Name: atom probe tomography analysis of exsolved mineral phases
Uploaded: 2019-10-25T12:30:00
Description: Watch this Scientific Journal Video about Atom Probe Tomography Analysis of Exsolved Mineral Phases at JoVE.com

This work was supported by funding from the National Science Foundation (NSF) through grants EAR-1560779 and EAR-1647012, the Office of the VP for Research and Economic Development, the College of Arts and Sciences, and the Department of Geological Sciences. Authors also acknowledge Chiara Cappelli, Rich Martens and Johnny Goodwin for technical assistance and the Montserrat Volcano Observatory for providing the ash samples.

1. Sourcing, selection, and preparation of mineral grains
NOTE: Samples were obtained from the catalogued collection at the Montserrat Volcano Observatory (MVO) and derived from falling deposits originating from a vigorous ash venting episode at Soufri&#232;re Hills Volcano that occurred on 5 October, 2009; this was one of 13 similar events over a course of three days14. This ash venting preceded a new phase of lava dome growth (phase 5) that commenced on 9 October. Previ...

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3D APT data reconstructions allow a precise measurement of the interlamellar spacing in the analyzed crystal at a resolution three orders of magnitude higher than those measured from conventional SEM images. This indicates that atomic variations in chemistry occur over a spatial extent three orders of magnitude smaller than optically observable mineralogical changes. Also, the measured interlamellar distances (29 nm and 14 nm), are consistent with the length scale for oxyexsolution as opposed to that for nucleation and g...

The study of chemical mineralogy has been a major source of information within the field of Earth Sciences for more than a century, as minerals actively record geological processes during and after their crystallization. Physio-chemical conditions of these processes, such as temperature changes during volcanism and metamorphism, are recorded during mineral nucleation and growth in the form of chemical zonation, striations, and lamellae, among others. Exsolution lamellae form when a phase unmixes into two separate phases in the solid state. The analysis of the orientation, size, morphology, and spacing of such exsolution lamellae can provide essential information to understand temperature and pressure changes during volcanism and metamorphism1,2,3 and the formation of ore mineral deposits4.
Traditionally, the study of exsolution lamellae was conducted with the observation of micrographs by simple scanning electron imaging5. More recently, this has been substituted by the use of energy-filtered transmission electron microscopy (TEM) providing detailed observations at the nanoscale level1,2,3. Nevertheless, in both cases, the observations are made in two dimensions (2D), which is not fully adequate for three dimensional (3D) structures represented by these exsolution lamellae. Nanotomography6 is emerging as a new technique for the 3D observation of nanoscale features inside minerals grains, yet there is insufficient information about the composition of these features. An alternative to these approaches is the use of atom probe tomography (APT), representing the highest spatial resolution analytical technique in existence for the characterization of materials7. The strength of the technique lies in the possibility of combining a 3D reconstruction of nanoscale features with their chemical composition at the atomic scale with a near part-per-million analytical sensitivity7. Previous applications of APT to the analysis of geological samples have provided excellent results8,9,10,11, particularly in the chemical characterization of element diffusion and concentrations9,12,13. Yet, this application has not been used for the study of exsolution lamellae, abundant in some minerals hosted in metamorphic and igneous rocks. Here, we explore the use of APT, and its limitations, for the analysis of the size and composition of exsolution lamellae, and interlamellar spacing in volcanic titanomagnetite crystals.

<table>
	<tbody>
		<tr>
			<td>InTouchScope Secondary Electron Microscope (SEM)</td>
			<td>JEOL</td>
			<td>JSM-6010PLUS/LA</td>
			<td></td>
		</tr>
		<tr>
			<td>Focus Ion Beam (FIB) Secondary Electron Microscope (SEM)</td>
			<td>TESCAN</td>
			<td>LYRA XMU</td>
			<td></td>
		</tr>
		<tr>
			<td>Local Electrode Atom Probe (LEAP)</td>
			<td>CAMECA</td>
			<td>5000 XS</td>
			<td></td>
		</tr>
		<tr>
			<td>Integrated Visualization and Analysis Software (IVAS, version 3.6.12).</td>
			<td></td>
			<td></td>
			<td>processing software</td>
		</tr>
	</tbody>
</table>

