Sourcing and Nuclear Magnetic Resonance: new applications for old materials
Résumé
Determining the geographical origin of materials used for precious artifacts such as cherts, ancient monuments, statues ... is a challenge that makes possible, after thorough investigations, to get more insight into essential historical features. The archaeometrist looks for clues, such as traces of some minerals, crystalline phases, fossils ... and associates more and more statistical tools to analyze the data collected. The methodological approach proposed in this work is to combine these traditional analysis methods with characterization tools commonly used in the study of modern materials. In particular, Nuclear Magnetic Resonance (NMR) has many advantages for the study of archaeomaterials: it is a non-destructive and non-dedicated technique since it allows us to probe the raw material without any previous preparation. It provides invaluable chemical information which cannot be obtained with other techniques as almost any element of the periodic table may be analyzed. The present work concerns two kinds of lithic materials, marbles and cherts which NMR spectra are used with the aim to provenance them since getting information about the chemical organization of lithic materials is undoubtedly an interesting geographic site marker.
For marbles, the target was its major component, calcium carbonate, which NMR signals response depends on its local environment, such as the presence of secondary elements, and its crystallization state (dolomite vs calcite). Two different nuclei of the calcium carbonate were aimed: Carbon (13C, which is the isotope detectable using NMR and is present at 1% naturally) and Calcium (43Ca, natural abundance, 0.1%). The 13C NMR signal of the main carbonate function of marbles contains information about both the Fe content in its area and the presence of calcium substitution in the calcite crystal in its linewidth. 43Ca, whose NMR detection is a challenge per se due to the low natural abundance of this calcium isotope and its low NMR sensitivity, provides complementary information: The two crystalline forms of marbles, dolomite and calcite, can be distinguished and quantified.
For cherts, the geologic history information of two geological formations from the Pyrénées (France) with similar petrographic features (1) provided by their analysis by 29Si (natural abundance, 4%) and 27Al NMR were investigated. Particularly, two specific data were of interest to discriminate cherts depending on their location: 29Si NMR gives access to the identification of their framework and their Si/Al ratio, and 27Al NMR permits to distinguish and quantify the two most common classes of aluminosilicates, tectoaluminosilicates and layer-lattice aluminosilicates.
To test the potential of NMR for provenance purposes, NMR spectra of archaeological items were recorded and analyzed. For marbles, two portraits (Roman emperor Hadrien , 130 c AD, Roman female portrait, 1st to 2nd c AD) from the National Museum of Tarragona (MNAT, Spain) were made of Carrara and Göktepe marbles, respectively, when the Ithacius sarcophagus (Oviedo cathedral, Spain, 5th c AD) was made of Estremoz white marble confirming previous observation (1, 2). For cherts, a lithic tool recovered at the Magdalenian levels from Montlleó open-air-site and petrographically similar to the two studied geological formations (3) was selected.
To conclude, this work highlights the power of NMR spectroscopy in its capability to identify the geographic origin of lithic artefacts.
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(1) Sánchez de la Torre, et al., Archaeological and Anthropological Sciences, published online 13-12-2017. https://doi.org/10.1007/s12520-017-0581-7.
(2) Vidal, et al., DigitAR, 2016, 3, 119-128.
(3) Sánchez de la Torre, M., 2015, Las sociedades cazadoras-recolectoras del Paleolítico superior final pirenaico: territorios económicos y sociales, Monografies del SERP, 11, 314 p.