Cosmochemistry 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). 

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. 

Most of the thousands of meteorites in our collections are bits and pieces of rocky or metallic asteroids. Because we can analyze these meteorites in the laboratory, they play a pivotal role in cosmochemistry. In this chapter we will focus on the compositions of meteorites that were anhydrous, or nearly so. The hydrated carbonaceous chondrites, in particular the Cl and CM chondrites, which sample bodies that once contained ices and fluids, will be considered in Chapter 12. 

Chondrules comprise the major portion of most chondrites, the most abundant type of meteorites. If the achondrites and terrestrial planets formed from chondrite-like precursors, then much, perhaps most of the solid matter in the inner solar system once existed as chondrules. Even if chondrules were restricted to the chondrites, the process that formed them was important in that region. The origin of chondrules is an important unsolved problem in cosmochemistry. Chondrules formed in the Sun s accretion disk through some sort of transient flash-heating event(s). Some CAIs apparently also were melted in the disk. What was the process (or processes) that melted the chondrules and CAIs Whatever it was, it dominated the disk for at least a few million years. 

The Cl chondrites represent one of the most curious paradoxes of cosmochemistry. Despite their unfractionated compositions, the Cl chondrites are the most altered of all chondrites, with water contents of —19.5 wt.% (Nagy et al., 1963). Anhydrous phases (olivines and pyroxenes) represent less than 1 vol.% of these meteorites (Leshin et al., 1997). Cl chondrites are complex meteorites that consist of a dark, fine-grained matrix comprised of phyllosilicates with magnetite, sulfides, carbonates, and sulfates embedded within it (e.g., DuFresne and Anders, 1962 Nagy, 1966). They have experienced extensive breccia-tion on their asteroidal parent bodies that caused... 

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