Jun-ichi Matsuda

Anomalous Ne enrichment in obsidians and Darwin glass: Diffusion of noble gases in silica-rich glasses

Abstract We have determined noble gas concentrations in seven obsidians and a Darwin glass by stepwise heating. The pattern of noble gases relative to air showed that Ne was enriched in most of the obsidians and Darwin glass. The release temperatures of Ne were 400–500°C. The occurrence of Ne excess in Darwin glass and related impact glasses can be explained by diffusion into the glass from the atmosphere. Diffusion coefficients and activation energies of noble gases in obsidians and a Darwin glass were determined. It is also conceivable that the anomalously high Ne Ar ratios observed in some submarine glasses are from sea water or atmosphere rather than derived from the mantle.

Noble gas studies on the host phase of high 3He4He ratios in deep-sea sediments

Abstract Elemental abundances and isotopic compositions of noble gases were determined for a magnetic separate and chemically leached samples from deep-sea sediments (14°29.61′S, 158°52.98′W). High 3 He 4 He ratios suggesting the presence of cosmic material were observed in all samples, and especially enriched in magnetic fractions. Ne isotopes and light noble gas abundances suggested that the high 3 He 4 He ratio was of solar wind origin and not due to cosmic ray spallation. However, heavy noble gases (Kr and Xe) were of atmospheric origin. Altogether the noble gases were well explained by a mixture of solar wind implanted on interplanetary dust particles (IDPs) and terrestrial noble gas fractionated from the atmosphere. The degree of contamination of IDPs in the magnetic fractions was estimated to be about 100 ppm.

Geochemical implications from Sr isotopes and K-Ar age determinations for the Cook-Austral islands chain

Abstract Sr isotopes and K-Ar ages were determined for volcanic rocks from three islands, Samoa, Rarotonga and Rurutu in the Austral-Gilbert-Marshall chain. We have established that Rurutu originated from the same “hot spot” as Tubuai and Raivavae in the Cook-Austral chain as indicated by its 11 m.y. K-Ar age and ( 87 Sr 86 Sr ) ratios. The ( 87 Sr 86 Sr ) ratios for Rurutu, Tubuai and Raivavae lie in the restricted range 0.7026–0.7035. K-Ar ages of all volcanic rocks from Samoa and Rarotonga are all less than 4 m.y. and their ( 87 Sr 86 Sr ) ratios are much higher than those in Rurutu, Tubuai and Raivavae. These findings suggest that Samoa and Rarotonga did not originate in a single hot spot for the Cook-Austral islands.

Noble gas emplacement in shock-produced diamonds

Abstract Amounts of noble gases in shock-produced diamonds were considerably higher than those in the raw material graphite, and their elemental ratios were similar to those of air. It is evident that the shock-produced diamonds trapped the noble gases, without significant mass fractionation, from the atmosphere during the phase transition. These noble gases were mainly released at temperatures within the range of 1700 to 2000°C, suggesting that noble gases were tightly incorporated within the diamonds and were being released during graphitization. A diamond synthesized under hydrostatic pressure was found to have noble gas abundances much lower than those in the shock-produced diamonds. The elemental abundances of noble gases in shock-produced diamonds are discussed in relation to those in ureilite diamonds.

Thermal conductivity variation in a deep-sea sediment core and its relation to H2O, Ca and Si content

Abstract A total of 146 thermal conductivity values were determined on a single deep-sea sediment core of about 9 m length from the eastern equatorial Pacific (1°02.2′N, 121°10.4′W). The variation of thermal conductivity values ranges from about 0.6 to 1.0 Wm −1 K −1 , and has no simple relation to the depth below the sea floor. The relative proportions of H 2 O in this sediment core show a strong negative correlation with CaCO 3 and positive correlation with SiO 2 . These correlations reflect the differences in grain size distributions and/or in skeletal shapes of calcareous and siliceous microfossils in the deep-sea sediment. The relationship of thermal conductivity to the proportions of these constituents fits remarkably well to a simple analytical expression used to estimate conductivity of rocks from their mineral components.

Noble gases in mesozoic cherts from the U.S.A. and Japan

Abstract Noble gases in cherts of various origins were measured by means of the stepwise heating technique. The elemental abundance patterns of noble gases in the samples showed monotonic fractionation from 20 Ne to 132 Xe relative to the atmosphere; this pattern is designated as type-1 and is similar to that observed in water, natural gases and sedimentary rocks. The noble gas concentrations in cherts were low compared with those in other sedimentary rocks and related samples, and showed scarcely apparent correlations with chemical components. Crystallization from amorphous silica to quartz was responsible for the degassing loss of noble gases in cherts.