Geochemistry at DSDP Leg 59

Deep Sea Drilling Project Legs 59 and 60 drilled 15 sites along an east-west transect at 18°N from the West Philippine Basin to the Mariana Trench (Fig. 1) in order to study the nature and genesis of the back-arc, marginal basins and the remnant and active arcs of the region. Leg 59 drilled at five sites at the western end of the traverse: Site 447 in the West Philippine Basin; Site 448 on the Palau-Kyushu Ridge; Sites 449 and 450 in the Parece Vela Basin; and Site 451 on the West Mariana Ridge. Penetration into basaltic basement of these sites was 183.5 meters at 447 (8 basalt flows); 623 meters at 448 (46 basalt flows, sills, and dikes and volcaniclastic units); 40.5 meters at 449 (2 basalt flows); 7 meters at 450 (1 basalt intrusion); and 4 meters of basalt breccia at 451 overlain by 861 meters of volcaniclastic sedimentary rocks.

Radiolarians at DSDP Leg 58 Holes

Radiolarians were observed at all five sites drilled during DSDP Leg 58. Three sites (442, 443, 444) are south of Japan in the Shikoku Basin. The remaining two sites (445, 446) are east of Okinawa, in the Daito Ridge and Basin areas. The observations made on radiolarians during Leg 58 are understood best by considering these two areas separately. The basement ages, preservation, diagenesis, and paleoecology are similar within each area, but different between the two areas. The radiolarian zones of Riedel and Sanfilippo (1978) were used to determine the sediment age. Because of the mixed nature of the fauna, there was an opportunity to test the tropical zonation in middlelatitude sediments. A middle- to high-latitude biostratigraphy for the Pliocene and Pleistocene has been formulated (Hays, 1970; Kling, 1973; Foreman, 1975), but there is no Miocene radiolarian zonation for these latitudes. The tropical elements of the present fauna are sufficient to use the low-latitude zonation, although there is a loss of resolution in the Pleistocene. Because of poor preservation, zone boundaries are indistinct in much of the cored sediment. Determination of abundance in any sample is always subjective and varies among investigators. This work was in its final stages at the publication of Westberg and Riedel (1978), and the guidelines outlined therein are not closely followed. The abundances recorded in Tables 1 through 5 are based on strewn slides which were searched entirely if an individual of a species was found, or for 8 to 10 minutes if the species was not found.

Stable chlorine isotopes in pore waters from DSDP Hole 79-546 and ODP Hole 131-808C

Stable Cl isotope ratios, measured in marine pore waters associated with the Barbados and Nankai subduction zones, extend significantly (to ~-8 per mil) the range of d37Cl values reported for natural waters. These relatively large negative values, together with geologic and chemical evidence from Barbados and Nankai and recent laboratory data showing that hydrous silicate minerals (i.e., those with structural OH sites) are enriched up to 7.5 per mil in 37Cl relative to seawater, strongly suggest that the isotopic composition of Cl in pore waters from subduction zones reflects diagenetic and metamorphic dehydration and transformation reactions. These reactions involve clays and/or other hydrous silicate phases at depth in the fluid source regions. Chlorine therefore cannot be considered geochemically conservative in these systems. The uptake of Cl by hydrous phases provides a mechanism by which Cl can be cycled into the mantle through subduction zones. Thus, stable Cl isotopes should help in determining the extent to which Cl and companion excess volatiles like H2O and CO2 cycle between the crust and mantle.