Kazuo Nakashima

Niobian and zincian ilmenites in syenites from Cape Ashizuri, Southwest Japan

SummaryIlmenite in alkali feldspar quartz syenite from Cape Ashizuri contains up to 4.4 wt.% Nb2O5. Niobium substitutes for Ti in the octahedral site of the ilmenite structure. Substitution of Nb for Ti may involve a coupled exchange to maintain charge balance, and an exchange of 2Nb + 2Fe3+ = 3Ti + 2Fe2+ is advocated. An Fe-Ti oxide geothermometer obtained from mineral pairs of granular and lamellar intergrowths indicates a subsolidus re-equilibration temperature of 510–640 °C andfO2 between the FMQ and MW-buffers, implying that it is very undersaturated with respect to water. On the other hand, Zn-ilmenite, containing up to 5.4 wt.% ZnO, occurs in miarolitic cavities in peralkaline rhyolite which cuts the quartz syenite. The Zn-ilmenite is one of the last crystalline phases of the Ashizuri magmatic activities under volatile-rich conditions. Nb-oxides, such as fergusonite, samarskite, columbite and a pyrochlore-like mineral, are Ta- and Mn-poor, which corresponds to those of less-fractionated rocks of anorogenic alkali granite and pegmatitic granite in the continental situation. Ta- and Mn-poor Nb-oxides in F- and Li-rich alkaline felsic magmas such as the Ashizuri syenites are unusual; this may be related to a rapid emplacement and cooling of mantle-derived small-volume magma in the island are situation.ZusammenfassungIlmenit in Alkalifeldspat-Quarz-Syenit von Cap Ashizuri enthält bis zu 4,4 Gew. % Nb2O5. Niob ersetzt Ti in der oktaedrischen Position der Ilmenit-Struktur. Substitution von Nb für Ti erfolgt über einen gekooppelten Austausch zum Erhalt der Ladungsgleichgewichte und hier wird der Austausch von 2Nb + 2Fe3+ = 3Ti + 2Fe2+ vorgeschlagen. Ein Fe-Ti-Oxid Geothermometer auf der Basis von Mineral-Paaren körniger und lamellarer Verwachsungen weist auf Subsolidus Re-Equilibrationstemperaturen von 510 bis 640 °C undfO2 zwischen den QFM und MW-Puffern hin. Dies bedeutet Wasser-Untersättigung. Andererseits kommt Zn-Ilmenit mit bis zu 5,4 Gew.% ZnO in miarolitischen Hohlräumen in peralkalinem Rhyolit vor, der den Quarzsyenit durchschlägt. Der Zn-Ilmenit ist einer der am spätesten gebildeten kristallinen Phasen der magmatischen Aktivitäten von Ashizuri, bei Bedingungen, die an volatilen Phasen reich sind. Nb-Oxide, wie zum Beispiel Fergusonit, Samarskit, Columbit und ein Pyrochlor-ähnliches Mineral sind Ta- und Mn-arm; dies entspricht weniger fraktionierten Gesteinen aus dem Bildungsberich anorogener Alkali-Granite und pegmatitischer Granite in einer kontinentalen Situation. Ta- und Mn-arme Nb-Oxide in F- und Li-reichen alkalischen felsischen Magmen, wie die Ashizuri Syenite, sind ungewöhnlich; dies mag mit rascher Platznahme und Abkühlung von kleinen Volumina von Magmen, die aus dem Mantel stammen, in einer Inselbogen-Situation zusammenhängen.

