Masami Noto

Origin of waters responsible for serpentinization of the Izu-Ogasawara-Mariana forearc seamounts in view of hydrogen and oxygen isotope ratios

Many serpentinite seamounts occur over a region 20–120 km west of the trench axis in the Izu-Ogasawara-Mariana forearc regions. The hydrogen and oxygen isotopic compositions of serpentine from these regions indicate that there are at least two kinds of waters responsible for serpentinization: seawater and water derived from dehydration of the descending slab. Serpentine from two Mariana and two Torishima samples with microscopically ductile and sheared texture (sheared-type) have lowerδD(−63to−52‰) and slightly higherδ18O values (+6.1 to +8.2‰) than that of other nine Ogasawara samples with mesh texture (mesh-type) (δD= −43to−49‰ andδ18O= +5.8to+6.7‰). This suggests that the sheared-type serpentine with lowerδD and slightly higherδ18O values was formed within the wedge mantle by interaction with water derived from a descending slab. The sheared texture is likely to have been produced during diapiric uplift. The unaltered portion of the ultramafic bodies later interacted with seawater after emplacement at or near the seafloor, resulting in formation of the mesh-type serpentine with higherδD values.

Oxygen isotope composition of quartz in soils developed on late Quaternary volcanic ashes in Japan

Oxygen isotope compositions of quartz isolated from 25 soil samples developed on late Quaternary volcanic ashes in Japan were determined. The δ18O values for aerosol sized quartz (1–10 μm in diameter) ranged from +10.9 to +17.5%. The variation is attributed to mixing of long-distance aeolian dust (δ18O=+16 to +17%) with variable amounts of local volcanic quartz (δ18O=+6 to + 8%). The δ18O values of soil quartz decreased systematically with the increasing particle size. The contribution of quartz derived from long-distance aeolian dust is higher in fine fractions ( 53 μm). Fine-grained quartz is often associated with hornblende in soils derived from volcanic ashes. Because large oxygen isotope fractionations (∼ 79%) between fine quartz (δ18O=+13%) and hornblende (δ18O=+6%) in bulk soil samples are unlikely in a normal magmatic equilibrium process, such fractionations are considered to result from post-depositional (secondary) mixing of the fine quartz incorporated by the eolian process. On the other hand, small oxygen isotope fractionations (< 2%) between coarse-grained quartz (+8%) and hornblende (+6%) indicate a volcanic origin.

Eolian influence on oxygen isotope abundance and clay minerals in soils of Hokkaido, northern Japan

Abstract Oxygen isotope geochemistry and clay mineralogy of five pedons developed on Pleistocene terraces in Hokkaido were studied to evaluate the eolian dust contribution from inland China. Soil quartz derived from the dust has fine and uniform particle-size distribution. The particle-size distribution is distinguishable from that of the underlying local terrace deposits which have a variable texture. Oxygen isotope compositions (δ 18 O) of fine quartz (1–10 μm) from surface and subsurface horizons fell within a narrow range of + 16.3 to + 17.0ℵ. The values are consistent with those of quartz present in long-distance eolian dusts. By contrast, higher δ 18 O values were obtained for quartz of the same size from the underlying Pleistocene terrace deposits ( δ 18 O = + 17.5 to + 20.0ℵ ), indicating a larger contribution of low-temperature authigenic quartz derived from local sedimentary rocks. Clay minerals of the surface horizons are characterized by well-defined X-ray diffractograms consisting mainly of micaceous minerals with minor amounts of kaolinite. The layer silicates are inherited from eolian micas. With depth, halloysite, with or without random interstratification with 2:1 expandable layer silicates, becomes common. High-charge smectite and halloysite are major constituents in some clays from the underlying local alluvium which represent detrital materials from weathering products of local parent rocks. The eolian addition was not obvious in a residual pedon on low hills of green tuff, probably due to frequent rejuvenation of the landscape surface by erosion.

