In the preceding sections, we have shown how isotopes play an important role in tackling a variety of problems concerned with tracing the Earth’s evolution, with an emphasis on radioactive isotopes as a unique time marker. Let us recall, for example, that argon consists of three isotopes argon-36, argon-38, and argon-40, all of which are stable isotopes. Argon-40 is a radiogenic stable isotope, and its amount increases with time through the radioactive decay of potassium-40. The other two isotopes of argon are non-radiogenic stable isotopes, and their abundances have not changed since their birth in a star. We have discussed radiometric geochronology, in which the amount of a radiogenic stable isotope such as argon-40 yields an absolute time marker of rock or mineral formation age.

Preface to the second edition Preface to the first edition 1. Heat from within: energy supporting the dynamic Earth 2. At the time of Earth's birth 3. Formation of the layered structure of Earth 4. Time scale of Earth's evolution 5. Plate tectonics evolution 6. Evolution of the mantle 7. Origin of the atmosphere and oceans 8. Isotopes as DNA of nature 9. Earth's magnetism 10. Moon: a looking glass to mirror the ancient Earth 11. The past and future of the evolving Earth References Index.

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A clear understanding of the Earth's past evolution can provide the key to its possible future development. The Earth: Its Birth and Growth explores the evolution of the Earth over 4.6 billion years using basic reasoning and simple illustrations to help explain the underlying physical and chemical principles and major processes involved. Fully updated and revised, this rigorous but accessible second edition includes three completely new chapters. It incorporates exciting developments in isotope geology, placing results within a wider framework of Earth evolution and plate tectonics. Some background in physics and chemistry is assumed, but basic theories and processes are explained concisely in self-contained sections. Key research papers and review articles are fully referenced. This book is ideal as supplementary reading for undergraduate and graduate students in isotope geochemistry, geodynamics, plate tectonics and planetary science. It also provides an enjoyable overview of Earth's evolution for professional scientists and general readers.

Introduction: The isotopic composition of noble gases is a key reference parameter in discussing the evolution of the solar system. Currently, two widely occurring noble gas components are identified in the early solar system, one is the Solar Wind noble gas (SWnoble gas, hereafter) and another is the Qnoble gas in primitive meteorites. Both noble gases are characterized by their ubiquitous occurrence and high isotopic homogeneity. Since the SW-noble gas is directly ejected from the Sun, it has been assumed to be good proxy for the average noble gas isotopic composition of the Sun, namely the solar noble gas. The systematic enrichment of the heavier isotopes in the Q-noble gas relative to the SW-noble gas is then commonly attributed to its isotopic fractionation from the SWnoble gas. Here, we show that the post Dburning Q-noble gas (see below) is better proxy for the solar noble gas in contrary to a conventional view. Results: The most distinct difference between the Qand the SW-noble gas is apparent in a He/He isotopic ratio; 1.23 x 10 in Q-He [1], whereas 4.64 x 10 in SW-He [2]. The difference is attributed to the conversion of deuteron (D) to He (D + p → He) in the Sun, namely the D-burning [3], due to high temperature during the pre-main sequence stage of the Sun. With the use of recent data on H/He ratio (11.75± 0.27) from helioseismology [4] and D/H ratio ((23.1± 2.4) x 10) from spectroscopic observation of the inter-stellar cloud [5], we estimate that the He/He ratio in the post D-burning He in the Sun is (3.98 ± 0.3) x 10. The latter value is considerably smaller than the recent estimate of the SW-He ratio by the GENESIS mission of He/He = (4.64 ± 0.09) x 10 [2]. We conclude that this difference is due to isotopic fractionation during the ejection of the Solar Wind from the upper solar atmosphere (accelerating region). In Figure 1, we show schematically the isotopic evolution of He/He in the early solar nebula.

Surface-correlated noble gases in lunar soils are primarily implanted SW (solar wind) noble gases. However, they also include apparently orphan radiogenic 40Ar, 129Xe, and 244Pu-derived fission Xe in excess of plausible primordial solar origin. These orphan radiogenic components are usually assigned a lunar origin, in a scenario in which radiogenic noble gases produced in the lunar interior were degassed into the transient atmosphere and then re-implanted to the lunar surface together with SW. There are some quantitative difficulties with this scenario, however, and it requires special constraints on the degassing history of the Moon that have not emerged from more general thermal history models. We therefore urge consideration of alternative hypotheses. As a possible source for the orphan radiogenic noble gases, we have examined planetary pollution of the Sun, as suggested by studies of extrasolar planetary systems (e.g., Murray et al., 2001, Astrophys. J. 555, 801–815; Israelian et al., 2001, Nature 411, 163–166). Pollution of the Sun by 2M⊕ (two Earth mass) planetary materials (Murray et al., 2001, Astrophys. J. 555, 801–815) is likely not significant for Ar but could be important to account for orphan Xe in the Moon.