Atom Probe Tomography Analysis of Exsolved Mineral Phases

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 Soufrière 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. Previous analysis of this sample showed it to be a combination of dense dome rock fragments, glassy particles, and accidental lithics¹⁴.

1. Pour 1 g of volcanic ash sample into a 10 cm diameter glass Petri dish.
2. Wrap a small (3 cm x 3 cm) sheet of weighing paper around a 10 G magnet.
3. Use a paper-wrapped magnet to pull magnetite-rich grains between 100 µm and 500 μm in diameter from the ash sample and place in a 32 μm (5 φ), 8 cm diameter stainless-steel sieve.
4. Rinse in deionized water for 20–30 s using a squeeze bottle to remove smaller, adhering ash particles (which will pass through the sieve), and air-dry for 24 h.
5. Affix clean and dry ash particles to sample mounts suitable for the secondary electron microscope (SEM). Image in secondary electron mode at a 15–20 kV accelerating voltage and at a working distance of 10 mm to select the 5–10 best candidates for further analysis. Selected grains should be predominantly (>50%) magnetite (Figure 1a,b).
6. Affix selected ash grains to clear tape, surround with one-inch diameter hollow mold (metal, plastic, or rubber) that has been internally coated with vacuum grease, and pour in epoxy resin to fill the mold (2 cm³). Allow epoxy to cure according to the specific epoxy instructions.
7. Once epoxy has cured, remove from mold and peel tape from the bottom. Ash grains should be partially exposed.
8. Polish the epoxy-cast ash grains using SiC grinding paper of five different grit sizes (400, 800, 1200, 1500 and 2000 grit).
   1. Polish the sample with each grit size, from highest (400) to lowest (2000) in a figure eight motion for at least 10 min. In between grit sizes, sonicate the sample in a bath of deionized water for 10 min.
   2. Check the sample under a microscope to ensure that the polishing grit is not present, and the sample surface is free from scratches. If scratches are present, repeat the polishing procedure with the previous grit size before sonicating and moving to the next grit size.
9. Polish the epoxy-cat ash grains with alumina polishing suspensions of 1.0 μm and then 0.3 µm on polishing cloths.
   1. Polish the sample with each suspension in a figure eight motion for at least 10 min. In between suspension sizes, sonicate the sample in a bath of deionized water for 10 min.
   2. Check the sample under a microscope to ensure that the suspension is not present and the sample surface is free of scratches. If scratches are present, repeat the polishing procedure with the previous suspension before sonicating and moving to the next suspension size. At the end of the polishing procedure, the epoxy surface should be smooth and the ash grains should be flat and well-exposed.
10. Coat the sample surface with a conducting coat of ~10 nm thick carbon using an available sputter coating device.
11. Obtain backscattered electron images of the ash grains with the electron microscope at a 15–20 kV accelerating voltage and a working distance of 10 mm to determine the location of exsolution lamellae in the magnetite (Figure 1c), as conducted in previous studies⁵.

Figure 1: Example of magnetite-rich ash grains from venting episodes at the Soufrière Hills volcano. (a, b): Backscattered electron images (BSE) of both reacted and unreacted textures in magnetite grains. (c) BSE image of a polished magnetite grain showing the presence of exsolution lamellae (light grey laths; red arrows) of potential ilmenite composition. (d) Secondary electron image of a polished magnetite grain prepared for atom probe tomography (APT) analysis, showing the location of some exsolution lamellae (dashed red lines), which are distributed all along the grain surface, and the location of the wedge extraction (blue arrow). Please click here to view a larger version of this figure.

2. Atom probe tomography (APT) sample preparation

1. APT samples will be prepared from a wedge of material that reveals the presence of exsolution lamellae (Figure 1d) employing a FIB-based lift-out protocol¹⁶ (Figure 2). Prior to FIB work, sputter coat the sample surface with 15 nm layer of Cu to avoid electron charging and sample drifting.
2. Using a focused ion beam in a dual beam scanning electron microscope (FIB-SEM), deposit a rectangle of platinum (Pt) (3 μm in thickness) on the polished section of interest containing the lamellae over a 1.5 µm x 20 µm region using a gallium (Ga⁺) ion beam at 30 kV and 7 pA.
   NOTE: This platinum layer is deposited to protect the region of interest (ROI) from ion beam damage.
3. Mill three wedges of material below three sides of the Pt rectangle using the ion beam (30 kV, 1 nA). See Figure 1d and Figure 2.
4. Insert the gas injection system (GIS) and weld the wedge to an in situ nano-manipulator using GIS-deposited Pt before cutting the final edge free (Figure 2a).
5. Using the Ga⁺ ion beam (5 kV and 240 pA), cut ten 1–2 μm wide segments from the wedge and sequentially affix them with Pt to the tops of Si posts of a microtip array coupon (Figure 2b–d).
6. Shape and sharpen each specimen tip using annular milling patterns of increasingly smaller inner and outer diameters (Figure 3). Initially, perform the milling at 30 kV to produce the specimen geometry necessary for APT (Figure 3, left panel).
7. Perform the final milling at an accelerating voltage of 5 kV in order to reduce Ga⁺ implantation and obtain a consistent tip-to-tip shape (Figure 3, right panel).
8. By applying the measuring tools of the SEM to the figure, ensure that the diameter at the top of the tips ranges 50–65 nm, while the shank angle of the tips ranges from 25° to 38°.
   NOTE: Further details have been previously published, describing the conventional lift-out protocol¹⁶.

