Abstract:
Novel imaging methods such as X-ray microtomography (μCT) are becoming increasingly popular tools in the geosciences to understand 3D aspects of geological processes.  Microtomography not only provides relatively easy visualization of the rock or grain structure, but can also reveal features that would be time consuming or impossible to quantify with traditional thin-sectioning methods.  The technique can be utilized with two very different types of X-ray source: 1) so-called ‘desktop’ μCT scanners, which are based on an X-ray tube and best suited for analysis of relatively large samples with micrometer resolution [1], and 2) synchrotron radiation sources, where the significantly higher flux enables a wider choice of beam mode and resolution [2].  A key advantage provided by a synchrotron source is the possibility to perform multimodal experiments, correlating the micro- or nanotomography scan with chemical or crystallographic -information. At the European Synchrotron (ESRF), the nanoprobe beamline ID16B [3] allows phase-contrast nanotomography to be complemented by X-ray diffraction and fluorescence scanning with a beam size of approximately 50 nm.  In this work, the capabilities of ID16B for geoscientific research are illustrated via two example minerals, along with discussion of the practical challenges to be overcome to obtain high quality imaging, fluorescence, and diffraction results of ~ 100 μm –sized grains.  In the first example [2], nanotomography is combined with desktop μCT in a multiresolution approach.  Nanotomography is used to visualize the spatial distribution and mineral associations of refractory gold locked inside arsenopyrite crystals originating from the largest primary gold producer in Europe (Kittilä mine in northern Finland).  Desktop μCT is used to quantify the sulphide orientation and size distributions at the drill core scale.  The results help understand the geological context of the deposit and to constrain the timing of sulfide mineralization with respect to known metamorphic events in the Paleoproterozoic deposit.  In the second example [4], 3D-nanotomography results are correlated with X-ray diffraction and fluorescence analyses of Paleoproterozoic zircon (Central Finland Granitoid Complex), in order to link the 3D patterns of growth zoning and inclusions within the crystal to their chemical and crystallographic composition.  Due to its high melting point, hardness, and ubiquity in the Earth’s crust, zircon is the most important mineral used as a geochronometer via U-Pb dating.  Dating results, however, can be affected by metamorphic events that lead to lead loss from the zircon, which makes detailed information about the uranium and lead distributions crucial for interpreting dating results.  In addition, zoning pattern and distribution of trace elements like Hafnium and the rare-earth elements (REE), as well as inclusion composition, provide geologists with clues to the zircon’s origin and history of the host rock.  All of these features in zircon can be non-destructively visualized by the combination of different Xray imaging modalities available on ID16B.  The last part of the presentation will discuss absolute quantification of X-ray fluorescence tomography results of minerals, and provide preliminary results on applying these concepts to aid in dating impact-shocked zircon.

References:
[1] Sayab M et al. (2015) Geology 43(1): 55-58
[2] Sayab M et al. (2016) Geology 44(9): 739-742
[3] Martínez-Criado G et al. (2016) Journal of Synchrotron Radiation 23: 344-352
[4] Suuronen JP & Sayab M (2018) Scientific Reports 8:4747