Geochemistry of historical-age silicic tephras in Iceland

The major element chemistry of nine silicic tephras of historical age from Iceland is assessed as a key step in the development of the recent tephrochronology of the North Atlantic region. The tephras include the largest such layers produced by each of the five central volcanoes Hekla, Oraefajokull, Eyjafjallajokull, Torfajokull and Askja since the ninth century AD (H 1104, O 1362, E 1821, Landnam tephra c. 870, A 1875) and four other tephras (H 1158, H 1510, H 1947, O 1727). The determination of grain discrete major element chemistry of the glass fraction is a fundamental stage in the identification and correlation of tephra, and allows links to be made between Icelandic source areas (with precise dating evidence) and distal deposits elsewhere in the North Atlantic region. Although major element data can be used to discriminate between tephra layers produced by the different central volcanoes, on its own it cannot be used to identify all the Holocene layers produced by each central volcano. However, integration with other stratigraphic and chronological data can resolve ambiguous cases, permitting the confident identification of precise isochrones.