40Ar-39Ar geochronological studies on rocks at DSDP Leg 58 Holes

The technique of 40Ar-39Ar step-heating dating was applied to three rock samples from core of DSDP Site 443, one sample from Site 445, and four samples at Site 446. All sites were drilled during DSDP Leg 58. At Site 443 (Shikoku Basin), about 116 meters of basalt basement was drilled. Three samples were chosen for dating from different levels in the basalt; two samples are aphyric basalt, and the other is subophitic dolerite. At Site 445 (Daito Ridge), no basement rock was drilled; however, conglomeratic sandstone was cored in the lower part of the hole. 40Ar-39Ar dating was applied to a basalt pebble in the conglomerate. At Site 446 (Daito Basin), the lower cored sequence is clay stone interlayered with 16 basalt sills. Four samples were chosen from sills at different levels.

Grain-size, carbona and calcium carbonate at DSDP Leg 58 Holes

Sand-silt-clay distribution was determined on 10-cm**3 sediment samples collected at the time the cores were split and described. The sediment classification used here is that of Shepard (1954), with the sand, silt, and clay boundaries based on the Wentworth (1922) scale. Thus the sand, silt, and clay fractions are composed of particles whose diameters are 2000 to 62.5 µm, 62.5 to 3.91 µm, and less than 3.91 µm, respectively. This classification is applied without regard to sediment type and origin; therefore, the sediment names used in this table may differ from those used elsewhere in this volume; e.g., a silt composed of nannofossils may be called a nannofossil ooze in a site chapter.

Organic carbon and nitrogen at DSDP Leg 58 Holes

The sediments penetrated on Leg 58 of the Deep Sea Drilling Project in the Philippine Sea represent long periods of geologic time during which depositional conditions apparently remained very constant. Organic carbon and nitrogen contents of the sediments decrease with increasing depth of burial, before leveling off at minimum values of about 0.05 to 0.10 per cent and 0.01 per cent, respectively. The depth at which the minimum values are reached varies from site to site, but ages of sediments corresponding to the minima are all about 5 m.y. We infer that slow bacterial diagenesis is responsible for the gradual depletion of organic carbon and nitrogen. It is likely that the rate of bacterial metabolism is controlled by the rate of diffusion of electron acceptors within the sediments. These results suggest that bacterial ecosystems in deep-water sediments play a much more important role in diagenesis than has previously been thought.

Geochemistry and minerals at DSDP LEg 58 Holes

Leg 58 successfully recovered basalt at Sites 442, 443, and 444, in the Shikoku Basin, and at Site 446 in the Daito Basin. Only at Site 442 did penetration reach unequivocal oceanic layer 2; at the other sites, only off-axis sills and flows were sampled. Petrographic observations indicate that back-arc basalts from the Shikoku Basin, with the exception of the kaersutite-bearing upper sill at Site 444, are mineralogically similar to basalts being erupted at normal mid-ocean ridges. However, the Shikoku Basin basalts are commonly very vesicular, indicating a high volatile content in the magmas. Site 446 in the Daito Basin penetrated a succession of 23 sills which include both kaersutite-bearing and kaersutite-free basalt varieties. A total of 187 samples from the four sites has been analyzed for major and trace elements using X-ray-fluorescence techniques. Chemically, the basalts from Sites 442 and 443 and the lower sill of Site 444 are subalkaline tholeiites and resemble N-type ocean-ridge basalts found along the East Pacific Rise and at 22° N on the Mid-Atlantic Ridge (MAR), although they are not quite as depleted in certain hygromagmatophile (HYG) elements. They do not show any chemical affinities with island-arc tholeiites. The basalts from Site 446 and from the upper sill at Site 444 show alkaline and tholeiitic tendencies, and are enriched in the more-HYG elements; they chemically resemble enriched or E-type basalts and their differentiates found along sections of the MAR (e.g., 45°N) and on ocean islands (e.g., Iceland and the Azores). Most of the intra-site variation may be attributed to crystal settling within individual massive flows and sills, to high-level fractional crystallization in sub-ridge magma chambers, or, where there is evidence of a long period of magmatic quiescence between units, to batch partial melting. However, the basalts from Sites 442 and 443 and from the lower sill at Site 444 cannot easily be related to those from Site 446 and the upper sill at Site 444, and it is possible that the different basalt types were derived from chemically distinct mantle sources. From comparison of the Leg 58 data with those already available for other intra-oceanic back-arc basins, it appears that the mantle sources giving rise to back-arc-basin basalts are chemically as diverse as those for mid-ocean ridges. In addition, the high vesicularity of the Shikoku Basin basalts supports previous observations that the mantle source of back-arc-basin basalts may be contaminated by a hydrous component from the adjacent subduction zone.