Primitive versus differentiated

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 

Some meteorites, and all planetary samples, have undergone melting and differentiation at some stage. Hence, the compositions of differentiated materials do not resemble solar system abundances. These samples can, however, tell us about various geochemical processes within asteroids and planets. 

We will now describe each of the various kinds of meteoritic samples available for cosmochemical investigation, progressing from primitive materials to samples from differentiated bodies. Presolar grains extracted from meteorites have already been described in Chapter 5, and interplanetary dust particles (IDPs) and returned comet samples will be described in Chapter 12. 

Chondrites are physical aggregates of various components, most of which formed in the nebula. Before describing the chondrites themselves, let s take a look at these components. 

Components of chondrites, (a) Photomicrograph of porphyritic chondrule showing olivine crystals in glassy mesostasis (dark), in the Clovis ordinary chondrite, field of view (FOV) 1.8 mm. 

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The simplest way of thinking about the relationship between the continental crust and mantle is to use a three reservoir box model, in which an initial primitive mantle composition (reservoir 1) is progressively differentiated through time into a depleted mantle reservoir (reservoir 2) and the continental crust (reservoir 3). This simple approach provides a good explanation of the Nd- and Sr-isotope compositions of the mantle and crust. On a Nd-Sr isotope plot, crustal compositions are complementary to the mantle isotopic array (Fig. 4.21a) relative to the composition of the primitive mantle. Expressing the Nd-isotope data in a slightly different manner, an eNd versus time... 

FIGURE 11.6 Markers of interest in the differential diagnosis of primitive neuroectodermal tumor (PNET) versus small-cell neuroendocrine carcinoma of the mediastinum (SCNC). 

To plot the kinetic curve of the fiactional extent versus time, it is enough to integrate [18.9] between 0 and orand 0 and t. This differential equation has separate variable the integral in a is carried out by posing u = - a and taking the primitive of u . We obtain ... 

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