The eolian origin of silty mantle in sedentary soils from Korea and Japan

Abstract The concentration, particle-size distribution and oxygen isotope composition ( δ 18 O SMOW ) of quartz were examined in order to determine the extent to which the accretion of long-distance eolian dusts from the interior of China had formed the sedentary soils overlying diverse parent materials in Korea and Japan. The quartz content of the surface soils developed on quartz-free basaltic materials varies from 8.9 to 47.7%. Generally, the soil quartz found in surface horizons is finer than of the subsurface horizons and/or substrata. The particle-size of the soil quartz decreases from the west coast to eastern coastal margins of Korea and to the Japanese islands. This follows the major trajectory of westerly winds which carry the eolian dusts from the interior of China to mid-latitude regions of the North Pacific. The oxygen isotope composition of the fine quartz (1–10 μm in diameter) when separated from surface and subsurface horizons, is found in a relatively narrow range of + 15.9 to + 17.7‰ The values are indiscernible from those of recent eolian dusts from the interior desert and loess plateau of China. The δ 18 O values from the same sized quartz found in C horizons, + 10.4 and + 19.2%, consisting of saprolitic andesites and shales, respectively, differ from those of the overlying soils. The δ 18 O values of soil quartz developed on igneous materials systematically decrease with increasing particle-size to 53 μm (∼ 14%). They abruptly decrease to + 8 to + 12% for fractions larger than 53 μm. A large apparent oxygen isotope fractionation (up to 10%) - between fine (+ 17%) and coarse (+ 8 to + 12%) grained quartz observed in the samples - does not represent an equilibrium oxygen isotope fractionation for normal magmatic processes. The data provide evidence for an eolian origin of the fine quartz incorporated during soil development.

An experimental study of oxygen isotope fractionation between wairakite and water

Abstract The analytical technique for determining oxygen isotopic ratios of wairakite has been carefully examined. Channel water in wairakite was separated from its aluminosilicate framework by dehydration in vacuum at various temperatures from 250 to 950°C. Dehydration at temperatures greater than 400°C resulted in erroneous isotopic ratios of the framework oxygen because of oxygen isotopic exchange between the framework oxygen and dehydrating water. The optimum conditions are dehydration of the channel water at 300°C for 24 h, followed by stepwise heating at 350°C for 12 h, 400°C for 5 h, and finally at 700† for 15 min. With this stepwise heating, isotopic measurements of wairakite framework oxygen can be satisfactorily achieved within an accuracy of ±0.1‰. The oxygen isotope fractionation factor for the wairakite-water system has been experimentally determined at temperatures between 250 and 400°C and pressures between 0.5 to 1.5 kbar. The equilibrium fractionation factor between framework oxygen and external water is expressed by the following equation: 10 3 1n α framework oxygen-external water = 2.46 ( 10 6 / T 2 ) − 1.76 The fractionation factor for the channel water-external water is given by 10 3 1n α channel water-external water = 0.79 ( 10 6 / T 2 ) − 3.07 In the temperature range studied, the framework-water fractionation of wairakite is very similar to that of analcime (Karlsson and Clayton, 1990a) and stilbite (Feng and Savin, 1993b) . The negative fractionation for the channel water-external water of wairakite (−3 to 0‰) is consistent with that for analcime and stilbite, but the magnitude is less than that of analcime and stilbite. Almost complete oxygen isotopic exchange during the hydrothermal runs indicates that the exchange between wairakite framework oxygen and water is rapid compared to that of the other silicate-water systems. This implies that the δ 18O value of natural wairakite is controlled by retrograde re-equilibration under hydrothermal conditions. The grain size did not change during the hydrothermal runs, suggesting that the exchange mechanism was dominated by sorption-exchange-desorption processes as proposed by Karlsson and Clayton (1990a) . However, scanning electron microscopy images of the run products showed that the grain surfaces of the exchanged wairakite were covered by newly deposited crystals, suggesting that dissolution-crystallization was also involved to a small extent.

