Sedimentary Crust and Metallogeny of Granitoid Affinity: Implications from the Geotectonic Histories of the Circum‐Japan Sea Region,Central Andes and Southeastern Australia

Metallogeny of granitoid affinity was reviewed from the aspect of geotectonic history of the continental crust, particularly of the genesis of sedimentary crust involved in magmatism. The redox state of granitoids and related mineralization shows a remarkable contrast between the east and west sides of the Pacific Rim, but if examined closely, the reduced‐type and oxidized‐type granitoid provinces are juxtaposed in three regions: the circum‐Japan Sea region, the central Andes, and the Lachlan Fold Belt in southeastern Australia. Comparative study of these regions revealed that the reduced‐type magmatism associated with Sn mineralization generated in thick sedimentary crust which formed in three geotectonic environments: (i) accretionary terrane along a subduction zone (e.g. Jurassic East Asia), (ii) continental rift (e.g. Early Paleozoic Andes), and (iii) mega‐fan (e.g. Early Paleozoic southeastern Australia). A collisional orogen can provide large amounts of clastic sediment to these environments. The age gap between the magmatism and sedimentation varies depending on the tectonic evolution of individual regions. Thin sedimentary crust may not play an essential role for the reduced‐type magmatism. The oxidized‐type magmatism associated with porphyry Cu and other mineralization generated in the crust which was initially carbon‐free igneous crust or modified from sedimentary crust by magmatism. Subduction‐related basaltic magmas are relatively oxidized, and may enhance fO₂ conditions of granitoid activity. Repeated magmatism in a monotonous convergent margin may be favorable for porphyry Cu mineralization as exemplified in the eastern Pacific Rim.     

Subsurface thermal and hydrothermal characterization based on geothermal resources map, drill hole thermal gradients, Curie point depths and hypocenter distribution — Examples of Tohoku district, Japan

Subsurface thermal structure in Tohoku district are characterized by existing data such as geothermal resources maps, drill hole thermal gradients, Curie point depths and hypocenters distribution maps. The collected data are registered in a database system, then, compared in plan view, cross-section and bird's-eye pictures. The comparison indicates that subsurface temperatures extrapolated from drill hole thermal gradients are generally concordant to the Curie point depth, assumed to be 650 °C. Tohoku district is generally divided into 5 type areas; fore arc lowland, fore arc mountain country, Quaternary volcanic terrain, back arc lowland and back arc mountain country. The surface thermal manifestations in Quaternary volcanic terrain are mainly controlled by the magma chambers as heat sources, while, surface thermal features such as hot springs in non-volcanic areas are controlled by degrees of heat flows, and hydrothermal flows in permeable Cenozoic formations and along permeable fault zones.     

Metal-organic complexes in seawater pumped up from under the ground

Metal-organic complexes of transition elements removed by Amberlite XAD-2 resin from seawater pumped up from under the ground were determined. The proportions of iron, copper and zinc retained on the resin to dissolved forms of these metals were about 70, 40 and 5%, respectively, while manganese, cobalt and nickel were not retained on the resin. These results suggest that although iron may be retained in colloidal form, a significant fraction of copper is present in some organic form(s).     

Fast Oxidation Reaction of Nitrite by Dissolved Oxygen in the Freezing Process in the Tropospheric Aqueous Phase

Nitrite oxidation in the tropospheric aqueous phase by freezing was evaluated by freezing a field sample. Nitrite oxidation by dissolved oxygen in the freezing process is much faster than by other oxidation processes, such as reactions with ozone, hydrogen peroxide or dissolved oxygen in an aqueous solution at pHs 3 to –6. At pH 4.5 and 25°C, the lifetime of nitrite in the aqueous phase is ca. 1 hr in oxidation by ozone (6×10^-10 mol dm^-3), ca. 10 hr in oxidation by H₂O₂ (2×10^-4 mol dm^-3), and 7.5 hr (Fischer and Warneck, 1996) in photodissociation at midday in summer. Under the same conditions at a temperature below 0°C, the lifetime of nitrite in the freezing process is estimated as ca. 2 sec when the droplets are frozen within a second. The reaction by freezing is affected by the presence of salts, such as NaCl or KCl, or orgnaic compounds, such as methanol or acetone. The results of freezing a field rain or fog sample showed that nitrite oxidation proceeds below pH 6, and the conversion ratio of nitrate from nitrite increases with decreasing pH. The oxidation of nitrite by freezing was also observed in freezing fog particles generated by an ultrasonic humidifier. The ratios of the concentrations of ions in the winter sample to those in the summer sample (or those in the fog sample) were almost the same values. However, the concentration of nitrite in the winter sample was lower than that estimated by the ratios of other ions. From the present study, it seems that the freezing process plays an important role in the nitrite sink process in the tropospheric aqueous phase.     

