Tuhinga 27: 20-49, doi: 10.3897/tuhinga.27.e34225

Obsidian floater washed up on a beach in the Chatham Islands: geochemical composition and comparison with other volcanic glasses

Foss Leach, Hamish Campbell, Nelson Eby, Katherine Holt, Marcel Regelous, Rhys Richards, Steve Weaver

This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Citation: Leach F, Campbell H, Eby N, Holt K, Regelous M, Richards R, Weaver S (2016) Obsidian floater washed up on a beach in the Chatham Islands: geochemical composition and comparison with other volcanic glasses. Tuhinga 27: 20-49. https://doi.org/10.3897/tuhinga.27.e34225

Abstract

A large block of pumice with a thick layer of volcanic glass attached to one side was found on a beach in the Chatham Islands. The geochemical signature of the specimen was most unusual: it proved to be a peralkaline phonolite with a negative europium anomaly. Since there was no obvious eruptive event that might have been the source of the floating object, identification of its geographic source involved a series of steps that progressively narrowed in on fewer and fewer potential sources. This process eventually pointed most strongly to McDonald Island in the Antarctic region southwest of Australia. This was confirmed only after unpublished geochemical data for the island were found. The process of identification described could have wider application when trying to find the volcanic source of obsidian artefacts with greater certainty.

Keywords

Obsidian, pumice, Chatham Islands, McDonald Island, geochemistry, Pacific archaeology, sourcing model.