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Characterising volcanic glass sources in the Banks Islands, Vanuatu.

Article Excerpt
Abstract

In 2006 volcanic glass deposits on Vanua Lava and Gaua Islands were re-visited and systematically sampled. Twenty-nine source samples were analysed using EDXA-SEM and LA-ICP-MS with a focus on detecting possible intrasource variation. The results show both Banks Islands deposits are readily distinguishable from each other and from other sources in the region and their chemical compositions are highly homogenous. Surface survey of other prospective areas established that these two are the only volcanic glass sources in the Banks Islands.

Keywords: Western Remote Oceania, Obsidian, Provenance studies, Laser ablation ICP-MS, EDXA-SEM

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Due to its strategic location in the Western Pacific, Northern Vanuatu has acted as a crossroad between other archipelagos from the time of initial human colonisation. Although this island group is very important for the understanding of human colonisation of the Pacific, few archaeologists have visited it. Compared to the long successful history of volcanic glass provenance studies in the North-Western Pacific (Leach 1996; Ambrose 1978; Summerhayes 2003; Torrence et al. 1996), which identified and unambiguously distinguished major sources and their exchange systems (Bird et al. 1997; Duerden et al. 1987; Torrence 2004; Specht 2002; White 1996), little is known about the distribution of northern Vanuatu volcanic glasses (Ambrose 1976).

Initial research in the Northern islands of Vanuatu in the early 1970s indicated there were additional sources to the previously-located sources in Papua New Guinea and Western Polynesia (Ambrose 1976; Bird et al. 1981; Ward 1979). A small number of samples of Banks Islands material were analysed using XRF and PIXE-PIGME with characteristically high Al, Mn and Fe concentrations detected (Duerden et al. 1987).

Artefacts from the southern Solomon Islands and Fiji, which had similar elemental composition to these Vanuatu sources, were the basis for assuming there had been a regional exchange system (Kirch & Yen 1982; Hedrick 1980; Best 1984). This interpretation was complicated by the proposition that more than two sources of volcanic glass might be found in the Banks Islands (Smith et al. 1977). After analysis of geological maps (Ash et al. 1980; Mallick & Ash 1975) and field exploration with the help of local guides on six most likely islands of the Banks Islands group, two volcanic glass exposures were identified and sampled.

This paper describes the detailed chemical composition and intra-source variation of the Northern Vanuatu volcanic glass sources for future fingerprinting archaeological artefacts and discusses the comparison of these new data with the chemical composition of major volcanic glass sources of the Western Pacific using multivariate statistics.

Methods

Geological background and sampling methods

Following recommendations by previous scholars (Glascock et al. 1998; Ambrose pets. comm.; Ambrose 1976), a relatively large suite of samples (14 from Vanua Lava and 15 from Gaua) were collected from different locations once the source areas were identified.

The material analysed from Vanua Lava was collected from a geological formation (a 2 km ridge, which strikes in a roughly north-westerly direction and terminates at the top of a volcanic cone) approximately 3 km inland from the village of Ambek (Fig 1; S13 44.610, E167 25.624). In this general locality, volcanic glass of poor quality has been washed out by rain and caught between big boulders of mixed fine-grained types of andesitic rocks and iron-rich tuff. No volcanic glass outcrop was visible on the surface probably due to a landslide which occurred sometime in the last century. Surface collection from riverbeds and river profiles were instead used to systematically sample the whole area.

[FIGURE 1 OMITTED]

Samples up to >15 cm in diameter were collected. Most of the larger samples were heavily shattered and were unsuitable as raw material for tool production. However, smaller pieces (~10 cm diameter) of good quality were also observed and collected.

The material from Gaua derives from a similar geological situation to the one found on Vanua Lava (Fig 1; S14 15.158, E167 33.095--S14 12.676, E167 34.068). The whole source area in northern Gaua is crisscrossed by several creeks which feed the Namasari River. These creeks cut through one extensive lava flow and unearth volcanic glass pebbles of different sizes in several locations. One primary deposit could be found where samples were collected in situ. Additionally, samples were collected along a transect starting from beach deposits, and moving south along the main riverbed and anabranches. Six test pits were excavated along the lava formation and in most cases small samples of volcanic glass were recovered below 1.5m.

Detailed description of the geology and sampling methods of both sources can be found in Reepmeyer (in prep.).

Elemental analysis

In this study Laser...

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