Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis

[1] Zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences. Modern microbeam techniques such as SIMS and LA-ICPMS have been successfully applied to in situ U-Pb zircon age determinations, at spatial resolutions of 20–30 μm or less. Matrix-matched calibration by external standardization of well-characterized natural zircon references is a principal requirement for precise microbeam U-Pb zircon age determination due to fractionation effects between Pb and U, which usually result in an external age error exceeding 1%. Alternatively, zircons with a closed U-Pb system can be directly dated by measurement of 207Pb/206Pb isotopic ratio without external standardization, which has been a common practice for zircons older than 1.0 Ga, but not for relatively young (<1.0 Ga and particularly Phanerozoic) ones because of limitations of analytical precision. We describe in this paper a method of 207Pb/206Pb measurement on Phanerozoic zircons using a new generation of large radius magnetic sector multicollector Cameca IMS-1280 SIMS. In combination with multicollector mode, a Nuclear Magnetic Resonance (NMR) magnet controller and oxygen flooding techniques, we achieve precisions of 207Pb/206Pb ratio of <0.1% and 0.1 ∼ 0.2%, propagating to Pb/Pb age errors <0.4% and 1–3% (excluding U decay constant uncertainties), for zircons of latest Neoproterozoic and late Paleozoic to Mesozoic age, respectively. Therefore, the multicollector SIMS is capable of direct determination of zircon Pb/Pb ages as young as Mesozoic age with uncertainties of geological significance. This technique is useful for direct dating of zircons in thin sections. Moreover, it has significance for dating of some other U-rich minerals (i.e., baddeleyite and zirconolite) that are not suitable for SIMS U-Pb dating by external standardization.

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Precise determination of Phanerozoic zircon Pb/Pb age by multicollector SIMS without external standardization

Age significance of U-Th-Pb zircon data from early Archaean rocks of west Greenland - a reassessment based on combined ion-microprobe and imaging studies - reply

We integrate petrography and SIMS REE analyses of garnet and polyphase zircon from a pelitic granulite adjacent to the Ronda peridotite, Betic Cordillera, southern Spain to constrain the significance of zircon U–Pb geochronology. Sillimanite inclusions in garnet rims suggest that they grew during decompression, and Ca enrichment in their rims records initiation of partial melting. Chondrite-normalised REE profiles of zircon cores are typically magmatic (positive La to Lu slope and Ce anomaly), whereas overgrowths have flat or negatively sloping heavy-REE profiles (Gd–Lu). The presence of rimmed zircon grains only in the garnet rims and the matrix suggests that this zircon phase grew after garnet had already sequestered heavy REEs, a process documented here by progressive depletion of heavy REE in the garnets from centre to rim. Combined with the textural evidence, we suggest that the U–Pb age of 21.3±0.3 Ma obtained from the zircon rims dates a point on this decompression path rather than the peak metamorphic pressure.

Dating high-grade metamorphism—constraints from rare-earth elements in zircon and garnet

Secondary ion mass spectrometry (SIMS) is a versatile technique for measuring the chemical and isotopic composition of solid materials on a scale of a few microns. A beam of high-energy primary ions is focused onto the polished target surface, sputtering (ablating) atoms and molecules, and in the process ionizing a small fraction. These secondary ions, which reflect the target composition, are analyzed by mass spectrometry. The SIMS instrument most common in geoscience is the ion microprobe, which uses a focused primary ion beam in either static or scanning mode to sample target areas usually 10- to 50-μm diameter. Total sampling depth is typically less than 5 μm and the sampled mass only a few nanograms, making the analysis for most samples effectively non-destructive. Coupled with surface imaging techniques such as backscattered electron (BSE) and cathodoluminescence (CL), SIMS enables finely targeted chemical and isotopic analysis of specific domains exposed on a crystal surface.

Zircon is particularly suitable for SIMS U–Th–Pb geochronology. It is a physically and chemically robust mineral that crystallizes under a range of geological conditions, incorporating trace U and Th, but little or no Pb. Zircon grains are commonly composite, having survived and grown during several geological events. This growth record is sometimes visible under an optical microscope, but is best revealed by CL and BSE imaging of polished sectioned grains (Fig. 1⇓). The U–Th–Pb closure temperature of unaltered zircon is very high (>900°C), and the growth domains commonly can preserve an isotopic record of thermal events spanning tens to thousands of millions of years. This record of provenance, and igneous and metamorphic history, is accessible only to microanalytical techniques such as SIMS.



 Figure 1. 
Both zircon crystals appear homogeneous in transmitted light but cathodoluminescence images show the true growth structures: (a) an igneous zircon with simple euhedral oscillatory zoning, …

T. Sunde

Papers

Ion-microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland