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

Arc magmas and the continental crust share many chemical features, but a major question remains as to whether these features are created by subduction or are recycled from subducting sediment. This question is explored here using Th/La, which is low in oceanic basalts ( 0·25) and varies in arc basalts and marine sediments (0·09–0·34). Volcanic arcs form linear mixing arrays between mantle and sediment in plots of Th/La vs Sm/La. The mantle end-member for different arcs varies between highly depleted and enriched compositions. The sedimentary end-member is typically the same as local trench sediment. Thus, arc magmas inherit their Th/La from subducting sediment and high Th/La is not newly created during subduction (or by intraplate, adakite or Archaean magmatism). Instead, there is a large fractionation in Th/La within the continental crust, caused by the preferential partitioning of La over Th in mafic and accessory minerals. These observations suggest a mechanism of ‘fractionation & foundering’, whereby continents differentiate into a granitic upper crust and restite-cumulate lower crust, which periodically founders into the mantle. The bulk continental crust can reach its current elevated Th/La if arc crust differentiates and loses 25–60% of its mafic residues to foundering.

http://www.mantleplumes.org/WebDocuments/Plank.Th-La.Jpet05.pdf

Constraints from Thorium/Lanthanum on Sediment Recycling at Subduction Zones and the Evolution of the Continents

We present newly acquired trace element compositions for more than 300 zircon grains in 36 gabbros formed at the slow-spreading Mid-Atlantic and Southwest Indian Ridges. Rare earth element patterns for zircon from modern oceanic crust completely overlap with those for zircon crystallized in continental granitoids. However, plots of U versus Yb and U/Yb versus Hf or Y discriminate zircons crystallized in oceanic crust from continental zircon, and provide a relatively robust method for distinguishing zircons from these environments. Approximately 80% of the modern ocean crust zircons are distinct from the fi eld defi ned by more than 1700 continental zircons from Archean and Phanerozoic samples. These discrimination diagrams provide a new tool for fi ngerprinting ocean crust zircons derived from reservoirs like that of modern mid-ocean ridge basalt (MORB) in both modern and ancient detrital zircon populations. Hadean detrital zircons previously reported from the Acasta Gneiss, Canada, and the Narryer Gneiss terrane, Western Australia, plot in the continental granitoid fi eld, supporting hypotheses that at least some Hadean detrital zircons crystallized in continental crust forming magmas and not from a reservoir like modern MORB.

Trace element chemistry of zircons from oceanic crust: A method for distinguishing detrital zircon provenance

Contrasting geochemical and Sm-Nd isotopic compositions of Archean metasediments from the Kongling high-grade terrain of the Yangtze craton: evidence for cratonic evolution and redistribution of REE during crustal anatexis

This chapter focuses on phosphates that are significant in metamorphic rocks A quick survey of phosphate mineral descriptions at a commercial mineral web-site revealed over 500 phosphate mineral names Remarkably, only three are common in metamorphic rocks: apatite, monazite, and xenotime, and this chapter is restricted to discussion of these minerals

Apatite, monazite, and, to a lesser extent, xenotime, have enjoyed intensive study during the previous half-century Following World War II, considerable study was made of minerals that contain fissionable materials, and the sometimes large concentrations of U and Th in monazite culminated in a number of seminal papers on the occurrence of that mineral (eg, Overstreet 1967) During the 1970s and early 1980s, the study of apatite and monazite turned towards their role as a sink for REEs and other trace elements in rocks (eg Watson 1980, Watson and Capobianco 1981) Throughout this period, monazite (and to some extent xenotime) enjoyed attention as a geochronometer (eg, Parrish 1990), and through today this attention has continued to grow

Accessory minerals such as monazite, xenotime, and apatite have come to the forefront of research in metamorphic petrology in recent years for two reasons First, it has recently been recognized that trace elements in metamorphic rocks, and especially trace element zoning in garnet and other major phases, contain considerable detailed information about the reaction history a rock has experienced Coupled with the fact that diffusion of many trace elements in metamorphic minerals is, in general, considerably slower than diffusion of major elements in those same minerals, the possibility has recently emerged of recovering details of metamorphic petrogenesis that were previously unattainable Inasmuch as accessory minerals play a dominant role in the mass budget of many trace elements, attention has naturally turned to their paragenesis It has also recently been discovered that …

Apatite, Monazite, and Xenotime in Metamorphic Rocks

Mineral/leucosome trace-element partitioning in a peraluminous migmatite (a laser ablation-ICP-MS study)

Microanalysis of minerals by an Excimer UV-LA-ICP-MS system

A new ICP-MS system was used for successful determination of REE, Th, and U at ultratrace level concentrations in ultramafic rocks. Typical concentrations for these elements in the analyzed samples were found to vary between 0.01 and 1 μg/g in the solid rock material. The samples were digested using acid sample treatment in a conventional microwave oven. Accuracy and precision of the described method were tested with international standard reference materials Serpentine (UB-N) and Anorthosite (AN-G). Stability of the instrument was investigated in clean as well as in synthetic rock solution (500 mg/L Fe, Mg, and Ca) over a period of 2 h using a 100-ng/L multielement standard solution. Detection limits were calculated from 10 measurements of the acid blank and the intensities of 100 ng/L standard solution in the synthetic rock matrix, which vary between 0.12 and 0.62 ng/L

A. Stroh

Papers

Mineral/leucosome trace-element partitioning in a peraluminous migmatite (a laser ablation-ICP-MS study)

Microanalysis of minerals by an Excimer UV-LA-ICP-MS system

Ultratrace-level determination of rare earth elements, thorium, and uranium in ultramafic rocks by ICP-MS