The solar system element and isotope abundances

Environments, processes and behaviour of the elements: some phenomenology

The Sun, our closest star, together with the planetary system allows an average elemental and isotope composition of matter typical of the solar neighbourhood (a 4.5-Gyr-old segment of the Milky Way Galaxy near the Sun) to be established. This so-called standard or reference composition is widely used in the astronomical literature for comparison with other objects or regions. The Sun is a typical low-mass star situated within the main sequence in the Hertzsprung-Russell (H-R) diagram (Fig. 5.1). The chemical composition of the Sun appears to be typical for a number of stars having similar size and evolution, so the solar composition may be considered as a convenient reference.

400 -1—mTTTTl-1—I I I lllll-1—I I 1111-1—I I I lllll

1

I 1 1111 1

1 1

Mil 1 1

inn

| 1 1 1 1 Mil

O Graphite

A SiCX

Solar

-

-

• A

-

-

-

-

A

A

A "

A

1

1

1 1

1 1

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Table 3.1 Chemical classification of the elements'a using volatility and affinity to metallic or silicate phases as criteriab

Elements

Lithophile (silicate)

Siderophile and/or chalcophile

refractory

Al, Ca, Ti, Be, Ba, Sc,

V, Mo, Ru, Rh, W, Re, Os, Ir, Pt

Tc = 1400 to 1850 Kc

Sr, Y, Zr, Nb, REE, Hf,

Ta, Th, U, Pu

transitional

Mg, Si, Cr, Li

Fe, Ni, Co, Pd

TC = 1250 to 1350 K

moderately volatile

Mn, P, B, Na, Rb, K,

Au, Cu, Ag, Ga, Sb, Ge, Sn, Se,

TC = 700 to 1230 K

F, Zn

S, Te, As

highly volatile

Cl, Br, I, Cs,

Cd, In, Tl, Bi, Pb, Hg

TC < 700 K

H, C, O, He, Ne, Ar,

Kr, Xe

a Elements are not listed in order of increase of the given characteristics.

b After Wasson (1985), McDonough and Sun (1995), Palme (2000) and Palme and Jones

c TC is the condensation temperature at a pressure of 10-4 bar.

a Elements are not listed in order of increase of the given characteristics.

b After Wasson (1985), McDonough and Sun (1995), Palme (2000) and Palme and Jones

c TC is the condensation temperature at a pressure of 10-4 bar.

Several independent approaches can be applied to determine the chemical and isotopic composition of the solar system: spectroscopy of the Sun's photosphere; measurements of solar-wind implantations in manmade and natural materials; the study of objects orbiting the Sun, e.g. planets, satellites, meteorite parent bodies and comets.

Deriving an inventory from all these data sources is not straightforward, for many reasons. Regarding the raw material, interstellar grains are greatly heterogeneous (Section 3.3) and differ in isotope composition from interstellar gas. Even after substantial mixing, traces of initial heterogeneity are observed in present-day solar system materials and the compositions depend on the origin and evolution of each given object. During the accretion of solid bodies in the solar nebula, chemical fractionation generally took place, at least in the inner segment where the terrestrial planets were formed. If the behaviour of a given element is known, it is possible to predict the effect that such fractionation processes could have on it. There are several classifications of the elements based on their behaviour in different natural fractionation processes (Table 3.1). For example, during condensation and crystallization from a hot gas in circumstellar or nebula environments, minerals formed by refractory elements such as Zr (which forms zircon) or Al (which forms corundum) grow first while other elements are still in the gas phase (Chapter 10). If some process subsequently separates the gas and solid phases, elements having different volatilities will be fractionated. Another example is the separation of metallic iron from silicate material in meteorite parent bodies or planetary interiors. In this process siderophile elements were presumably partitioned into the metal, leaving the silicate mantles depleted in these elements (Chapter 18).

Some bodies in the asteroid belt (Fig. 14.1) have undergone only minor processing and therefore may be used to reconstruct the bulk solar system composition. Collisions have allowed samples of these bodies to be delivered to our planet as meteorites. The class of meteorites known as chondrites contains small spheroids called chondrules (hence their name) and have a complicated, non-equilibrium, texture, indicating relatively primitive material. Traditionally, C1, carbonaceous, chondrites are considered to be the most primitive, and their composition is used as a reference even though unfortunately they have not avoided hydrous alteration (Chapter 11). Even though C1 meteorites contain volatile-rich low-temperature material, as solid bodies they have preserved only a tiny portion of gases. Therefore the solar abundances of the highly volatile elements H, C, N and of the noble gases have to be derived from spectroscopic measurements on the solar atmosphere, solar-wind studies and investigations of the giant planets.

