Solar composition

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. 2-4 The sequence of condensation of solids from a solar composition gas at a nebular pressure of 10 Pa (ca. 10 atm). (Modified with permission from J. A. Wood, "The Solar System," p. 162, Copyright C 1979, Prentice-Hall, Englewood Cliffs, NJ.)...

Fig. 1. Left panel. Post-explosive yields versus mass of the Ge isotopes for a 25 M of solar composition by [10]. Arrows represent a production factor of 200 over the initial mass fraction of each isotope. Right panel. Logaritmic abundances relative to O and to solar ratio observed in the DLA-B/FJ0812+32 System (dust corrected) [5]. The observed [Zn/O] value is represented by a full square.

Fig. 1. Post—explosive yields versus mass of 63Cu and 6BCu for a 25 M model of solar composition. The arrows indicate a production factor of 200 with respect to the initial composition. For 16 O the production factor is nearly 100.

Fig. 3.44. Metallicities in gas-poor galaxies (open symbols) and oxygen abundances at a representative radius in gas-rich disk galaxies (filled symbols), as a function of galaxy luminosity in blue light. The dotted lines in each panel represent identical trends for [Fe/H] and [O/H] and the ordinate 0.0 represents solar composition. Adapted from Zaritsky, Kennicutt and Huchra (1994).

Fig. 5.1. Opacity of stellar material with X = 0.7, Z = 0.02 (roughly solar composition) as a function of temperature and the parameter log R, where R = p/T is approximately constant throughout a main-sequence star, corresponding to a polytrope with n = 3 (see Appendix 4) e.g. at 1 M0 log/ varies from —1.5 at the centre to 0.0 in the envelope, while at 10 M0 the corresponding range is from -3.5 to -4.0. (Density in units of gmcm-3, T(, in units of 106K, opacity in units of cm2 gm-1.) OP and OPAL refer to two independent opacity calculation projects. After Badnell, Bautista, Butler et al. (2005).

Fig. 5.14. Element production in winds and supernova ejecta from stars affected by strong mass loss, as a function of initial mass. Upper panel stars with about 1/20 solar heavy-element abundance. Lower panel stars with approximately solar composition, for which the effects of mass loss are believed to be more drastic. Horizontal shadings indicate outer layers that are expelled in winds prior to SN explosion. After Maeder (1992).

Fig. 5.20. Convective regions during the 15th and 16th pulses in a model of mass 7 Mq and solar composition. Shading indicates where convection occurs, and the dashed lines indicate the location of the H-He discontinuity before dredge-up begins. After Iben and Renzini (1983). Copyright by Annual Reviews, Inc.

Inclusions of the CV3 led to the search for isotopic signatures of individual nucleosynthetic processes, or at least for components closer to the original signature than average solar compositions. They have also begun to demonstrate the isotopic variability of matter emerging from these processes in agreement with astrophysical and astronomical expectations. The principal features of inclusions are an up to 4% 0 enriched reservoir in the early solar system, variations in a component produced in a nuclear neutron-rich statistical equilibrium, and variations in the contribution of p- and r-process products to the heavy elements. 

Magnesium. Corundum-hibonite associations are what eould be the first eondensates from a solar composition gas. Mg is not a refractory element and is strongly depleted in... 

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). 

The (,P+ v) reactions are essentially instantaneous and so are much faster than the other reactions at the temperatures in low- and intermediate-mass stars. The fastest proton reaction in this series is 15N(p,a)12C, while the slowest reaction is 14N(p,y)150. As a result, extensive CN cycling converts much of the 15N, 12C, and 13C into 14N. In the CN cycle, 12C is destroyed more rapidly than 13C by about a factor of three. In the solar composition,12C is 89 times more abundant than 13C, so initially more 13C is produced from the destruction of 12C than is destroyed by proton reactions. The 12C/13C ratio decreases until it reaches an equilibrium value equal to the inverse of the reaction rates, where as much 13C is being destroyed as is being produced. From this point on the 12C/13C ratio remains the same as carbon is gradually converted to 14N. 

Their table included isotopic abundances as well as elemental abundances. Since the Suess and Urey table was published, subsequent work has primarily refined the determinations of the cosmic abundances through improved measurements of meteorites, a better understanding of which meteorites should be considered for this work, improved measurements of the solar composition, and a better understanding of nuclear physics. 

Grevesse, N. and Sauval, A. J. (1998) Standard solar composition. Space Science Reviews, 85, 161-174. 

Theoretical modeling provides strong evidence that most presolar silicon carbide grains come from 1.5 to 3 M stars. As discussed in Chapter 3, stellar modeling of the evolution of the CNO isotopes in the envelopes of these stars makes clear predictions about the 12C/13C, 14N/15N, 170/160,180/160 ratios as a star evolves. For example, in the envelopes of low- to intermediate-mass stars of solar composition, the 12C/13C ratio drops to 40 (from a starting value of 89), and 14N/15N increases by a factor of six as carbon and nitrogen processed by... 

Titanium isotopic data for mainstream silicon carbide grains versus 829Si. The correlation between excesses of minor titanium isotopes and minor silicon isotopes most likely reflects galactic chemical evolution. The offset of the S50Ti trend to pass above the solar composition probably reflects 5-process nucleosynthesis in the parent stars, which most strongly affects 50Ti. Data from Huss and Smith (2007) and references therein. 

Two different kinds of metals are found in chondrites. Small nuggets composed of highly refractory siderophile elements (iridium, osmium, ruthenium, molybdenum, tungsten, rhenium) occur within CAIs. These refractory alloys are predicted to condense at temperatures above 1600 from a gas of solar composition. Except for tungsten, they are also the expected residues of CAI oxidation. 

Vapor-solid and vapor-liquid transformations (condensation of a gas, or its reverse, evaporation) can fractionate elements and sometimes isotopes. Each element condenses over a very limited temperature range, so one would expect the composition of the condensed phase and vapor phase to change as a function of the ambient temperature. Many of the chemical fractionations that took place in the early solar system are due, in one way or another, to this phenomenon. It is convenient to quantify volatility by use of the 50% condensation temperature, that is, the temperature by which 50% of the mass of a particular element has condensed from a gas of solar composition. Table 7.1 lists the 50% condensation temperatures for the solid elements in a gas of solar composition at a pressure of... 

Table 7.1 Equilibrium 50% condensation temperatures for a gas of solar composition at 10 4 atm ...

In Chapter 1 and again above, we introduced the cosmochemical classification of elements based on their relative volatilities in a system of cosmic (solar) composition. In a cooling solar gas, elements condense in a certain order, depending on their volatility (Table 7.1). Condensation and evaporation partition elements between coexisting gas and solid (or liquid) phases, and the removal of one or the other of these phases can fractionate element abundances of the system as a whole from their original cosmic relative proportions. 

Calculated condensation sequence for a gas of solar composition at 10-4 atm. Condensed minerals are labeled in italics and curves show the fraction of each element condensed as a function of temperature. Modified from Grossman and Larimer (1974). 

Aluminum and calcium can become volatile if they form hydroxides. Depletions of these elements compared to others with similar volatility in a gas of solar composition can help us to investigate the redox conditions under which certain objects formed. 

Describe the important minerals in the condensation sequence, and their order of appearance from a cooling gas of solar composition. 

---