CHARACTERIZATION OF PRECAMBRIAN ORGANIC-WALLED MICROFOSSIL ULTRASTRUCTURE USING FOCUSED ION BEAM ELECTRON MICROSCOPY

Examination of microfossil ultrastructures using electron microscopy (EM) frequently involves tedious and painstaking mechanical preparation and presents only restricted viewing perspectives. Even so, field emission scanning electron microscopy (FE-SEM) is able to provide detailed, nanometer-scale surficial analyses and, in conjunction with transmission electron microscopy (TEM), allows for simulated three-dimensional views of surface and subsurface structures. However, this combination of EM techniques offers little control over orientation and selection of cross-sectioning sites, and therefore provides sub-optimum three-dimensional ultrastructural information. On the other hand, integrated sub-nanometer resolution FE-SEM, focused ion beam (FIB) sectioning capabilities, and energy dispersive x-ray spectroscopy (EDS) available in dual beam FIB systems allow for real-time, three-dimensional ultrastructural analysis and compositional mapping with precision site-selectivity and little to no mechanical sample preparation requirements.

The capabilities and methodologies for using a FEI DualBeam Helios NanoLab field-emission scanning electron/field-emission ion beam system constitute the majority of this presentation, utilizing multiple Precambrian organic-walled microfossils as subjects for ultrastructural analysis. While FIB techniques have been utilized in in-situ examination of microfossils (Kempe et al. 2005, Precambrian Res., 140, 36–54; Cavalazzi 2007, Astrobiology, 7, 402–415), this study focuses on analyzing palynologically extracted microfossils for thorough investigation of both surface and subsurface microstructures. The microfossils highlighted here include sphaeromorphic and acanthomorphic acritarchs from the Mesoproterozoic Ruyang Group of North China, including Dictyosphaera delicata and Shuiyoushpaeridium macroreticulatum; large sphaeromorphic acritarchs from the Paleoproterozoic Chuanlinggou Formation of the Changcheng Group in North China; and possible graphitized sphaeromorphic vesicles from the late Archean–early Paleoproterozoic Jingangku Formation of the Wutai Group in North China (Schiffbauer et al., Astrobiology, in press).