Chondrite

In contrast to D/H and ratios, an anomalous carbon isotope composi- 

These resnlts snggest that the interplanetary dust particles are among the most primitive samples available for laboratory stndies. Isotopically anomalous material constitutes only a small fraction of the investigated particles. Thus, it appears that the isotopic composition of these anomalons particles is not different from those observed in minor components of primitive meteorites. 

Chondrites are the oldest and most primitive rocks in the solar system. They are hosts for interstellar grains that predate solar system formation. Most chondrites have experienced a complex history, which includes primary formation processes and secondary processes that inclnde thermal metamorphism and aqneons alteration. It is generally very difficult to distinguish between the effects of primary and secondary processes on the basis of isotope composition. Chondrites display a wide diversity of isotopic compositions including large variations in oxygen isotopes. 

The first observation, that clearly demonstrated isotopic inhomogeneities in the early solar system, was made by Clayton et al. (1973a). Earlier, it had been thonght that all physical and chemical processes mnst produce mass-dependent O-isotope fractionations yielding a straight line with a slope of 0.52 in a plot of vs. 

Bulk meteorites, the Moon and Mars lie within a few percentile above or below the terrestrial fractionation line on a three-oxygen isotope plot (see Fig. 3.1). Therefore, the oxygen isotope composition of the Sun has been assnmed to be the same as that of the Earth. This view has changed with the suggestion of Clayton (2002) that the Sun and the initial composition of the solar system are 0-rich com- 

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Percentage of meteorites seen to fall. Chondrites. Over 90% of meteorites that are observed to fall out of the sky are classified as chondrites, samples that are distinguished from terrestrial rocks in many ways (3). One of the most fundamental is age. Like most meteorites, chondrites have formation ages close to 4.55 Gyr. Elemental composition is also a property that distinguishes chondrites from all other terrestrial and extraterrestrial samples. Chondrites basically have undifferentiated elemental compositions for most nonvolatile elements and match solar abundances except for moderately volatile elements. The most compositionaHy primitive chondrites are members of the type 1 carbonaceous (Cl) class. The analyses of the small number of existing samples of this rare class most closely match estimates of solar compositions (5) and in fact are primary source solar or cosmic abundances data for the elements that cannot be accurately determined by analysis of lines in the solar spectmm (Table 2). Table 2. Solar System Abundances of the Elements ... 

Element Solar system Mean CI chondrite, ppb Orgued, ppb Element Solar system Mean CI chondrite, ppb Orgued, ppb... 

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...

Fig. 2. The plot of total reduced iron, Fe, and oxidized iron, Fe, normalized to Si abundance shows how the chondrite classes fall into groups distinguished by oxidation state and total Fe Si ratio. The soHd diagonal lines delineate compositions having constant total Fe Si ratios of 0.6 and 0.8. The fractionation of total Fe Si is likely the result of the relative efficiencies of accumulation of metal and siUcate materials into the meteorite parent bodies. The variation in oxidation state is the result of conditions in the solar nebula when the soHds last reacted with gas. Terms are defined in Table 1 (3).

Chondrite classes are also distinguished by their abundances of both volatile and refractory elements (3). For volatile elements the variation among groups results from incomplete condensation of these elements into soHd grains that accrete to form meteorite parent bodies. Volatile elements such as C,... 

Within each chondrite class there are petrographic grades that relate to alteration processes that occurred within the meteorite parent body. The... 

Pig. 6. A 0.3-mm-diameter cosmic spherule coUected from the ocean floor. The particle is composed of oUvine, glass, and magnetite and has a primary element composition similar to chondritic meteorites for nonvolatile elements. The shape is the result of melting and rapid recrystaUi2ation during... 

Extraterrestrial dust particles can be proven to be nonterrestrial by a variety of methods, depending on the particle si2e. Unmelted particles have high helium. He, contents resulting from solar wind implantation. In 10-)J.m particles the concentration approaches l/(cm g) at STP and the He He ratio is close to the solar value. Unmelted particles also often contain preserved tracks of solar cosmic rays that are seen in the electron microscope as randomly oriented linear dislocations in crystals. Eor larger particles other cosmic ray irradiation products such as Mn, Al, and Be can be detected. Most IDPs can be confidently distinguished from terrestrial materials by composition. Typical particles have elemental compositions that match solar abundances for most elements. TypicaUy these have chondritic compositions, and in descending order of abundance are composed of O, Mg, Si, Ee, C, S, Al, Ca, Ni, Na, Cr, Mn, and Ti. 

