5th Swiss Geoscience Meeting, Geneva 2007
Changes in microstructures and cathodoluminescence
fabrics of calcite marbles from Naxos and their
relevance for provenance analyses of ancient white
marbles.
Andreas Ebert*, Karl Ramseyer*, Edwin Gnos**, Danielle Decrouez**
*Institut für Geologie, Baltzerstrasse 1+3, CH-3012 Bern (ebert@geo.unibe.ch)
**Muséum d'Histoire naturelle, Ville de Genève, 1 route de Malagnou, 1211 Genève 6
Provenance analyses of marbles are often applied in archaeology to determine
the origin of marbles used for ancient artworks and buildings. Since data of
single analysis of provenance overlap, investigations are based on multimethod approaches. Frequently used techniques, which give satisfying results,
are isotope analysis and electron paramagnetic resonance spectroscopy.
However, they don’t allow a discrimination of origin within a tectonic setting, like
Naxos or Carrara. Additionally, detailed quantitative microstructural studies
coupled with cathodoluminescence (CL) of one region are missing. In this study
we close this gap on the basis of marbles from Naxos.
The analyses show that calcite grain sizes tend to increase with metamorphic
degree from south (150µm) to the metamorphic core (2mm) of Naxos. This is in
agreement with observations of Covey-Crump and Rutter (1989). Additionally,
the observed trend of grain size increase fits into the trends of grain coarsening
of carbonates of the Helvetic Alps (Herwegh et al., 2005; Ebert et al., 2007a).
This is an important observation because it shows that microstructures can be
used to estimate temperature conditions during deformation, although the
tectonic settings are different (e.g. strain rate, fluid flow).
Variations in microstructure within an outcrop or sample are caused by second
phases. As recently presented by Herwegh et al. (2005) and Ebert et al.
(2007a), pinning of second phases (e.g. dolomite, sheet silicates) retards grain
growth of calcite. Therefore, varying contents and types of second phases result
in different microstructures. Quantitative analysis of the second phase influence
provides the possibility to correct for a defined content and size of second
phase to estimate the calcite grain size of an equivalent pure marble. This in
turn precises provenance analyses and is necessary to define deformation
and/or metamorphic conditions.
Likewise, the grain size distribution and grain shape of calcite change 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,
which is in contrast to Covey-Crump and Rutter (1989) but in agreement with
e.g. Urai et al. (1990). Bimodal grain size distributions and intensely lobate grain
boundaries, which are dominant in the core, point to later overprinting by
deformation under retrograde conditions (see also Ebert et al., 2007b).
Additionally, the colour of calcite marbles changes with temperature. The colour
becomes whiter and the spectrum narrows towards the core. This can be
explained by the breakdown (oxidation) of graphite due to elevated
temperatures and fluid flow (e.g. Herwegh and Berger, 2003). To precise
provenance analyses, latter parameters should be added to the grain size
provenance analyses.
5th Swiss Geoscience Meeting, Geneva 2007
The CL microfabrics also change along the sampled profile. The maximum CL
intensity decreases continuously with rising temperatures but increases again in
the zone of anatexis (metamorphic core of Naxos). The colour spectrum
changes from yellow, orange, and blue colours in the south of Naxos to blueviolet colours, where intensities are the lowest, and reach again mainly intense
orange colours in the core. Latter colours can be explained by metamorphic
mineral reactions or migmatization resulting in higher fluid flow penetrating the
fabrics and providing different ratios of Mn and Fe, which change the CL colour
and intensity. Simultaneously to the change in CL intensity, the CL fabrics
become more homogenous towards the core. The differences in CL intensity
and fabrics depend of the dolomite content of the calcite marbles and the
homogenization of microfabrics. Latter one is faster with increasing
temperatures during deformation inducing faster diffusion processes and grain
boundary mobility due to dynamic recrystallization.
The investigations indicate that provenance analyses using the microstructure
and CL fabric are possible. Furthermore, this helps to narrow down the origin
within a region. In addition, we are now able to provide a good database of
microstructures for geologists and archaeologists. Since we can interpret the
variations of microstructures within an outcrop, the large grain size variations of
Covey-Crump and Rutter (1989) for constant temperature conditions can be
explained by the influence of second phases.
REFERENCES
Covey-Crump, S. J. & Rutter, E. H. 1989: Thermally-induced grain growth of
calcite marbles on Naxos Island, Greece. Contributions to Mineralogy and
Petrology 101(1), 69-86.
Ebert, A., Herwegh, M., Evans, B., Pfiffner, A., Austin, N. & Vennemann, T.
2007a: Microfabrics in carbonate mylonites along a large-scale shear zone
(Helvetic Alps). Tectonophysics In Press.
Ebert, A., Herwegh, M. & Pfiffner, A. 2007b: Cooling induced strain localization
in carbonate mylonites within a large-scale shear zone (Glarus thrust,
Switzerland). Journal of Structural Geology 29(7), 1164-1184.
Herwegh, M. & Berger, A. 2003: Differences in grain growth of calcite: a fieldbased modeling approach. Contributions to Mineralogy and Petrology 145(5),
600-611.
Herwegh, M., Berger, A. & Ebert, A. 2005: Grain coarsening maps: A new tool
to predict microfabric evolution of polymineralic rocks. Geology 33(10), 801-804.
Urai, J.L., Schuiling, R.D. & Jansen, B.H. 1990: Alpine deformation on Naxos
(Greece). Deformation mechanisms, rheology and tectonics. Geological Society
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