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The sedimentary sequence recovered from Hole 487 documents northeast seafloor spreading on the Cocos Plate. A basal 65-meter unit of upper Miocene-Pliocene brown clay was deposited on the subsiding east flank of the East Pacific Rise. The overlying 105-meter unit of gray hemipelagic silt and mud was deposited when Site 487 drifted within reach of terrigenous sediment derived from the Mexican continental margin. Analyses of 21 elements in 45 samples, taken at regular intervals up Hole 487, give a geochemical profile which shows metal enrichment in basal sediment similar to that observed in previously recovered basal sediment sections in the Eastern Pacific (e.g., von der Borch & Rex, 1970; von der Borch et al., 1971). This verifies that the basalt cored at the base the hole is oceanic basement, since the first sediment to be deposited on newly formed ocean crust is characteristically enriched in metals, particularly Fe and Mn. The metals precipitate from circulating hydrothermal-exhalative solutions which are an integral part of active ridge volcanism (summary in Jenkyns, 1978). In this chapter I discuss the geochemistry of the basal metalliferous sediment from Hole 487. The geochemical data may also prove useful in studies of onshore volcanism: mass-balance calculations show that much (possibly all) of the incoming Cocos Plate sediment has been subducted since the Miocene (Watkins et al., this volume). It may have contributed to the genesis of magmas erupted in the Mexican volcanic arc.

Geochemistry of Cocos Plate Pelagic-Hemipelagic Sediments in Hole 487, Deep Sea Drilling Project Leg 66