Zircon is the main mineral in the majority of igneous and metamorphic rocks with Zr as an essential structural constituent. It is a host for significant fractions of the whole-rock abundance of U, Th, Hf, and the REE (Sawka 1988, Bea 1996, O’Hara et al. 2001). These elements are important geochemically as process indicators or parent isotopes for age determination. The importance of zircon in crustal evolution studies is underscored by its predominant use in U-Th-Pb geochronology and investigations of the temporal evolution of both the crust and lithospheric mantle. In the past decade an increasing interest in the composition of zircon, trace-elements in particular, has been motivated by the effort to better constrain in situ microprobe-acquired isotopic ages. Electron-beam compositional imaging and isotope-ratio measurement by in situ beam techniques—and the micrometer-scale spatial resolution that is possible—has revealed in many cases that single zircon crystals contain a record of multiple geologic events. Such events can either be zircon-consuming, alteration, or zircon-forming and may be separated in time by millions or billions of years. In many cases, calculated zircon isotopic ages do not coincide with ages of geologic events determined from other minerals or from whole-rock analysis. To interpret the geologic validity and significance of multiple ages, and ages unsupported by independent analysis of other isotopic systems, has been the impetus for most past investigations of zircon composition. Some recent compositional investigations of zircon have not been directly related to geochronology, but to the ability of zircon to influence or record petrogenetic processes in igneous and metamorphic systems.

Sedimentary rocks may also contain a significant fraction of zircon. Although authigenic zircon has been reported (Saxena 1966, Baruah et al. 1995, Hower et al. 1999), it appears to be very rare and may in fact be related to …

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The Composition of Zircon and Igneous and Metamorphic Petrogenesis

As a significant carrier of uranium, thorium and the rare earth elements (REEs) at ppm level, and hafnium at the percent level, zircon hosts a remarkable number of long-lived radioactive isotopes and their stable decay products. These include 238U, 235U and 232Th which decay via intermediate steps to 206Pb, 207Pb and 208Pb, respectively, 176Lu which decays to 176Hf, 147Sm which decays to 143Nd, and 138La which undergoes branched decay to 138Ce and 138Ba. This combination makes zircon one of the most versatile minerals available to the geochronologist. Whilst the majority of rock-dating studies involving zircon have utilized the U–Pb and Th–Pb decay schemes, as discussed in previous chapters, an increasing number of investigations are targeting the complementary age information available in the lesser known decay schemes. This chapter reviews the history of application of the Lu–Hf and Sm–Nd isotope systems in zircon and, in its conclusions, speculates on potential future use of the La–Ce system.

176Lu is an unstable radionuclide that undergoes spontaneous β− decay to stable 176Hf, with a half-life of approximately 35 billion years. Variations in the abundance of 176Hf are conventionally expressed with respect to 177Hf whose natural abundance is constant. Thus, the basic age equation for the Lu–Hf dating method, as applied to any closed system, is as follows:

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[(^{176}Hf/^{177}Hf)\_{t}\ =\ (^{176}Hf/^{177}Hf)\_{initial}\ +\ (^{176}Lu/^{177}Hf)_{t}\ {\cdot}\ (e^{{\lambda}t}\ {-}\ 1)\] \end{document} 

where t is the elapsed time, and λ is the 176Lu β− decay constant. The exact value of the decay constant λ is currently under review. Most geological studies have either used the value 1.94 × 10−11 y−1, originally calculated from the slope of a Lu–Hf isochron for the 4.56 Ga eucrite achondrite meteorites (Patchett and Tatsumoto 1980a, Tatsumoto et al. 1981), or …

Lu–Hf and Sm–Nd isotope systems in zircon

Granitic pegmatites of the rare_element (and miarolitic) class display extreme fractionation and accumulation.of rare lithophile elements, beyond the limits observed in other igneous and postmagmatic assemblages. Enrichment in Li and -Cs leads to precipitation of rock-forming spodumene, petalite, Li-phosphates or lepidolite, and pollucite. These gqeoes-can be interpreted as crystallochemically conditioned Li- and-Csbased feldspathoids, crystallizing along with (and after) the last generations of primary K- and Nalfeldspars. Extreme fractionation is encountered in K-feldspar gi,znb !,9.rK/Qs 22.4, K/Tt 236, Rb,zCs 6.5, Rb,/Tt 130 io 35, K,/Ba 18,0@, BalRb <0.002); muscovite and lepidolite q/R! !.4, K,/Cs 4.0, K/Tl3ZO, Rb,/Cs 4.4, nb,ril SO ro 3.6, N/Ga239): pollucite (KAl 10, Rb/Tl 60io l3); microute, srmpsonite and other late oxide minerals of Ta (Nb/Ta < 0.001); manganoranialire (Fe,/Mn 0.@3): spes_ sartine @e,/Mn < 0.@3); lithiophilite (FeZVn < O.oii: iraf_ yoy (ZrlHf 0.014), and hawteyite and dernjite (in/Cd, 0.067)- Classic crystallochemical concepts of Goldichmictr and Ringwood alone are not adequate tb explain the above values. Selective extraction and transport in a volatile phase (by diffusion or in an exsolved hydrous fluid), complex_ ing stable to relatively low late- to postmagmaiic temieratures, and crystallochemical for specific mineral structures operate in conjunction with crystal,/melt fractionation; the environment evolves from that of a volatile_ saturated residual granitic melt, through that ofa melt coex_ isting with exsolved supercritical fluid, to that of an aque_ ous solution and possibly a gas phase, in a closed oirestricted system.

Extreme fractionation in rare-element granitic pegmatites; selected examples of data and mechanisms

Geochemical and petrogenetic features of mineralization in rare-element granitic pegmatites in the light of current research



High-precision data on U and Th contents and Th/U ratios of zircon obtained using secondary ion mass spectrometry analysis have been collected from the literature. Zircon in the granitic rocks has median values of 350 ppm U, 140 ppm Th, and Th/U=0.52; the recommended zircon-melt partition coefficients are 81 for DU and 8.2 for DTh. In zircon from mafic and intermediate rocks, the median values are 270 ppm U, 170 ppm Th, and Th/U=0.81, and the recommended zirconmelt partition coefficients are 169 for DU and 59 for DTh The U and Th contents and Th/U ratios of magmatic zircon are low when zircon crystallizes in equilibrium with the melt. Increasing magma temperature should promote higher Th contents relative to U contents, resulting in higher Th/U ratios for zircon in mafic to intermediate rocks than in granitic rocks. However, when zircon crystallizes in disequilibrium with the melt, U and Th are more easily able to enter the zircon lattice, and their contents and Th/U ratios depend mainly on the degree of disequilibrium. The behavior of U and Th in magmatic zircon can be used as a geochemical indicator to determine the origins and crystallization environments of magmatic zircon.

U and Th Contents and Th/U Ratios of Zircon in Felsic and Mafic Magmatic Rocks: Improved Zircon-Melt Distribution Coefficients

A mineralogical nomenclature applicable to the natural binary isomorphous series ZrSiO4-HfSiO4 is proposed. It is suggested that (Hf, Zr)SiO4 with more than 90 mol.-% hafnium component be called hafnon.

J. E. Lopes Nunes

Papers

High hafnium members of the zircon-hafnon series from the granite pegmatites of Zambézia, Mozambique