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Salters, V.J.M. and Hart, S.R. (1991) The mantle sources of ocean ridges, islands and arcs: the Hf-isotope connection. Earth Planet. Sci. Lett., 104, 364-80.
HAFNIUM Element 72, with an atomic weight of 178.485, is chemically a very close analog of zirconium, and is almost always enriched or depleted to the same degree. Erlank et al. (1978) have summarized all aspects of Hf geochemistry. The most important mineral host by far in the Earth's crust is zircon (Zr,Hf)Si0 4 , where Hf averages 1%, corresponding to the terrestrial Zr/Hf ratio of ca. 37. Hf has acquired a new importance in recent years due to the development of Hf isotope geochemistry (see Lutetium-hafnium decay system). Although dating applications are possible (Patchett, 1983; Faure, 1986), the utility of variations in radiogenic 176 Hf lies mainly in petrogenesis and crust-mantle evolution. 176Hfj 177 Hf ratios vary from 0.28005 for 4.2 Ga zircon from Australia (Kinny et a!., 1991) to a maximum of 0.2835 for Mid ocean ridge basalts of today (Patchett, 1983; Salters and Hart, 1991). In the petrogenetic and crustal evolutionary context, zircons are important because they are also directly U-Pb dated and the Hf isotopic composition is more easily determined than in most rock samples (e.g. Patchett, 1983; Corfu and Noble, 1992). P. Jonathan Patchett
Bibliography Corfu, F. and Noble, S.R. (1992) Genesis of the southern Abitibi greenstone belt, Superior Province, Canada: evidence from zircon Hf isotope analyses using a single filament technique. Geochim. Cosmochim. Acta, 56, 2081-97. Erlank, A.J. et a!. (1978) Section 72 - Hafnium, in Handbook of Geochemistry (ed. K.H. Wedepohl). Berlin: Springer. Faure, G. (1986) Principles ol Isotope Geology, 2nd edn. New York: Wiley, 589 pp. Kinny, P.D., Compston, W. and Williams, I.S. (1991) A reconnaissance ion-probe study of hafnium isotopes in zircons. Geochim. Cosmochim. Acta, 55, 849-59. Patchett, P.J. ( 1983) Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution. Geochim. Cosmochim. Acta, 47, 81-91.
Cross-references Elements: high field strength Elements: lithophile Elements: trace Elements: transitional Lutetium-hafnium decay system
HELIUM After hydrogen, helium is the most abundant element in the universe ( ~ 6.5%) and is observed spectroscopically in particular in hot stars such as Rigel in the constellation of Orion. The first evidence of its occurrence was found 1868 by P. Janssen in solar spectral lines during a solar eclipse. He observed a new yellow line close to the D 1 and D 2 double line of sodium. The same observation was made independently in the same year by J.N. Lockyer, who proposed the name helium, derived from the Greek helios = sun. On the Earth, the helium D 3 spectral line was probably observed for the first time by the Italian L. Palmieri in 1881 in gases extracted from solid materials deposited on the rim of a fumarole on Mt Vesuvius, but he did not realize the importance of this observation. In 1891 W.F. Hillebrand extracted gases from uranites
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