Frontiers and Opportunities in Antarctic Geosciences * Certosa di Pontignano * 29-31 July 2004
3-D Petrography of Cores of Rocks and Glacigenic Sediments through an
Innovative Tomographic System for Paleoclimatic Investigations
F. FASANO*, F.M. TALARICO
Università degli Studi di Siena, Dipartimento di Scienze della Terra, Via del Laterino 8. 53100 – Siena
*Corresponding author (fasano@unisi.it)
High-resolution X-ray computed tomography (XR-CT) (Ketcham & Carlson, 2001) has only recently found
applications in the Earth Sciences, but its potential is growing. Three-dimensional models of the fabric (planar and
linear anisotropies, preferred orientation, porosity, etc.) of analysed sample can be acquired through this methodology,
allowing 3D viewing (10 micron resolution) and the acquisition of digital and vector data for detailed quantitative
textural analyses (Van Geet et al., 2001). Another advantage is the non-destructive nature of specimen analysis that
solves multi thin sections; this is extremely important in the case of unique specimens, samples preserved for future
investigations (e.g. archived cores), or samples of poorly consolidated, coarse-grained aggregates for which it is
particularly difficult to make the many large thin sections necessary for traditional microstructural analyses.
In application to glacigenic sediments XR-CT can further contribute to a precise definition of the microfabric of
analysed samples and of structures associated with deformation and post-sedimentary alteration processes (plasmic
fabric, rotational structures, shear structures, flow structures, etc.). Selected samples for preliminary XRCT applications
include cores of glaciomarine sediments recovered by the CRP project and diamicts from on-shore glacial deposits of
the TransAntarctic Mountain of the Northern Victoria Land. Some samples of glacial sediments from the Alps are also
under investigation to compare different facies and different structures.
The application of XR-CT methodology to these materials is a novelty. The study of these materials (which focuses on
their use as palaeoclimatic indicators) would benefit from the use of XR-CT for various reasons: 1) acquisition of a
larger quantities of higher-quality microstructural and micromorphological data than would be possible through 2D
analysis of just a few large-format thin sections; 2) the construction of 3D quantitative models; 3) the ability to analyse
poorly consolidated specimens; 4) the ability to complete non-destructive analyses and preserve the specimen intact
(preservation of archived cores and of samples of interest for their fossiliferous content).
The instrument is composed by a xfor the sample movement and a XCCD camera as detector. Current used is generally less than 30 mA with a tension of
140kV. Sample is rotated of 360° and for every step of 1° an image is collected. All the data are processed through
specific software to obtain a 3D dataset, for processing, 3D rendering and image analysis. The correct identification and
interpretation of glacial sedimentary facies plays an important role in every palaeoclimatic model and the study of XRCT applications are particularly important for the above-mentioned reasons.Data from this study will better constrain
detailed reconstruction of glacigenic sedimentary cycles. Three-dimensional models will allow a precise
characterisation of sedimentary facies and of depositional mechanisms which will in turn be used in palaeoclimatic
investigations to document the evolution of glacial dynamics.
XR-CT data will be integrated with sedimentological and petrological data in a preferably unified model representing
the interaction of glacigenic sedimentary processes, tectonic evolution and climatic forcing during the Cenozoic and
Quaternary. We shall present some models and images, based on preliminary results from on-going investigations,
obtained on samples coming from different basal till, ablation till and glacio-fluvial deposits.
REFERENCES
Ketcham R.A. & Carlson W.D., 2001. Acquisition, optimization and interpretation of X-ray computed tomographic imagery: applications to the
geosciences. Computers and Geosciences, 27, 381-400.
Van Geet M., Swennen R. & Wevers M., 2001. Towards 3-D petrography: application of microfocus computer thomography in geological science.
Computers and Geosciences, 27, 1091-1099.