Chondrule

Fig. 1. A "barred olivine" chondrule from the Allende-type CV chondrite that feU in Mexico in 1979. The transmitted polarized light image of the 0.5 mm-diameter chondrule was taken from a poHshed thin section. The bars are composed of oHvine, (Mg,Fe)2Si04. The interstitial material is glass quenched...

Undifferentiated meteorites these are derived from asteroids which never underwent the heating which leads to fusion. They consist of millimetre-sized spherules (chondrules) embedded in a matrix. 

The chondrules contained in the chondrites contain olivine, pyroxene, plagiok-lase, troilite and nickel-iron they can make up 40-90% of the chondrites. Chondrules are silicate spheroids, fused drops from the primeval solar nebula. Because of their differing constitution, chondrites are further subdivided one group in particular is important for the question of the origin of life, and has thus been intensively studied—that of the carbonaceous chondrites. 

Achondrite A meteorite subclass that does not contain chondrules - small globules of once-molten mineral. 

Carbonaceous chondrite A meteorite containing once-molten globules of rock called chondrules that are surrounded by carbon-containing species. 

Chondrites Meteorites containing once-molten droplets of rock called chondrules. 

Chondrules Once-molten droplets of rock that are thought to derive from the proto-planetary disc of the forming solar system. 

Kerogen The carbonaceous material found around minerals and chondrules in meteorites such as the Murchison meteorite. 

The application of the laser probe to meteorite chronology is illustrated by a study of Ca-Al-rich inclusions from the Allende meteorite [7]. This study was able to show that the K in the inclusions studied mainly concentrated in veins and rims with very little, if any, K in the major minerals. The limit obtained is something of the order of 10 ppm. On the other hand, the major minerals do contain appreciable 40Ar. Individual chondrules and the matrix were also studied in the Allende meteorite from places adjacent to the Ca-Al-rich inclusions. For these samples the ages varied from 3.3 to 4.4 G.y. There appears to be evidence that the Allende meteorite has been subjected to numerous metamorphic events, presumably of a collisional origin. 

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. 

In ordinary chondrites, Ca-Al rich inclusions are also present and give the canonical value (Russell et al. 1996), but other object like chondrules or mineral grains give reduced values by a factor of 5 to 100 (Hinton and Bischoff 1984 Hutcheon and Hutchison 1989). Delayed formation relative to CAIs is a probable cause. High precision ICPMS measurements of Mg have been used to address the timing of chondrule formation and show the importance of gas during the formation process (Galy et al. 2000). 

Galy A, Young ED, Ash RD, O Nions RK (2000) The formation of chondrules at high gas pressures in the solar nehula. Science 290 1751-1753... 

Gooding JL, Mayeda TK, Clayton RN, Fukuoka T (1983) Oxygen isotopic heterogeneities, their petrological correlations, and implications for melt origins of chondrules in unequilibrated ordinary chondrites. Earth Planet Sci Lett 65 209-224... 

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