Coarse-silt-fraction mineralogy at DSDP Legs 56 and 57 Holes

Soviet sedimentologists use the term "coarse silt" to denote the size fraction 0.1 to 0.05 mm (50-100 µm). Petelin (1961) has shown that this fraction is most diagnostic for terrigeneous and volcanogenic mineral assemblages and provinces in Recent deep-sea sediments, because of its greatest variability of both heavy and light non-opaque minerals, which may be easily identified by the common immersion method. We believe that the fraction is suitable for mineralogical study of unconsolidated and friable sediments from DSDP cores as well, if the objective is to investigate their source area and transporation tracks. In the case of fine-grained oceanic sediments, mineral composition of the coarse silt does not differ markedly from that of the "coarse fraction" (>62 µm).

(Table 1) Concentrations and isotopic ratios of sulfur and carbon in acid conglomerates from DSDP Hole 57-439

The sulfur content of one rhyolite and four dacite conglomerates was found to be low - from 9 to 97 ppm - similar to that of Quaternary andesites and basalts of the Japanese Islands. However, the d34S values of these samples are unexpectedly high - +23 to +35 per mill - relative to troilite from the Canon Diablo meteorite. The sulfide/sulfate ratios vary among the five samples from 0 to 13. No significant isotope fractionation seems to exist between sulfate and sulfide sulfurs. Carbon in these samples is predominantly in the form of carbonate (and probably CO2). It ranges in concentration from 128 to 721 ppm and in d13C from -2.5 to -20.7 per mill relative to PDB.

Mineralogy and geochemistry of volcanic ash at Japan Trench from DSDP Legs 87 and 57

Volcanogenic sediments were obtained from Site 584, located on the midslope of the Japan Trench. Occurrences of volcanic ash in the diatomaceous mudstones increase within sediments dated 6-3 Ma. The frequency pattern and the sediment accumulation rate obtained at Site 584 are similar to those of Site 440 and to those of Sites 438 and 439, located on the upper slope basin. Explosive volcanism increased during the Pliocene and late Miocene in relation to the intrusion of Tertiary granites and uplift of the Tohoku Arc (northeastern Japan Arc). Hygromagmaphile element concentration shows that the glass does not belong to a unique series, and a comparison with Nankai Trough data distinguishes at least two different evolutionary lines.