Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...2..493b&link_type=abstract
Treatise on Geochemistry, Volume 2. Editor: Richard W. Carlson. Executive Editors: Heinrich D. Holland and Karl K. Turekian. pp
Mathematics
Logic
6
Scientific paper
The mantle is the Earth's largest chemical reservoir comprising 82% of its total volume and 65% of its mass. The mantle constitutes almost all of the silicate Earth, extending from the base of the crust (which comprises only 0.6% of the silicate mass) to the top of the metallic core at 2,900 km depth. The chemical compositions of direct mantle samples such as abyssal peridotites (Chapter 2.04) and peridotite xenoliths (Chapter 2.05), and of indirect probes of the mantle such as basalts from mid-ocean ridge basalts (MORBs) and ocean island basalts (OIBs) (Chapter 2.03), and some types of primitive granites, tell us about the compositional state of the modern mantle, with ever increasingly detailed information providing strong evidence for chemical complexity and heterogeneity at all scales (Chapter 2.03). This chemical heterogeneity must reflect the complex physical interplay of a number of distinct long-lived geochemical reservoirs that are identified primarily by their radiogenic isotopic compositions.Many of the chapters in this volume provide detailed images of the current chemical and physical state of the Earth's mantle, whereas other contributions examine the starting composition for the Earth (Chapter 2.01). This chapter attempts to link these two areas by tracking the composition of the mantle through time. The first part of this chapter is a summary of the empirical evidence for secular change in the chemical composition of the mantle from the formation of the Earth at 4.56 Ga throughto the present day. The emphasis is on results from the long-lived radiogenic isotopic systems, in particular 147Sm-143Nd, 176Lu-176Hf, 87Rb-87Sr, and 187Re-187Os systems as these isotopic data provide some of the best constraints on the composition of the mantle in the first half of Earth history, and the timing and extent of chemical differentiation that has affected the mantle over geologic time. Selected trace element data and the "short-lived" 146Sm-142Nd isotopic systems are also considered. Understanding the origin of chemical heterogeneity in the Earth's mantle remains a fundamental focus in Earth science. Thus, the second part of this chapter is devoted to the implications of these observations for some of the key questions in mantle geochemistry such as, what are the major chemical reservoirs, when did they form, and how do they interact with each other? Was the Archean mantle substantially different from the modern mantle? How much, if any, of what we see in the modern mantle is a result of early planetary differentiation processes such as those inferred for the Moon and Mars, and how much is a result of the prolonged effects of plate tectonic processes? Has the mantle become more or less chemically heterogeneous with time? All of these questions remain active areas of research, and the intention of this chapter is primarily to present an overview of the current "state of play," which will undoubtedly evolve rapidly with further study.
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