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Andreas Ebert, University of Bern (Switzerland)
Karl Ramseyer, University of Bern (Switzerland)
Edwin Gnos, Muséum d'histoire naturelle (Switzerland)
Decrouez Danielle, Muséum d'histoire naturelle (Switzerland)
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Provenance analyses of marbles are often applied in archaeology. However, data derived from specific single analytical techniques like isotope analysis or electron paramagnetic resonance spectroscopy to define the provenance of marbles coming from different regions commonly overlap. Even more, commonly used techniques are not diagnostic to determine the marble's origin within one geodynamic region.
This study presents a new approach of detailed quantitative microstructural analysis coupled with cathodoluminescence (CL) of marbles from Naxos, which allows a clear determination of the marble's origin within one geodynamic region. The obtained mean calcite grain size continuously increases with metamorphic degree from south (150μm) to the metamorphic core (2mm) of Naxos. However, a clear allocation of the marble's origin is only possible, if the microstructures are corrected for a similar second-phase influence. Due to the pinning effect of second phases on the calcite grain boundaries, calcite grain growth is inhibited and therefore, grain sizes are smaller in case of second-phase controlled microstructures. Additionally, grain size distributions and grain shapes of calcite change along the sample profile with temperature and second phase influence. Slightly right-sided skewed distributions and recrystallization microfabrics (strong crystallographic preferred orientations, lobate grain boundaries, subgrains) point to deformation fabrics that are typical for marbles from Naxos. Bimodal grain size distributions and intensely lobate grain boundaries are dominant in the core and point to later overprinting by deformation under retrograde conditions.
Besides the change in microstructures, marble colours become whiter and the colour spectrum narrows towards the core. This is related to the breakdown of graphite with temperature and fluid flow. These facts and additional correlations with CL microfabrics improve the provenance analyses. The maximum CL intensity decreases continuously with rising temperature but increases again in the metamorphic core of Naxos. Simultaneously, the CL colour spectrum and the colour homogeneity vary along the sample profile allowing further narrowing of the marble's origin. These changes are related to metamorphic mineral reactions, fluid flow, amount of dolomite in the marbles and recrystallization.
Summarized, the calcite microstructure, if corrected for a similar second-phase influence, is a suitable tool to determinate the origin of a marble even on a km- scale.
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