Presolar grains silicates

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.

Matrix minerals are complex mixtures of silicates (especially olivine and pyroxene), oxides, sulfides, metal, phyllosilicates, and carbonates. The bulk chemical composition of matrix is broadly chondritic, and richer in volatile elements than the other chondrite components. Some chondrules have rims of adhering matrix that appear to have been accreted onto them prior to final assembly of the meteorite. Small lumps of matrix also occur in many chondrites. Presolar grains, described in Chapter 5, occur in the matrix. 

Figure 2.5 Presolar grains from primitive meteorites. Left presolar SiC from the Murchison meteorite scale bar is 200 nm. Middle presolar graphite from Murchison scale bar is 1 pm. Right presolar silicate grain (within the white circle) in the matrix of the Acfer 094 meteorite scale bar is 500 nm. Photo credit Max Planck Institute for Chemistry.

Compared to carbonaceous presolar grains much less isotope information, mostly for the major elements, is available for O-rich presolar dust (Lodders Amari 2005 Zinner 2007). This has several reasons. Other than presolar SiC and graphite, presolar oxides have only low trace-element concentrations. Moreover, it is not possible to produce chemical separates that consist mostly of presolar oxide grains. Presolar silicates can be found only in situ by ion imaging (see Appendix 2), a time-consuming task. The O-isotopic data of refractory oxides (corundum and other... 

Chondrite matrices are also the carriers of presolar grains. These phases cover a broad mineralogy and include diamonds and graphite, silicon carbide (SiC), titanium carbide (TiC), silicon nitride (SisN, ), corundum, spinel, silicates, and even rare metal grains. 

Many different processes were involved in making each chondritic component. Unaltered chondrite matrices may contain at least six different types of micrometer-to-nanometer-sized components, which formed in diverse environments amorphous FeO-rich silicate, forsterite and enstatite grains, refractory grains, presolar grains, carbonaceous material, and iron-rich olivine. Chondrules formed by several nebular processes (closed-system melting, condensation, and possibly evaporation) and at least one asteroidal process (impact melting in regoliths). CAIs may be condensates, residues or processed versions of both. An exception to this preference for complexity is provided by the amoeboid olivine inclusions all AO As could have formed by the same basic process nebular condensation. Aluminum-rich chondrules may provide a second exception, at least within carbonaceous chondrites. 

Most presolar silicon carbide and oxide grains and a significant fraction of presolar silicate grains found in meteorites come from low- to intermediate-mass stars in the asymptotic giant branch (AGB) phase (see Chapter 3). Evidence for this conclusion derives from two sources (1) spectroscopic observations of the envelopes of these stars and (2) comparison... 

Laboratory studies of presolar dust grains also show that dust is formed in the mass ejected after an SN explosion, as will also be discussed in Section 2.2. Observations show that the ejected mass shells occasionally do form dust some time after the SN explosions (e.g. Bianchi Schneider 2007), but generally the efficiency of dust production seems to be rather low (Bianchi Schneider 2007 Zhukovska et al. 2008). Other important sources of stardust are red supergiants (mostly silicate dust). Most of the dust from red supergiants, however, is not expected to survive the shock wave from the subsequent SN explosion of the star (Zhukovska et al. 2008). Some dust is also formed by novae (Amari et al. 2001b), Wolf-Rayet stars (WRs, Crowther 2007), and luminous blue variables (LBVs, Voors et al. 2000), but the dust quantities formed by these are very small. Stardust - i.e. dust that is formed in stellar outflows or ejecta - in the interstellar medium is dominated by dust from AGB stars. 

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