Timing of clockwise rotation of Southwest Japan: constraints from new middle Miocene paleomagnetic results

Southwest Japan rotated clockwise during the late stage of the opening of the Japan Sea, although the exact timing of the rotation is controversial. A recent biostratigraphic study has revealed that the Miocene Oidawara Formation in eastern Southwest Japan was deposited just before 15 Ma; consequently, its paleomagnetic direction may help constrain the timing of rotation. For this purpose, we collected fine felsic tuffs and siltstones at 71 stratigraphic sites (horizons) in the Oidawara Formation. An analysis of alternating field and thermal demagnetization results yielded characteristic remanent magnetization (ChRM) directions for 177 samples. Approximately 80 % (142) of the samples exhibit reverse polarity ChRM directions that are thought to be paleofield directions of reverse polarity Chron C5Br. Normal polarity ChRM directions in 35 samples include primary paleofield records as well as records of secondary magnetization. The data suggest that a short normal polarity interval (microchron or cryptochron) at ~15.8 Ma is present within the dominantly reverse polarity interval of Chron C5Br. Reliable site-mean directions for 19 sites yield a tilt-corrected formation-mean direction of D = 10.5°, I = 41.1°, α95 = 7.0°, and k = 23.9, indicating virtually no rotation with respect to a reference paleomagnetic direction for the Asian continent. A rotation versus age plot for Southwest Japan indicates that the clockwise rotation started after 17.5 Ma and ceased largely before 15.8 Ma, yielding a rotation rate of ~23°/Myr.

Regional and local variations in the composition of the wolframite series from SW Japan and possible factors controlling compositional variations

The regional and local variations in the composition of the wolframite series associated with the ilmeniteseries and magnetite-series granitoids in the Inner Zone of SW Japan were investigated using X-ray diffraction and microprobe analyses. It is concluded that the variation in Mn/Fe ratios in the source materials (granitic magmas in most cases) is the dominant factor controlling the regional variation in the wolframite compositions, while, as exemplified by the Kaneuchi mine, Kyoto Prefecture, the wallrock chemistry and pH of ore fluids are responsible for the local variation within a single vein system. Within a fresh euhedral crystal from the mine, only slight compositional variation of less than 3 mole% MnWO4 is found in the form of oscillatory zoning. This study, combined with the fluid inclusion research, indicates that the wolframite compositions (Mn/Fe ratios) cannot be used as a geothermometer.

Pyrophanite and high Mn ilmenite discovered in the cretaceous Tono pluton, NE Japan

Pyrophanite and high Mn ilmenite have been discovered in the Tono plutonic rocks of Kitakami Mountains, NE Japan. The rocks in the pluton are classified into four rock facies; Marginal, Main, Arakawa-type and Central facies. The former three facies which are composed of quartzdiorite and granodiorite are believed to represent a series of continuous differentiation and the last facies, composed of tonalite and granodiorite, is similar but slightly different from the above differentiation. Magnetite, ilmenite and hematite are present in these rock facies, although ilmenite is absent in the Arakawa-type facies. Manganese content of ilmenite increases from the Marginal facies to the Main facies as differentiation proceeds. Especially, in the Central facies, the highest MnO content of 35 wt.% (74 mol. % MnTiO 3 ) was detected in a composite-type ilmenite which exsolved hematite. This is the maximum value that has ever been reported from calc-alkaline igneous rocks, except for pegmatite. The presence of euhedral discrete hematite, myrmekite, euhedral biotite and intergranular muscovite in the Central facies, and an equilibrium temperature obtained by a two-feldspar geothermometer, show that crystallization of the Tono granitic magma continued to a low-temperature subsolidus stage under volatile-rich highly oxidized conditions. The pyrophanite crystallized in the greatly differentiated facies which solidified from an intrinsically volatile-rich magma.

Crystal chemistry of Ti-rich ferriallanite-(Ce) from Cape Ashizuri, Shikoku Island, Japan

