Chondritic meteorites matrix

Transmitted-light photomicrograph of the Tieschitz chondritic meteorite. Horizontal field of view is 3.5 mm. The rounded, millimeter-size chondrules contain crystals of olivine and pyroxene, and the chondrules are set in a fine-grained, opaque matrix. 

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. 

Figure 1.2 Components of chondritic meteorites fluffy CAI (upper left), compact CAI (upper right), chondrule (lower left), and matrix (lower right).

Table 7.1 Summary of key properties of matrix and fine-grained rims in pristine chondritic meteorites...

The terrestrial planets and the Moon are differentiated, with dense iron-rich cores and rocky mantles. The uncompressed densities of Earth and Venus are similar. Mercury has a high density which suggests it has relatively large core. Conversely, the Moon has a low density, indicating a very small core. There is little observational evidence that asteroids are differentiated except for Vesta and Ceres (Thomas et al. 2005). However, iron meteorites from the cores of differentiated asteroids are quite common, and the irons found to date come from several dozen different parent bodies (Meibom Clark 1999). Most meteorites come from asteroids that never differentiated. These chondritic meteorites consist of intimate mixtures of heterogeneous material millimeter-sized rounded particles that were once molten, called chondrules, similarly sized calcium-aluminum-rich inclusions (CAIs), and micrometer-sized matrix grains. 

Brearley A. J. (1997) Phyllosilicates in the matrix of the unique carbonaceous chondrite, LEW 85332 and possible imph-cations for the aqueous alteration of Cl chondrites. Meteorit. Planet. Sci. 32, 377-388. 

Chondritic meteorites, characterized by their relatively unfractionated chemical compositions, and usually consisting of chondrules and some matrix, can be subdivided into classes, as follows carbonaceous chondrites—Vigarano-type (CV), Ornans-type (CO), Mighei-type (CM), Renazzo-type (CR), Karoonda-type (CK), Bencubbin-type (CB), and ALH 85085-type (CH) (see Chapter 1.05 for details) ordinary chondrites—high-iron (H), low-iron (L), and low-iron, low-metal (FF) and enstatite chondrites—high-iron (EH), low-iron (EL) R-chondrites, characterized by olivine with very high ferrous iron content. 

Klerner S. and Palme H. (2000) Large titanium/aluminum fractionation between chondrules and matrix in Renazzo and other carbonaceous chondrites. Meteorit. Planet. Sci. 35, A89-A89. 

Figure 2 Backscattered electron photograph of a Wark-Lovering rim sequence surrounding a type A CAI in the Vigarano (CV3) chondrite. Some of the minerals in the rim duplicate minerals occurring in the interior of the CAI, notably melilite, spinel, perovskite, and hibonite. Abbreviations Al-Diop—aluminum diopside Fo—forsterite Hib—hibonite Kir—kirsch-steinite Mel—melilite Mtx—meteorite matrix Pv— perovskite Sp—spinel.

Figures 3 and 13 show two spinel-pyroxene-rich CAIs from Mighei (CM) that are typical of the kinds of seen in many chondrite types, including CO, CR, ordinary, and enstatite chondrites. Besides being much smaller than their CV3 kindred, these CM examples typically are deficient in melilite and anorthite. Figure 3 shows what MacPherson et al. (1983) called a nodular spinel-pyroxene inclusion, in which the central spinel region consists of numerous spinel crystals in a dense overgrowth. In contrast, the CAl in Figure 13 is a chain-like inclusion (MacPherson and Davis, 1994) that consists of numerous small linear chains of spinel and even some individual spinel crystals. Each has its own individual pyroxene rim. Although clearly the ensemble is part of a single object, its individual components are separated from one another by up to 50 p,m of intervening meteorite matrix.

Keller L. P. (1998) A transmission electron microscope study of iron-nickel carbides in the matrix of the Semakona unequilibrated ordinary chondrite. Meteorit. Planet. Set 33, 913. 

Chondrites—meteorites from parent bodies in the asteroid belt that never melted—represent the most primitive samples available of material that formed in the protoplanetary nebula. Chondrites are mainly composed of chondrules, with smaller amounts of refractory inclusions and a fine grained matrix of silicate, metal, and sulfide. Chondrules are roughly spherical objects, typically —1 mm in size, and largely composed of olivine and pyroxene (Taylor, 2001). They appear to have formed from melt droplets that cooled on timescales of... 

Chondrules from Chainpur LL3 meteorite have somewhat lower 5 Mg values than Chainpur LL3 matrix. Cl, and terrestrial mantle clinopyroxene. Chondrules from the Bjurbole L/LL4 ordinary chondrite also have slightly lower 8 Mg values than Cl and LL3 matrix. Chondrules, matrix, and whole rock samples from the Allende CV3 meteorite span a larger range in 5 Mg that overlaps the Cl and ordinary chondrite data. 

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