Osamu Matsubaya

Antarctic saline lakes—stable isotopic ratios, chemical compositions and evolution

Abstract About 90 saline and fresh water lakes as well as glaciers and their melt waters from the ice-free areas of the Soya Coast, the Vestfold Hills and the Southern Victoria Land of Antarctica have been analyzed for hydrogen and oxygen isotopic ratios. These results and the chemical compositions so far published indicate four types of saline lakes. 1. (1) Three saline lakes on the beaches of the first two areas which still are receiving sea water as the major inflow. 2. (2) Other lakes of these two areas which are isolated from sea water inflow. The isotopic ratios of their waters are higher and are plotted farther to the right of the meteoric water line, on the δD vs δ 18 O diagram, with increasing salinity. This is because the higher the salinity of a lake, the lake is ice free for a longer period of year, and thus the lake water is more significantly affected by the isotopic effect of evaporation from liquid water. 3. (3) Lake Bonney in the Taylor Valley of the Southern Victoria Land. The west and east lobes of this lake chemically are stratified but only the east lobe shows stratification in the isotopic ratios. Both lobes started as shallow saline lakes similar to some of the saline lakes classified in (2). Fresh water flooding into the lakes and subsequent diffusion mixing formed the present features. By solving diffusion equations under certain assumptions the evolutionary history of Lake Bonney was modeled. 4. (4) Lake Vanda and Don Juan Pond in the Wright Valley of the Southern Victoria Land. Lake Vanda is strongly stratified in both the salinity and isotopic ratios and seems to have a similar evolution history to east lobe of Lake Bonney. Chemical composition of the lakes in (2) and (3) differs variously from that of sea water but can be interpreted by different degrees of low temperature concentration of sea water by evaporation or freeze-drying. On the other hand, the high concentration of Ca 2+ relative to Mg 2+ and Na + in the lakes of the Wright Valley cannot be interpreted by this way.

Oxygen isotopic study of the Cretaceous granitic rocks in Japan

The (δ18O values of nine Cretaceous granitic rocks from the low P/T type regional metamorphic zone of Japan are +10.0 to +13.2‰ relative to SMOW, while ten Cretaceous granitic rocks from the non-metamorphic zone are +7.9 to +9.8‰. The 18O-enrichment in the former rocks is mainly attributed to oxygen isotopic exchange between the granitic magma and the surrounding metamorphic rocks during regional metamorphism. The assimilation of 18O-rich country rocks is also possible in the cases such as gneissose granite and migmatite.The oxygen isotopic ratios of quartz-biotite pairs in the granitic rocks indicate that they are isotopically in near-equilibrium with each other. The quartz-biotite isotopic equilibrium temperatures estimated for these rocks range from 550° to 670° C. Feldspar is occasionally isotopically in disequilibrium with other minerals. This may be caused by exchange of oxygen isotopes between feldspar and hydrothermal or meteoric water after crystallization.

Oxygen isotope variations in magmatic differentiation processes of the volcanic rocks in Japan

The 18O/16O ratios were analyzed for 35 volcanic rocks from the Izu-Hakone region, Oki-Dogo Island, Asama Volcano and Kiso-Ontake Volcano in the Japanese Islands. The 18O-enrichment in magma during crystallization differentiation is recognized in every rock series. The magnitude of solid-liquid isotope fractionation in the magma of tholeiite series is similar to that of alkali rock series, while the apparent isotope fractionation of calc-alkali rock series is larger than that of the above two series. The δ18O value of the tholeiite magma in the Izu- Hakone region is +5.7‰ relative to SMOW, which suggests its origin from the fresh upper mantle material. The primary magma of the calc-alkali rock series in Asama Volcano is assumed to have the δ18O value similar to or slightly higher than that of the Izu-Hakone primary magma. The alkali rock series of Oki-Dogo Island in the continental side of the island arc is by 1‰ heavier than the Izu-Hakone rock suite. This may imply that Oki-Dogo rock series might have exchanged their oxygen with 18O-rich crustal materials, or they might have originated from somewhat 18O-rich upper mantle material.

Na-Ca-Cl-SO4-type submarine formation waters at the seikan undersea tunnel, Japan. chemical and isotopic documentation and its interpretation

Abstract Formation waters were collected from the submarine volcanogenic Kunnui formation about 200 m below the sea floor and zero to 4 km off the shore in shafts of the Seikan Undersea Tunnel between Honshu and Hokkaido, Japan. The concentration of Cl − ranges from 20 to 500meq/l, more than 90% of the concentration in seawater. Stable isotopic and chemical evidence indicate that the waters having Cl − less than 70–80 meq/1 are essentially of meteoric origin, whereas those of higher Cl − contents are mixtures of the meteoric formation waters and seawater. Aquifers of the mixed formation waters overlie those of the meteoric formation waters. The former seem to have expanded into the latter by infiltration of seawater as a result of excavation of the tunnel. Sulfur and oxygen isotopic ratios of dissolved sulfates in the meteoric formation waters are +32 -+ 25 per mil and +9 –+15 per mil, respectively, and approach the values of dissolved oceanic sulfate as the proportion of the seawater component in the mixed formation waters increases. Thus, two sources of sulfate exist; one is gypsum in the Kunnui formation which is highly enriched in 34 S and 18 O compared to the other source, modern marine sulfate. Both the meteoric and mixed formation waters are rich in Na + , Ca 2+ , Cl − and SO 4 2− and extremely depleted in Mg 2+ and K + . They are highly enriched in Ca 2+ and SO 4 2− compared to simple mixtures of meteoric and seawater. The observed chemical composition of the formation waters of Cl − less than 150 meq/1 can be satisfactorily reproduced by a model calculation, if we assume: 1. (1) the formation waters are in cationic exchange equilibrium with montmorillonite-rich tuffs and 2. (2) they are saturated with gypsum and calcite.