(Table 1) Geochemistry of realatively fresh rocks of ODP Sites 127-794 and 127-797

The flows and sills drilled at Sites 794 and 797 in the Yamato Basin of the Japan Sea are subalkalic, olivine, and/or plagioclase phyric basalts. Compositionally, the rocks can be divided into a depleted, low-K type and an enriched, relatively high-K type. In addition, two contrasting evolution trends are reflected in the rock compositions, which allow four different magmatic suites to be identified. It is suggested that the depleted or enriched nature of these suites represent primary characteristics, while the different evolution trends are related to fractionation processes in crustal magma chambers. A tholeiitic evolution trend, with increasing FeO and TiO2 and decreasing Al2O3, can be modelled by fractional crystallization of 40%-50% plagioclase, olivine, and augite. A mildly calc-alkalic evolution trend, with decreasing FeO, increasing Al2O3, and nearly constant TiO2, can be modelled by 8%-12% olivine fractionation. Mineralogical evidence suggests that these differences may be related to the effect of small amounts of water during crystallization of the calc-alkalic suites. The tholeiitic suites occur in the lower parts of the drill cores, while the calc-alkalic suites occur in the upper parts. This suggests a complex tectonic and magmatic evolution, perhaps reflecting a transition between calc-alkalic magmatism related to subduction zone activity and tholeiitic magmatism related to back-arc spreading. Furthermore, any magmatic model must be able to account for the range in parental magmas from depleted to enriched throughout the tectonic history of the Yamato Basin.

Composition of sedimentary material from the bottom and the area surrounding the Lake Untersee, East Antarctica

This paper presents data on geographic and geologic conditions of modern sedimentation in the Lake Untersee, the largest lake in the East Antarctica. Geochemical and sedimentation data indicate that the leading mechanism supplying aluminosilicate sedimentary material to the surface layer of bottom sediments is seasonal melting of the Anuchin glacier and the mountain glacier on the southeastern part of the valley hosting the lake. Strongly reduced conditions in the lowermost 25 m of the water column in the smaller of two depressions of the lake bottom were favorable for enrichment of the bottom sediments in bacteriogenic organic matter, Mo, Au, and Pd. H2S-contaminated water results to significant enrichment of the sediments only in redox-sensitive elements that are able to migrate in anionic complexes and precipitate (co-precipitate) as sulfides.