A Raman spectroscopic study of zircons on micro-scale and Its significance in explaining the origin of zircons

TL;DR

This study uses Raman spectroscopy to distinguish magmatic from metamorphic zircons by analyzing their spectral variations, revealing insights into their origin, metamictization degree, and U/Th distribution, which aids in understanding geological history.

Contribution

It introduces a Raman spectroscopic method to differentiate zircon origins and interpret their formation and metamorphic history based on spectral variations.

Findings

Magmatic zircons show decreasing Raman peak intensity from core to rim.

Metamorphic zircons exhibit two spectral variation patterns indicating different metamorphic conditions.

Spectral differences help identify zircon genesis and geological evolution.

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.