A New Automatic Subsurface Gas Monitoring System for Seismogeochemical Studies, Installed in Haruno Borehole, Shizuoka Prefecture, Central Japan

The results of subsurface gas monitoring by application of gas chromatography (GC) to the gas composition of bubbles associated with groundwater for seismogeochemical studies are reported. An automated gas monitoring system was used to determine gas compositions in a 500-m borehole at the Haruno Crustal Movement Observation Site (HOS), central Japan during period 1, from December 1999 to December 2000. The average ± two standard deviation (2SD) compositions of gases in this period were He = 82 ± 29 ppmV, H₂ = 170 ± 62 ppmV, Ar = 0.05 ± 0.07%, N₂ = 50 ± 8%, and CH₄ = 45 ± 6%. A new automated gas monitoring system equipped with a micro-GC was installed in the borehole at the HOS, and gas bubbles from the borehole were monitored during period 2, from December 2006 to March 2007. The average ± two standard deviation (2SD) compositions of gases in this period were He = 8 ± 7 ppmV, H₂ = 13 ± 15 ppmV, Ar = 0.6 ± 0.3%, N₂ = 66 ± 7%, and CH₄ = 14 ± 14%. The gas concentration ratios (He/Ar, H₂/Ar, N₂/Ar, and CH₄/Ar) fluctuated significantly over time and repeatedly showed abrupt spike-like increases during period 2. The gas compositions obtained in period 1 and 2 were markedly different. Over the period from 2006 to 2007, the gas bubbles were depleted in He, H₂, and CH₄ of deep origin, but enriched in Ar and N₂ of atmospheric origin. This difference can be interpreted as being due to an irreversible change of the aquifer/gas system. The present deep component in the HOS gas is estimated to have composition He = 63 ppmV, H₂ = 37 ppmV, Ar = 0.17%, N₂ = 63%, and CH₄ = 37%. The new monitoring system is able to analyze the gas composition using a smaller volume of sample gas and with greater precision than the previous system. During the 3-month monitoring period 2, the separation capacity of the capillary column of the micro-GC was sufficiently maintained to determine gas-chromatographic peak areas for the five gaseous species examined. This study confirms that the new monitoring system with micro-GC is promising for continuous subsurface gas monitoring for earthquake prediction studies.     

Geographical distribution of helium isotope ratios in northeastern Japan

In order to study the precise geographical distribution of helium isotope ratios in northeastern Japan and compare it with geophysical data, we collected 43 gas and water samples from hot and mineral springs in the region where the ratio had never been reported, and measured the ³He/⁴He and ⁴He/²⁰Ne ratios of these samples. It was found that the ³He/⁴He ratios show clear contrasts between the forearc and the back-arc regions in the Tohoku district in northeastern Japan. In the forearc region, the ratios are smaller than 1 R_A (1 R_A = 1.4 × 10⁻⁶; R_A means the ³He/⁴He ratio of the atmosphere). On the other hand, those along the volcanic front and in the back-arc region are apparently higher. Moreover, we found a variation in the ³He/⁴He ratios along the volcanic front. In Miyagi Prefecture (38–39°N), the ratios range from 2 to 5 R_A. On the other hand, the ratios are less than 1 R_A in and around the southern border between Iwate and Akita Prefectures (39–39.5°N). Comparing the distribution of helium isotope ratios to results of recent geophysical studies, we found that the features in geographical distribution of helium isotope ratios are similar to those of seismic low-velocity zone distributions and high Qp⁻¹ distributions in the uppermost mantle. These observations strongly suggest that the helium isotope ratios reflect the distribution of melts in the uppermost mantle and are a useful tool for investigating the origin, behavior, and distribution of deep fluids and melts.     

A stochastic model of fractional crystallization and its application to the Japanese geosynclinal basalt

A stochastic model of the magmatic differentiation by fractional crystallization is given. The probability distribution of the quantity of crystallized solid removed from silicate melt was assumed to be binomial at any stage of the differentiation. According to this model, when concentrations of an element are transformed into their powers by using the reciprocal of the difference between bulk-partition coefficient for the element and unity as an exponent, the resultant frequency-distribution pattern becomes absolutely normal. The patterns of concentrations, however, cannot be expressed by a particular type of function but are variable according to the values of bulk-partition coefficients. Frequency distributions of element concentrations were computed under selected conditions on the basis of the model. The result shows that the skewed pattern often observed in frequency distributions of minor-element concentrations is explained by this fractional crystallization process. The frequency distributions of Ni and Cr concentrations in the geosynclinal basalt of southwestern Japan were examined in terms of this model. It was concluded that the model can be applied to the formation of the basalt concerning at least these two elements.     

