Mineralogical and textural variation of silica minerals in hydrothermal flow-through experiments: Implications for quartz vein formation

We conducted hydrothermal flow-through experiments at 430 °C and 31 MPa to investigate the mechanism of silica precipitation on granite under crustal conditions. Two experiments were performed using different input solutions: a single-component Si solution, and a multi-component solution with minor Al, Na, and K. The degree of supersaturation with respect to quartz, Ω = C_Si/C_Si,Qtz,eq, where C_Si and C_Si,Qtz,eq indicate Si concentration in solutions and the solubility of quartz within water, respectively, decreased from 3-3.5 to <1.1 along the flow path. A variety of silica minerals formed during the experiments (opal-A, opal-C, chalcedony, and quartz), and their occurrences and modal abundances changed in response to Ω and the presence of additives in the solution.For near-equilibrium solutions (Ω < ∼1.2), silica precipitation occurred in a simple way in both experiments, being restricted to overgrowths on pre-existing quartz surfaces in the granite. At higher saturation levels (Ω > ∼1.2), silica minerals were deposited on other surfaces in addition to quartz. In the single-component experiment, the dominant silica minerals changed in the order of opal-A → opal-C → quartz with decreasing Si concentration along the flow path. In contrast, in the multi-component experiment, quartz and minor chalcedony formed throughout the entire reaction vessel. This finding indicates that impurities (Na, K, and Al) in the solutions inhibited the precipitation of opal and enhanced the direct nucleation of quartz. The systematic appearance of metastable silica minerals were examined by nucleation processes and macroscopic precipitation kinetics. Our experimental results indicate that different precipitation mechanisms yield contrasting textures, which in turn suggests that vein textures can be used as indicators of solution chemistry within the fracture.     

Garnet-amphibolites at Yokokawa in the Abukuma metamorphic belt,Japan

The mineralogy and petrochemistry of the garnet-amphibolites from the highgrade part of the Abukuma metamorphic belt have been studied, using five analyses of rocks, five of hornblendes, three of garnets and one analysis of cummingtonite, Garnetiferous amphibolites are rich in Fe, whereas non-garnetiferous ones are rich in Mg, especially in cummingtonite-amphibolite. The chemical composition of hornblendes associated with garnet is pargasitic and rich in FeO and poor in CaO, but that of non-garnetiferous rocks is rich in MgO. The garnets are rich in almandine molecule. Mg/Mg + Fe²⁺ ratios of both hornblendes and garnets correspond with those of the host rocks. The development of garnet in the Adirondack metabasites belonging to the upper almandine-amphibolite and granulite facies is observed in Mg-rich rocks as well as in Fe-rich rocks, in which both garnet and hornblende are rich in Mg respectively. However, under the conditions of the andalusite-sillimanite type metamorphism as shown in the Abukuma Plateau, Fe-rich garnet occurs in Fe-rich basic rocks, but cummingtonite occurs in Mg-rich ones instead of Mg-rich garnet. Finally, the problem of polymetamorphism is discussed. The cummingtonite-amphibolite may be the product of polymetamorphism, and Mg-rich garnet which had been present previously was decomposed to cummingtonite and plagioclase by the subsequent regional metamorphism of andalusite-sillimanite type.     

Overview of the MITI Nankai Trough Wells: A Milestone in the Evaluation of Methane Hydrate Resources

Abstract. Bottom-simulating reflectors suggestive of the presence of methane hydrates are widely distributed below the ocean floor around Japan. In late 1999, drilling of the MITI Nankai Trough wells was conducted to explore this potential methane hydrate resource and a Tertiary conventional structure. The wells are located in the Northwest Pacific Ocean off Central Japan at a water depth of 945 m. A total of six wells were drilled, including the main well, two pilot wells, and three post survey wells at intervals of 10–100 m. All wells except the first confirmed the occurrence of hydrates based on logging-while-drilling, wire-line logging and/or coring using a pressure and temperature coring system in addition to conventional methods. Based on the various well profiles, four methane hydrate-bearing sand-rich intervals in turbidite fan deposits were recognized. Methane hydrates fill the pore spaces in these deposits, reaching saturation of up to 80 % in some layers. The methane hydrate-bearing turbiditic sand layers are less than 1 m thick, with a total thickness of 12–14 m. The bottom depth of high hydrate concentration correlates well with the depth of the bottom-simulating reflector. Based on these exploration results, the Japanese government inaugurated a 16-year methane hydrate exploitation program in 2001.     

Nanoscale occurrence of Pb in an Archean zircon

We report, for the first time, a direct, atomic-scale characterization of Pb in zircon (4.4-3.1 Ga) from the early Archean Yilgarn craton in Australia using high-resolution HAADF-STEM. Two forms of Pb have been identified: Pb concentrated at ∼3 atom% as a nanoscale patch in zircon structure, and Pb concentrated within the amorphous domain created by fission fragment damage. The first result suggests that the Pb atoms directly substitute for Zr⁴⁺ in the zircon structure, and the latter observation demonstrates that Pb diffusion can occur through amorphous regions created by radiation damage, although volume diffusion is typically considered to be the dominant mechanism for Pb diffusion. Beyond the first percolation point, i.e., when the amorphous domains overlap and form a fully interconnected network of amorphous domains, there is a new pathway for the diffusion of Pb that is faster than volume diffusion through crystalline zircon.     

