Osamu Matsubayashi

Rare gas isotopic compositions in natural gases of Japan

Isotopic and elemental compositions of rare gases in various types of gas samples collected in the Japanese Islands were investigated. Excess3He was found in most samples. Many samples showed a regionally uniform high3He/4He ratio of about 7 times the atmospheric ratio. The He concentrations varied from 0.6 to 1800 ppm, and they were low in CO2-rich gases and high in N2-rich gases. Ne isotopic deviations from the atmospheric Ne were detected in most volcanic gases. The deviations and the elemental abundance patterns in volcanic gases can be explained by a mixing between two components, one is mass fractionated rare gases and the other is isotopically atmospheric and is enriched in heavy rare gas elements. Ar was a mixture of mass fractionated Ar, atmospheric Ar and radiogenic Ar, and the contribution of radiogenic40Ar was small in all samples. Except for He, elemental abundance patterns were progressively enriched in the heavier rare gases relative to the atmosphere. Several samples were highly enriched in Kr and Xe relative to the abundance pattern of dissolution equilibrium of atmospheric rare gases in water. The component which is highly enriched in heavy rare gases may be released from sedimentary materials in the crust.

Isotopic anomalies of rare gases in the Nigorikawa geothermal area, Hokkaido, Japan

Isotopic and elemental compositions of rare gases were determined for gas samples enriched in CO2 collected in the Nigorikawa geothermal area, Hokkaido, Japan. A high isotopic ratio of 3He/4He= (9.95 ± 0.25) × 10−6 was obtained for a sample collected from bubbles naturally seeping out. This sample has a rare gas elemental abundance pattern attributable to low-temperature solubility of atmospheric rare gases into water. Soil gas samples yield evidence for mass fractionation of atmospheric rare gases, in addition to the originally high 3He/4He ratio. The highest 20Ne/22Ne ratio found, 10.33 ± 0.10, was accompanied by the highest 36Ar/38Ar ratio, 5.60 ± 0.08, and a systematic enrichment of the light isotopes of Kr and Xe. The enrichment of the light isotopes in each gas in the soil gas samples was also accompanied by a systematic elemental enrichment of the light gases. Migration of rare gas in the crust is responsible for this isotopic and elemental fractionation. Primordial He from the mantle is extensively present in CO2-rich gases in this area.

Relations between the thermal properties and porosity of sediments in the eastern flank of the Juan de Fuca Ridge

The empirical relations of the thermal properties (thermal conductivity, heat capacity, specific heat, and thermal diffusivity) to the porosity and mineral composition of clay and sandy sediments recovered in the eastern flank of the Juan de Fuca Ridge are examined using the observed thermal properties, index properties, and mineral composition of the sediments. Observed thermal conductivity-porosity relations are explained using the geometric mean model. The observed relations of heat capacity and specific heat, respectively, to porosity are given by the arithmetic mean formula. A new model for the sediment thermal diffusivity-porosity relation is proposed based on models of thermal conductivity and heat capacity. This model, expressed by the geometric mean model with a correction function for the porosity and heat capacities of grain sediment and pore-filling fluid, explains the observed thermal diffusivity-porosity relations. These thermal property models are applicable to thermal properties of other sediment lithology types and are useful as standard models for estimating the thermal properties of marine sediment.

Conductive heat flow at the TAG Active Hydrothermal Mound: Results from 1993–1995 submersible surveys

We report 70 measurements of conductive heat flow at the 50-m-high, 200-m-diameter TAG active hydrothermal mound, made during submersible surveys with Alvin in 1993 and 1995 and Shinkai 6500 in 1994. The stations were all measured with 5-thermistor, 0.6- or 1-m-long Alvin heat flow probes, which are capable of determining both gradient and thermal conductivity, and were transponder-navigated to an estimated accuracy of ±5–10 m relative to the 10-m-diameter central complex of black smokers. Within 20 m of this complex, conductive heat flow values are extremely variable (0.1- > 100 W/m²), which can only be due to local spatial and possible temporal variability in the immediate vicinity of the vigorous discharge sites. A similar local variability is suggested in the “Kremlin” area of white smokers to the southeast of the black smoker complex. On the south and southeast side of the mound, there is very high heat flow (3.7- > 25 W/m²) on the sedimented terraces that slope down from the Kremlin area. Heat flow is also high (0.3–3 W/m²) in the pelagic carbonate sediments on the surrounding seafloor within a few tens of meters of the southwest, northwest, and northeast sides of the mound. On the west side of the sulfide rubble plateau that surrounds the central black smoker peak, there is a coherent belt of very low heat flow (<20 mW/m²) 20–50 m west of the smokers, suggestive of local, shallow recharge of bottom water. The three submersible surveys spanned nearly two years, but showed no indication of any temporal variability in conductive heat flow over this time scale, whether natural or induced by ODP drilling in 1994.

