Chondritic meteorite

Source posted on Flickr by Shiny Things and used in accordance with a CC BY 2.0 license. 

There are three main classes of chondrites (Table 10.1) that are most readily distinguished by whole-rock chemical and O isotopic compositions [14, 17]. The ordinary chondrites are the most abundant type of meteorite, as they account for about 80% of all meteorite falls. Enstatite chondrites have very reduced compositions and they are rich in reduced Fe and Fe-poor silicate phases, such as enstatite (MgSiOa). Most carbonaceous chondrites have high contents of carbon and other volatile elements that only condensed into solids at very low temperatures in the solar nebula. 

The carbonaceous chondrites are a particularly important meteorite group. Petrographically they record the lowest temperatures of thermal metamorphism ( 150°C) and the highest degree of aqueous alteration of all meteorites, particularly in CIl chondrites. Hence almost all carbonaceous chondrites are of petrological types 1, 2, and 3 [15], The chemical compositions render the carbonaceous chondrites especially noteworthy, as they have the highest abundances of volatile elements and are hence considered to feature the most pristine chemical compositions that most closely approximate the bulk composition of 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... 

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... 

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

Murchison meteorite A carbonaceous chondrite meteorite landing 100 miles north of Melbourne in a town called Murchison. 

The historical background is presented for the asteroid-impact theory that is based on the iridium anomaly found in rocks frm the Cretaceous-Tertiary boundary. Recent measurements of Ir, Pt, and Au abundances from such rocks in Denmark have shown that the element abundance ratios are different from mantle-derived sources and agree with values for chondritic meteorites within one standard deviation of the measurement errors (7-10%). Rare-earth patterns for these rocks are... 

Evensen, M.N., Hamilton,P.J., Onions.P.K., 1978.Rare earth abundance in chondrite meteorites. Geochimlca et. Cosmochimica Acta, 42, 1199-1212. 

Table 9.1. Abundance of REE in a chondrite meteorite (ppm) used for normalization of REE data (reported in Henderson 1984).

Huss GR, MacPherson GJ, Wasserburg GJ, Russell SS, Srinivasan G (2001) Aluminum-26 in calcium-aluminum-rich inclusions and chondrales from unequilibrated ordinary chondrites. Meteorit Planet Sci... 

Virag A, Zinner E, Lewis RS, Tang M (1989) Isotopic compositions of H, C, and N in C8 diamonds from the Allende and Murray carbonaceous chondrites. Lunar Planet Sci XX 1158-1159 Volkening J, Papanastassiou DA (1989) Iron isotope anomalies. Astrophys J 347 L43-L46 Volkening J, Papanastassiou DA (1990) Zinc isotope anomalies. Astrophys J 358 L29-L32 Wadhwa M, Zinner EK, Crozaz G (1997) Manganese-chromium systematics in sulfides of unequilibrated enstatite chondrites. Meteorit Planet Sci 32 281-292... 

Figure 20. Lithium isotopic compositions of chondritic meteorites relative to their petrologic type (James and Palmer 2000a McDonough et al. 2003). The extant data indicate lighter and more variable isotopic compositions in higher petrologic types of meteorites, those with the least record of hydrous parent body alteration.

Consensus should be reached on the interpretation of Li isotope data for chondritic meteorites. Can bulk planetary Li isotopic compositions be accurately estimated from meteorites, or do these objects preserve only parent body near-surface or metamorphic histories Are either of these possibilities viable Detailed, high precision studies should permit this assessment. 

Figure 10. A Mg vs. 5 Mg plot showing that the chondrite meteorite data define a trend inconsistent with mass-dependent fractionation. A single whole-rock CAI sample with the lowest A Mg value plots off the diagram to the lower right.

Variability in Mg isotope ratios among chondritic meteorites and their constituents is dominated by mixing between a radiogenic CAI-like reservoir and a reservoir resembling ordinary chondrites. The mixing is evident in 5 Mg and 8 Mg, Al/Mg, and A 0 values but... 