atom probe tomography analysis of exsolved mineral phases

Element diffusion rates and temperature/pressure control a range of fundamental volcanic and metamorphic processes. Such processes are often recorded in lamellae exsolved from host mineral phases. Thus, the analysis of the orientation, size, morphology, composition and spacing of exsolution lamellae is an area of active research in the geosciences. The conventional study of these lamellae has been conducted by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and more recently with focused ion beam (FIB)-based nanotomography, yet with limited chemical information. Here, we explore the use of atom probe tomography (APT) for the nanoscale analysis of ilmenite exsolution lamellae in igneous titanomagnetite from ash deposits erupted from the active Soufri&#232;re Hills Volcano (Montserrat, British West Indies). APT allows the precise calculation of interlamellar spacings (14&#8211;29 &#177; 2 nm) and reveals smooth diffusion profiles with no sharp phase boundaries during the exchange of Fe and Ti/O between the exsolved lamellae and the host crystal. Our results suggest that this novel approach permits nanoscale measurements of lamellae composition and interlamellar spacing that may provide a means to estimate the lava dome temperatures necessary to model extrusion rates and lava dome failure, both of which play a key role in volcanic hazard mitigation efforts.

Like many titanomagnetite crystals from various stages of the Soufri&#232;re Hills Volcano (SHV) eruption, the crystal analyzed here contains exsolution lamellae &#60;10 &#181;m in thickness, visible in secondary SEM images (Figure 1d), which separate zones of Ti-rich magnetite, indicating a C2 stage of oxidation18. Based upon the SEM images, spacing between these lamellae ranges from 2 to 6 &#181;m (n = 15). Four titanomagnetite specimen tips, referred as 20...

Watch this Scientific Journal Video about Atom Probe Tomography Analysis of Exsolved Mineral Phases at JoVE.com

Atom Probe Tomography Analysis of Exsolved Mineral Phases

Analysis of the morphology, composition, and spacing of exsolution lamellae can provide essential information to understand geological processes related to volcanism and metamorphism. We present a novel application of APT for the characterization of such lamellae and compare this approach to the conventional use of electron microscopy and FIB-based nanotomography.

Element diffusion rates and temperature/pressure control a range of fundamental volcanic and metamorphic processes. Such processes are often recorded in ...

