Extreme lithium isotopic fractionation in three zircon standards (Plešovice, Qinghu and Temora).
Extreme lithium isotopic fractionation in three zircon standards (Plešovice, Qinghu and Temora).
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DOI：
10.1038/srep16878
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发表时间：
2015-11-23
影响因子：
4.6
通讯作者：
O'Reilly SY
中科院分区：
文献类型：
--
作者：
Gao YY;Li XH;Griffin WL;Tang YJ;Pearson NJ;Liu Y;Chu MF;Li QL;Tang GQ;O'Reilly SY
为了了解Li在锆石中的行为，我们分析了广泛用于微束U-Pb年龄分析和O-Hf同位素分析的三个锆石标准(PLEšOvices、清湖和Thema)中Li的丰度和同位素组成。我们使用Cameca 1280HR二次离子质谱仪(SIMS)绘制了大颗粒上的Li浓度([Li])。所有的锆石都有一个5-20 μm宽的边缘，其中[Li]比核心高5到20倍。在单个锆石颗粒的边缘观察到小范围的高达20‰的同位素分馏。测得的δ_7Li值：Ple-‰为-14.3~3.7‰，清湖为-2 2.8~1.4‰，托尔玛锆石为-4.7~16.1‰。[Li]和δ7Li在晶界变化较大，但在晶核中相对均匀。从锆石边缘到核心，[Li]迅速降低，而δ7Li迅速增加，表明Li在锆石中的较大同位素变化可能是由扩散引起的。我们的数据表明，锆石核心中均一的δ7Li可以保留岩浆的原始同位素特征，而矿物分离物和块状岩石样品中Li同位素的整体分析可能会产生误导数据。
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.