Optical microscopy photos of thin sections of subglacial carbonate samples collected from Boggs Valley and Elephant Moraine, East Antarctica

Brief description

The dataset relates to:

Optical microscopy photos of thin sections of samples archived in: https://data.aad.gov.au/metadata/records/AAS_4558_Subglacial_Carbonates_Samples

The observations were carried out with transmitted light in both planar mode and cross-polarized light mode.

From the optical microscopy observations, samples were selected for Epifluorescence and Cathodoluminescence microscopy with the aim to observe: 1) presence of organic compounds (epifluorescence - EPI); 2) presence of diagenetic alterations (Cathodoluminescence).
Epifluorescence was carried out at an optical microscope equipped with an Hg vapour lamp and filters for blue light excitation (450–490 nm). The filter set was composed of a diachromatic beam splitter (510 nm) and a barrier filter (515 nm). The first image in the EPI dataset is in plane polarized light (pl), the second image is taken with blue light excitation, and labelled “bl”.

Cathodoluminescence was conducted with a Nuclide ELM-3R cold cathodoluminescence device operating at 10 kV.

The lamellae for High Resolution Transmission Electron Microscopy (HRTEM) observations were cut by Focused Ion Beam (FIB) (cf. Frisia et al., 2018). The FIB preparation implies that during EDS element analyses there will be Cu (from the sample holder) and Ga (from the beam) peaks that must be discarded in the interpretation of results. Each sample holder holds two lamellae, less than 100 nm in thickness. The “broad” areas where the FIB lamellae were taken are highlighted in the photos of the specimens. Note that the selection of each lamella (size is 5 micrometres in length) was guided by Scanning Electron Microscopy image in Back Scattered Electron (BSE) mode.
The HRTEM data were acquired with a 200 keV and 0.07-nm point resolution Thermo Fisher Scientific FEI THEMIS 200 electron microscope at the Institute of Technical Physics and Materials Science, Budapest, Hungary. Fast Fourier transform (FFT) lattice spacings were calculated using the Gatan Digital Micrograph 3.6.1 software.
Scanning transmission electron microscope (STEM) on delicate samples (amorphous silica) was carried out by using a FEI Talos F200X G2 operated at 200 kV accelerating voltage.
High-resolution TEM (HRTEM) images show lattice spacings, their selected-area electron diffraction (SAED) patterns show the crystallinity (dots=crystals, rings = amorphous phases, rings + dots = nanocrystalline + amorphous phases) and the phase (calcite or silica), their energy-dispersive X-ray spectrometry (EDS) show the composition of the area in the image. The EDS system is semi-quantitative and elements whose concentration is below 100 ppm are not detected.

The dataset consists of the following folders:

EPI-Cathodo folder. Contains images in Epifluorescence and cathodoluminescence. The folder contains sub-folders. Each sub-folder groups the images relative to one sample. The location of microscope images (in planar and epifluorescent modes or planar and cathodoluminescence mode) are provided for each sample.

TEM Folder. TEM and HR-TEM Images with some interpretation to facilitate understanding of images. For this reason, the images are uploaded as pdf presentations.
File PRR23259 shows the backscattered electron images of the areas that were selected to obtain two lamellae with FIB. The locations where the inner and outer lamellae were taken is shown in the photo with the platinum deposit on the FIB area.
The lamellae, when extracted from the sample, are placed in a copper sample holder. There are two lamellae in the sample holder we used, one was named “ outer window” and the other “inner window” just as a working tool (so that one knew what part of the sample one was looking at).
Because observations at the TEM are at the nano-scale, each step is being documented to allow recognition of the site from which the lamella came from. Each lamella is 5 micrometres across, and then the observations are carried out on areas of 100s of nanometres.
The file “PR23259 inner window” shows the images of the lamella located in the inner window as seen in file PR23259. The file “PR23259 outer window” shows the images of the sample from the outer window.

Additional Chemical data and BSE images for sample PR23259 are archived in https://data.aad.gov.au/metadata/records/AAS_4558_Microprobes

In PRR 16794 all images (BSE, FIB site, HR-TEM) for that sample are shown.