Aaron Torpy

A study of cathodoluminescence and trace element compositional zoning in natural quartz from volcanic rocks: Mapping titanium content in quartz

This article concerns application of cathodoluminescence (CL) spectroscopy to volcanic quartz and its utility in assessing variation in trace quantities of Ti within individual crystals. CL spectroscopy provides useful details of intragrain compositional variability and structure but generally limited quantitative information on element abundances. Microbeam analysis can provide such information but is time-consuming and costly, particularly if large numbers of analyses are required. To maximize advantages of both approaches, natural and synthetic quartz crystals were studied using high-resolution hyperspectral CL imaging (1.2-5.0 eV range) combined with analysis via laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS). Spectral intensities can be deconvolved into three principal contributions (1.93, 2.19, and 2.72 eV), for which intensity of the latter peak was found to correlate directly with Ti concentration. Quantitative maps of Ti variation can be produced by calibration of the CL spectral data against relatively few analytical points. Such maps provide useful information concerning intragrain zoning or heterogeneity of Ti contents with the sensitivity of LA-ICPMS analysis and spatial resolution of electron microprobe analysis.

Hyperspectral cathodoluminescence imaging and analysis extending from ultraviolet to near infrared.

The measurement of near-infrared (NIR) cathodoluminescence (CL) with sufficient sensitivity to allow full spectral mapping has been investigated through the application of optimized grating spectrometers that allow the ultraviolet (UV), visible, and NIR CL spectra to be measured simultaneously. Two optical spectrometers have been integrated into an electron microprobe, allowing simultaneous collection of hyperspectral CL (UV-NIR), characteristic X-rays, and electron signals. Combined hyperspectral CL spectra collected from two natural apatite (Ca5[PO4]3[OH,F]) samples from Wilberforce (Ontario, Canada) and Durango (Mexico) were qualitatively analyzed to identify the emission centers and then deconvoluted pixel-by-pixel using least-squares fitting to produce a series of ion-resolved CL intensity maps. Preliminary investigation of apatite has shown strong NIR emissions associated primarily with the rare-earth element Nd. Details of growth and alteration were revealed in the NIR that were not discernable with electron-induced X-ray mapping. Intense emission centers from Nd3+ and Sm3+ were observed in the spectra from both apatites, along with minor emissions from other 3+ rare-earth elements. Quantitative electron probe microanalysis was performed on points within the mapped area of the Durango apatite to produce a calibration line relating cathodoluminescent intensity of the fitted peak centered at 1,073 nm (1.156 eV) to the Nd concentration.

Hyperspectral cathodoluminescence examination of defects in a carbonado diamond.

Hyperspectral cathodoluminescence mapping is used to examine a carbonado diamond. The hyperspectral dataset is examined using a data clustering algorithm to interpret the range of spectral shapes present within the dataset, which are related to defects within the structure of the diamond. The cathodoluminescence response from this particular carbonado diamond can be attributed to a small number of defect types: N-V0, N2V, N3V, a 3.188 eV line, which is attributed to radiation damage, and two broad luminescence bands. Both the N2V and 3.188 eV defects require high-temperature annealing, which has implications for interpreting the thermal history of the diamond. In addition, bright halos observed within the diamond cathodoluminescence, from alpha decay radiation damage, can be attributed to the decay of 238U.

Systematics of Cathodoluminescence and Trace Element Compositional Zoning in Natural Quartz from Volcanic Rocks: Ti mapping in Quartz

Relations between trace element contents and cathodoluminescence (CL) in natural minerals potentially have many applications in the geological sciences [1-3]. For example, recent experimental calibration of Ti concentration in quartz as a function of temperature [4] provides the basis for a quantitative geothermometer for many natural rocks. However, intricate crystal growth and/or reaction textures observed in CL images often preserve a record of complex interaction between quartz crystals and their host magmas or fluids. The processes and thermal history attending such mineral growth can be discerned only by detailed microanalytical work that is both timeconsuming and limited by available spatial resolution of the analytical methods used relative to the scale of compositional zoning. Here we explore the relation between compositional zoning and CL spectral properties to evaluate (a) the underlying causes for the CL radiation, and (b) the potential of CL images in mapping specific compositional variations over a range of spatial scales. This work stems from petrologic investigations of magmatic temperatures in rhyolites from the Yellowstone supervolcano and its precursory activity in the Snake River Plain (S. Idaho). The samples investigated consist of quartz phenocrysts from some 30 samples, for most of which temperature estimates (ca. 750 to 1050°C) are available based on thermodynamic equilibrium between other mineral pairs [5-6].

Collecting and Analysing - 1.6eV - 20keV Emission Spectra in an EPMA

The process of soft x-ray generation involves transitions from electron orbitals that are involved in bonding, and thus the peak shape and position of soft x-ray lines are sensitive to the local chemical environment. X-ray transitions from KLM and N shell are now routinely observed and in the case of SXES higher order reflections are commonly observed. This creates both problems and benefits for microanalysis: quantification becomes problematic when the x-ray line shape and position varies with composition, but the x-ray line shape and position carry information about the chemical bonding allowing the microanalysis to yield more than just the chemical concentrations.

