Palagonite tuffs (hyaloclastites) and the products of post-eruptive processes

Abstract

A synoptic review of the studies of well-known occurrences of palagonite tuffs is presented. Included are palagonite tuffs from Iceland, and pillow-lava palagonite complexes from Columbia River basalts and from the central Oregon coast. Additional petrologic and x-ray defraction data for selected samples are presented. Petrologic evidence shows that basaltic glass of aqueous tuffs and breccias consists of sideromelane, which is susceptible to palagonitization. It is shown that palagonitization is a selective alteration process, involving hydration, oxidation and zeolitization. Some of the manganese nodules dredged from the Pacific Ocean floor contain nucleus of palagonite-tuff breccias or of zeolite. A brief megascopic and microscopic description of nodules from the south Pacific, the Mendocino ridge and the «Horizon» Nodule from the north Pacific is presented. Petrographic studies of palagonite-tuff breccias of manganese nodules and other palagonites suggest that migration and segregation of metallic elements occur during and subsequent to palagonitization. During the palagonitization of sideromelane, nearly 30 per cent of sea water is absorbed. The hydration of sideromelane is also accompanied by oxidation of iron and other elements. These oxides may be released either in colloidal form or in true solution and tend to precipitate first from the unstable palagonite. Available data on manganese deposits associated with submarine volcanism and the petrologic studies presented here show that a genetic relationship exists between palagonite tuffs and ferromanganese minerals of the nodules. It is suggested that palagonitization is an important step in the dissolution and precipitation of the manganese minerals and the associated minerals contained in the nodules. It is also suggested that some of the red clays associated with manganese minerals are alteration products of palagonite tuffs. The present study indicates that zeolites and montmorillonite are important post-eruptive products of palagonite. It appears that phillipsite is the dominant zeolite and is genetically associated with palagonite in north Pacific sediments. The mechanism of solution, migration and re-deposition of ferromanganese minerals is not clearly understood. However, the association of zeolites, carbonates and manganese minerals seems to be controlled by a delicate balance betweenpH andEh variations. Mineralogic associations suggest that these minerals are deposited under high alkaline conditions, which seems to be consistent with observations made by earlier workers. Pertinent results from the studies of palagonitized basalt from the experimental Mohole drilling are also discussed.