Chondrules composition

Perhaps the most well-known, unsolved problem in cosmochemistry is the question of the mechanism whereby dust grain aggregates were thermally processed to form chondrules and some rounded refractory inclusions. Chondrule compositions and textures require rapid heating and somewhat slower cooling for their explanation a globally hot nebula is inconsistent with these requirements... 

Chondrules may have two types of rims finegrained matrix rims and coarse-grained igneous rims both are missing in CH and CB chondrites. Matrix rims vary in composition but the variations are uncorrelated with chondrule composition (Section 1.07.5.7). However, the compositions of igneous rims are related to those of the host chondrule type I chondrules have igneous rims that are FeO poor, while type II chondrules have FeO-rich rims (Krot and Wasson, 1995). The igneous rims formed by collisions between chondrules and smaller partly molten particles, or solid particles that were accreted and later melted. 

Hewins R. H., Yu Y., Zanda B., and Bourot-Denise M. (1997) Do nebular fractionations, evaporative losses, or both, influence chondrule compositions Antarct. Meteorit. Res. 10, 275-298. 

The O isotopes show signihcant heterogeneity between the different meteorite classes (Fig. 8a Clayton et al. 1976, 1977). Differences are small, but, each chondrite group has a distinct bulk O isotopic composition. O isotopes also indicate the close ties between the Earth and the Moon. O therefore can be used to identify members of a family that formed from a common reservoir, which is the definition of a tracer. Such differences are also formd between chondrules within the same meteorites related to their size (Gooding et al. 1983). This is a survival of the initial isotopic heterogeneity in already high temperature processed materials like chondrules. 

Many more high-precision whole-rock analyses of both Mg and O isotope ratios for chondrite components would help to establish whether or not the mixing trends in Figure 15 are valid, or even if the concept of mixing is useful. Bulk objects are desirable because their isotopic compositions are not affected by secondary inter-mineral exchange reactions that are know to be important for both the Mg and O isotopic systems in CAIs and chondrules. 

Mullane E, Russel SS, Gounelle M (2002) Iron isotope fractionation within a differentiated asteroidial sample suite. 65 Annual Meteoritical Society Meeting, abstract number 5157 Mullane E, Russel SS, Gounelle M, Mason TED (2003a) Iron isotope composition of Allende and Chainpur chondrules effects of equilibration and thermal history. Lunar Planet Sci Conf XXXIV, abstract number 1027... 

In the cosmochemistry literature, you will often see data normalized to (that is, divided by) solar system abundances (most commonly those of Cl chondrites). An important reason for doing this is illustrated in Figure 4.6. The top panel of this figure shows a plot of the composition of a chondrule with the elements arranged in order of their volatility from most... 

Cosmochemistry is the study of the chemical compositions of various solar system materials. Chondrites are the most abundant primitive samples. They are essentially sedimentary rocks composed of mechanical mixtures of materials with different origins (chondrules, refractory inclusions, metal, sulfide, matrix), which we will call components. Chondrites formed by the accretion of solid particles within the solar nebula or onto the surfaces of growing planetesimals. They are very old (>4.5 billion years, as measured by radioactive chronometers) and contain some of the earliest formed objects in the solar system. Chondrites have bulk chemical compositions very similar to the solar photosphere, except... 

Chondrules exhibit a bewildering variety of compositions and textures (F ig. 6.1 a,b). Most are composed primarily of olivine and/or pyroxene, commonly with some glass. (For a crash course in mineral names and compositions, see Box 6.1.) If melt solidifies so quickly that its atoms cannot organize into crystalline minerals, it quenches into glass. Iron-nickel metal and iron sulfide occur in many chondrules, often clustered near the peripheries. The textures of... 

The chemical compositions of individual chondrules have been determined by neutron activation of extracted samples or by electron microprobe analyses of chondrules in situ. Some, but not all chondrules are depleted in moderately volatile elements. There is a compositional continuum between the olivine-rich and aluminum-rich chondrules. Original concentrations of the short-lived radionuclide 26A1 in chondrules suggest they formed very early, before all of this isotope decayed, but as much as 2-5 million years after the formation of CAIs (see Refractory Inclusions, below). 

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. 

Oxygen isotopic compositions of individual chondrules in various chondrite groups, after Clayton (2004). 

The winonaites are compositionally similar to silicate inclusions in some IAB irons (described below). They have chondritic compositions, and relict chondrules have been found in some meteorites. They consist of olivine, pyroxenes, plagioclase, metal, troilite, and other minor minerals (Benedix et al., 1998), and most have been recrystallized. Like the acapulcoites, they have experienced only small degrees of melting. 

Scott, E. R. D., Jones, R. H. and Rubin, A. E. (1994) Classification, metamorphic history, and pre-metamorphic composition of chondrules. Geochimica et Cosmochimica Acta, 58, 1203-1209. 

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