Solar system abundance elements

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 ... 

Fig. 5.25. Nucleosynthetic outcome (after radioactive decay) of model W7 for Type la supernovae (Nomoto, Thielemann Yokoi 1984, and Thielemann, Nomoto Yokoi 1986), compared to Solar-System abundances. Dominant isotopes of multi-isotope elements are circled. Adapted from Tsujimoto (1993).

Fig. 5.5. Decomposition of Solar System abundances into r and s processes. Once an isotopic abundance table has been established for the Solar System, the nuclei are then very carefully separated into two groups those produced by the r process and those produced by the s process. Isotope by isotope, the nuclei are sorted into their respective categories. In order to determine the relative contributions of the two processes to solar abundances, the s component is first extracted, being the more easily identified. Indeed, the product of the neutron capture cross-section with the abundance is approximately constant for aU the elements in this class. The figure shows that europium, iridium and thorium come essentially from the r process, unlike strontium, zirconium, lanthanum and cerium, which originate mainly from the s process. Other elements have more mixed origins. (From Sneden 2001.)... 

Anders, E. and Ebihara, M., 1982. Solar-system abundances of the elements. Geochim. Cosmochim. Acta, 46 2363-2380. 

The term cosmochemistry apparently derives from the work of Victor Goldschmidt (Fig. 1.6), who is often described as the father of geochemistry. This is yet another crossover and, in truth, Goldschmidt also established cosmochemistry as a discipline. In 1937 he published a cosmic abundance table based on the proportions of elements in meteorites. He used the term cosmic because, like his contemporaries, he believed that meteorites were interstellar matter. Chemist William Harkins (1873-1951) had formulated an earlier (1917) table of elemental abundances - arguably the first cosmochemistry paper, although he did not use that term. As explained in Chapter 3, the term solar system abundance is now preferred over cosmic abundance, although the terms are often used interchangeably. 

In the final section of this chapter, we discussed the formation of galaxies and the formation and chemical evolution of the Milky Way. This sequence of events set the stage for the formation of the solar system. In Chapter 4, we will look at the resulting abundances of the elements and isotopes, both in the solar system and in the galaxy. The solar system abundances of the elements are a fundamental constraint for understanding the Sun, the planets, and the smaller bodies in the solar system. 

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. 

In recent decades, spectroscopy has revealed that the elemental and isotopic abundances in the galaxy vary with radial position and that the Sun has a somewhat different composition than the molecular clouds and diffuse interstellar medium in the solar neighborhood. For this reason, we can no longer think of the solar system abundances as truly cosmic abundances. 

Element Mean Cl chondrites Solar photosphere Solar system abundance ... 

As understanding of nuclear physics and stellar nucleosynthesis has improved, it has become possible to use that knowledge along with measured abundances to infer the solar system abundances of elements that cannot be directly measured, either by solar spectroscopy because their ionization potentials are too high, or in meteorites because they were not efficiently incorporated into the chondrites. In particular, the noble gases cannot be directly measured and must be inferred from theoretical considerations or other indirect means. 

Table 4.1 shows the solar system abundances of the elements as determined by the methods discussed above. For some elements, the photospheric abundances provide the best estimate, whereas for others, the meteorite data must be used. In some cases, the data are equally reliable and an average of the values determined from the solar photosphere and Cl chondrites is used. The abundances of the noble gases come from indirect measurements or theoretical considerations. The method for determining each abundance is indicated in the far right column of Table 4.1. 

The solar system abundances of the elements are the result of the Big Bang, which produced hydrogen and helium, 7.5 billion years of stellar nucleosynthesis, which produced most of the rest of the elements, and the physical processes that mixed the materials together to form the Sun s parent molecular cloud. The unique features of the solar system composition may also reflect the stochastic events that occurred in the region where the Sun formed just prior to solar system formation. 

In the cosmochemistry literature, you will often see data normalized to (that is, divided by) solar system abundances (most commonly those of Cl chondrites). An important reason for doing this is illustrated in Figure 4.6. The top panel of this figure shows a plot of the composition of a chondrule with the elements arranged in order of their volatility from most... 

