La signature chimique des inclusions minérales comme traceur de l’origine des céramiques: l’apport des analyses par LA-ICP-MS

Abstract : The analysis of ceramic raw materials, whether through petrographic observations of the inclusions using a polarizing microscope or through general chemical analysis, may, in some cases, encounter difficulties related to the nature of the clay used by potters. This is particularly the case in the crystalline massifs, such as the Massif Central or the Armorican Massif, where the ubiquity of certain rocks such as granite or gabbro (weathered clays of which have been used by potters to make vessels) does not allow us to accurately identify the origins of the products. In fact, ceramics fashioned using granitic clays from locations several hundred kilome-tres apart, may have similar mineral inclusions and chemical signatures. The same is true for pottery made with rock alteration products such as gabbro and serpentinite, whose outcrops (although less numerous) may also cause doubt as to the exact geographical origin of the clays.These methodological obstacles curtail discussion regarding the exchanges which may have taken place between communities, and thus hamper our understanding of the relationships between human groups. It is, therefore, necessary to develop new approaches to the study of ceramic raw materials, in particular using tools such as the plasma mass spectrometer coupled with a laser ablation sampling system (LA-ICP-MS). This technique has allowed renewed studies of ceramic inclusions, as demonstrated by the work on the sourcing of fluvial shell tempers or the dating of detrital zircon inclusions using U/Pb methods. However, it also allows the sources of ceramic raw materials to be identified through analysis of the clay matrix, including mineral inclusions.This article presents the latest methodological advances made using the LA-ICP-MS technique for distinguishing the origin of ceramic pastes that are petrographically and geochemically similar. By means of several examples, located on the Armorican Massif and chro-nologically spanning the Early Neolithic to the Late Iron Age, we will demonstrate that some of the minerals included in these pastes can be used as tracers to differentiate between productions and to precisely identify clay sources and parent rocks. We will describe how the chemical composition of biotite tablets allows us to distinguish the origins of ceramics whose inclusions correspond to the mineral assemblage of granitic rock. Secondly, we will present the results obtained from the analysis of the biotites included in the clay paste of a Bronze Age urn discovered on Belle-Ile-en-Mer (Morbihan), an island whose geology does not feature granitic outcrops. This urn is, therefore, an import, which may have come from the neighbouring islands of Houat or Hoedic, or from the mainland. Next, we will present the results obtained from the analyses of amphibole grains included in pottery made from alteration clay derived from gabbroic rocks and we will see that this method allows us to identify and distinguish between two main pottery productions from workshops dating to the Late Iron Age. In fact, during this period in Brittany, two production areas used gabbroic clay to produce ceramics: these are located on two different gabbroic massifs, one at Trégomar (Côtes-d’Armor) and the other at Saint-Jean-Du-Doigt (Finistère). Their products were exported over several hundred kilometres, as far as the site of Hengistbury Head in southern England (Morzadec, 1995). However, until analysis of the amphibole grains using LA-ICP-MS, it was very difficult to identify the precise origin of the products.Furthermore, through the unique example of a Bronze Age ‘paléo-onctueuse’ ceramic discovered on the site of Kermenguy (Finistère), we will see that the singularity of a paste does not always allow us to identify its origin. This vessel was made with clay which derived from serpentinite, an ultra-basic rock, only two outcrops of which exist in Brittany: one located at Ty-Lan (Finistère) and the other at Belle-Isle-en-Terre (Côtes-d’Armor). However, since Kermenguy is equidistant from the two outcrops, it is difficult to identify the exact origin of the raw material used. Furthermore, clay from Ty-Lan was used by potters during the late Iron Age to make ‘proto-onc-tueuses’ ceramics. We will outline the results of the analyses of the opaque minerals present in the paste of the ‘paléo-onctueuse’ and ‘proto-onctueuse’ ceramics and in the two serpentinites, in order to identify their chemical signatures.In sedimentary basins, such as the Caen Plain, certain ceramic workshops may have existed that used clays derived from the decal-cification of shelly limestone. These vessels are naturally tempered with fossil shells which are more or less cemented by limestone. This is one of the main clues permitting the origin of fossil shells to be determined. Examples of this type of pottery are common in the sedimentary basin of the Caen Plain, where Bronze Age and Early Iron Age potters produced pottery known as Caen Plain ware (Manson et al., 2011). However, in the case of shell-tempered pottery discovered in a crystalline massif we need to determine whether the inclusions are fossils shells derived from shelly limestone, or if the potters deliberately added crushed shells to their pastes. This is the case on the Late Iron Age site of La Batterie-Basse (Manche) where several ceramics containing crushed shell temper were found on the shore. We will see that the LA-ICP-MS can help us to determine the nature of these inclusions on the basis of their chemical compositions.Finally, the application of the methods presented in this article to ceramics from the Early Neolithic site of Kervouyec-Nevez (Finistère) provides us with an example of data crossover which can be obtained from analysis of several types of mineral inclusions. In fact, we will see that the inclusions within a section of wattle and daub can be used as a reference to determine the chemical compositions of several local minerals which can then be compared to the inclusions in the pottery discovered on the site.
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Submitted on : Monday, April 1, 2019 - 3:45:43 PM
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Gehres Benjamin, Guirec Querré. La signature chimique des inclusions minérales comme traceur de l’origine des céramiques: l’apport des analyses par LA-ICP-MS. Actes de la table ronde de Namur, May 2015, Namur, Belgique. pp.177-197. ⟨hal-02086783⟩

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