Mituko Ozima

Noble gases in submarine pillow volcanic glasses

Fifteen submarine glasses from the East Pacific Rise (CYAMEX), the Kyushu-Palau Ridge (DSDP Leg 59) and the Nauru Basin (DSDP Leg 61) were analysed for noble gas contents and isotopic ratios. Both the East Pacific Rise and Kyushu-Palau Ridge samples showed Ne excess relative to Ar and a monotonic decrease from Xe to Ar when compared with air noble gas abundance. This characteristic noble gas abundance pattern (type 2, classified by Ozima and Alexander) is interpreted to be due to a two-stage degassing from a noble gas reservoir with originally atmospheric abundance. In the Kyushu-Palau Ridge sample, noble gases are nearly ten times more abundant than in the East Pacific Rise samples. This may be attributed to an oceanic crust contamination in the former mantle source. There is no correlation between the He content and that of the other noble gas in the CYAMEX samples. This suggests that He was derived from a larger region, independent from the other noble gases. Except where radiogenic isotopes are involved, all other noble gas isotopic ratios were indistinguishable from air noble gas isotopic ratios. The3He/4He in the East Pacific Rise shows a remarkably uniform ratio of (1.21±0.07)×10−5, while the40Ar/36Ar ranges from 700 to 5600.

Noble gas state of the ancient mantle as deduced from noble gases in coated diamonds

Cores and coats of five coated diamonds, one from Botswana and four from Zaire, were separately analyzed for their noble gases. Noble gases in the diamonds are essentially of a trapped origin, including radio- and nucleogenic components such as4He, 40Ar, 21Neexcess and excesses in Xe isotopes (129, 131–136). The fairly precise elemental and isotopic abundances allow us to infer the noble gas state in the ancient mantle. 20Ne/22Ne ratios are fairly constant (11.8 ± 0.4), and very close to that of SEP (solar energetic particle)-Ne, but distinctly different from the atmospheric ratio. 21Ne/22Ne ratios range from 0.028 to 0.06, which is attributed to nucleogenic 21Ne from 18O(α, n)21Ne and 24Mg(n, α)21Ne reactions. The difference in 20Ne/22Ne between atmosphere and mantle can be attributed to the hydrodynamic escape of hydrogen from the primitive atmosphere during the very early stage in the Earths history. 38Ar/36Ar and Kr isotopic ratios are identical to the atmospheric values within 1%. After correction for 238U- or 244Pu-fission Xe, the 131–136Xe abundance ratios are indistinguishable from atmospheric ratios. Lighter Xe isotopes (124–128Xe) are also likely to be atmospheric, but a final conclusion must wait until better data are obtained. In a 136Xe/130Xe−129Xe/130Xe diagram, diamond data lie on the same line as defined for MORB. The observed identical correlation for both diamonds and MORBs appears to suggest that the progenitor of the excess131–136Xe is 244Pu, but not238U, though the direct Xe isotopic measurements was not precies enough to decide unanimously the progenitor.

Experimental studies of He and Ar degassing during rock fracturing

Abstract We studied degassing of He and Ar from granite, basalt and volcanic tuff samples which were subjected to uniaxial compression. From the samples which were fractured in the dilatant region He was always degassed, while degassing of Ar dependent not only on the dilatancy, but also on the type of rock and the compressional conditions such as wet, room dry or vacuum conditions. It is concluded that degassing of rare gases from the compressed samples depends primarily on the generation of new surface area by dilatancy.

Noble gas distribution between basalt melt and crystals

We studied the partition of Ar and Kr between synthetized crystals (clinopyroxene, olivine) and a basalt-melt. Clinopyroxene-glass pairs were synthetized in a platinum capsule containing air with a piston-cylinder-type high-pressure apparatus at 1.5 GPa at1250∼1175°C; olivine-glass pairs were synthesized at1350∼1300°C in an electric furnace with Ar flow. Distribution coefficients (noble gas content in crystal/noble gas content in melt) measured for these crystal-melt pairs are0.2∼0.5 for clinopyroxene-glass for Ar and Kr and0.08∼0.18 for olivine-glass for Ar.

Mantle helium flux from the bottom of Lake Mashu, Japan

Abstract Most of the 52 water samples from Lake Mashu, Hokkaido, Japan show large anomalies in their3He/4He ratios (up to 4.6 times the atmospheric ratio) and4He/20Ne ratios, indicating continuous injection of mantle He due to hot spring activity at the lake bottom. The3He/4He ratio of the injected mantle He is estimated at3He/4He= 9.43 ± 0.17 × 10−6 , which is close to the highest value reported for volcanic gases from the Japanese Islands. Accumulation of mantle He in the near-bottom water can be reasonably explained by a diffusion process with a constant He flux from the lake bottom off4He = 9.2 × 107 atoms/cm2/s andf3He = 8.7 × 102 atoms/cm2/s. The heat/3He ratio ( 1.7 × 10−7J/atom ) falls in the range of values reported for submarine hydrothermal systems (0.4–26 × 10−7 J/atom), whereas the C3He/ ratio ( 1.8 × 1011 ) is about two orders of magnitude higher than the MORB average ( 2 × 109 ). The mantle He injected from the lake bottom has a fairly short mean residence time in the lake water, and is estimated at about 2–4 years. Overturn of the lake water, which occurs twice a year, probably plays an important role in the rapid transportation of the mantle He from the bottom to the surface.

