Visualization of trace-element zoning in fluorapatite using BSE and CL imaging, and EPMA and μPIXE/μPIGE mapping

Katarzyna Gros,Frans Munnik,Hans-Jürgen Förster,Przemysław P Michalak,Ewa Słaby,Jens Götze,Dieter Rhede

doi:10.1007/s00710-016-0452-4

Abstract

In this paper, zonation patterns of trace elements in fluorapatite are discussed that were visualized using four analytical techniques, namely back-scattered electrons (BSE) and cathodoluminescence (CL) imaging, electron probe micro-analysis (EPMA), and micro-proton-induced X-ray/gamma ray emission (μPIXE/μPIGE) mapping. Each method demonstrates the in-grain compositional variations in a slightly different way. Both BSE and CL provide qualitative data, and the internal textures are displayed in most detail. Additionally, CL points to specific elements enriched in certain growth zones. Qualitative EPMA maps show detailed zonation patterns for specific elements (with high spatial resolution), which are in general correspondence with the patterns observed in BSE and CL images. The μPIXE/μPIGE maps are fully quantitative and the detection limits are relatively low compared to EPMA mapping. In present spot measurements μPIXE demonstrates lower detection limits than EPMA, however, the latter could be considerably improved by extending the acquisition times. There is no significant overlap of REE (rare earth elements) peaks in the acquired μPIXE energy spectra, however, when multiple REEs are present with sufficiently high concentrations, peak deconvolution may pose some difficulties. Spatial resolution of μPIXE/μPIGE images is not sufficiently high to reflect minor textural features, which also result from the greater interaction depth of the proton beam. However, major growth zones are distinguishable. Even though each method has their advantages and limitations, when applied together, they provide an almost complete characterization of compositional variability in trace-element-bearing minerals.

Highlights

Trace elements, including the rare-earth-elements (REEs), are important petrogenetic indicators and provide useful information about a wide range of processes taking place in magmatic −hydrothermal as well as metamorphic and sedimentary environments
The following conclusions can be drawn from the acquired results: 1) For the separation of REE X-ray lines, electron probe micro-analysis (EPMA)-WDS is a well suitable technique whereas extensive overlapping of peaks may pose problems in the application of μPIXE/ μPIGE-EDS techniques
2) The EPMA element distribution maps produced in this study give only a qualitative image of compositional variations in fluorapatite. μPIXE/μPIGE maps are quantitative, meaning that element concentrations may be calculated for each spot represented by an energy spectrum

Summary

Introduction

Trace elements, including the rare-earth-elements (REEs), are important petrogenetic indicators and provide useful information about a wide range of processes taking place in magmatic −hydrothermal as well as metamorphic and sedimentary environments. Ginibre et al 2002; Reed 2005; Götze et al 2013 and references therein; North-Valencia et al 2014; Wiedenbeck et al 2014) All of those permit visualizing of growth textures in minerals, the information given by each of them is unique due to different physical principles each method is based on. Cathodoluminescence imaging reveals the presence of specific structural defects, e.g. vacancies or impurities in the form of trace elements. These cause electron excitation during interaction with an electron beam and induce luminescence of varying energy (Götze et al 2013). E.g. REEs, significantly longer acquisition times are required

Methods

Results

Discussion

Conclusion