Glassy Abyssal Basalts, Atlantic Sea Floor near St. Paul's Rocks: Petrography and Composition of Secondary Clay Minerals

摘要

Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement. Hyaloclastite and pillow lavas were dredged a short distance northwest of St. Paul's Rocks. These rocks are more alkaline than the basalts erupted from spreading ridges. In addition to olivine and plagioclase phenocrysts, the rocks contain complex sector-zoned pyroxene phenocrysts. Fe₂O₃/FeO of about 0.20 was calculated for the glass from FeO-MgO partitioning between olivine and liquid. Clay minerals, mainly potassian smectite, are abundant in both rock types. These are divided into three groups on the basis of their composition, optical properties, and mode of occurrence. All these groups have structural formulas that are intermediate between dioctahedral and trioctahedral smectite. Potassium, sodium, and calcium, in order of abundance, are the dominant interlayer cations. These smectites occur with small amounts of analcite and even less abundant phillipsite. The abundance and composition of these clays suggest that they are important traps for potassium and magnesium from sea water.