[论文解读] A Raman spectroscopic study of zircons on micro-scale and Its significance in explaining the origin of zircons

Xuezhao Bao, Huiming Li+1 more|ArXiv.org|Jul 23, 2007

Geological and Geochemical Analysis参考文献 7被引用 4

一句话总结

本研究利用微区拉曼光谱，通过分析锆石从核心到边缘的拉曼峰强度和H/W比值变化趋势，区分岩浆成因与变质成因锆石。结果表明，岩浆锆石中强度和H/W比值的降低，表明其边缘处蚀变程度（metamictization）和U/Th浓度逐渐升高；而变质锆石中强度和H/W比值的增加，则反映其U/Th浓度降低且蚀变程度减小。该方法可有效识别锆石成因，并为母岩演化历史提供重要信息。

ABSTRACT

The magmatic and metamorphic zircons were investigated with Raman spectrum microprobe analysis. We found notable differences between these two kinds of zircons exhibited by the variation trend of Raman peak intensity from core to rim of a crystal. In magmatic zircons, the intensity and the ratio H/W of Raman spectrum peaks gradually decrease from core to rim of a crystal, which is produced by an increase in metamictization degree and suggests an increase in U and Th concentrations from core to rim. In metamorphic zircons, there are two kinds of crystals according to their Raman spectra: the first group of zircons exhibits a variation trend opposite to those of magmatic zircons, tending to increase in the Raman peak intensity and H/W value from core to rim of a crystal, which is produced by a decrease in metamictization degree and indicates a decrease of U and Th concentrations from core to rim of a crystal. The second group of zircons exhibits no change in Raman peak intensity and H/W value through a crystal. The data of infrared and Raman spectra of these crystals show that they are well crystallized and have no lattice destruction induced by metamictization, and are thought to crystallize in high temperature stages of metamorphism. During these stages, the U and Th ions have been removed by metamorphic fluids from the parent rocks of these zircons. The other difference between magmatic and metamorphic zircons is the background level of their Raman spectra, which is high and sloped in magmatic zircons, but low and horizontal in metamorphic zircons. The differences between magmatic and metamorphic zircons can be used to identify the genesis of zircons and understand the origin and evolution history of their parent rocks.

研究动机与目标

• 利用微区拉曼光谱区分岩浆成因与变质成因锆石。
• 研究锆石晶体从核心到边缘的拉曼峰强度和H/W比值变化特征。
• 将观测到的拉曼光谱趋势与蚀变程度及U/Th浓度梯度相关联。
• 通过光谱背景和强度分析，推断含锆石岩石的结晶条件与流体相互作用。
• 建立一种基于光谱学的方法，用于确定锆石成因并重建母岩演化历史。

提出的方法

• 对岩浆岩和变质岩中单颗粒锆石晶体进行拉曼光谱微区探针分析。
• 测量从核心到边缘各截面的拉曼峰强度和H/W比值（高频峰与低频峰强度之比）。
• 分析红外光谱与拉曼光谱，评估由蚀变引起的结晶度和晶格损伤程度。
• 对比光谱背景特征：岩浆锆石表现为高且倾斜的背景，变质锆石则为低且平坦的背景。
• 将光谱趋势与根据蚀变程度推断出的U和Th浓度梯度进行关联。
• 根据光谱行为将变质锆石分为两组：从核心到边缘强度和H/W比值呈上升趋势的组，以及保持恒定的组。

实验结果

研究问题

• RQ1 岩浆锆石与变质锆石从核心到边缘的拉曼光谱趋势有何差异？
• RQ2 拉曼峰强度和H/W比值的变化在锆石晶体中反映了U和Th浓度梯度的何种信息？
• RQ3 锆石晶体中蚀变程度如何变化？其变化揭示了U/Th分布的何种特征？
• RQ4 为何部分变质锆石从核心到边缘的拉曼强度和H/W比值无明显变化？
• RQ5 拉曼光谱背景水平如何帮助区分岩浆成因与变质成因锆石？

主要发现

• 在岩浆锆石中，拉曼峰强度和H/W比值从核心到边缘逐渐降低，表明边缘处蚀变程度增加，且U和Th浓度升高。
• 在第一类变质锆石中，拉曼强度和H/W比值从核心到边缘逐渐升高，表明蚀变程度降低，且U和Th浓度向边缘减少。
• 在第二类变质锆石中，拉曼强度和H/W比值从核心到边缘保持恒定，表明结晶度高，未受蚀变引起的晶格损伤影响。
• 这些第二类变质锆石被解释为在高温变质阶段结晶，当时U和Th被变质流体有效迁移去除。
• 岩浆锆石表现出高且倾斜的光谱背景，而变质锆石则呈现低且平坦的背景，反映出结构无序程度的差异。
• 这些独特的拉曼光谱特征可实现锆石成因的可靠识别，并为母岩的热历史与流体活动历史提供关键信息。