20. Origin and Alteration of Volcanic Ash and Pelagic Brown Clay, Deep Sea Drilling Project Leg 62, North-central Pacific

Volcanic ash was recovered from lower Aptian to Albian deposits from DSDP Sites 463, 465, and 466; pelagic clay of the upper Pleistocene to Upper Cretaceous was recovered mainly from Site 464, with minor amounts at Sites 465 and 466 (Figs. 1 and 2). We present X-ray-mineralogy data on pelagic clay and altered volcanic ash recovered from the four Leg 62 sites (Table 1; Fig. 1). In addition, two ash samples from Sites 463 and 465, a pelagic clay from Site 464, and a clay vein from the basaltic basement at Site 464 each were analyzed for major, minor, and trace elements (Table 2). Our purpose is to describe the mineralogy and chemistry of altered ash and pelagic clays, to determine the sources of their parent material, and to delineate the diagenetic history of these clay-rich deposits. Correlation of chemistry and mineralogy of ash and pelagic clay with volcanic rocks suspected to be their parent material is not always straightforward, because weathering and diagenetic alteration caused depletion or enrichment of many elements. Cenozoic and Cretaceous sediments, and sedimentary and volcanic rocks were recovered at Site 463, in the Mid-Pacific Mountains, and at Sites 464, 465, and 466, on Hess Rise (Fig. 1). Site 463 cores contain nannofossil ooze, chalk, and limestone interbedded with minor porcellanite, chert, and, in the lower part, minor volcanic ash (Fig. 2). Site 464 sediments are mainly siliceous and brown pelagic clays in the upper section, and interbedded chert, chalk, and mudstone in the Cretaceous section; basaltic basement rock was reached at 308 meters. Calcareous ooze interbedded with minor chert and pelagic clay characterizes the section at Site 465. Cores recovered from the lowermost sections of Site 465 contain abundant pyritic limestone and barite overlying altered ash that in turn directly overlies trachytic flow rocks. Site 466 sediments are mainly nannofossil and cherty nannofossil ooze, with pelagic clay beds in the upper part and limestone with minor chert and porcellanite beds in the lower part. An ash bed was cored near the bottom of the hole. Smectite minerals commonly form from the elements released by breakdown of volcanic rocks and pyroclastic debris. Smectite is the internationally accepted general term for the group of clay minerals that include montmorillonite, nontronite, saponite, etc. (Bridley and