Multi-spectral Electron Microprobe - Now and the Future

Hyperspectral datasets are now routinely collected in electron microprobes during mapping with an increasing amount of the energy spectrum being covered. Energy dispersive spectrometers (EDS) are now giving spectral information on X-rays down to Li [1], with the development of soft X-ray emission spectroscopy (SXES) giving high resolution spectroscopy down to energies of 50 eV [2], while cathodoluminescence (CL) detectors are giving spectral information in the ultra-violet to the infra-red range [3]. The SXES detector gives direct measurement of Si-L (100eV), Al-L (70eV) and importantly Li-K (52eV) using first order lines, while higher order x-ray reflections can be used to probe other elements which do not have first order lines able to be directly measured using this spectrometer. The high energy resolution of the detector, down to 0.2eV [2], can differentiate many of the overlapping xray lines observed using wavelength dispersive spectrometers, as well as resolve peak shape and energy changes which aid in the determination of the chemical bonding state of various compounds. CL also captures bonding and valence information having a high energy resolution over a comparatively small energy range which, depending on configuration, can be down to 10 meV. Both the SXES and CL techniques are capable of detecting trace element information down to the ppm in mapping mode.

Characterization of Complex Industrial Specimens by Hyperspectral EPMA Mapping

The characterization of mineral or metallurgical specimens from industry can present a number of significant challenges. Often, geological or mineral processing specimens contain both many elements (>10) and many mineral phases (>10), some of which may differ in composition in small yet critical degrees. Both metallurgical and mineral systems can exhibit significant solid solution chemistry, meaning that phases do not have discrete or well-defined compositions. Furthermore, mineral and mineral processing specimens display compositional or morphological heterogeneity over length scales from the sub-micron (e.g. nano-precipitation / intergrowths) to the macroscopic (e.g. metasomatism), requiring techniques that can rapidly characterize large specimens at fine spatial resolution. Moreover, the opportunity to characterize industrial problems typically only arise when a process has unexpectedly failed, such as may occur with the presence of unexpected elements and/or unexpected phases, requiring the problems to be solved with little (or sometimes incorrect) a priori knowledge. In such instances, the urgency of the problem may also be great, requiring the problems to be solved efficiently with short turnaround times, and without the ability to reproduce the faulty process under controlled conditions.

EPMA Characterisation of Quartz and Quartz-Cement from a Triassic Sandstone

The Southern Perth Basin of Western Australia has been proposed as a carbon capture and storage region for commercial quantities of CO2 in a deep saline aquifer hosted by the Triassic fluvial sandstone of the Lesueur Formation. As part of the site characterization four wells were drilled to confirm the stratigraphy of the area and retrieve core material for laboratory analysis. Current depth of the samples analysed here is approximately 1.5 km below surface although the widespread abundance of quartz overgrowths cement and the palaeo-temperatures estimated from fluid inclusion analysis suggest that maximum burial depth during the past geological history was at least 3 km. Quantitative cathodoluminescence and trace analysis has been used to establish the magnitude and relative timing of diagenetic processes affecting the Lesueur Sandstone and their chemical fingerprint at the microscopic scale. Key to the diagenetic and reservoir properties is understanding the dissolution of feldspar resulting in the precipitation of kaolinite and quartz cements which are common diagenetic reactions that can significantly affect reservoir quality in sandstones containing detrital feldspars. The prosed mechanism being:

Standardization of the MSA/MAS/AMAS Hyper-Dimensional File Format

In 2010, the Standards Committee of the Microscopy Society of America (MSA) formed a working group comprising members of the MSA, the Microbeam Analysis Society (MAS) and the Australian Microbeam Analysis Society (AMAS) to develop a standardized file format to facilitate the exchange of microscopy datasets with high dimensionality, such as hyperspectral maps. The proposed file format, known as the MSA/MAS/AMAS hyper-dimensional data file (HMSA, for short), was presented to the community for comment at the M&M 2011 meeting in Nashville, TN [1], and revisions incorporating feedback from researchers and vendors was presented at subsequent meetings [2-5]. After a further round of generalisation and simplification, a finalised specification (HMSA v1.01) was approved by the MSA Standards Committee in 2016 [6], which have now commenced the formal standardisation process with the International Standards Organisation (ISO).

Nitrogen Defects in Diamond Examined by an Electron Microprobe

Hyperspectral examination of diamonds in the electron probe microanalyser (EPMA) has proved a useful technique in the past to map nitrogen-vacancy defects and correlate these with elemental data[1]. In recent years, the data collection ability of the EPMA has greatly increased with the advent of new detectors, such as the soft x-ray emission spectrometer (SXES) [2] have not only lowered the limit of detectable x-rays to as low as Mg-L (49 eV), but also come with a high energy resolution, which can be as low as 99 meV for Mg L. The importance of having high energy x-ray resolution is that peak shape and position changes reflecting changes in the local chemical environment and can be easily observed for x-ray lines that are involved with bonding, such as the C Kα line. SXES spectra are collected in parallel using a CCD camera, and in our case are collected simultaneously with WDS, EDS and CL spectral data.