Cl-meteorite, solar and terrestrial element and isotope abundances: a comparison

Except for the highly volatile elements (the last row in Table 3.1) the agreement between the two reservoirs, C1 meteorites and the solar photosphere, appears to be remarkably good for the majority of elements. The similarity for the refractory siderophile and lithophile elements, measured with relatively good precision, is the most impressive (Fig. 3.6). Anders and Grevesse (1989) discussed reasons for the discrepancies observed for several elements.

In contrast with C1 meteorites, the terrestrial planets are highly differentiated bodies that have experienced a complicated evolution, as discussed below in Parts III and IV of the book, and no terrestrial sample could ever be imagined to represent the bulk Earth composition. To some degree, the refractory lithophile elements present an exception. Although the abundances of Al, Zr, Nb, the rare Earth elements (REEs), U, Th and some others are found to be fractionated, in the Earth's accessible reservoirs (EARs), they nevertheless enable us to postulate a tight affinity of terrestrial and chondritic matter.

Isotopic arguments reinforce the affinity of solar, meteoritic and terrestrial matter. The similar chemical behaviour of the isotopes of one element tends to restrict their relative fractionation in chemical processes. This is the case for most elements, especially those that are refractory. Figure 3.7 illustrates the similarity of solar and chondritic isotopic compositions for several abundant elements; the solar data

Abundance of the elements: meteoritic and solar

L¡| AISc V | Y ^b Ru Ba Ce Nd EuTb HoTm Lui W| Ir Th Mg PFeNiAu B Na K CuGaRbSn CI In Pb

refractor

«Ml siderophile

moderately volatile

Be Co Ti Sr Zr Mo Rh La PrSmGd Dy Er Yb Hf Os Pt Si Cr Co Pd

highly volatile*

Fig. 3.6 Comparison of solar and meteoritic abundances of the elements. Agreement between the solar and CI abundances of the elements appears to be remarkably good, especially taking into account that both reservoirs, the solar photosphere and the CI parent bodies, represent a negligibly small portion of solar system material. From Anders and Grevesse (1989), © Pergamon Press 1989, reproduced by permission of Elsevier Science.

ro cc

ro CC

25Mg/24Mg 26Mg/24Mg 29SI/28SI 30Si/28Si 40Ca/44Ca 40Ca/42Ca 54Fe/56Fe

Fig. 3.7 The isotope compositions of several abundant elements in solar emissions and in C1 chondrites are indistinguishable (within the precision of the measurements). SSW, slow solar wind; FSW, fast solar wind; ASW, average solar wind; CME, coronal mass ejection. After Anders and Grevesse (1989), Wimmer-Schweingruber et al. (1998, 1999a, b) and Kallenbach (2001).

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L iL s . t %

1

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25Mg/24Mg 26Mg/24Mg 29SI/28SI 30Si/28Si 40Ca/44Ca 40Ca/42Ca 54Fe/56Fe

Fig. 3.7 The isotope compositions of several abundant elements in solar emissions and in C1 chondrites are indistinguishable (within the precision of the measurements). SSW, slow solar wind; FSW, fast solar wind; ASW, average solar wind; CME, coronal mass ejection. After Anders and Grevesse (1989), Wimmer-Schweingruber et al. (1998, 1999a, b) and Kallenbach (2001).

were obtained from solar-wind analyses. Even though solar-wind propagation itself probably causes some fractionation, the observed agreement is quite good. These abundant elements are among the major building blocks of the Earth, together constituting more than half the Earth's mass. Their terrestrial, lunar and meteoritic isotopic compositions are indistinguishable except in presolar grains, as discussed above. Moreover, the initial ratios of daughter radiogenic and stable reference isotopes, e.g. 87Sr/86Sr or 143Nd/144Nd, comprise a self-consistent set of systematics for meteorites and the Earth.

The comparison of elemental abundances together with the isotopic arguments leaves little doubt that the solar system as a whole was made from the same relatively well (but not completely) mixed material (Figs. 3.6, 3.7).

Solar system elemental and isotope abundances

Following the above discussion, a reference solar system composition may be synthesized from two principal sets of objects:

• the solar atmosphere and neighbouring interstellar clouds, which are especially important for constraining the abundances of H, the noble gases and some other volatile elements;

• meteoritic and planetary materials, as well as the matter in other objects within the solar nebula.