Anomalous changes of CH4/Ar ratio in subsurface gas bubbles as seismogeochemical precursors at Matsuyama,Japan

The results of seismogeochemical monitoring of subsurface gases associated with ground-waters from deep artesian wells in Matsuyama, Japan, are presented. The weekly collections of bubble-gas samples from the two borehole wells and gas chromatographic determinations of their He/Ar, H₂/Ar, N₂/Ar, and CH₄/Ar ratios for the recent nineteen months have revealed that the bubble-gas CH₄/Ar ratios at the two stations are correlated with the seismic activity in this area. Positive anomalies of the CH₄/Ar ratio were accompanied by five earthquakes with relatively large magnitudes and/or relatively small epicentral distances among thirty earthquakes that occurred during the monitoring period. The anomalous increases in the CH₄/Ar ratio had evidently begun prior to the respective earthquakes.The occurrence of the geochemical anomaly appears to depend strongly upon the earthquake magnitude and distance, because the earthquakes that accompanied the anomalies and those that did not can be reasonably distinguished in the plot of magnitude vs. distance. Similar anomalies can be also seen in the variations of He/Ar and N₂/Ar ratios, but their magnitudes are much smaller than those of the CH₄/Ar ratio. These features have been discussed in relation to the possible mechanism of the seismogeochemical phenomena.     

Chemical compositions of refractory inclusions in the Murchison C2 chondrite

Samples from ten refractory inclusions in Murchison, some of which are splits of inclusions whose mineralogical and petrographic characteristics are known, have been analysed for thirty-six elements by neutron activation. Six inclusions have group II or group III patterns or variants of such patterns. Two inclusions, BB-5 and MUCH-1, have large negative Yb anomalies unaccompanied by correspondingly large negative Eu anomalies. It is possible that the latter condensed originally with group III patterns and preferentially took up Eu in later exchange processes under reducing conditions. One inclusion, SH-2, has heavy REE enrichment factors that increase with the refractoriness of the REE, indicating the presence of an extremely high-temperature, or ultrarefractory, REE condensate, but it also has a heavy REE/light REE ratio that indicates mixing of that component with a lower-temperature REE condensate. The frequency of highly fractionated REE patterns and absence of group I patterns suggest that refractory inclusions in Murchison stopped equilibrating with the nebular gas at higher temperatures than most Allende coarse-grained inclusions. The lower Ir/Os and Ru/Re ratios of some Murchison inclusions compared to those of Allende coarse-grained inclusions indicate that condensate alloys that contributed noble metals to the former also stopped equilibrating with the nebular gas at higher temperatures than those that contributed noble metals to the latter. Murchison inclusions tend to be lower in non-refractory elements than Allende coarse-grained inclusions, suggesting that, on average, the former underwent less severe secondary alteration than the latter.     

地壳表层砷的循环与污染地下水模式

为了更好地监测和预测地下水砷污染,揭示由自然原因引起的地下水砷污染规律和机制,通过多年来对内蒙古河套平原地下水砷污染的调查研究,在充分了解和掌握砷的化学与地球化学性质以及世界各国地下水砷污染状况与分布特征的基础上,总结了砷在自然环境中的分布和污染地下水的机理。文章阐述了4种地下水砷污染类型的形成机理,并提出了地壳表层砷的循环与污染地下水模式,通过该模式在基本掌握某一地区地质环境背景的情况下,可以初步预测其可能发生地下水砷污染的地段。地下水砷污染是砷在地壳表层循环过程中发生的一种环境水文地球化学现象,污染区的分布与特定的地质、地理背景和环境条件有关。在还原解吸污染类型中,封闭的环境、硫元素和有机碳的存在是地层中砷溶出的主要控制因素。今后应加强对封闭的冲积或湖积平原、河流入湖或入海三角洲等地区地下水砷污染的调查与监测。