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Focused Ion Beam Scanning Electron Microscope (FIB-SEM)

Subject Area Mineralogy, Petrology and Geochemistry
Term Funded in 2014
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 257745926
 
Final Report Year 2018

Final Report Abstract

There have been two main functions of the focused ion beam (FIB) system at the Bayerisches Geoinstitut, both of which are broadly aimed at studying the physical and chemical state of the Earth’s interior. The first has been to prepare samples and components for high pressure experiments in the diamond anvil cell (DAC). In this device conical opposing diamond anvils with flattened tips of the order of 0.2 mm diameter are used to compress a metal gasket in which a sample hole is drilled. Pressures inside the gasket hole, which may be as small as 0.05 mm in diameter, can reach those in the mega bar range, and way beyond those at the centre of the Earth. Samples held under these conditions can then be examined using a range of x-ray diffraction and spectroscopic techniques, with analytical access to the sample through the diamonds. Samples can also be heated at high pressure to several thousand degrees by employing a laser. Such experiments often require samples with highly parallel surfaces and/or known thicknesses generally < 0.02 mm and of suitably small diameter to fit into the gasket hole, which can only be prepared accurately using an FIB. Petitgirard et al. (2017), for example, employed the FIB to prepare SiO2 glass samples from a double polished plate of the correct diameter such that x-ray absorption measurements could be used to measure the density of the glass in the DAC to pressures of 110 GPa. The data show density discontinuities related to changes in silicon coordination state and indicate that the density of SiO2 liquid at the base of the mantle is similar to that of MgSiO3 liquid. Such chemical variations, therefore, have no effect on melt density whereas the melt iron content is likely to be decisive. Kurnosov et al. (2017) used the FIB to prepare similar DAC samples from single crystals of the dominant lower mantle mineral bridgmanite. Brillouin spectroscopy measurements were then performed on the crystals in the DAC at high pressures to determine the velocity of seismic waves through this mineral at lower mantle conditions. By preparing semi-circular shaped parallel polished crystal plates from different crystallographic orientations two crystals could be placed in the same DAC gasket chamber such that the full elastic tensor could be determined in one experimental series. Such preparation would be impossible without the FIB. The results could be compared with seismic velocity determinations to show that the lower mantle has a chemical composition similar to that of the upper mantle but that iron must be semi equally distributed between metallic, ferric and ferrous oxidation states. Schulze et al. (2017) similarly used the FIB to machine 4 parallel polished slices of different single crystals of the mineral ringwoodite, a dominant mineral in Earth’s mantle, each with a different iron or H2O content. All 4 slices were placed together in the gasket hole of a DAC with a combined diameter of the 4 slices being approximately 0.2 mm. Subtle differenced in elastic properties could be determined between the crystals and it was shown that the solubility of H2O as hydroxyl defects has a much smaller effect on the seismic wave velocities through this mineral than previously proposed. The results indicate that the presence of H2O in ringwoodite in the Earth’s transition zone would have no discernible effect on the velocity of seismic waves in this region. Similar work partially reported in Buchen et al. (2017) comes to the same conclusion for the mineral wadsleyite. FIB machining has also been used to increase the sample volume in the DAC by creating cavities in the flattened diamond tips. A recess of about 80 μm diameter by 15 μm depth has been used to increase, for example, the sample volume for high pressure nuclear magnetic resonance measurements, NMR. By increasing the sample volume and also machining a so called Lenz lens, from a 1 µm thick layer of copper with the FIB, it has been possible to measure NMR spectra on organic materials up to 72 GPa. This opens up an enormous range of conditions to this remarkably flexible and sensitive technique. The second function of the FIB has been to recover and prepare thin lamellas from high pressure experiments and some natural samples for analysis with the transmission electron microscope (TEM). Determining the rheological properties of mantle minerals and rocks, for example, is vital for understanding the origin of plate tectonics and the convective driving forces behind its operation. High pressure and temperature deformation experiments are crucial in this respect but the samples recovered from such experiments are generally extremely fragile and the FIB is one of the only tools that can prepare such samples for TEM analyses. The nano-manipulator lift-out system has been crucial for this application as it allows the machined slices to be removed from the sample and attached to a TEM foil without danger of loss. Kawazoe et al. (2016) for example was able to deform ringwoodite in a deformation multianvil device at transition zone conditions. TEM analysis on the recovered samples prepared by FIB showed a microstructure that demonstrated deformation by dislocation glide via the Peierls mechanism. The results indicate that ringwoodite has a relatively low creep strength compared to other minerals. Furthermore the FIB has been used to prepare samples extracted from exactly defined areas of interest such as interfaces or inclusions. This allowed Marquardt and Faul (2018), for example, to determine the grain boundary character distribution for olivine aggregates, a fundamental starting point for any quantitative model for transport properties such as grain boundary sliding or grain boundary diffusion. Using the selective carbon milling system of the FIB it has also been possible to extract specific inclusions from natural diamond samples and prepare them for TEM analysis. This allowed Palot et al. (2016), for example, to identify exsolution of a hydrous mineral from an inclusion of ferro-periclase in a diamond, which provides some of the only direct evidence for the existence of H2O in the Earth’s lower mantle.

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