Preparation of TEM samples by focused ion beam (FIB) techniques: applications to the study of clays and phyllosilicates in meteorites

Abstract

AbstractTransmission electron microscope samples were prepared of ALH 78045 and ALH 88045, two clay-and phyllosilicate-bearing Antarctic meteorites, using argon ion milling and focused ion beam (FIB) techniques. ALH 78045 contains clay- and phyllosilicate-filled veins that have formed by terrestrial weathering of olivine, orthopyroxene and metal. Very narrow (∼10 nm) intragranular clay-filled veins could be observed in the TEM samples prepared by argon ion milling, whereas differential thinning and lack of precision in the location of the electron-transparent areas hindered the study of wider (5 — 15 μm) phyllosilicate-filled intergranular veins. Using the FIB instrument, electron-transparent slices were cut from specific parts of the wider veins and lifted out for TEM study. Results show that these veins are occluded by cronstedtite, a mixed-valence Fe-rich phyllosilicate. This discovery shows that silicates can be both dissolved and precipitated during terrestrial weathering within the Antarctic ice. ALH 88045 is one of a small number of known CM1 carbonaceous chondrites. This meteorite is largely composed of flattened ellipsoidal aggregates of serpentine-group phyllosilicates. To determine the mineralogy and texture of phyllosilicates within specific aggregates, TEM samples were prepared by trenching into the cut edge of a sample using the FIB instrument. Results show that Mg-rich aggregates are composed of lath-shaped serpentine crystals with a ∼0.73 nm basal spacing, which is typical of the products of low temperature aqueous alteration within asteroidal parent bodies. Results of this work demonstrate that the FIB has enormous potential in a number of areas of Earth and planetary science.