Mineralogy of the planets: a voyage in space and time

Abstract

Equilibrium progressive condensation of the solar nebula is regarded as a useful theoretical boundary, but complex processes involving crystal-liquid differentiation in, and collisions between, planetesimals are used to interpret the properties of meteorites and terrestrial planets. Chemical differentiation in the nebula begins with condensation and aggregation of dust, which can yield oxidized and reduced products depending whether C/O is less or greater than unity. Simple models for direct accretion of condensed materials into planets are reviewed but not adopted. For the Earth, the early history is constrained by Archaean rocks dating from -3.8 x lO 9 yr whose properties indicate a non-reducing atmosphere, and a mantle that yielded volcanic rocks mostly similar to recent ones. Physical interactions involving small bodiesThe upper mantle (above 200 km depth) contains peridotitic rocks attributable to crystal-liquid differentiation and metamorphism. Volatile elements exist in mica and other minerals, but are sparse. Abundances of sider0Phile and chalcophile elements are high enough to require late accretion of material rich in these elements, the presence of a barrier between upper mantle and core, and some extraction by sinking sulphide. The mantle (deeper than 200 kin) and core are inaccessible to direct study but interpretation of seismic data coupled with high-pressure laboratory studies requires inversion to dense phases in the mantle (especially perovskite?) and presence of light elements in the core (mainly S?). The bulk composition is modelled by cosmochemical analogy constrained by geophysical and geochemical parameters. The early condensate may be augmented by 1. 5 +0.5? over (Mg+Si), and metal by 1.2++O.I?, while alkalies are probably depleted six-fold. Radial heterogeneity from a reduced interior to an oxidized exterior is suggested. For the Moon, sections cover observations, petrologic interpretations, and bulk chemical composition. For the origin of the

Speculations on the composition and origin of the earth.

“…The retention of all of its primordial com plements of Cr and Mn in the Earth, as entailed by the principles of elemental distribution out lined in LIu (1982a), is not in line with conven tional ideas regarding the abundances of these elements in the Earth (e.g., MASON, 1966;ANDERS, 1977;RINGWOOD, 1979;SMITH, 1979). However, the present model is supported by the results of recent investigations of DREIBUS and WANKE (1980) on the parent body of eucrites.…”

“…Ni, Al and Ca each amounts to 1-2%. S can amount to about 3 % if it is assumed to be the major light element in the core (SMITH, 1979). No other element has ever been estimated to be more than 1 %.…”

“…This rials in the pressure range shown in Fig. 4 should leaves 0 and S as the most likely candidates all represent their corresponding high-pressure (e.g., BRETT, 1976;SMITH, 1977SMITH, , 1979RING modifications. In comparison with all other WOOD, 1979).…”

“…WASHINGTON (1925), NIGGLi, (1928, MASON (1966), GANAPATHY and ANDERS (1974), ANDERS (1977), RINGWOOD (1959RINGWOOD ( , 1979 and SMITH (1977SMITH ( , 1979 together with a large num ber of other pertinent papers concerning the composition of the Earth's crust, mantle and core.…”

Tectonics and composition of Venus

“…The density of Venus is 1.2 to 1.9% less than that of the Earth after correcting for the difference in pressure [Ringwood and Anderson, 1977;Smith, 1979]. This has been attributed to differences in iron content [e.g., Kovach and Anderson, 1965], sulfur content [Lewis, 1972] or oxidization state of the mantle [Ringwood and Anderson, 1977].…”