Examining variations in mineral chemistry can shed light on changes in volcanic activity, and allow researchers to obtain timescales of volcanic processes to better understand any potential hazards. Atom probe tomography allows an unprecedented 3D visualization of mineral exsolved phases while measuring their chemical composition at atomic scale. We are currently applying the technique to the characterization of pathological mineralization, for example, in the case of kidney stones.This method can be applied to volcanic systems where eruption transitions can occur over small timescales. These transitions are recorded in the minerals over very small spatial scales. Begin by pouring one gram of the sample into a 10-centimeter glass Petri dish and wrap a three by three-centimeter sheet of weight paper around a 10-gauss magnet.Use the magnet to pull magnetite-rich grains between 100 and 500 micrometers in diameter from the ash sample and place the grains in a 32-micrometer pore, 8-centimeter diameter stainless steel sieve. Use a squeeze bottle of deionized water to flush the smaller adhering ash particles through the sieve for 20 to 30 seconds and allow the grains to air dry for 24 hours. The next day, affix any clean and dry ash particles to sample mounts suitable for a secondary scanning electron microscope and image the particles in secondary electron mode at a 15 to 20-kilovolt accelerating voltage and at a working distance of 10 millimeters to select the five to 10 best candidates for further analysis.The selected grains should be predominantly magnetite. Affix the selected ash grains to a piece of clear tape and surround the samples with a one-inch diameter hollow mold that has been internally coated with vacuum grease, then fill the mold with epoxy resin mold. Once the epoxy is cured, remove the sample from the mold and peel tape from the bottom.The ash grains should be partially exposed. Polish the epoxy-cast ash grains with silicon carbide grinding paper of five different grit sizes, from the highest to lowest grit size in a figure eight motion for at least 10 minutes per grinding paper. In between grit sizes, sonicate the sample in a bath of deionized water for 10 minutes.After each last polish, check the sample under a microscope to ensure that no polishing grit is present and that the sample surface is free from scratches. Next use polishing cloths to polish the epoxy-cast ash grains with consecutive one and 0.3-micrometer alumina polishing suspensions in a figure eight motion for at least 10 minutes, sonicating the sample in deionized water for 10 minutes between suspension sizes. After the second suspension polish, check the sample under the microscope to ensure that no suspension is present and that the sample surface is free of scratches.At the end of the polishing procedure, the epoxy surface should be smooth and the ash grains should be flat and well exposed. Using an available sputter coating device, coat the sample surface with an approximately 10-nanometer thick carbon-conducting coat and obtain backscattered electron images of the ash grains with the electron microscope at a 15 to 20-kilovolt accelerating voltage and a working distance of 10 millimeters to determine the location of exsolution lamellae in the magnetite. Before beginning the focused ion beam procedure, sputter coat the sample surface with a 15-nanometer layer of copper to avoid electron charging and sample drifting.Next use a focused gallium-ion beam in a dual-beam scanning electron microscope on the polished section of interest containing the lamellae over a 1.5 by 20-micrometer region at 30 kilovolts and seven pascals. Use the ion beam to mill three wedges of material below three sides of the platinum rectangle and insert the gas-injection system to weld the wedge to an in situ nanomanipulator using gas-injection system-deposited platinum before cutting the final edge free. Using the gallium-ion beam, cut 10 one to two micrometer-wide segments from the wedge and sequentially affix the wedges with platinum to the tops of silicon posts of a microtip array coupon.Shape and sharpen each specimen tip using annular milling patterns of increasingly smaller inner and outer diameters, starting at 30 kilovolts to produce the specimen geometry necessary for atom probe tomography and finishing at an accelerating voltage of five kilovolts to reduce the gallium implantation and to obtain a consistent tip-to-tip shape. For atom probe tomography acquisition, mount the micro coupon with the sharpened tips welded to the silicon posts onto a specimen puck and load the puck into a carousel for placement inside the local electrode atom probe. Insert the carousel inside the buffer chamber of a local electrode atom probe equipped with a picosecond 355-nanometer ultraviolet laser and turn the head of the laser.After calibration, achieve a vacuum in the analysis chamber at or below six by 10 to the negative 11 torr and use a transfer rod to insert the puck specimen into the main analysis chamber. Then move the specimen puck to align the micro coupon with the local electrode to select the tip and update the database to indicate the tip number. In this analysis, four titanomagnetite specimen tips were successfully extracted from the single crystal and analyzed by atom probe tomography.Two of the specimens demonstrated homogenous concentrations of both iron and titanium throughout, indicating that lamellae were not intersected. The other two specimens exhibited zones with variable concentrations in iron, oxygen, and titanium. 3D reconstructions of the atom probe tomography data permit a precise measurement of the intralamellar spacing and provide length scales that average between 14 to 29 nanometers with a one-sigma value of two nanometers for both specimens.In addition to these measurements, atom probe tomography permits the extraction of chemical information across these lamellae at a high spatial resolution through the analysis of proxigrams using the point zero as the intersection between the lamella and the host mineral. Atomic concentrations of titanium in the crystal confirmed that the fragment is indeed a titanomagnetite and are consistent with previous petrologic analyses of Soufriere Hills Volcano eruptive products. These proxigrams also confirm that the composition of the lamella matches that of ilmenite.Preparing the wedges to capture the lamellae is critical to the FIB SEM sample preparation as well as sharpening the tips to the correct dimension. Transmission electro microscopy can also be performed to verify the lamellae dimensions and the interlamellar spacing. This technique could allow vulcanologists to calculate the timescales of eruptive activity to better understand the potential hazards of active volcanoes.

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