In this chapter, we have introduced solar system abundances and cosmic abundances of the elements and isotopes and briefly discussed their importance in cosmochemistry. The idea of cosmic abundances has been around for over a century and is rooted in measurements of many different celestial objects. Only in the last 20-30 years has our knowledge of the compositions of the solar system, of stars and molecular clouds in our galaxy, and of other galaxies grown sufficiently that we can now detect clear compositional differences between our solar system and other objects in the galaxy and the universe. 

Solar system abundances are a cornerstone of our understanding of the origin and evolution of the solar system and the elements from which it formed. To understand... 

Lodders, K. (2003) Solar system abundances and condensation temperatures of the elements. Astrophysical Journal, 591, 1220-1247. A comprehensive discussion of the solar system abundances of the elements from the perspective of a cosmochemist. 

Now that we have gained some appreciation of the value and limitations of the various kinds of samples available for cosmochemical analysis, we will begin to consider what these materials really tell us. In Chapter 4, we saw how solar system abundances of elements and isotopes are determined using the most primitive chondrites. In subsequent chapters, we will see how elements are fractionated in space, planetesimals, and planets. 

The solar system formed from a well-mixed collection of gas and dust inherited from its parent molecular cloud. The bulk composition of this material, as best we can know it, is given by the solar system abundances of elements and isotopes (Tables 4.1 and 4.2). From this bulk material, the planets, asteroids, and comets formed, each with its own unique composition. The processes that produced these compositions separated, or fractionated, elements and isotopes from one another. By studying these elemental and isotopic fractionations, we can potentially identify the processes that separated the elements and can leam about the physical conditions involved. This is particularly important for understanding the early solar system, because its processes and conditions are not directly observable. 

Planetary differentiation is a fractionation event of the first order, and it involves both chemical fractionation and physical fractionation processes. Planetary crusts are enriched in elements that occur in silicate minerals that melt at relatively low temperatures. Recall from Chapter 4 that the high solar system abundances of magnesium, silicon, and iron mean that the silicate portions of planetesimals and planets will be dominated by olivine and pyroxenes. Partial melting of sources dominated by olivine and pyroxene ( ultramafic rocks ) produces basaltic liquids that ascend buoyantly and erupt on the surface. It is thus no surprise that most crusts are made of basalts. Remelting of basaltic crust produces magmas richer in silica, eventually resulting in granites, as on the Earth. 

Noble gas abundances in lunar soils and chondrites, (a) Elemental abundance patterns for trapped solar wind in lunar soils, normalized to solar system abundances, (b) Elemental abundance patterns for planetary trapped noble gases, normalized to solar system abundances. This diagram is intended to illustrate patterns only vertical positions are arbitrary. Modified from Ozima and Podosek (2002). 

Palme, H. and Jones, A. (2004) Solar System abundances of the elements, in Treatise on Geochemistry Volume 2 The Mantle and Core (eds R.W. Carlson, H.D. Holland and K.K. Turekian Editors-in-Chief), Elsevier Science, pp. 41-61. 

A main concern of geochemistiy is the investigation of the abundance and the distribution of the elements on the surface and in deeper layers of the earth, and of transport processes. The components of the geosphere are the lithosphere, the hydrosphere and the atmosphere. The relative abundance of the elements on the surface of the earth is plotted in Fig. 15.1 as a function of the atomic number. This relative abundance is similar within the solar system. The elements H, O, Si, Ca and Fe exhibit the highest abundances and maxima are observed at the magic numbers Z = 8, 20, 50 and 82. The abundances of the elements and their isotopes are determined by the nuclear reactions by which they have been produced and by their nuclear properties, whereas the chemical properties of the elements are only responsible for distribution and fractionation processes. 

Figure 1 The abundances of the isotopes present in solar system matter are plotted as a function of mass number A (the solar system abundances for the heavy elements are those compiled by Palme and Jones (see Chapter 1.03).

Figure 2 The s-process and r-process abundances in solar system matter (based upon the work by Kappeler et aL, 1989). Note the distinctive s-process signature at masses A —88, 138, and 208 and the corresponding r-process signatures at A — 130 and 195, all attributable to closed-shell effects on neutron capture cross-sections. It is the r-process pattern thus extracted from solar system abundances that can be compared with the observed heavy element patterns in extremely metal-deficient stars (the total solar system abundances for the heavy elements are those compiled by Anders and Grevesse, 1989), which are very similar to those from the compilation of Palme and Jones (see Chapter 1.03).

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