Water and carbon play critical roles in many of the Earth s chemical and physical cycles and yet their origin on the Earth is somewhat mysterious. Carbon and water could easily form solid compounds in the outer regions of the solar nebula, and accordingly the outer planets and many of their satellites contain abundant water and carbon. The type I carbonaceous chondrites, meteorites that presumably formed in the asteroid belt between the terrestrial and outer planets, contain up to 5% (m/m) carbon and up to 20% (m/m) water of hydration. Comets may contain up to 50% water ice and 25% carbon. The terrestrial planets are comparatively depleted in carbon and water by orders of magnitude. The concentration of water for the whole Earth is less that 0.1 wt% and carbon is less than 500 ppm. Actually, it is remarkable that the Earth contains any of these compounds at all. As an example of how depleted in carbon and water the Earth could have been, consider the moon, where indigenous carbon and water are undetectable. Looking at Fig. 2-4 it can be seen that no water- or carbon-bearing solids should have condensed by equilibrium processes at the temperatures and pressures that probably were typical in the zone of fhe solar... 

Mullie, F. and Reisse, J. Organic Matter in CarbonaceoTO Chondrites, 139, 83-117 (1986). Murakami, Y. Functionalited Cyclophanes as Catalysts and Enzyme Models. 115, 103-151 (1983). Mutter, M., and Pillai, V. N. R. New Perspectives in Polymer-Supported Peptide Synthesis. 106, 119-175(1982). 

A few REE data on hydrothermal solutions are available (Fig. 2.34). Chondrite normalized REE patterns of hydrothermal solutions from Vienna Wood, Pacmanus and Desmos, Manus Basin exhibit positive Eu anomaly and LREE enrichment are similar to midoceanic ridge solution and Kuroko ore fluids. This positive Eu anomaly (Fig. 2.35) may have been caused by the selective leaking of Eu due to the interaction of an ascending hydrothermal solution and footwall volcanic rocks (Gena et al., 2001). It is interesting to note that altered basaltic andesite has a negative Eu anomaly and this feature is the same as that found in the Kuroko mine area (Shikazono, 1999). 

Blichert-Toft J, Alberede F (1997) The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crast system. Earth Planet Sci Lett 148 243-258... 

New computer simulations of the accretion process of the protoearth indicate that only a few large bodies with a high water concentration collided with the Earth during the later bombardment. They apparently came from the same region of the asteroid belt as the carbonaceous chondrites. 

Chondrites Achondrites Stony iron meteorites Iron meteorites... 

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. 

Carbonaceous chondrites (C-chondrites) account for only 2-3% of the meteorites so far found, but the amount of research carried out on them is considerable. C-chondrites contain carbon both in elemental form and as compounds. They are without doubt the oldest relicts of primeval solar matter, which has been changed only slightly or not at all by metamorphosis. C-chondrites contain all the components of the primeval solar nebula, apart from those which are volatile they are often referred to as primitive meteorites . 

Table 3.2 The commonness of elements (log n) in the solar system, in the sun and in carbonaceous chondrites of type Cl, with respect to hydrogen log n(H) = 12, i.e., n( ) = 1012 (Unsold and...

Now and then, projectiles from outer space cause excitement and surprises, as in January 2000, when a meteorite impacted the frozen surface of Lake Targish in Canada. It was a new type of C-chondrite with a carbon concentration of 4-5%, and probably came from a D-type asteroid (Hiroi et al., 2001). More exact analysis of the Targish meteorite showed the presence of a series of mono- and dicarboxylic acids as well as aliphatic and aromatic hydrocarbons (Pizzarello et al., 2001). Aromatic compounds and fullerenes were detected in the insoluble fraction from the extraction this contained planetary helium and argon, i.e., the 3He/36Ar ratio was... 

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