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2.01 Cosmochemical Estimates of Mantle Composition H 2.01 Cosmochemical Estimates of Mantle Composition H. Palme Universita¨tzuKo¨ln, Germany and Hugh St. C. O’Neill Australian National University, Canberra, ACT, Australia 2.01.1 INTRODUCTION AND HISTORICAL REMARKS 1 2.01.2 THE COMPOSITION OF THE EARTH’S MANTLE AS DERIVED FROM THE COMPOSITION OF THE SUN 3 2.01.3 THE CHEMICAL COMPOSITION OF CHONDRITIC METEORITES AND THE COSMOCHEMICAL CLASSIFICATION OF ELEMENTS 4 2.01.4 THE COMPOSITION OF THE PRIMITIVE MANTLE BASED ON THE ANALYSIS OF UPPER MANTLE ROCKS 6 2.01.4.1 Rocks from the Mantle of the Earth 6 2.01.4.2 The Chemical Composition of Mantle Rocks 7 2.01.4.2.1 Major element composition of the Earth’s primitive mantle 11 2.01.4.2.2 Comparison with other estimates of PM compositions 13 2.01.4.2.3 Abundance table of the PM 13 2.01.4.3 Is the Upper Mantle Composition Representative of the Bulk Earth Mantle? 20 2.01.5 COMPARISON OF THE PM COMPOSITION WITH METEORITES 21 2.01.5.1 Refractory Lithophile Elements 21 2.01.5.2 Refractory Siderophile Elements 23 2.01.5.3 Magnesium and Silicon 24 2.01.5.4 The Iron Content of the Earth 25 2.01.5.5 Moderately Volatile Elements 26 2.01.5.5.1 Origin of depletion of moderately volatile elements 28 2.01.5.6 HSEs in the Earth’s Mantle 31 2.01.5.7 Late Veneer Hypothesis 32 2.01.6 THE ISOTOPIC COMPOSITION OF THE EARTH 33 2.01.7 SUMMARY 34 REFERENCES 35 2.01.1 INTRODUCTION AND HISTORICAL scientists begin to consider Chladni’s hypothesis REMARKS seriously. The first chemical analyses of meteor- ites were published by the English chemist In 1794 the German physicist Chladni pub- Howard in 1802, and shortly afterwards by lished a small book in which he suggested the Klaproth, a professor of chemistry in Berlin. extraterrestrial origin of meteorites. The response These early investigations led to the important was skepticism and disbelief. Only after conclusion that meteorites contained the same additional witnessed falls of meteorites did elements that were known from analyses of 1 2 Cosmochemical Estimates of Mantle Composition terrestrial rocks. By the year 1850, 18 elements the Earth. The presence of a central metallic core had been identified in meteorites: carbon, to the Earth was suggested by Wiechert in 1897. oxygen, sodium, magnesium, aluminum, silicon, The existence of the core was firmly established phosphorous, sulfur, potassium, calcium, tita- using the study of seismic wave propagation by nium, chromium, manganese, iron, cobalt, nickel, Oldham in 1906 with the outer boundary of the copper, and tin (Burke, 1986). A popular hypo- core accurately located at a depth of 2,900 km by thesis, which arose after the discovery of the first Beno Gutenberg in 1913. In 1926 the fluidity of asteroid Ceres on January 1, 1801 by Piazzi, held the outer core was finally accepted. The high that meteorites came from a single disrupted density of the core and the high abundance of planet between Mars and Jupiter. In 1847 the iron and nickel in meteorites led very early to the French geologist Boisse (1810–1896) proposed suggestion that iron and nickel are the dominant an elaborate model that attempted to account for elements in the Earth’s core (Brush, 1980; see all known types of meteorites from a single planet. Chapter 2.15). He envisioned a planet with layers in sequence of Goldschmidt (1922) introduced his zoned Earth decreasing densities from the center to the surface. model. Seven years later he published details The core of the planet consisted of metallic iron (Goldschmidt, 1929). Goldschmidt thought that surrounded by a mixed iron–olivine zone. The the Earth was initially completely molten and region overlying the core contained material separated on cooling into three immiscible liquids, similar to stony meteorites with ferromagnesian leading on solidification to the final configuration silicates and disseminated grains of metal gradu- of a core of FeNi which was overlain by a sulfide ally extending into shallower layers with alumi- liquid, covered by an outer shell of silicates. nous silicates and less iron. The uppermost layer Outgassing during melting and crystallization consisted of metal-free stony meteorites, i.e., produced the atmosphere. During differentiation eucrites or meteoritic basalts. About 20 years elements would partition into the various layers later, Daubre´e (1814–1896) carried out experi- according to their geochemical character. ments by melting and cooling meteorites. On the Goldschmidt distinguished four groups of basis of his results, he came to similar conclusions elements: siderophile elements preferring the as Boisse, namely that meteorites come from a metal phase, chalcophile elements preferentially single, differentiated planet with a metal core, a partitioning into sulfide, lithophile