INTRASAMPLE VARIATIONS IN STABLE ISOTOPE COMPOSITION OF CARRARA MARBLE: IMPLICATIONS FOR PROVENANCE STUDIES

Stable isotope analysis is a common tool for provenance analysis of marble artifacts. This study examined Carrara marble samples from 7 quarries from the Colonnata, Miseglia and Torano valleys in the Apuan Alps, near the town of Carrara in Tuscany, Italy. Carrara marble was formed by metamorphism of Triassic to Early Jurassic limestones during the Tertiary, and it is one of the best known architectural, decorative and statuary stones in the world. The primary objective of this study was to establish our own database for comparison with preexisting data and to use in future provenance studies. Samples for stable isotope analysis were obtained by drilling a small amount of powder (1-2 mg) from cut and polished marble slabs using a microscope-mounded drill. The analyses of multiple samples collected from the same small piece of marble (<10 cm in diameter) documented significant intrasample variations in isotopic composition among some samples, with potentially significant implications for the use of this technique in provenance studies.

Carbon isotope values of Carrara marbles generally do not vary by much, and tend to cluster around 2 ‰ VPDB, but oxygen isotope values range between -2 and -8 ‰ VPDB, with some reaching as low as -15 ‰, and differing by almost 12 ‰ in samples collected only 1-2 cm apart. We also examined the results for any trends in marble color and crystal size. In some cases, coarse-crystalline and lighter color (white) marble samples have lower isotope values compared to associated finer-crystalline, darker (gray) samples. In other instances, there were no obvious trends and variations in isotopic composition were recorded in samples of very similar appearance.

In general, and not surprisingly, the samples of pure white marble, including statuary marble or statuario, had lower intrasample variations than multicolored examples. While this is certainly advantageous for provenance analysis of white Carrara marble, it calls for caution in studies that involve other Carrara varieties. Sampling of precious artifacts for stable isotope analysis is often limited in number and amount due to its destructive nature. Collecting a small amount of mineral powder from just one area and regarding it as a representative sample, however, may be misleading for Carrara marbles with heterogeneous isotopic compositions.