Abstract
To understand the behavior of Li in zircon, we have analyzed the abundance and isotopic composition of Li in three zircon standards (Plešovice, Qinghu and Temora) widely used for microbeam analysis of U-Pb ages and O-Hf isotopes. We have mapped Li concentration ([Li]) on large grains, using a Cameca 1280HR Secondary Ion Mass Spectrometer (SIMS). All zircons have a rim 5–20 μm wide in which [Li] is 5 to 20 times higher than in the core. Up to ~20‰ isotopic fractionation is observed on a small scale in the rims of a single zircon grain. The measured δ7Li values range from –14.3 to 3.7‰ for Plešovice, –22.8 to 1.4‰ for Qinghu and –4.7 to 16.1‰ for Temora zircon. The [Li] and δ7Li are highly variable at the rims, but relatively homogenous in the cores of the grains. From zircon rim to core, [Li] decreases rapidly, while δ7Li increases, suggesting that the large isotopic variation of Li in zircons could be caused by diffusion. Our data demonstrate that homogeneous δ7Li in the cores of zircon can retain the original isotopic signatures of the magmas, while the bulk analysis of Li isotopes in mineral separates and in bulk-rock samples may produce misleading data.
Highlights
In this study, we have analyzed Li isotopes (δ 7Li) and abundance ([Li]), in both whole-rock samples and in situ in zircon, and have imaged the distribution of seven trace elements (Li, P, Ti, Yb, Y, U, and Th), in three well-characterized zircon standards commonly used for U-Pb and Hf-O isotopic analysis
Zircons with euhedral crystal shapes were analyzed for both Li isotopic composition and element distribution
The abundance and isotopic composition of Li were measured along 12 traverses across seven zircon grains (2 Plešovice, 4 Qinghu and 1 Temora)
Summary
We have analyzed Li isotopes (δ 7Li) and abundance ([Li]), in both whole-rock samples and in situ in zircon, and have imaged the distribution of seven trace elements (Li, P, Ti, Yb, Y, U, and Th), in three well-characterized zircon standards commonly used for U-Pb and Hf-O isotopic analysis. Our aim is to shed light on the Li-isotope characteristics of these zircons, to find out whether variations in the Li isotopes reflect differences in source values and/or is caused by later diffusion. Three standard zircons (Plešovice[9], Qinghu[10] and Temora9,11,12) and have been chosen for this study, as they (i) are homogeneous and concordant in terms of radiogenic Pb/U ratios, (ii) are homogeneous in Hf and O isotopic composition, (iii) have crystalline (nonmetamict) structures, and (iv) occur as mm-sized grains. The Plešovice zircon is homogeneous in terms of U-Pb age and Hf isotopes, and is widely used as a standard for microbeam isotopic analyses[9]. Previous studies indicate that Qinghu zircon is quite homogeneous and can be used as a working reference standard for SIMS U-Pb and Hf-O isotopic analysis[10]. Temora zircons have been widely used as standards for U-Pb age and Hf-O isotope microanalyses[9,11,12,19]
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