Oxygen isotope ratios of opaline silica and plant opal in three recent volcanic ash soils

Abstract Opaline silica (1– μm) from tropical and temperate soils developed in recent volcanic ash was concentrated by chemical purification and heavy liquid separation. Oxygen isotope ratios (δ18OSMOW) of the opaline silica samples were estimated from the isotopic mass balance of the fractions before and after selective chemical dissolution with heated 0.5M NaOH. The δ18O values range from +24.7 to +33.6%, indicating an authigenic origin. The values are higher than those of crystalline silica polymorphs (cristobalite: +5 to + 11%, fine-grained quartz: +16%) commonly found in soils derived from volcanic ash. The contribution of biogenic processes in the formation of opaline silica is limited, as indicated by a substantial difference in the isotope values of opaline silica and plant opal in the 10–53 μm size fractions. Possible mechanisms of formation of the opaline silica are discussed. These are based on the equilibrium oxygen isotope fractionation of amorphous silica and water, and on the estimated δ18O values of the solution from which the silica formed. We suggest that authigenic silica arises from the evaporation of water at the soil surface in tropical regions, and from the freezing of soil solutions containing monomeric silicic acid in temperate regions.

Oxygen isotope geochemistry of geothermal wairakite

Abstract Oxygen and hydrogen isotopic compositions of wairakite were determined for drillcore samples from exploration wells in the active geothermal systems in Japan (Takigami and Kirishima) and in New Zealand (Wairakei and Ngatamariki) and for samples from some extinct Japanese hydrothermal systems. The δ 18 O values range from −0.4 to 7.8‰ for wairakite from the active systems, whereas they are generally higher (ranging from 8.0 to 14.1‰) for the mineral from the extinct systems, indicative of retrograde re-equilibration with local meteoric water at low temperatures. The δ D values of channel water are lower than those of hydrothermal fluids or local meteoric water. The D/H fractionation between channel water and external water becomes more negative as temperature lowers. The monomineralic oxygen isotopic temperatures based on the framework oxygen-channel water fractionation of wairakite are in good agreement with the measured, present-day downhole temperatures and/or calculated geochemical temperatures for the Takigami and Wairakei geothermal fluids. At Takigami, wairakite and associated calcite are found to be in oxygen isotopic equilibrium at the measured fluid temperature of around 220°C. The Kirishima geothermal system is thought to be in a waning stage of geothermal activity as recorded by fluid inclusion temperatures. The wairakite isotopic temperatures are consistent with the above thermal history, reflecting precipitation of wairakite from the most recent, relatively cool water. The disequilibrium oxygen isotopic relationship between some calcite-wairakite pairs for the Kirishima geothermal system suggests that the two minerals were precipitated from the waters with different δ 18 O values at different stages in the evolution of the system, consistent with paragenetic relations of the minerals in core samples. Oxygen isotopic composition of wairakite can be used as a new, single-mineral isotopic geothermometer for active geothermal systems and be applied to elucidate their thermal evolution.

18O/16O ratio determination of framework oxygen of apophyllite and wairakite by the preferential isotopic exchange of their water of crystallization

Abstract A new analytical method has been developed for the determination of the oxygen isotopic compositions of the Si8O20 framework of apophyllite and the Al2Si4O12 framework of wairakite. The method consists of the preferential oxygen isotopic exchange of the water of crystallization with isotopically different external water, followed by the determination of the bulk isotopic composition of the minerals. The δ180-value of the silicate framework can be obtained by subtracting the δ18-O-value of the water of crystallization from that of the bulk mineral in proportion to the oxygen atomic fraction of the water of crystallization. Optimum conditions of the exchange for apophyllite have been found to be ∼ 160° C for ∼ 10 days during which the water of crystallization is almost completely exchanged with the external water, whereas the framework oxygen remains unchanged. For wairakite analysis, a graphical method utilizing a three-isotope plot (17O/16 vs. 18O/160) has been used. The advantage of the three-isotope method is that the δ18O-value of the silicate framework can be graphically estimated with an accuracy better than ± 2‰, even if the complete exchange of external and crystallization waters is not attained.