Sediments ranging in age from Eocene (50 m.y.) to Holocene have been sampled at or near the sea floor in the equatorial Pacific. The mineralogy of fine fractions of these deposits shows that a component derived in large part from tholeiitic debris is the principal constituent of all middle Eocene (50-m.y.-old) to late Miocene (10-m.y.-old) deposits. This component is characterized by abundant montmorillonite, plagioclase of intermediate composition, and the zeolites phillipsite and clinoptilolite-heulandite. It persists as a major constituent of many late Cenozoic southern equatorial deposits but forms a minor portion of Quaternary sediments from the North Pacific.

Southern and western equatorial sediments younger than late Miocene (10 m.y.) are rich in pyroxene (including orthopyroxene) and chlorite and have high pyroxene:plagioclase ratios (typically 1.4:1) in 2- to 20-μ fractions. The abundance of pyroxene may in part reflect late Cenozoic changes in the tenor of oceanic volcanism. Most of the pyroxene-rich debris is derived from the island arcs that form the western margin of the Pacific basin, however, and to a much lesser extent from Andean volcanoes. The appearance of the “island-arc” material coincides with a period of heightened tectonism in many marginal areas of the Pacific, and also with an interval when it has been suggested that the rate or pattern of sea-floor spreading was undergoing a marked change.

Northern equatorial deposits of Quaternary age are dominated by windborne continental debris rich in quartz and illite. Fine fractions of periglacial loess deposits associated with Pleistocene ice masses in the northern hemisphere probably constitute the main source of such debris. Tertiary equatorial sediments (with the exception of a few northwestern equatorial occurrences of Pliocene age) consist of less than 20 percent “continental” material.

Of the minerals studied, only chlorite appears to be modified by prolonged exposure to pore water. The crystallinity of chlorite deteriorates over a period of 5 to 10 m.y. but then changes little over additional periods of tens of millions of years. The only other observed pronounced diagenetic effect is a systematic increase in the grain size of older sediments due to dissolution of opaline tests and reprecipitation of the silica as cement.

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