Solar system mantle

A collision with a Mars-sized object may have resulted in the formation of the Earth s moon. Our moon is by no means the largest satellite in the solar system, but it is unusual in that it and the moon of Pluto are the largest moons relative the mass of the planets they orbit. Geochemical studies of returned lunar samples have shown that close similarities exist between the bulk composition of the moon and the Earth s mantle. In particular, the abimdances of sidero-... 

Ice mantles are important constituents of interstellar grains in molecular clouds, and icy bodies dominate the outer reaches of the solar system. The region of the solar system where ices were stable increased with time as the solar system formed, as accretion rates of materials to the disk waned and the disk cooled. The giant planets and their satellites formed, in part, from these ices, and probably also from the nebular gas itself. 

Ices formed as mantles on silicate grains in interstellar space, trapping noble gases and providing sites for the synthesis of organic compounds. As the solar system formed, these ices were vaporized, particularly in the warmer regions near the Sun. Water ice recondensed outside the snowline and combined with rocky material and surviving interstellar material to form planetesimals. 

Many asteroids are dry, as evidenced by meteorites in which water is virtually absent. These samples include many classes of chondrites, as well as melted chunks of the crusts, mantles, and cores of differentiated objects. Anhydrous bodies were important building blocks of the rocky terrestrial planets, and their chemical compositions reveal details of processes that occurred within our own planet on a larger scale. The distributions of these asteroids within the solar system also provide insights into their formation and evolution. 

The so-called late heavy bombardment, discussed more fully in Chapter 14, was a period of intense impacts by large planetesimals, concentrated in the Nectarian and Early hnbrian periods. The Procellarum and Imbrium impacts exposed subsurface rocks in the PKT. The SPA terrane formed during this time and represents not only the largest impact basin on the Moon, but also the biggest basin in the solar system. It exposes lower crust and perhaps mantle materials. 

Palme, H. and Jones, A. (2004) Solar System abundances of the elements, in Treatise on Geochemistry Volume 2 The Mantle and Core (eds R.W. Carlson, H.D. Holland and K.K. Turekian Editors-in-Chief), Elsevier Science, pp. 41-61. 

Nb decays by electron capture to Zr with a half-life of 36 3 Myr. At one time it was thought to offer the potential to obtain an age for the Moon by dating early lunar Umenites and the formation of Umenite-rich layers in the lunar mantle. Others proposed that it provided constraints on the time-scales for the earhest formation of continents on Earth (Miinker et al, 2000). In addition, it was argued that it would date terrestrial core formation (Jacobsen and Yin, 2001). There have been many attempts to utilize this isotopic system over the past few years. To do so, it is necessary to first determine the initial Nb abundance in early solar system objects accurately and various authors have made claims that differ by two orders of magnitude. 

Lee D.-C. and Halliday A. N. (2000b) Hf-W isotopic systematics of ordinary chondrites and the initial i82Hf/i80Hf of the solar system. Chem. Geol. 169, 35-43. Lee D.-C., Halliday A. N., Davies G. R., Essene E. L, Eitton J. G., and Temdjim R. (1996) Melt enrichment of shallow depleted mantle a detailed petrological, trace element and isotopic study of mantle derived xenohths and megacrysts from the Cameroon line. J. Petrol. 37, 415-441. 

Table 1 Composition of the mantle of the Earth assuming average solar system element ratios for the whole Earth.

The principal division of the Earth into core, mantle, and crust is the result of two fundamental processes, (i) The formation of a metal core very early in the history of the Earth. Core formation ended at —30 million years after the beginning of the solar system (Kleine et aL, 2002). (ii) The formation of the continental crust by partial melting of the silicate mantle. This process has... 

Earth s mantle must have occurred very early in the history of the solar system. Eigure 17 plots Cr/ Cr versus Mn/ Cr ratios of several chondritic meteorites and samples of differentiated planets. The data are taken from the work of Lugmair and Shukolyukov (1998) and Shukolyukov and Lugmair (2000, 2001). Samples from the metal-rich CH-chondrite HH237, the Earth, Allende (CV), Murray (CM), and Orgueil (Cl) define an approximately straight line in Eigure 17. If this line is interpreted as an isochron. 

The oldest samples that provide direct constraints on the osmium isotopic composition of the upper mantle are rare spinel peridotites contained within the Early Archean Itsaq gneiss complex of southwest Greenland that are interpreted to be —3.81 Ga abyssal peridotites (Friend et al., 2002). The measured and initial compositions determined from low-Re/Os spinel and olivine mineral separates from these peridotites are the most primitive, in the sense of closest to solar-system initial compositions, Os isotopic compositions yet obtained on any terrestrial material (Bennett et al., 2002). This shows that at least some, if not all, of the Early Archean upper mantle was characterized by chondritic Os/ Os isotopic compositions. Osmium isotopic constraints from this time period (—3.8-3.9 Ga) are of particular interest as they provide a rough constraint on the timing of the addition of the late veneer of... 

There are various terrestrial reservoirs that have distinct volatile characteristics. Data from midocean ridge basalts (MORBs) characterize the underlying convecting upper mantle, and are described here without any assumptions about the depth of this reservoir. Other mantle reservoirs are sampled by ocean island basalts (OIBs) and may represent a significant fraction of the mantle (Chapter 2.06). Note that significant krypton isotopic variations due to radiogenic additions are neither expected nor observed, and there are no isotopic fractionation observed between any terrestrial noble gas reservoirs. Therefore, no constraints on mantle degassing can be obtained from krypton, and so krypton is not discussed further. Comparison between terrestrial and solar system krypton is discussed in Chapter 4.12. 

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