THE PAST, PRESENT, AND FUTURE OF TEM IN EARTH SCIENCE STUDIES: A NEW ABERRATION-CORRECTED SCANNING TRANSMISSION ELECTRON MICROSCOPE FOR EARTH MATERIALS RESEARCH

Transmission electron microscope (TEM) was invented in early 1930's after electron diffraction (coherent scattering) phenomenon was observed and explained in 1920's. TEM that includes electron diffraction, amplitude contrast imaging, phase contrast imaging (or high-resolution TEM imaging), and associated techniques of X-ray energy-dispersive spectroscopy (EDS), electron energy-loss spectroscopy (EELS) and energy-filtering TEM imaging, has greatly improved our understanding in structures, defects, and local chemistry of minerals. Since application of HRTEM in minerals studies in 1970's, extended defects in minerals and their implications to petrological studies have been extensively investigated. Scanning transmission electron microscope (STEM) with high-angle annular dark field (HAADF) imaging (or Z-contrast imaging) system can directly reveal structure and chemistry of minerals at the atomic scale. Z-contrast imaging is a composition sensitive technique because the intensity is approximately proportional to square of atomic number (Z) of element. The Z-contrast images from e-plagioclase (or, intermediate plagioclase) show that the incommensurate structure is resulted from Al-Si ordering only, instead of coupled Al-Si ordering and Ca-Na segregation. Using Z-contrast imaging and specially resolved EELS spectroscopy, it is very powerful to study interface / boundary structures and nano-phases in earth materials. Traditionally, spherical aberration of magnetic lenses limits both HRTEM and Z-contrast imaging resolution. The new invention of aberration corrector (or, Cs-corrector) breaks the resolution barriers. New STEM system with Cs-corrector can resolve sub-Angstrom features. The Cs-corrector also improves the electron beam intensity dramatically. Minerals of omphacite and rutile from ultra-high pressure (UHP) metamorphic rocks in Eastern China were studied using a Cs-corrected STEM with associated EELS and EDS systems. The results show that nanometer size lamellae-like precipitates in rutile are relatively rich in Si (with smaller unit cell parameters) with respect to the rutile matrix. The precipitates were formed during rapid exhumation of subducted continental slab. We can foresee that new Cs-corrected TEM and STEM will benefit earth materials research in the new era.