Feldspar distributions in South Pacific pelagic sediments

Abstract

The relative abundances and types of feldspars from pelagic sediments of the South Pacific have been studied mainly by X-ray powder diffraction and also by optical methods. Geographic provinces based on the distributions of these feldspars and volcanic quartz have been delineated. Every sample studied contained some feldspars, demonstrating a resistance to submarine weathering, even in conditions of slowest deposition. Most of the feldspars are of volcanic origin and may be used to characterize roughly the predominant type of volcanism in the area. Basic volcanism is dominant over most of the region and is associated with generally linear arrays of large volcanoes. A more heterogeneous suite of feldspars is found near South America and in the island arc province of the western South Pacific. A province of acidic volcanism, apparently related to the profile of the East Pacific rise, has been partially delineated south of Easter Island. The alkali feldspars are predominantly anorthoclases, both homogeneous (near Or35) and perthitic; they are somewhat more sodic than alkali feldspars from common continental volcanic assemblages, and quite unlike the dominant plutonic feldspar in granitic rocks. Quartz of typical volcanic morphology is present in fair abundance. The volcanic ejecta apparently were erupted explosively from small cones about 1 km high. Dissemination appears to have been of the order of less than a few hundred kilometers for the fraction larger than 32 μ and one or two thousand kilometers for the size fraction between 4 and 8 μ. Most of the feldspars of pelagic sediments from the southeast Pacific Ocean must be derived from within the oceanic basin itself.