COMPARISON OF PETROGENETIC PROCESSES BETWEEN THE WEST VALLEY SEGMENT OF JUAN DE FUCA RIDGE AND THE ADJACENT HECK CHAIN OF SEAMOUNTS: DETAILED ELECTRON-MICROPROBE STUDY AND NOMARSKI INTERFERENCE IMAGING OF PLAGIOCLASE

NANCY A. VAN WAGONER and MATTHEW I. LEYBOURNE*
Department of Geology, Acadia University, Wolfville, Nova Scotia B0P 1X0

THOMAS H. PEARCE and CATHY E. TIMMS
Department of Geological Sciences, Queen's University, Kingston, Ontario K7L 3N6

* Present address: Ottawa-Carleton Geoscience Centre, Department of Geology, University of Ottawa, 161 Louis Pasteur Drive, Ottawa, Ontario K1N 6N5.

Abstract

We studied plagioclase phenocrysts from West Valley, the Endeavour Ridge and the Heck chain of seamounts of the northern Juan de Fuca Ridge using Nomarski imaging and detailed electron-microprobe scans in order to further document differences between magma-chamber processes in ridge and seamount environments. Chemical and textural variations in the zonation of plagioclase phenocrysts and megacrysts provide insights into the pre-eruptive history of the relevant magmas. Both seamount and ridge samples exhibit texturally complex patterns of zoning. Plagioclase phenocrysts from the seamounts, however, vary by less than 9% An content, and there is little measurable variation in An content within zones or between zones. The textures and chemistry can be accounted for by changes in temperature and pressure as the magma rises to the surface, or back mixing involving very similar magmas. In contrast, ridge samples vary by up to 21% An content. In some cases, it is possible to correlate resorption surfaces with chemical variations, providing evidence for the timing of mixing and the minimum number of magmas involved. The Endeavour Ridge rocks have not preserved mineralogical evidence for mixing, possibly owing to more efficient mixing associated with robust volcanism.

Keywords: plagioclase, Nomarski, mineralogy, Juan de Fuca Ridge, seamounts, rift, magma mixing.