Degree of preservation of igneous zonation in zircon as a signpost for concordancy in U/Pb geochronology
Degree of preservation of igneous zonation in zircon as a signpost for concordancy in U/Pb geochronology
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DOI：
10.1016/s0009-2541(00)00234-5
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发表时间：
2001-02-01
期刊：
影响因子：
3.9
通讯作者：
Connelly, JN
中科院分区：
文献类型：
--
作者：
Connelly, JN
从长英质岩浆中结晶出来的锆石通常(如果不是无处不在的话)发育出很容易用阴极发光(CL)成像的成分生长带状。这种分带代表了本质上不稳定的化学梯度，只有通过缓慢的扩散才能保存下来。来自经历了单一变质事件的火成岩的CL图像表明，在变质过程中发生了化学扩散，导致岩浆锆石中锐利的原生分带逐渐模糊并最终消失。用化学发光成像表征单个锆石种群中的分带保存范围，可以提供对该种群内发生的相对扩散程度的定性估计。化学发光图像和常规U/Pb数据的结合表明，在化学扩散的规模足以改变原生分带的情况下，发生了晶外铅的损失(因此不一致)。虽然扩散程度和伴随而来的铅损失无法定量估计，但这一综合数据定义了不一致性与锆石中分带化的相对保存程度之间的强烈关系。这一联系为常规U/Pb年代学提供了一个有用的工具，可以更好地预测变质火成岩样品的不一致程度。这也证实了铅的穿透性、热驱动的铅损失发生在积累了变质岩化的锆石中。(C)2000 Elsevier Science B.V.保留所有权利。
Zircon crystallized from felsic magmas typically (if not ubiquitously) develop compositional growth zonation that is easily imaged by cathodoluminescence (CL). This zonation represents inherently unstable chemical gradients that are preserved only by virtue of slow diffusion. CL images from igneous rocks that have experienced single metamorphic events suggest that chemical diffusion occurs during metamorphism and causes progressive blurring and eventual eradication of sharp-edged, primary zonation in magmatic zircon. Characterizing the range of zonation preservation in a single zircon population using CL imaging can provide a qualitative estimate of the relative degree of diffusion that occurred within that population. The integration of CL imagery and conventional U/Pb data indicates that extra-crystalline Pb loss (and therefore discordance) has occurred where chemical diffusion has operated at scales sufficient to modify primacy zonation. Whereas the degree of diffusion and attendant Pb loss cannot be quantitatively estimated, this integrated data defines a strong relationship between discordance and the relative degree of preservation of zonation in zircon. This link provides a useful tool for conventional U/Pb geochronology to better predict the degree of discordance in samples of meta-igneous rocks. It also confirms that penetrative, thermally driven Pb loss occurs in zircon that has accumulated metamictization. (C) 2000 Elsevier Science B.V. All rights reserved.