Figure 2: Example of FIB-SEM sample preparation protocol for APT analysis. (a) Wedge (W) lift-out extraction with the nanomanipulator (Nm). (b) Lateral view of the micro-coupon array of silicon posts mounted on a copper clip. (c) Top view of the micro-coupon array of silicon posts showing the nanomanipulator for mounting the wedge sections. (d) Wedge fragment (S), showing a portion of the protective platinum cap (Ptc), mounted on a silicon post after welding with platinum (Ptw). Please click here to view a larger version of this figure.

Figure 3: Example of tips prepared for APT analysis. (Left) Image of tip after the first stage of sharpening. (Right) Image of the same tip after low kV cleaning, indicating the tip radius (67.17 nm) and the shank angle (26°). Please click here to view a larger version of this figure.

3. APT data acquisition

1. Perform analysis with a local electrode atom probe (LEAP) equipped with a pico-second 355 nm UV laser. See specific LEAP running parameters in Table 1.
   NOTE: Analyses were performed with a LEAP equipped with a pico-second 355 nm UV laser, housed in the Central Analytical Facility (CAF) at The University of Alabama.
2. Mount the micro-coupon with the sharpened tips, welded to the Si posts, in a specimen puck and load into a carousel for placement inside the LEAP.
3. Insert the carousel inside the buffer chamber of the LEAP.
4. Turn the laser head on and perform a laser calibration.
5. After achieving vacuum in the analysis chamber at or below 6 x 10^-11 Torr, insert the puck specimen into the main analysis chamber. For this use a transfer rod; this is operated with a series of automatic steps and manual insertion.
6. Before analysis, select the tip by moving the specimen puck to align the micro-coupon with the local electrode and update the database to indicate the tip number.
   NOTE: Four out of six tips were analyzed successfully (two fractured during analysis) and resulted in a variable amount of acquired data ranging from 26 to 92 million total detected ions, which corresponded to the removal of a 160–280 nm-thick layer of magnetite (see specific LEAP running parameters in Table 1).

Table 1. Atom probe tomography data acquisition settings and run summary.

4. APT data processing

1. Open the dataset in the processing software (see the Table of Materials) and perform the following steps for data analysis.
   1. Review the information setup.
   2. Select ion sequence range based on the voltage history plot. Select the detector region of interest (ROI).
   3. Perform time-of-flight (TOF) correction. Use peaks corresponding to oxygen and iron for the TOF correction.
   4. Perform mass calibration with the identification of main peaks.
   5. Perform the ranging of ions for their assignments to specific masses.
   6. Perform the reconstruction of the tip profile.
2. Show the data into the two main formats: 1) mass-to-charge state ratio (Da) chemical spectra (Figure 4); and 2) 3D reconstructions of tip specimens (Figure 5).
3. Define peak ranges as the entire visible peak, or adjust manually when large thermal tails are present, for each mass-to-charge state ratio spectrum (Figure 4). These peaks represent single elements or molecular species, and the decomposition of peaks provides the overall chemical composition for each tip or feature (i.e., clusters and exsolution lamellae) inside each tip (Table 2).
4. Perform three-dimensional (3D) tip reconstructions using the “voltage” tip profile method¹⁷ to determine the reconstructed radius as a function of analyzed depth (Figure 5 and Movie 1).
5. Reconstruct isosurfaces of exsolution lamellae to conduct interlamellar spacing measurements (Figure 5) and establish the host mineral-lamella chemical relationship using proxigrams^17,18 (Figure 6).
6. Measure interlamellar spacings with image analysis software.

Figure 4: Example of a representative APT mass-to-charge spectrum. Spectrum for the analyzed magnetite crystal with individual ranged peaks showing examples of the identification of peaks corresponding to single elements (e.g., oxygen (O) or iron (Fe)) or molecules (e.g., FeO). Please click here to view a larger version of this figure.

Table 2. Atom probe tomography bulk compositional data for all analyzed specimens.