Radiolarian Age of Chert-hosted Bedded Manganese Deposits from the Gen-otani Mine in the Tamba District, Northern Kyoto City, Japan

Abstract. Numerous bedded manganese deposits sporadically distributed throughout the Tamba district, southwestern Japan are intercalated within chert sequence. It is well known that radiolarian remains are commonly included in both bedded manganese deposits and host cherts. The Gen‐otani mine, one of these deposits, is located at Otani, Keihoku‐Shimonaka, northern Kyoto City. Chemical composition and age of the chert sequence at the mine were examined. Mainly according to SiO₂ and MnO contents together with lithology, the chert sequence is divided into three sections; lower massive chert, middle bedded manganese deposit and upper bedded chert sections. Radiolarian faunas consisting of middle Jurassic species such as Eucyrtidiellum unumaense, Dictyomitrella(?) kamoensis, Parvicingula dhimenaensis, Sethocapsa aitai, Sethocapsa kodrai, Transhsuum brevicostatum, Tricolocapsa plicarum, Unuma echinatus and others were extracted from both the middle manganese section and overlying bedded chert of the upper section. This examination reveals that the bedded manganese deposit at the Gen‐otani mine formed until Bajocian to early Bathonian (middle Middle Jurassic) in age.     

Molecular dynamics study of the crystal structure and phase relation of the GeO2 polymorphs

A two-body interatomic potential model for GeO₂ polymorphs has been determined to simulate the structure change of them by semi-empirical procedure, total lattice energy minimization of GeO₂ polymorphs. Based on this potential, two polymorphs of GeO₂; α-quartz-type and rutile-type, have been reproduced using the molecular dynamics (MD) simulation techniques. Crystal structures, bulk moduli, volume thermal expansion coefficients and enthalpies of these polymorphs of GeO₂ were simulated. In spite of the simple form of the potential, these simulated structural values, bulk moduli and thermal expansivities are in excellent agreement with the reliable experimental data in respect to both polymorphs. Using this potential, MD simulation was further used to study the structural changes of GeO₂ under high pressure. We have investigated the pressure-induced amorphization. As reported in previous experimental studies, quartz-type GeO₂ undergoes pressure-induced crystalline-to-amorphous transformation at room temperature, the same as other quartz compounds; SiO₂, AlPO₄. Under hydrostatic compression, in this study, α-quartz-type GeO₂ transformed to a denser amorphous state at 7.4 GPa with change of the packing of oxygen ions and increase of germanium coordination. At higher pressure still, rutile-type GeO₂ transformed to a new phase of CaCl₂-type structure as a post-rutile candidate. Received: 29 July 1996 / Revised, accepted: 30 April 1997     

High-Speed Data Acquisition System Messia for Subaru

We have developed a high-speed data acquisition system, Messia-III (Modularized Extensible SyStem for Image Acquisition) for Subaru, Japanese 8.2m telescope. Messia has a direct 1-Gbps link to a host UNIX workstation. Messia is a VMEbus-based system but it does not have local OS nor CPU's in order to minimize software development and maintenance.     

A momentum distribution of high energy particles around Crab pulsar

We determine the momentum distribution of the relativistic particles near the Crab pulsar from the observed X- and -ray spectra (10³10⁹ eV), provided that the curvature radiation is responsible for it. The power law spectrum for the relativistic electrons,f() ^–5, reproduces a close fit to the observed high-energy photon spectrum. The theoretically determined upper limit to the momentum (due to radiation damping), _M 8×10⁶, corresponds to the upper cut-off energy of the -ray spectrum, 10⁹ eV. The lower limit to the momentum, _m 1.8×10⁵, is chosen such that flattening of the X-ray spectrum below 10 keV is simulated. The number density of these electrons is found to be much higher than the Goldreich-Julian density. We also discuss pulse shape and polarization of high-energy photons. The extremely high density of particles and the steep momentum spectrum are difficult to understand. This may imply that another, more efficient, mechanism is in operation.     

Mineralogical and Genetical Aspects of the Doyashiki Kuroko Deposits, Hokuroku Basin, Japan

Abstract. A detail investigation of ore and gangue minerals was performed on the Doyashiki Kuroko deposits, Hokuroku basin, Japan for the first time. Main ore minerals are sphalerite, galena, pyrite, chalcopyrite, tetrahedrite-tennantite and digen-ite. Small amounts of enargite, wittichenite, electrum, covellite, bornite, marcasite and hematite are also observed. Quartz, barite and gypsum are common gangue minerals. Homogenization temperatures and salinities of fluid inclusions in quartz, sphalerite and barite range from 190 to 240C and 3.0 to 5.5 wt% NaCl equivalent, respectively. The FeS contents of sphalerite and Ag contents of electrum were 0.12 to 0.18 mol %, 39.0 to 39.6 atom %, respectively. The chemical composition of digenite as a primary mineral shows high sulfur contents.
These data indicate that ore fluid responsible for digenite and associated ore minerals was characterized by a range of high sulfur fugacity with a moderate formation temperature. This is concordant with the mineral assemblage of bornite-pyrite and chalcopyrite, which shows high sulfur fugacity conditions. It seems that the mineralization closely associated with acidic volcanism has occurred around 13 Ma of Middle Miocene on the seafloor at the depth of about 1500 m.