Presolar grains isotope anomalies

In practice, it is not sufficient for an object to have an isotopic composition that cannot be explained by radioactive decay or mass-dependent fractionation effects. The object must also have physical and chemical characteristics making it unlikely to be a product of solar system processes. For example, millimeter- to centimeter-sized refractory inclusions from primitive chondrites have been shown to contain small (parts in 103 to 104) isotopic anomalies in many elements. However, based on the size, composition, physical characteristics, and abundance of the inclusions, it is generally believed that these objects formed within the solar system. They preserve small isotopic anomalies because they did not form from a representative sample of the bulk solar system (see Chapters 7 and 14). So, isotopic anomalies can indicate either that an object is itself presolar or that it formed in the solar system from precursor material that was not fully homogenized in the solar system. As mass spectrometry has become more precise, small isotopic anomalies of the second type have shown up in a wide variety of chondritic materials. As we discuss below and in Chapter 7, these anomalies and bona fide presolar grains can be used as probes of processes in the early solar system. 

As noted previously, most of the presolar grains so far identified are circumstellar condensates (stardust), but some grains formed in interstellar space. The interstellar grains are not likely to contain large isotopic anomalies. So how can we recognize these interstellar grains in meteorites ... 

Bulk techniques still have a place in the search for presolar components. Although they cannot identify the presolar grain directly, they can measure anomalous isotopic compositions, which can then be used as a tracer for separation procedures to identify the carrier. There are several isotopically anomalous components whose carriers have not been identified. For example, an anomalous chromium component enriched in 54Cr appears in acid residues of the most primitive chondrites. The carrier is soluble in hydrochloric acid and goes with the colloidal fraction of the residue, which means it is likely to be submicron in size (Podosck el al., 1997). Measurements of molybdenum and ruthenium in bulk primitive meteorites and leachates from primitive chondrites show isotopic anomalies that can be attributed to the -process on the one hand and to the r- and /7-processes on the other. The s-process anomalies in molybdenum and ruthenium correlate with one another, while the r- and /7-process anomalies do not. The amounts of -process molybdenum and ruthenium are consistent with their being carried in presolar silicon carbide, but they are released from bulk samples with treatments that should not dissolve that mineral. Thus, additional carriers of s-, r-, and/ -process elements are suggested (Dauphas et al., 2002). 

CAIs are composed of a variety of minerals, primarily hibonite, perovskite, melilite, spinel, aluminum- and titanium-rich diopside, anorthite, forsterite, and occasionally corundum or grossite. They also show significant enrichments in refractory trace elements. CAIs exhibit a host of isotopic anomalies inherited from incorporated presolar grains or from the early nebula itself. 

Huss, G. R. (2004) Implications of isotopic anomalies and presolar grains for the formation of the solar system. Antarctic Meteorite Research, 17, 132-152. 

Presolar grains of several different types reveal sometimes a deficiency and sometimes an excess of this isotope relative to l8Si. Itis common to representthe anomaly by the fractional difference of the 29Si/28Si ratio from its solar value ... 

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. 

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