Petrology of palagonite tuffs of Koko Craters, Oahu, Hawaii

Abstract

Tuff deposits of the Koko Crater group consist largely of alkali basalt glass, either fresh or palagonitized. Most of the deposits are progressively palagonitized at depth, and topographic relations of palagonite on Koko Crater indicate that the palagonite was formed after the cone had been deeply eroded. The principal authigenic minerals in the palagonite tuffs were deposited in following sequence: phillipsite, chabazite, analcime, montmorillonite together with opal, and calcite. The amount of authigenic minerals in a given sample is generally proportional to the amount of palagonite, indicating that the authigenic minerals are produced in palagonitization of glass. Chemical analyses of sideromelane and associated palagonite by the electron microprobe show that about a quarter of the SiO2, half of the Al2O3 and MgO, and three quarters or more of the CaO, Na2O, and K2O are lost in converting sideromelane to an equal volume of palagonite. A substantial proportion of these components lost from the sideromelane are precipitated nearby in zeolites, montmorillonite, opal, and calcite. Reaction of sideromelane with percolating ground water at low temperatures accounts for the vertical zoning from relatively fresh tuffs down into palagonite tuffs. The pH and ionic strength of percolating water probably increased with depth by solution and hydrolysis of glass, and where the pH and ionic strength became sufficiently high, the glass reacted to form palagonite and zeolites. Palagonite was formed by a microsolution-precipitation mechanism rather than by hydration or devitrification.