Low frequency electrical impedance of partially molten gabbro: The effect of melt geometry on electrical properties

Electrical impedance of partially molten gabbro was measured as a function of the frequency of the applied electric field. The impedance of the partially molten gabbro was mainly controlled by the volume and geometry of partial melt, which were observed with an optical microscope study of the quenched sample. The experimental results are interpreted by a theory that formulates the electrical property of rocks containing partial melt, taking ionic diffusion in the melt into account. The theory evaluates the contributions from two types of melt-geometry distributed in the rock matrix, namely:

1. (a) nearly isolated melt pockets, and

2. (b) connected melt along grain boundaries.

According to the theory, connected melt mainly contributes to the increase of net conductivity, while melt pockets increase displacement current and cause the frequency dependence of impedance. The characteristic ionic diffusivity in melt pockets was determined from the peak frequency of the imaginary part of impedance. The magnitude of diffusion coefficient is estimated to be of the order of 10⁻⁷ to 10⁻⁶ cm²/sec. This suggests that sodium, iron, magnesium and/or calcium ions mainly carry the electric charge in partial melt. The observed frequency dependence of impedance is considered to reflect ionic polarization in melt pockets due to the migration of these alkali ions toward solid-melt interfaces.The present experimental and theoretical work suggests that electrical properties of the earth's upper mantle may involve an intrinsic frequency dependence associated with ionic diffusion in the frequency range of geomagnetic observation, if a partial melt zone exists. The analysis of the earth's conductivity structure may be inadequate if such intrinsic frequency dependence of mantle media is neglected.     

A practical numerical method for large strain liquefaction analysis of saturated soils

Accurate prediction of the liquefaction of saturated soils is based on strong coupling between the pore fluid phase and soil skeleton. A practical numerical method for large strain dynamic analysis of saturated soils is presented. The u–p formulation is used for the governing equations that describe the coupled problem in terms of soil skeleton displacement and excess pore pressure. A mixed finite element and finite difference scheme related to large strain analysis of saturated soils based on the updated Lagrangian method is given. The equilibrium equation of fluid-saturated soils is spatially discretized by the finite element method, whereas terms associated with excess pore pressure in the continuity equation are spatially discretized by the finite difference method. An effective cyclic elasto-plastic constitutive model is adopted to simulate the non-linear behavior of saturated soils under dynamic loading. Several numerical examples that include a saturated soil column and caisson-type quay wall are presented to verify the accuracy of the method and its usefulness and applicability to solutions of large strain liquefaction analysis of saturated soils in practical problems.     

Experimental Study of Fe-Mg Exchange Reaction Between Orthopyroxene and Sapphirine and its Calibration as a Geothermometer

The exchange of Fe²⁺ and Mg²⁺ between orthopyroxene and sapphirine has been investigated at pressures 7-16 kbar and temperatures 850-1300°C using a piston cylinder apparatus for the synthetic and natural systems. This reaction is temperature-dependent and would be a good geothermometer. The equilibration temperature T is approximately expressed by the distribution coefficient as follows: T(°C)=1515/(ln K_D+0.943)-273.

This empirical equation was applied to some Antarctic granulites and associated rocks. The new orthopyroxene-sapphirine geothermometer gives consistent results with those estimated from the Fe-Mg orthopyroxene-garnet geothermometer.     

Some instabilities in the 3/1 and 2/1 resonances

In a very simple way, it is possible to show the existence of small regions of instability, inside the observed 3/1 and 2/1 Kirkwood Gap, by using the classical Laplace-Lagrange secular theory.     

Holocene lake development and glacial-isostatic uplift at Lake Skallen and Lake Oyako,Lützow-Holm Bay,East Antarctica: Based on biogeochemical facies and molecular signatures

The paleolimnology of two lakes which were isolated as a result of the crustal uplift during the late Holocene along the Soya Coast, Lützow-Holm Bay, East Antarctica were studied. The focus was on temporal variations in the biogeochemical composition of sediment cores recovered from Lake Skallen at Skallen and Lake Oyako at Skarvsnes. Both sets of lake sediments record environmental changes associated with a transition from marine to lacustrine settings, as indicated by analyses of C and N contents, nitrogen isotopic compositions (δ¹⁵N), and major element concentrations. Changes in the dominant primary producers during the marine–lacustrine transition (marine diatom to cyanobacteria) at L. Skallen was clearly revealed by biogenic opal-A, diatom assemblages, and molecular signature from denaturing gradient gel electrophoresis (DGGE) with 16S ribosomal RNA (rRNA) gene analysis. Radiocarbon dating of acid-insoluble organic C suggested that the environmental transition from marine to fresh water occurred at 2940 ± 100 cal yr BP at L. Skallen and 1060 ± 90 cal yr BP at L. Oyako. Based on these data, a mean crustal uplift rate of 3.2 mm yr⁻¹ is inferred for the history of marine–lacustrine transition via brackish conditions. The geological setting causing glacio-isostatic uplift was the primary factor in controlling the transition event in sedimentary and biological facies.