Iwao Kawabe

REE(III) adsorption onto Mn dioxide (δ-MnO2) and Fe oxyhydroxide: Ce(III) oxidation by δ-MnO2

The adsorption experiments of rare-earth elements (REE) onto δ-MnO2 and Fe oxyhydroxide precipitates suspended in aqueous NaCl solutions have been conducted. The distribution coefficients, Kd(REE: precipitate/solution) between suspended δ-MnO2 and solutions show extremely large positive Ce anomalies unlike those between Fe oxyhydroxide precipitates and solutions. The iodometric titration of δ-MnO2 precipitates containing different amounts of scavenged Ce reveals that Ce(III) is oxidized to Ce(IV) by δ-MnO2. A convex tetrad effect observed in the series variations of log Kd(REE) between suspending δ-MnO2 and solutions becomes conspicuous with increasing pH in the range of 4.8 < pH < 6.8. This reflects the changing ratio of hydroxyl ion to water molecule coordinating REE(III) adsorbed on δ-MnO2 suspensions with increasing experimental pH in the solution.

Partitioning of elements between majorite garnet and melt and implications for petrogenesis of komatiite

Partitioning of elements between majorite garnet and ultrabasic melt has been studied at 16 GPa and 1950° C. Ca, Ti, La, Sm, Gd, Zr, Hf, Fe, Ni, Mn, K, and Na are enriched in the melt, whereas Al, Cr, V, Sc and Yb are concentrated in majorite garnet. Thus, majorite garnet fractionation by partial melting could produce chemical heterogeneities in these elements deviating from chondritic abundance. Using the partitioning behaviour of elements between majorite garnet and ultrabasic melt, the petrogenesis of komatiite is discussed. A simple model to explain the chemical varieties of komatiites is as follows. Aluminadepleted komatiite was generated by partial melting of the primitive mantle at 200–650 km depth, and alumina-enriched komatiite is the product of remelting of the residual solid at the same depths, whereas alumina-undepleted komatiite was formed by partial melting of the primitive upper mantle at depths shallower than 200 km. We suggest the possibility of large-scale chemical layering or heterogeneity in the early Archean upper mantle as an alternative model for komatiite genesis; shallower mantle depleted in majorite garnet and the underlying mantle enriched in majorite garnet. Alumina-depleted and alumina-enriched komatiites in the early Archean might be generated by a high degree of partial melting of the layered mantle. Such chemical layering could have been homogenized by the late Archean. This explains the observations that alumina-depleted and alumina-enriched komatiites were generally formed in the early Archean but alumina-undepleted komatiite was erupted in the late Archean.

Calculation of oxygen isotope fractionation in quartz-water system with special reference to the low temperature fractionation

Reduced partition function ratios for α and β-quartz have been calculated at various temperatures using the modified Urey-Bradley force field and Debye and Einstein functions. The error estimation for the reduced partition function ratios has also been made with the use of the standard deviations of force constants calculated by a least squares procedure. Theoretical values for the oxygen isotope fractionation factor between quartz and water vapor up to 1000°C have been calculated. The oxygen isotope fractionation factor between quartz and liquid water below 100°C has been obtained by combining the theoretical reduced partition function ratios for α-quartz and water vapor and experimental data on the liquid-vapor frationation of water. The obtained quartz-liquid water fractionations can be expressed as 103lnαqtz−H2O(1) = −18.977 + 8.582(1010T) + 1.9189(1010T)2 It was also found that the theoretical calculation was inevitably accompanied by a relatively large error. The isotopic temperature scales obtained by other workers for the pair of quartz and liquid water lie within the error range of the present calculation. Although the slopes of all scales including the present one on a plot of 103 ln α vs 106T2 are almost identical, the empirical scale by Knauth and Epstein is the one that is closest to the present result.

Gas geochemical changes at mineral springs associated with the 1995 southern Hyogo earthquake (M = 7.2), Japan

Abstract Gas concentrations monitored at a mineral spring in central Japan over the past 15 years showed similar changes before and after two large earthquakes (September 1984, M = 6.9, 50 km distant: and January 1995, M = 7.2, 220 km distant); namely, a concurrent and abrupt drop in all three gas ratios, He Ar , N 2 Ar , and CH 4 Ar , several months before the events. Improved measurements of gas compositions since 1991 and the addition of gas discharge rate monitoring since 1993 disclosed the following features for anomalies in response to small earthquakes as well as the large 1995 event: (1) the gas ratios increased for both large and small earthquakes, but they did not return to the preseismic values for the large event, as they did within a few hours for the small events; (2) at the 1995 event, the gas discharge rate decreased remarkably 3 h before the event and then increased to higher than preseismic level for several months. In contrast, coseismic rates at small events generally increased for less than 15 min; (3) during a seismically quiet period, from January 1993 to December 1994, the variation in the He Ar ratio was well correlated with that of gas discharge rate. Such correlation disappeared in August and September, 1994, when the three gas ratios dropped before the large event; (4) gas compositions at an auxiliary monitoring site revealed a coseismic change similar to that described above for the 1995 event. These lines of evidence show that subsurface fluids may habitually respond to changes in physical conditions relating to earthquake occurrences, and thus suggest the usefulness of monitoring subsurface fluids for earthquake prediction.

