Author:

Keywords:

Science & Technology, Physical Sciences, Geochemistry & Geophysics, 0402 Geochemistry, 0403 Geology, 0404 Geophysics, Energy, 3703 Geochemistry, 3705 Geology

Abstract:

The alkalic suite of Jan Mayen is of the trachybasaltic type with a K2O/Na2O ratio of about 1·64. The suite includes all intermediate types of lavas between ankaramite and trachyte, with ankaramites being particularly prevalent. The major and trace element trends are well defined. A new method allows distinction between fractionated and accumulative ankaramites, and it is shown that the most primitive ankaramite contains 13-14 per cent MgO. Experimentally determined P-T phase relations of this composition suggest that it might be a primary composition, formed by partial melting of a spinel lherzolitic source at 19·5kb and 1415°C. The fractionation from ankaramite to trachybasalt occurred at low pressure, and was controlled by delayed gravitative settling of phenocrysts, while the fractionation from trachybasalt to trachyte is explained by crystallization on the walls of the magma chambers. © 1986 Oxford University Press.