Marbles from Roman Hispania: stable isotope and cathodoluminescence characterization
Introduction
The correct assignment of the provenance of marbles used for ancient artefacts (statues, inscriptions, etc.) provides valuable information for their historic study. Databases of chemical and/or physical parameters aimed at discriminating ancient quarrying areas can resolve important problems (e.g. associating broken or separated fragments, distinguishing between original sculptures and copies, elucidating ancient trade patterns) as the periods of operation are known for many of the major quarries of the Mediterranean basin (see Borghini, 1992, Dodge and Ward-Perkins, 1992).
Over the last 10 a, white marble provenance studies have made great progress (see, for example, Herz and Waelkens, 1988, Waelkens et al., 1992, Maniatis et al., 1995; and others). The main effort has been directed to characterizing the classical marbles from the central and eastern area of the Mediterranean region. Some works have also contributed to the identification of marbles from the Roman quarries of the French Pyrenees (Costedoat and Alvinerie, 1990, Costedoat, 1988, Costedoat, 1992, Costedoat, 1995, Blanc, 1995a) and some districts of the Iberian Peninsula (Lapuente et al., 1988, Lapuente, 1995, Lapuente and Turi, 1995). In this paper, C and O stable isotopes and cathodoluminescence (CL; in both cathodomicrofacies and spectrum response modes) have been derived for marbles from the Betic Chain and the Hesperic Massif in the Iberian Peninsula.
The use of O and C isotope ratios for determining the provenance of classical Greek marbles was pioneered by Craig and Craig (1972) and subsequently applied to other quarry localities of archaeological interest in the Mediterranean basin (Manfra et al., 1975, Coleman and Walker, 1979, Herz and Dean, 1986, Germann et al., 1988; and others). Databases have been developed (Herz, 1987, Herz, 1988, Herz, 1992, Moens et al., 1988, 1992; Gorgoni et al., 1998) which confirm that the signatures thus obtained are less ambiguous than those provided by other geochemical methods. Stable isotopes can also provide information on the authenticity of ancient sculptures through the study of the patinas developed on the marble surface by weathering agents. Isotopic analysis shows a progressive variation in the abundance of the C and O isotopes moving from the inner, fresh portions to the outer, weathered layer or patina (Margolis and Showers, 1988, Margolis and Showers, 1990, Margolis, 1989, Ulens et al., 1995).
Cathodoluminescence microscopy has been widely applied to several classical quarrying areas in Greece, Italy and Turkey (Barbin et al., 1989, Barbin et al., 1991, Barbin et al., 1992a, b) and is particularly useful when isotopic signatures overlap. The CL produces a set of coloured pictures, or cathodomicrofacies which are a powerful tool for characterizing white marbles. However, there are difficulties in reproducibility (stability of luminescence colour and the exposure time) which can be overcome by combining the cathodomicrofacies with the spectral response and recording the peak intensities in order to improve characterization.
Section snippets
Geological setting
A representative selection of 210 samples was systematically collected from quarries located in the Betic Chain (South Spain) and the Ossa Morena Zone of the Hesperic Massif, in the SW part of the Iberian Peninsula (Fig. 1). Detailed information on the geology of these areas can be found in Lapuente (1995). In some quarries, signs of ancient workings are still noticeable (faced blocks and marks of extraction) and petrographic analyses have been used to show that marbles from some of these
Analytical methods
Several techniques (XRD, petrography, cathodoluminescence and stable C and O isotopes) have been applied in order to characterize the Hispanic marbles. XRD was carried out on powdered samples using a PHILIPS diffractomer (PW 1729/00 X-Ray generator). The data thus obtained were corroborated by thin-section examination with the aid of a staining technique (Alizarin red S to distinguish calcite from dolomite). The polarizing microscope was systematically used for studying texture, crystal
Analytical results
The analytical data obtained from the application of the CL-Optical Microscopy technique are summarized in Table 1, Table 2, Table 3, Table 4. The ranges and average intensities of the cathodoluminescence emission spectra are shown in Table 5. Carbon and O isotopic data for the marbles are displayed in Table 6, Table 7, Table 8, Table 9, for each quarry district, respectively. It should be stressed that these isotopic data characterize the modern working surfaces of the various quarry areas and
Discriminating between the Iberian marbles
The key parameters for recognition of the origin of the white Hispanic marbles are summarized in Fig. 8. The first key parameter is the mineralogical composition, the second is always the isotopic signature and the third is a different combination of the more distinctive parameters in each case.
Fig. 9 shows the overall isotopic distribution of the white marbles of the Iberian Peninsula. Grey marbles have been excluded, but yellow spotted or slightly coloured white samples have been considered
Discriminating Iberian marbles from the classical marbles
The Hispanic marbles show some similarities in physical and chemical parameters to those exhibited for the main marbles exploited in ancient times, which are known to have been extensively involved in ancient trade. These marbles, referred to as classical marbles, comprise all the principal archaeologically described marbles of classical Greece and Rome, including those in western Anatolian, Cyclades, mainland Greece and Carrara in Italy. In order to complete the characterization of the white
Archaeological application
The Portuguese quarries of Estremoz Anticline supplied the Colony of Augusta Emerita, the capital of Lusitania in the Iberian Peninsula, with all the marbles used for the Imperial provincial temple of worship and a great part of those of the Forum Porticus (Lapuente et al., 1999; Nogales et al., 1999).
The petrographic characteristics and CL-patterns of the majority of these Portuguese marbles are distinctive, but the fine grained marbles from Borba are superficially similar to the Carrara
Conclusions
Petrographic, stable isotope and CL analyses were used to characterize the white marbles from 4 districts in the Iberian Peninsula. Marbles from Malaga district, Ossa Morena Zone and some of the Estremoz Anticline district are quite uniform in both petrography and CL-pattern. Several lithotypes, especially those of the Almeria district and some of the Estremoz Anticline, are characterized by various cathodomicrofacies.
The use of C and O isotope ratios for characterizing Hispanic marbles is a
Acknowledgements
The authors are indebted to Dr Machel and Dr Marshall for their critical reading of the manuscript and for their useful remarks. They wish to acknowledge the contribution of the associate editor Dr Raiswell in offering valuable suggestions for improving the style and structure of the text. They are also deeply grateful to the Consiglio Nazionale delle Richerche of Italy for making available the facilities of the Stable Isotope Laboratory of the Centro di Studio per il Quaternario e l’Evoluzione
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