A mechanism for Na incorporation in garnet; an example from garnet in orthogneiss from the Su-Lu Terrane, eastern China

Abstract To calibrate infrared (IR) spectroscopy for quantitative analysis of trace structural OH in specific minerals, we have determined concentrations of H in pure separates of mantlederived pyrope garnet (56 ± 6 ppm H2O by weight), augite (268 ± 8, ppm H2O), and enstatite (217 ± 11 ppm H2O) by manometry after heating the samples and extracting H2 gas under vacuum. IR spectroscopy confirmed the presence of intrinsic OH in these samples prior to extraction and indicated between 86 and 100% removal of H during the extraction procedure. The integral specific absorption coefficients of 1.39 ± 0.14 (1σ),7.09 ± 0.32, and 15.6 ± 0.94 /(ppm H2O·cm2) for pyrope, augite, and enstatite, respectively, allow precise spectroscopic determination of the OH content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million. Uncertainties in accuracy depend on mineral composition and characteristics of the OH absorption spectrum and are estimated to range between ± 10 and ± 50%.

Preseismic hydrogen gas anomalies caused by stress‐corrosion process preceding earthquakes

Simultaneous monitoring of He, H2, Ar, N2, CH4 and Rn in gas bubbles of groundwater from the 1,500 m deep Nagashima well in the active Yoro fault zone, central Japan, reveals that three microearthquakes with M=1.6–2.6 in the fault zone are accompanied and preceded by increases of H2, as is a moderately large event with M=5.4 in the same fault zone. All the focal distances of the earthquakes are within 25 km. No comparable anomalies were seen in the gas species other than H2. We suggest that the selective and preseimic increases of H2 in groundwater gas bubbles are geochemical signals of the earthquake nucleation process of stress-corrosion mechanism. Aseismic slips preceding earthquakes in the active fault zone are likely to have enhanced H2 production by mechanochemical reaction of fractured rock surface with water in pre-existing fault planes as groundwater conduits.

Polymorphic transformations of kaolin minerals in aqueous solutions

Dissolution of kaolinite, halloysite and amorphous hydrous aluminosilicate in a 0.001 N HCl solution at 47°C has been investigated to study polymorphic transformations of these minerals in aqueous solutions. Chemical analyses of the solutions obtained show that the concentrations of Al, Si and H+ change noticeably with time. However, the change of the concentrations are not described by any specific simple rate expressions. The loci of points on a log aAl3+ + 3 pH vs. log aH4SiO4 diagram indicate that the solution 1. (1) has not reached any partial equilibrium when dissolving phase is kaolinite, 2. (2) has reached equilibrium with kaolinite when dissolving phase is halloysite and 3. (3) has reached equilibrium with halloysite when the dissolving phase is amorphous hydrous aluminosilicate. It is suggested that these polymorphic transformations proceed by dissolution of the original solid and precipitation of the more stable polymorph. The transformation may continue in a solution saturated with the more stable polymorph until the original solid disappears. This transformation mechanism explains the zonal distribution of allophane, halloysite and kaolinite in a weathering profile and occurrence of kaolinite-rich sediments.

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.

Simplified gas chromatographic analysis of H2, He, Ne, Ar, N2 and CH4 in subsurface gases for seismo-geochemical studies

Abstract An improved method for gas chromatographic analysis of subsurface gases is presented. The concentrations of H2, He, Ne, Ar, N2 and CH4 in gas samples can be directly determined in a simple procedure within 1 hr., at room temperature. The optimum condition for the analysis is the use of an 8-m column packed with a 60–80 mesh 5A molecular sieve activated at 360°C for 12 hr. and the use of oxygen as carrier gas. By means of this method, the components can be separated chromatographically and determined with high sensitivities; with a preamplifier and the oxygen carrier, the determination limit for He and H2 is 10−3 μl. The advantage of this method is that accurate ratios of He/Ar and N 2 Ar , which are useful parameters for seismo-geochemistry, can be directly measured in a single procedure and that it can be easily programmed.

Influence of multi-electron excitation on EXAFS spectroscopy of trivalent rare-earth ions and elucidation of change in hydration number through the series

Abstract We have made a detailed study of the extended X-ray absorption fine spectra (EXAFS) at the K edge of aqueous Y ion and at L3 edges of aqueous lanthanide ions and thereby elucidated the systematic changes in their hydration structures. Anomalous peaks arising from double-electron excitation (2p, 4d → 5d, 5d) appear in the EXAFS signals of La3+-Tb3+ between 5-7 Å−1. We established a removal process of double-electron excitation from EXAFS spectra. Using that process, we confirmed that the intensity and energy position of the extracted double-electron excitation are comparable to previously reported data. The presence of double-electron excitation engenders a smaller error than the errors estimated in the fitting process. Consequently, double-electron excitation does not seriously affect the determination of the structures of REE3+ aquo ions in the first coordination sphere. Subsequent EXAFS analyses of hydrated REE3+ ions suggest that the hydration numbers, the interatomic distances, and the Debye-Waller factors decrease from 9.7, 2.55 Å, and 9.0 × 10−3 Å2 for La3+ to 7.9, 2.31 Å, and 5.7 × 10−3 Å2 for Lu3+. These parameters change as a sigmoid curve with increasing atomic number. The hydration structures of REE3+ ions are inferred to change from the nonahydrated structure for La3+-Nd3+ to the octahydrated structure for Tb3+-Lu3+ through intermediate structures for Sm3+, Eu3+, and Gd3+. In addition, the hydration state of Y3+ closely resembles that of Ho3+ because the two have almost identical ionic radii.