Journal of Conference Abstracts

Submarine Hydrothermal Fluid Composition in Relation to the Eruption at Montserrat

David S. Cronan (d.cronan@ic.ac.uk), Alan G. Johnson (agjohnson@ic.ac.uk) & Richard A. Hodkinson (r.hodkinson@ic.ac.uk)

Department of Geology, Imperial College of Science, Technology & Medicine, London, SW7 2BP.

Geochemical observations on submarine hydrothermal springs off Montserrat have shown that their composition has varied up to and during the early stages of the current eruption there. Average concentrations of the hydrothermally associated elements Mn, Fe and As increased in the springs by approximately forty-fold, three hundred-fold and sixty-fold respectively up to the early stages of the eruption, and then decreased to intermediate values as the eruption proceeded. These results are thought to be related to the leaching of fresh magmatic rock under the volcano prior to the eruption and could have important implications for predicting the eruptions of coastal and submarine volcanoes on the basis of the monitoring of hydrothermal fluid composition.

From Tertiary Basalt Lavas to Neolithic Porcellanite Axes: the Uses of Geochemistry in Provenancing Stone Artifacts

Joanne Curran (jm.curran@qub.ac.uk), Ian Meighan (fax: +44 (0) 232 321280) & Derek Simpson (fax: +44 (0) 232 321280)

School of Geosciences, The Queen's University of Belfast, BT7 1NN.

The manufacture of stone axes represents a major part of Ireland's prehistory. Although a variety of rock types were utilised during the Neolithic (c. 4000­2500 BC) porcellanite (the contact metamorphosed degradation product of Tertiary basalt) was the single most important raw material in axe production in Ireland accounting for over half the 20,000 known stone axes (Cooney & Mandal, Irish Stone Axe Project).

It is well established that porcellanite axe factories existed at two locations in the north of Ireland: a mainland site in County Antrim (TIEVEBULLIAGH) and an island site on Rathlin off the N. Antrim coast (BROCKLEY). Critically, however, distinguishing between the products from these two source regions has proved to be a major limitation to the interpretation of the production and distribution of porcellanite axes in Ireland.

The porcellanite formed in similar ways at Tievebulliagh and Brockley where parts of the Interbasaltic Formation (ferruginous/bauxitic material derived from the weathering of Tertiary basalts) underwent thermal metamorphism caused by the intrusion of dolerite plugs. However, despite the similarity in the mode of formation, there are notable differences in the elemental compositions (e.g. Sr; Meighan et al., Ulster J. of Archaeology, 56, 1993) of the porcellanites at these two axe factory sites. These geochemical contrasts are probably related to variations in the original basalt composition and/or subsequent alteration and weathering of the lavas. Magnetic susceptibility and differences in isotopic composition are also being investigated.

Here we present several geochemical discriminants that can distinguish porcellanite artifacts from these two axe factories. These allow definitive provenancing for the first time which could provide further insight into the organization of implement production in the Neolithic Society of Ireland.