The composition of the Earth

http://earthscience.rice.edu/wp-content/uploads/2015/09/AndersGrevesse.pdf

Abundances of the elements: Meteoritic and solar

Solar photospheric and meteoritic CI chondrite abundance determinations for all elements are summarized and the best currently available photospheric abundances are selected. The meteoritic and solar abundances of a few elements (e.g., noble gases, beryllium, boron, phosphorous, sulfur) are discussed in detail. The photospheric abundances give mass fractions of hydrogen (X ¼ 0:7491), helium (Y ¼ 0:2377), and heavy elements (Z ¼ 0:0133), leading to Z=X ¼ 0:0177, which is lower than the widely used Z=X ¼ 0:0245 from previous compilations. Recent results from standard solar models considering helium and heavy-element settling imply that photospheric abundances and mass fractions are not equal to protosolar abundances (representative of solar system abundances). Protosolar elemental and isotopic abundances are derived from photospheric abundances by considering settling effects. Derived protosolar mass fractions are X0 ¼ 0:7110, Y0 ¼ 0:2741, and Z0 ¼ 0:0149. The solar system and photospheric abundance tables are used to compute self-consistent sets of condensation temperatures for all elements. Subject headings: astrochemistry — meteors, meteoroids — solar system: formation — Sun: abundances — Sun: photosphere

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Solar System Abundances and Condensation Temperatures of the Elements

Zircon is the main mineral in the majority of igneous and metamorphic rocks with Zr as an essential structural constituent. It is a host for significant fractions of the whole-rock abundance of U, Th, Hf, and the REE (Sawka 1988, Bea 1996, O’Hara et al. 2001). These elements are important geochemically as process indicators or parent isotopes for age determination. The importance of zircon in crustal evolution studies is underscored by its predominant use in U-Th-Pb geochronology and investigations of the temporal evolution of both the crust and lithospheric mantle. In the past decade an increasing interest in the composition of zircon, trace-elements in particular, has been motivated by the effort to better constrain in situ microprobe-acquired isotopic ages. Electron-beam compositional imaging and isotope-ratio measurement by in situ beam techniques—and the micrometer-scale spatial resolution that is possible—has revealed in many cases that single zircon crystals contain a record of multiple geologic events. Such events can either be zircon-consuming, alteration, or zircon-forming and may be separated in time by millions or billions of years. In many cases, calculated zircon isotopic ages do not coincide with ages of geologic events determined from other minerals or from whole-rock analysis. To interpret the geologic validity and significance of multiple ages, and ages unsupported by independent analysis of other isotopic systems, has been the impetus for most past investigations of zircon composition. Some recent compositional investigations of zircon have not been directly related to geochronology, but to the ability of zircon to influence or record petrogenetic processes in igneous and metamorphic systems.

Sedimentary rocks may also contain a significant fraction of zircon. Although authigenic zircon has been reported (Saxena 1966, Baruah et al. 1995, Hower et al. 1999), it appears to be very rare and may in fact be related to …

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The Composition of Zircon and Igneous and Metamorphic Petrogenesis

Cosmochemistry of the rare earth elements: meteorite studies.

Geochim. cosmochim. acta: Rubin A.E. and Wasson J. T. (1987) Chondrules, matrix and coarsegrained chondrule rims in the Allende meteorite: Origin, interrelationships and possible precursor components 51, 1923–1937

A compilation of data on 78 elements in the nine groups of chondrites shows each to be isochemical with the exception of a few volatiles. With the exception of the most volatile elements, the groups have solar abundances to within a factor of two. The solar abundances and the chemical and physical properties of phases in the leastaltered chondrites indicate formation by grain agglomeration in the preplanetary nebula. Planets formed by the gradual growth of bodies in the solar nebula. Because there is no evidence for the formation of non-chondritic bodies in the nebula, the simplest model calls for the bulk compositions of the terrestrial planets to be chondritic. Mercury is enriched in metal, perhaps either because of high loss of silicates due to enhanced radial drag in the innermost part of the nebula, or because of enhanced accretion of metallic cores from disrupted asteroids. Chondritic compositions should be considered as boundary conditions for planetary models.

Compositions of chondrites

Progressive aqueous alteration of CM carbonaceous chondrites

The concept of 'urKREEP' (primeval KREEP), a magma residuum hypothetically produced early in lunar history by fractional distillation of the global magma ocean which hypothetically created the lunar crust, is used to explain the origin of KREEPy lunar rocks. The incompatible-rich last dregs of the magma ocean left their trace in the form of incompatible patterns that show no relative fractionation from site to site on the moon and that, with the exception of minor fractionals in two pristine clasts, are the same in pristine samples as in breccias. The high concentration on the lunar surface of these urKREEP remnants demands a high efficiency in upward transport of the incompatibles. This transport may have been enhanced by urKREEP's presumably low density and by high temperatures produced by radioactive decay in the K-, U-, and Th-rich residuum.

John T. Wasson

Papers

Geochim. cosmochim. acta: Rubin A.E. and Wasson J. T. (1987) Chondrules, matrix and coarsegrained chondrule rims in the Allende meteorite: Origin, interrelationships and possible precursor components 51, 1923–1937

Compositions of chondrites

Progressive aqueous alteration of CM carbonaceous chondrites

The origin of KREEP

The compositional classification of chondrites. vi: the cr carbonaceous chondrite group