The recovered samples were mounted in epoxy resin, sectioned and polished for analysis with scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), and electron probe microanalysis (EPMA). Before the SEM and EPMA measurements, the charges were coated with an 8 nm thick carbon layer to avoid electrical charging of the surface under electron beams.
2.3.1 Scanning electron microscopy
Textural observations, preliminary phase identification and semi-quantitative composition determination of the recovered run products were performed using a scanning electron microscope (ZEISS Gemini 1530) operating at 15 kV equipped with a field emission gun and energy-dispersive X-ray spectrometer (EDS). A working distance of 13-14 mm was normally applied.
In this technique, a focused electron beam generated from an electron gun, is scanned across a polished specimen. Depending on the interaction between the electron beam and the sample, different signals such as secondary electrons, back-scattered electrons and auger electrons can be generated. The secondary electrons with low energy are emitted from the near-surface regions of the sample due to inelastic interactions between the primary electron beam and the sample, which can be used for inspection of the topography of the sample’s surface. On the other hand, backscattered electrons (BSE), produced by elastic collisions of electrons with atoms originate from a wide region within the interaction
53 volume. Heavy atoms are stronger scatters of electrons compared with light atoms, therefore the intensity of the BSE is proportional to the average atomic number of the target sample, providing imaging with information on the sample’s composition and helping to distinguish between different phases. Moreover, when the electron beam collides with the sample, electrons from the inner shells are ejected and the resulting vacancies are filled later by outer shell electrons, emitting characteristic X-rays, which depend only on the type of elements and thus can be used for rapid qualitative chemical analysis of minerals.
2.3.2 Electron probe micro-analyzer (EPMA)
Precise quantitative analysis of major and minor element concentrations of coexisting phases were obtained using a JEOL JXA-8200 electron microprobe equipped with five wavelength-dispersive spectrometers.
The physical principles of electron microprobe are fundamentally the same as the ones of the SEM. When the sample is bombarded by an accelerated and focused electron beam produced by a tungsten filament, the electron-sample interactions yield both derivative electrons and X-rays. The secondary and back-scattered electrons, as discussed before, are useful for imaging a surface or obtaining an average composition of the material; while accurate quantitative elemental analyses are mainly based on measurement of characteristic X-rays. Electrons penetrate a volume of the sample whereby an inner-shell electron is ejected from its orbit by inelastic collisions of the incident electrons, leaving a vacancy. And electrons from the outer shell fall back to fill this vacancy and shed some energy as X-rays (Reed, 2005).
These X-rays are characteristic of the element and can be analyzed either by an energy dispersive spectrometer or by crystal spectrometers (wavelength dispersive mode). For precise quantitative analysis wavelength-dispersive spectroscopy (WDS) was employed. The electron microprobe is equipped with different crystal spectrometers (e.g. synthetic LiF, PET or TAP crystals) each with a specific d spacing, and the characteristic X-rays from the samples are selected based on their wavelength using the Bragg reflections from the
54 crystals. The position and intensity of each spectral line are then compared with those emitted by standards with known composition, allowing precise chemical composition determination after matrix corrections have been made. In EPMA analysis, matrix corrections are applied in order to obtain ‘true’ concentrations including atomic number, absorption and fluorescence corrections represented by the acronym ‘ZAF’ (Reed, 2005).
There are mainly four actual types of models used for matrix corrections in EPMA: (1) Empirical, the simplest, it is based on known binary experimental data; (2) ZAF: 1st generalized algebraic procedure, it assumes a linear relation between concentration and x-ray intensity; (3) Phi-rho-Z, it is based upon depth profile (tracer) experiments; (4) Monte Carlo, it is based upon statistical probabilities of electron-sample interactions and it is used particularly for unusual specimen geometries (Heinrich and Newbury, 2013).
In this study, an accelerating voltage of 15 kV and a beam current of 5 nA were employed. Counting times per element were 10 s on the peak and 5 s on the background with a defocused beam of 30 μm for melt and 3 μm for Brg grains larger than 5 μm. For smaller Brg grains and other mineral phases, focused beam was used. Enstatite for Mg and Si, Fe metal for Fe, corundum for Al, Ir metal for Ir, Pt metal for Pt, and Ru metal for Ru were used as standards. The composition of pure MgSiO3 akimotoite single crystals also was determined using the same settings as a benchmark analysis before each period of measurement to ensure that an accurate Mg/Si ratio was measured for these high-pressure phases. Only once the Mg/Si ratio obtained both for the enstatite and akimotoite standards was equal to 1.00 (1) the analyses of the samples were performed. The purpose of using low beam current, short counting time and defocused beam was to minimize the amorphization and damage of Brg induced by the electron beam. The Phi-rho-Z correction routine was applied for all analyses of this work. More than 20 points for each sample were measured in order to check the homogeneity of the synthetized bridgmanite crystals. The average compositions of the Brg crystals synthetized in this study are reported in Table 3.3 and 3.4.
2.3.3 Micro-focus X-ray diffraction
55 Micro-focus X-ray diffractions was performed for phase identification of the recovered samples. A micro-focused X-ray diffractometer (Bruker, D8 DISCOVER) equipped with a two-dimensional solid-state detector (VÅNTEC500) and micro-focus Co-K radiation source (IμS) operated at 40 kV and 500 μA was used (Figure 2.3). The X-ray beam was collimated to a minimum of 50 μm spot in diameter using an IFG polycapillary X-ray mini-lens. The unique 2-dimentional large-area detector enables the coverage of a larger reciprocal space and achievement of high diffraction angles. The patterns were collected for 2400 - 6000 s in the 2θ range between 25° and 85°.
Fig 2.3 D8 DISCOVER diffractometer equipped with a micro-focus Co-K radiation source (IμS) and two-dimensional solid-state detector (VÅNTEC500).