Glassy and basaltic fragments within graded volcaniclastic sediments at DSDP Site 89-585

Graded volcaniclastic sediments deposited by turbidity currents in the basal section at Site 585 are mixed vitric-lithic tuffs and lapilli-stones with minor Plagioclase and clinopyroxene crystal fragments. They represent the reworked products of basaltic hyaloclastite accumulations that developed on the flanks and tops of nearly emergent seamounts. They were subsequently swept to the abyssal depths of the southern Mariana Basin by turbitity currents, along with minor reef debris. Vesicles in glassy clasts are on average very small (0.1-0.2 mm diameter), although their volumes, as a percentage of the enclosing glass, are relatively high and suggest a maximum depth of initial lava intrusion of < 400 m. A shallow-water environment for the hyaloclastites is indicated by the commonly associated shallow-water faunal debris and ooids. There is no clear evidence of epiclastic or pyroclastic material associated with the hyaloclastites. Glass, rarely preserved as sideromelane, is generally replaced by palagonite or smectites, and ranges from highly vesicular aphyric to moderately vesicular phyric types, with various assemblages of olivine, Plagioclase, and clinopyroxene microphenocrysts and megaphenocrysts. Lithic clasts exhibit a range of textures—from opaque quench-textured tachylite glass to hypocrystalline and holocrystalline basalts—which are commonly Plagioclase- or plagioclase-clinopyroxene- phyric. Correlation of primary petrographic features between vitric and lithic clasts and textures, based on cooling history, allows the identification of five "lava types," each of which represents an extrusive episode. All the volcaniclastics are pervasively altered, with the development of palagonite, various colored smectites, celadonite, and zeolites typical of the lower zeolite facies. Two phases of alteration are recognized: submarine alteration after initial fragmentation, and subsequent alteration during burial after transportation to abyssal depths.