Bridges JC, Banks DA, Grady MM (2001) Stable chlorine isotope reservoirs in chondrites. Meteoritics Planet Sci 36 A29-A30... 

Urey HC (1947) The thermodynamic properties of isotopic substances. J Chem Soc 562-581 Urey HC, Greiff LJ (1935) Isotopic exchange equilibria. J Am Chem Soc 57 321-327 Van Schmus WR, Wook JA( 1967) A chemical petrologic classification for the chondritic meteorites. Geochim Cosmochim Acta 31 747-765... 

The Ca isotope ratios of meteoritic samples are of interest because they can give information on early solar system processes and because meteorites represent the materials from which the Earth accreted and hence relate to the expected values for the bulk Earth. Russell et al. (1978b) made the first measurements of stable Ca isotope variations in meteorites. They formd variations of about +l%o for the Ca/ Ca ratio in samples from six different meteorites. Although some of these samples were spiked after having separated the Ca with an ion exchange column and hence may contain artifacts, it is clear from their data that bulk meteorites have some variability in 8 Ca and that the average value is quite close to the terrestrial standard. No data on bulk meteorites have been reported since the Russell et al. (1978b) measurements, and since their one measurement of an ordinary chondrite had a poor Ca column yield, there exist no reliable measurements that can be used to verify the composition of typical chondritic meteorites. 

A cosmochemical periodic table, illustrating the behavior of elements in chondritic meteorites. Cosmic abundances are indicated by symbol sizes. Volatilities of elements reflect the temperatures at which 50°/o of each element would condense into a solid phase from a gas of solar composition. As in Figure 1.2, the chemical affinities of each element, lithophile for silicates and oxides, siderophile for metals, and chalcophile for sulfides, are indicated. Some of the most highly volatile phases may have remained uncondensed in the nebula. Stable, radioactive, and radiogenic isotopes used in cosmochemistry are indicated by bold outlines, as in Figure 1.2. Abundances and 50% condensation temperatures are from tabulations by Lodders and Fegley (1998). 

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. 

Short-lived radionuclides are those with half-lives sufficiently short that any atoms present in the early solar system would have completely decayed away. In 1960, John Reynolds found the first clear evidence that short-lived nuclides were present in the form of large excesses of Xe, the decay product of short-lived I, in chondritic meteorites. This discovery showed that elements had been synthesized in stars shortly before the formation of the solar system. A more important short-lived radionuclide, 26A1, was demonstrated to have been present in meteorites by Typhoon Lee and coworkers in... 

The interiors of planets, moons, and many asteroids either are, or have been in the past, molten. The behavior of molten silicates and metal is important in understanding how a planet or moon evolved from an undifferentiated collection of presolar materials into the differentiated object we see today. Basaltic volcanism is ubiquitous on the terrestrial planets and many asteroids. A knowledge of atomic structure and chemical bonding is necessary to understand how basaltic melts are generated and how they crystallize. Melting and crystallization are also important processes in the formation of chondrules, tiny millimeter-sized spherical obj ects that give chondritic meteorites their name. The melting, crystallization, and sublimation of ices are dominant processes in the histories of the moons of the outer planets, comets, asteroids, and probably of the Earth. 

In this chapter, we discuss the abundances of the elements and isotopes in the solar system. First, we look at the techniques used to determine solar system abundances, including spectroscopy of the stellar photosphere, measurements of solar wind, and analyses of chondritic meteorites. The solar system abundances of the elements and isotopes are then presented. These abundances are then compared to the abundances in the solar neighborhood of the galaxy and elsewhere. Finally, we introduce how solar system abundances provide a basis for much of what we do in cosmochemistry. 

The Earth and other planetary bodies have been heavily modified by planetary-scale differentiation, smaller scale melting and the resulting chemical fractionations, collisions that mix material with different histories, and other processes. Samples of these materials are thus not suitable for determining the solar system composition. More primitive objects, such as comets and chondritic meteorites, have compositions more similar to the composition of... 

---