddenly taken seriously. Indeed, the earlier predictions of the existence of refractory isotopi-cally anomalous presolar grains endowed this picture with experimental credibility. 

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