Microprobe element distribution guided by Back Scattered Electron Image mode, Elephant Moraine, East Antarctic Ice Sheet. Sample PRR 23259, Black calcite (older than 200,000 years)

Brief description

The dataset combines petrography as seen in backscattered electron mode (BSE) and point-by-point chemical analysis (semiquantitative) by electron microprobe analysis. The electron microprobe analysis is based on the production of X-rays and electrons when the specimen interacts with the incident electron beam. Two types of interactions were used to obtain the dataset: backscattered electrons (BSE) and the generation of characteristic X-rays (X-ray fluorescence) that are typical for each element. The electron microprobe was operated at an accelerating potential of 20keV.
In the BSE mode, the lighter the shade of grey, the heavier the elements that compose the sample: TiO2 (titanium oxide) for example appears almost white, calcite appears grey, silica is dark grey and organic matter is almost black. BSE mode is sensitive to the atomic weight of the compound, and SiO2 is lighter than CaCO3, whereby it appears darker in the images.

Sample preparation: a double polished slice from PRR 23259 was mounted on a standard glass slide, polished and cleaned (ultrasonicated) prior to being ready for analysis. Sample PRR23259 is typical of all the black, crystalline calcites retrieved from the from Elephant: PRR 23258, PRR23259, PRR 39222, PRR 13081 in the list of samples available at
https://data.aad.gov.au/metadata/records/AAS_4558_Subglacial_Carbonates_Samples

Their radiometric (U-series) ages spans from ca. 250,000 years to circa 500,000 years.

The oldest radiometric ages show large errors and, therefore, the samples could be younger or older than 500,000 years.

Sample PR 23259 was originally collected by G. Faure at a location known as “Elephant Moraine”. It was interpreted as a calcite crust that formed under a glacier (subglacial) at the contact between bedrock and ice. G. Faure et al (1988) presented a reconstruction of the temperatures of formation from the O isotope ratios data of about 27 degrees C, which points to the formation of the calcite influenced by subglacial hydrothermal fluids.

Part of this crust must have been plucked by the flowing ice and transported to the site where it was found. Therefore, there is no direct information about the bedrock onto which the calcite crust formed, or where exactly it formed. This can only be reconstructed from the analysis of the detritus embedded in the calcite. Most of the analyses of these detritus particle are presented in PRR23259-top dataset, sites 1 to 10.

The dataset consist of images, spectra and tables of chemical composition for 43 areas of sample PRR23259. The PRR23259 top includes from sites 1 to site 10, highlighted by yellow dots in Fig. 1, which span the part of the sample rich in detritus. The PRR23259 middle-bottom, from site 11 to site 43, highlighted by bright pink dots in Fig. 1, span the part of the sample mostly consisting of calcite.

In the dataset, sites 3, 4 and 5 are adjacent to each other. Therefore, the tree points are “summarized” by the maps of Site 3 and the spectrum for site 4.
For sites 13,16, 27, 34, 38, 40, 42 there are just the petrographic images. Sites 16 and 40 show the SiO2 spherulites. Site 15 is missing for a technical issue. Sites 19, 20, 35, 37, 39, 41 are maps showing the distribution of elements. They illustrate silica spherulites within a carbonate matrix. For Site 21 chemical maps are available.

The PRR23259 microprobe dataset highlights the existence of micrometre-scale spherulitic, amorphous silica within calcite crystals, which could not be observed by standard petrography.


How to read the dataset:
Site 1 is at the top (the youngest) of the sample and Site 43 at the bottom of PRR23259 (see Fig. 1).

Each Site (1, 2, 3. etc up to Site 43) has an image area ranging from a few square mm to several 100 square m. For each Site, the spot-sizes labelled Spectrum x (where x is a number from 1 to 93) indicate where each chemical analysis in electron fluorescence was taken. taken and the chemical analyses were carried out (for example: Spectrum 1, 2, 3, 4…etc). For each spot size the composition is provided as spectrum and the semiquantitative composition table was extracted from the spectrum.