Self-reversal of remanent magnetization in some dredged submarine basalts

Several cases of selfreversal of remanent magnetization in ferromagnetic minerals and rocks have been known [ 1-4] . However, occurrence of self-reversal in basalts is extremely rare in spite of numerous reports on their magnetic properties [5-71 . Lately, we had a chance to study several basalts dredged from deep sea mounts in the Pacific Ocean (table 1). We found that three of eight dredged basalts from various localities-showed self-reversal of remanent magnetization. Although very little has been studied about magnetic properties of dredged basalts [S, 91, a high percentage of occurrence of self-reversal of remanent magnetization in the dredged basalts would imply that ferromagnetic minerals in submarine basalts differ significantly from those in continental basalts. In this short note, we report results of some preliminary experiments on self-reversal of remanent magnetization in dredged basalts. Pig. 1 indicates an occurrence of self-reversal of thermo-remanent magnetization in WPDR-2, in which the ordinate represents an intensity of thermoremanent magnetization produced by cooling the sample to room temperature in the geomagnetic field in air of various temperatures. Thermoremanent magnetization produced below 3000C has a parallel direction to the ambient geomagnetic field, whereas those produced between 3000C and 33000 have a reversed direction indicating self-reversal of the thermoremanent magnetization, When the sample is cooled from above 33000, thermoremanent magnetization has again a parallel direction to the geomagnetic field .

Real-time detection of anomalous geoelectric changes

Abstract Aiming at an accurate and quick detection of anomalous changes of the electric field so that we can detect tectonic signals, we developed observation methods for use both on land and on the ocean floor. With these observation methods, that is, observation on land using electrodes and the underground cables of the Nippon Telegraph and Telephone Company, and on the ocean floor using the power-feeding arrangement of the ocean-bottom seismograph of the Japan Meteorological Agency, we can obtain more reliable data than those from ordinary observation methods. Using these data, we further developed two techniques of analysis in real-time which are based on a statistical model, eliminating components induced by the variations of the geomagnetic field and/or the movement of the seawater. For both cases, the geomagnetic field at the Kakioka Magnetic Observatory and/or the seawater level data observed on the ocean floor were used as associated data. The predominant induced component in the variation of the geoelectric field is almost completely separated in the cases of the geoelectric field in the Mito region and on the ocean floor. Owing to the characteristic periodicity of the monopolar variation of noise in the Numazu region, in spite of the large amplitude of the noise (mainly due to the leak-current of the d.c. electric trains) in the original data from the Numazu region, the noises were considerably separated, with these methods, into the induced component part or tidal component part. With these methods, consequently, the detectability of anomalous changes was improved about ten times and about five times in the cases of the geoelectric field in the Mito region and on the ocean floor, and in the case of the geoelectric field in the Numazu region, respectively.

Evidence of the extinct nuclide146Sm in “Juvinas” achondrite

Abstract The search for the isotopic anomaly of Nd due to the disintegration of 147 Sm and 146 Sm was carried out on separated mineral phases of Juvinas achondrite. An internal isochron of 4.3 ± 2.5 billion years was obtained for 147 Sm 143 Nd pair. This provides the possibility of dating the planetary materials. Anomalies were observed for 142 Nd, giving the isotopic ratio 146 Sm/ 144 Sm at the time of solidification to be 0.39 ± 0.14 . This is the first positive evidence of extinct 146 Sm which may be used as a p-process chronometer.

Temperature and pressure effects on40Ar39Ar systematics

Abstract The effects of thermal and compressional treatment on 40 Ar- 39 Ar systematics have been investigated on three artificially heated biotite samples (heated for 1 hour at 700°C and 860°C in air and 700°C in vacuum respectively) and uniaxially compressed granite ( p = 1400bar ) and basalt samples ( p = 1650bar ). The 40 Ar- 39 Ar results for the disturbed samples are compared with those for undisturbed samples. Except for the vacuum-heating case, the effects of the disturbances may be interpreted as the combined effect of a partial loss of radiogenic 40 Ar from the sample and an incorporation of air Ar into the sample. Common diagnostic effects are (1) reduction of the total fusion age, (2) distortion of the age spectrum and, if the degree of the partial Ar loss is small, (3) approximate preservation of the isochron age, and (4) reduction of the intercept value ( 40 Ar/ 36 Ar) in the isochron plot. The features observed in the age spectra of artificially disturbed samples are rather common in geologically disturbed samples, suggesting that the artificial disturbances simulate the effects of geological disturbances on 40 Ar- 39 Ar systematics.

Growth of nickel olivine single crystals by the flux method

Single crystals of nickel olivine (Ni2SiO4) up to maximum 10 mm in length were grown by the flux method. Lattice parameters of nickel olivine at room temperature were refined as a = 4.7285 (2) A, b = 10.1179 (7) A, c = 5.9132 (4) A. Magnetic measurements revealed that Ni2SiO4 olivine is antiferromagnetic below 30 K and the easy axis of the spin lies along the b-axis.