Presolar grains

Figure 8. Figure (a) after Clayton et al. (1976, 1977). The scales are as in Figure 1. The O isotopic compositions of the different meteorite classes are represented ordinary chondrites (H, L, LL), enstatite chondrites (EFl, EL), differentiated meteorites (eucrites, lAB irons, SNCs) and some components of the carbonaceous chondrites. As the different areas do not overlap, a classification of the meteorites can be drawn based on O isotopes. Cr (b) and Mo (c) isotope compositions obtained by stepwise dissolution of the Cl carbonaceous chondrite Orgueil (Rotaru et al. 1992 Dauphas et al. 2002), are plotted as deviations relative to the terrestrial composition in 8 units. Isotopes are labeled according to their primary nucleosynthetic sources. ExpSi is for explosive Si burning and n-eq is for neutron-rich nuclear statistical equilibrium. The open squares represent a HNOj 4 N leachate at room temperature. The filled square correspond to the dissolution of the main silicate phase in a HCl-EIF mix. The M pattern for Mo in the silicates is similar to the s-process component found in micron-size SiC presolar grains as shown in Figure 7.

Zinner E (1998) SteUar nucleosynthesis and the isotopic composition of presolar grains from primitive meteorites. Ann Rev Earth Planet Sci 26 147-188... 

The second interesting feature of this isotope is that minuscule grains of silicon carbide extracted from meteorites have been found to be very rich in calcium-44, as mentioned earlier. They have been identified with presolar grains that condensed in the ejecta of supernovas during their first few years of expansion. Could it be that supernovas have been throwing sand in our eyes Data gathered by the ISO (Infrared Space Observatory), yet another experiment with strong participation by the French CEA, clearly demonstrates that new dust condensed inside the Cas A remnant very soon after explosion of the supernova that caused it. °... 

Within some meteorites are also found minuscule presolar grains, providing an opportunity to analyze directly the chemistry of interstellar matter. Some of these tiny grains are pure samples of the matter ejected from dying stars and provide constraints on our understanding of how elements were forged inside stars before the Sun s birth. Once formed, these... 

In the next few chapters, we will investigate these topics further. For example, in Chapter 5, we will introduce presolar grains and show how they can be used to investigate stellar nucleosynthesis and processes operating in interstellar space and in the early solar system, and in Chapter 7, we will discuss chemical and isotopic fractionation processes that operate in the solar system. 

Presolar grains a record of stellar nucleosynthesis and processes in interstellar space... 

In recent years, a new source of information about stellar nucleosynthesis and the history of the elements between their ejection from stars and their incorporation into the solar system has become available. This source is the tiny dust grains that condensed from gas ejected from stars at the end of their lives and that survived unaltered to be incorporated into solar system materials. These presolar grains (Fig. 5.1) originated before the solar system formed and were part of the raw materials for the Sun, the planets, and other solar-system objects. They survived the collapse of the Sun s parent molecular cloud and the formation of the accretion disk and were incorporated essentially unchanged into the parent bodies of the chondritic meteorites. They are found in the fine-grained matrix of the least metamorphosed chondrites and in interplanetary dust particles (IDPs), materials that were not processed by high-temperature events in the solar system. 

There are two types of presolar grains. Circumstellar condensates condensed from hot gas ejected from dying stars in the immediate vicinity of their parent stars. They are also sometimes called stardust. Circumstellar condensates give snapshots of the life histories of the stars from which they formed. Although the stars themselves no longer exist, the... 

Examples of presolar silicon carbide from the Orgueil meteorite (a, b, c) and hibonite from the Semarkona meteorite (d). These are relatively large for presolar grains. Note the geometric outlines of crystal faces in images (a) and (d). Image (d) is reproduced by permission of the AAS. 

The potential of presolar grains to provide information about nucleosynthesis, stellar evolution, galactic chemical evolution, interstellar processes, and nebular processes is only beginning to be tapped. But as new instrumentation is developed, more and more of the information that they carry will be extracted. 

The discovery happened by accident. Lewis and Anders were frustrated by their failure to find the carrier of anomalous xenon in carbonaceous chondrites. They decided to try an extreme treatment to see if they could dissolve the carrier. They treated a sample of the colloidal fraction of an Allende residue with the harshest chemical oxidant known, hot perchloric acid. The black residue turned white, and to their surprise, when they measured it, the anomalous xenon was still there The residue consisted entirely of carbon, and when they performed electron diffraction measurements on it, they found that it consisted of tiny (nanometer sized) diamonds. After a detailed characterization that included chemical, structural, and isotopic studies, they reported the discovery of presolar diamond in early 1987 (Lewis et al., 1987). The 23-year search for the carrier of CCFXe (Xe-HL) was over, and the study of presolar grains had begun. 

The carriers of anomalous Ne-E (two forms of which were now known) and Xe-S were quickly identified. Neon-E(H), which is released at temperatures above 1200 °C in stepped heating experiments, and Xe-S were found to be carried in presolar silicon carbide (Tang and Anders, 1988). Neon-E(L), which is released below 900 °C, was found to be carried by presolar graphite (Amari et al., 1990). Once these presolar compounds were shown to be present in meteorites, studies were carried out to identify all of the different types of meteorites that carry presolar grains. Concentrated searches for other presolar phases were also initiated, and many new types of presolar grains have been found. This work is just beginning, however, and we cannot yet account for the majority of the presolar components that must have been present in the Sun s parent molecular cloud. 

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 ... 

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