The first-order trend seen in the integrated solar system abundances of the elements is a decrease in abundance as the atomic number increases: the more massive the element, the lower the abundance (Fig. 3.8 and Table 3.2). Against the

Table 3.2 Solar system abundance of the elements, from Anders and Grevesse (1989), © Pergamon Press 1989, reproduced by permission of Elsevier Science

Table 3.2 Solar system abundance of the elements, from Anders and Grevesse (1989), © Pergamon Press 1989, reproduced by permission of Elsevier Science

Z

Element

Atoms per 106 atoms of Si

1

H

2.79 x

1010

2

He

2.72 x

109

3

Li

57.1

4

Be

0.73

5

B

21.2

6

C

1.01 x

107

7

N

3.13 x

106

8

O

2.38 x

107

9

F

843

10

Ne

3.44 x

106

11

Na

5.74 x

104

12

Mg

1.07 x

106

13

Al

8.49 x

104

14

Si

1.00 x

106

15

P

1.04 x

104

16

S

5.15 x

105

17

Cl

5.24 x

103

18

Ar

1.01 x

105

19

K

3.77 x

103

20

Ca

6.11 x

104

21

Sc

34.2

22

Ti

2.4 x 103

23

V

293

24

Cr

1.35 x

104

25

Mn

9.55 x

103

26

Fe

9.0 x 105

27

Co

2.25 x

103

28

Ni

4.93 x

104

29

Cu

522

30

Zn

1.26 x

103

31

Ga

37.8

32

Ge

119

33

As

6.56

34

Se

62.1

35

Br

11.8

36

Kr

45

37

Rb

7.09

38

Sr

23.5

39

Y

4.64

40

Zr

11.4

41

Nb

0.698

42

Mo

2.55

44

Ru

1.86

45

Rh

0.344

46

Pd

1.39

Table 3.2 (cont.)

Z

Element

Atoms per 106 atoms of Si

47

Ag

0.486

48

Cd

1.61

49

In

0.184

50

Sn

3.82

51

Sb

0.309

52

Te

4.81

53

I

0.9

54

Xe

4.7

55

Cs

0.372

56

Ba

4.49

57

La

0.446

58

Ce

1.136

59

Pr

0.167

60

Nd

0.828

62

Sm

0.258

63

Eu

0.097

64

Gd

0.33

65

Tb

0.06

66

Dy

0.394

67

Ho

0.089

68

Er

0.251

69

Tm

0.038

70

Yb

0.248

71

Lu

0.037

72

Hf

0.154

73

Ta

0.021

74

W

0.133

75

Re

0.052

76

Os

0.675

77

Ir

0.661

78

Pt

1.34

79

Au

0.187

80

Hg

0.34

81

Tl

0.184

82

Pb

3.15

83

Bi

0.144

90

Th

0.0335

92

U

0.009

background of this trend several important features are readily recognized, some of which result from the particular binding energy characteristics of nuclei (Section 1.2). The light elements Li, Be and B, having low binding energies, are of low abundance. Iron and its neighbouring elements, which have the highest binding energies (Fig. 1.2), are overabundant, constituting the iron peak. Some other tightly

Atomic number Z

Fig. 3.8 Abundance of the elements in the solar system. Note the overall smooth decrease in abundance except for a severe underabundance of the light fragile elements and several peaks related to species with magic proton and neutron numbers. The Fe peak is especially distinct because its isotopes have the highest binding energy per nucleon (see Fig. 1.2). The inset shows the only gap in the abundance curve: all Tc isotopes are radioactive (t ~ 105 yr or less) and Tc is observed only in stellar photospheres, thus indicating that it is produced in stars. After Anders and Grevesse (1989).

Atomic number Z

Fig. 3.8 Abundance of the elements in the solar system. Note the overall smooth decrease in abundance except for a severe underabundance of the light fragile elements and several peaks related to species with magic proton and neutron numbers. The Fe peak is especially distinct because its isotopes have the highest binding energy per nucleon (see Fig. 1.2). The inset shows the only gap in the abundance curve: all Tc isotopes are radioactive (t ~ 105 yr or less) and Tc is observed only in stellar photospheres, thus indicating that it is produced in stars. After Anders and Grevesse (1989).

bound elements (e.g. Pb, with doubly magic 208Pb) are more abundant than their neighbours. The heaviest elements, having low binding energy, are unstable and show negligibly low natural abundances except for relatively long-lived U and Th. The short-lived element Tc is also absent (see the inset in Fig. 3.8).

The sawtooth shape of the abundance curve is also predicted. For the heavy elements it results primarily from the fact that elements with even Z can have many isotopes, whereas in the light elements the greater binding energy of even-even nuclei is the most important factor (Section 1.2).

These and other features are also seen in Fig. 3.9, which summarizes the solar system isotopic abundances (Anders and Grevesse, 1989). The odd-odd nuclei are characterized by quite low binding energy, and only four of these nuclei are stable. The abundances of the even-odd species can be approximated by a smoother curve than that for the even-even species. This results from the differing decay chains for odd and even isobars (Fig. 1.4). Most even-even nuclei are at the top of the isotope

10 10 i

CO 6

8 102-c

10-S

Sn A

Xe a

Te Xe Te(8430)

Xe a

Xe(8437)

Ba(8405)

123 124 125 126 127 128 129 130 131 132 Atomic mass number, A

oo a

Ba(8405)

123 124 125 126 127 128 129 130 131 132 Atomic mass number, A

60 80 100 120 140 160

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