elements rema- silicate mantle, and a crust. Both Daubre´e and ining in the silicate shell, and atmophile elements Boisse also expected that the Earth was composed concentrating into the atmosphere. The geochem- of a similar sequence of concentric layers (see ical character of each element was derived from its Burke, 1986; Marvin, 1996). abundance in the corresponding phases of At the beginning of the twentieth century meteorites. Harkins at the University of Chicago thought that At about the same time astronomers began to meteorites would provide a better estimate for the extract compositional data from absorption line bulk composition of the Earth than the terrestrial spectroscopy of the solar photosphere, and in a rocks collected at the surface as we have only review article, Russell (1941) concluded: “The access to the “mere skin” of the Earth. Harkins average composition of meteorites differs from made an attempt to reconstruct the composition of that of the earth’s crust significantly, but not very the hypothetical meteorite planet by compiling greatly. Iron and magnesium are more abundant compositional data for 125 stony and 318 iron and nickel and sulfur rise from subordinate meteorites, and mixing the two components in positions to places in the list of the first ten. ratios based on the observed falls of stones and Silicon, aluminum, and the alkali metals, irons. The results confirmed his prediction that especially potassium, lose what the others elements with even atomic numbers are more gain.” And Russell continued: “The composition abundant and therefore more stable than those with of the earth as a whole is probably much more odd atomic numbers and he concluded that the similar to the meteorites than that of its ‘crust’.” elemental abundances in the bulk meteorite planet Russell concludes this paragraph by a statement are determined by nucleosynthetic processes. For on the composition of the core: “The known his meteorite planet Harkins calculated Mg/Si, properties of the central core are entirely Al/Si, and Fe/Si atomic ratios of 0.86, 0.079, and consistent with the assumption that it is com- 0.83, very closely resembling corresponding ratios posed of molten iron—though not enough to of the average solar system based on presently prove it. The generally accepted belief that it is known element abundances in the Sun and in composed of nickel–iron is based on the CI-meteorites (see Burke, 1986). ubiquitous appearance of this alloy in metallic If the Earth were similar compositionally to the meteorites,” and, we should add, also on the meteorite planet, it should have a similarly high abundances of iron and nickel in the Sun. iron content, which requires that the major Despite the vast amount of additional chemical fraction of iron is concentrated in the interior of data on terrestrial and meteoritic samples and Composition of the Earth’s Mantle 3 despite significant improvements in the accuracy Pluto (39.5 AU) out to some 500 AU. This region of solar abundances, the basic picture as outlined is now called Kuiper belt, a disk of comets by Russell has not changed. In the following orbiting the Sun in the plane of the ecliptic. Lack sections we will demonstrate the validity of of a chemical gradient in the inner solar system Russell’s assumption and describe some refine- (2 AU) therefore, does, not allow any conclusions ments in the estimate of the composition of regarding the entire solar system. the Earth and the relationship to meteorites and The abundances of all major and many minor the Sun. and trace elements in the Sun are known from absorption line spectrometry of the solar photo- sphere. Accuracy and precision of these data have continuously improved since the early 1930s. In a 2.01.2 THE COMPOSITION OF THE EARTH’S compilation of solar abundances, Grevesse and MANTLE AS DERIVED FROM THE Sauval (1998) list the abundances of 41 elements COMPOSITION OF THE SUN with estimated uncertainties below 30%. A first The rocky planets of the inner solar system and approximation to the composition of the bulk Earth the gas-rich giant planets with their icy satellites is to assume that the Earth has the average solar of the outer solar system constitute the gross system composition for rock-forming elements, structure of the solar system: material poor in i.e., excluding extremely volatile elements such as volatile components occurs near the Sun, while hydrogen, nitrogen, carbon, oxygen, and rare the outer parts are rich in water and other volatile gases. components. The objects in the asteroid belt, The six most abundant, nonvolatile rock- between Mars at 1.52 AU and Jupiter at 5.2 AU forming elements in the Sun are: Si (100), Mg (1 AU is the average Earth–Sun distance, ca. (104), Fe (86), S (43), Al (8.4), and Ca (6.2).
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