Sub-bottom temperature anomalies detected by long-term temperature monitoring at the TAG Hydrothermal Mound

During Aug. 1994–Feb. 1995, geothermal measurements were carried out on the TAG hydrothermal mound (26°0′N, 44°49′W) of Mid-Atlantic Ridge. Two geothermal probes made reliable measurements. Six transient anomalies in subbottom temperatures were detected 20 m south of the central black-smoker complex (CBC), having durations 40 to 400 hours and magnitude 0.2°C to > 30°C. These anomalies are probably evidence for short-period fluctuations in the fluid circulation beneath the mound. The time variability in subbottom temperatures near CBC suggests that instantaneous heat flow measurements may not represent the time average of heat flow there. Although it is difficult to specify the origin of thermal anomalies during the drilling period by the Ocean Drilling Program, at least one of them seems to correlate with the drilling events because it was also accompanied by transient temperature anomalies at CBC and 50 m east of CBC observed by other instruments.

Thermal conductivities, thermal diffusivities, and volumetric heat capacities of core samples obtained from the Japan Trench Fast Drilling Project (JFAST)

We report thermal conductivities, thermal diffusivities, and volumetric heat capacities determined by a transient plane heat source method for four whole-round core samples obtained by the Japan Trench Fast Drilling Project/Integrated Ocean Drilling Program Expedition 343. These thermal properties are necessary for the interpretation of a temperature anomaly detected in the vicinity of the plate boundary fault that ruptured during the 2011 Tohoku-Oki earthquake and other thermal processes observed within the Japan Trench Fast Drilling Project temperature observatory. Results of measured thermal conductivities are consistent with those independently measured using a transient line source method and a divided bar technique. Our measurements indicate no significant anisotropy in either thermal conductivity or thermal diffusivity.

Geothermal constraints on the hydrological regime of the TAG active hydrothermal mound, inferred from long-term monitoring

Abstract During August 1994 to March 1995, a period that included ODP Leg 158 drilling, bottom-water and sub-bottom temperatures were continuously logged by a long-term temperature monitoring system ‘Daibutsu’ at the base of the central black-smoker complex (CBC) and within the low heat flow zone at the TAG hydrothermal mound on the Mid-Atlantic Ridge. The temperature of hydrothermal fluid at CBC was also measured with a small high-temperature probe ‘Hobo’. Bottom-water temperature variations measured with Daibutsu at both sites have predominant semi-diurnal periods, causing the sub-bottom temperatures to fluctuate at these periods with reduced amplitudes and phase delays at sub-bottom depths. Seawater entrainment into the mound has been previously suggested at the low heat flow zone. We quantitatively evaluate the seawater entrainment rate at both sites from a one-dimensional numerical model, combined with a heat conduction model for the semi-diurnal variations. The entrainment rate of seawater at the base of CBC is estimated as 1.3±0.5×10−5 m/s, at least from August 17 to 30, 1994. On the other hand, the seawater entrainment rate at the low heat flow zone was undetected by long-term temperature monitoring at shallow sub-bottom depth. Nevertheless an increase in heat flow observed at the low heat flow zone during ODP drilling can be interpreted as a decrease in the entrainment rate of seawater. Before ODP Leg 158, Daibutsu measured three sub-bottom temperature anomalies at the base of CBC not derived from bottom-water temperature variations and Hobo also detected a CBC fluid temperature anomaly, indicating some natural changes in fluid flow within the mound. Daibutsu and Hobo also measured temperature anomalies during and after drilling at the ODP TAG-1 area. The Hobo temperature anomalies are inferred to have occurred when the cold fluid entrained through the drill holes at TAG-1 site reached or cooled the main fluid path to CBC. The entrained seawater through the drill holes appears to have contributed to dissolution and precipitation of anhydrite within the mound and perhaps affected the local permeability structure inside the mound. The temperature anomalies measured with Daibutsu at the base of CBC may have been induced by the change in the fluid flow pattern as a result of such permeability changes within the mound.

Simulation of gas hydrate dissociation caused by repeated tectonic uplift events

Gas hydrate dissociation by tectonic uplift is often used to explain geologic and geophysical phenomena, such as hydrate accumulation probably caused by hydrate recycling and the occurrence of double bottom-simulating reflectors in tectonically active areas. However, little is known of gas hydrate dissociation resulting from tectonic uplift. This study investigates gas hydrate dissociation in marine sediments caused by repeated tectonic uplift events using a numerical model incorporating the latent heat of gas hydrate dissociation. The simulations showed that tectonic uplift causes upward movement of some depth interval of hydrate-bearing sediment immediately above the base of gas hydrate stability (BGHS) to the gas hydrate instability zone because the sediment initially maintains its temperature: in that interval, gas hydrate dissociates while absorbing heat; consequently, the temperature of the interval decreases to that of the hydrate stability boundary at that depth. Until the next uplift event, endothermic gas hydrate dissociation proceeds at the BGHS using heat mainly supplied from the sediment around the BGHS, lowering the temperature of that sediment. The cumulative effects of these two endothermic gas hydrate dissociations caused by repeated uplift events lower the sediment temperature around the BGHS, suggesting that in a marine area in which sediment with a highly concentrated hydrate-bearing layer just above the BGHS has been frequently uplifted, the endothermic gas hydrate dissociation produces a gradual decrease in thermal gradient from the seafloor to the BGHS. Sensitivity analysis for model parameters showed that water depth, amount of uplift, gas hydrate saturation, and basal heat flow strongly influence the gas hydrate dissociation rate and sediment temperature around the BGHS.