Abstract The crystal chemistry of Ti-rich ferriallanite-(Ce) in syenites from Cape Ashizuri, Shikoku Island, Japan, was studied using electron microprobe analysis (EMPA) and single-crystal X‑ray diffraction methods. Our specimens (AS001 and AS304) are characterized by high-Ti (-3.6 wt% TiO2) and Fe contents (-21.3 wt% FeO*; Fe2+/total Fe = 0.6-0.7). Structure refinements of two ferriallanite-(Ce) crystals converged to R1 = 1.50-1.57%. In both specimens, the dominant cations at A1, A2, and M3 are Ca, Ce, and Fe2+, respectively. However, depending on Ti assignment, two different schemes of the cation distributions at M1 and M2 can be considered: (1) (Fe0.72Ti0.23Al0.05)M1(Al0.82Fe3+0.18)M2 and (2) (Fe0.90Al0.10)M1(Al0.77Ti0.23)M2 for AS001, and (1) (Fe0.65Ti0.29Al0.06)M1(Al0.89Fe3+0.11)M2 and (2) (Fe0.76Ti0.13Al0.11)M1(Al0.84Ti0.16)M2 for AS304. The Si2O7 group is compressed along Sil-O9-Si2 direction because of the expansions of the M1O6 and M3O6 octahedra due to substitution of large octahedral cations for Al. This compression is monitored by the atom position of O9 that shifts away from the cation at A1 decreasing the Si1-O9-Si2 angle. In allanite-subgroup minerals, the A1-O9 distance is longer and the bridging angle is smaller than those of clinozoisite-subgroup minerals. Although the A1 polyhedron is generally described as a ninefold-coordinated site, it is appropriately described as sevenfold-coordinated in Ti-rich allanite-(Ce) because of the lengthened A1-O9 (×2) distance. The cation substitutions at M1 and M3 indirectly affect the topology of the A1 polyhedron.

Pure Silicate Fragment in a Recurrent Stone Former of Calcium Oxalate

Long-time oral intake of magnesium-silicate-containing drugs is thought to be a causative factor inducing silicate urolithiasis. Besides, magnesium seems to play an anti-urolithogenic part in the formation of calcium oxalate stones. We report a recurrent calcium oxalate former who had been treated with magnesium aluminometasilicate antacid for gastric ulcer for approximately 17 years. One of the fragments found during extracorporeal shock wave lithotripsy was identified as 100% silicate. Deposition of silica was also found on other fragments. A large dose of magnesium (given in a part of the drug) might have little influence on the formation of calcium oxalate.

Chemistry of Fe-Ti oxide minerals in the Hobenzan granitic complex, SW Japan: Subsolidus reduction in relation to base metal mineralization

SummaryThe mineralized stock of the Hobenzan granitic complex is composed of tonalite and a continuous differentiation series of biotite-hornblende granodiorite, hornblende biotite granite and biotite granite. Texture and mineral chemistry of the Fe-Ti oxide minerals in the Hobenzan granitic complex exhibit two different processes of magma evolution: one is an oxyexsolution process related to the magmatic and high temperature subsolidus stage, and the other is a reduction process of consecutive subsolidus stage. Rocks distributed in the northern part of the granitic complex preserve well the oxyexsolution process and show higher magnetic susceptibility, whereas those in the southern part of the complex, record the reduction process and show lower magnetic susceptibility.The magnetite-ilmenite geothermometer indicates temperatures of ca. 730°C for the oxide pairs of the early stage. Oxygen fugacity of one to three orders of magnitude higher than the annite-sanidine-magnetite (ASM) univariant curve, and an aqueous sulfur composition,fSO2/fH2S, of around 1.0 is indicated. This first stage corresponds to the crystallization of phenocrystic hornblende and plagioclase at depth. At about 700°C crystallization changed to biotite, K-feldspar and quartz, and continued to about 600°C. ThefO2 during this second stage is buffered by the ASM assemblage. This second stage defines the oxyexsolution process. Below about 600°C, a reduction process, caused by assimilation of carbonaceous matter of country rocks, overprinted the southern part of the complex. Oxide pairs show that thefO2 is about four orders of magnitude lower than the ASM univariant curve, andfSO2/fH2S is 10−8.0 or less at 550°C for this reduced assemblage. The drastic change in composition of sulfur-bearing aqueous species may be one of the principal factors allowing base metal mineralization.ZusammenfassungDer mineralisierte Hobenzan Granitkomplex setzt sich aus Tonaliten und einer kontinuierlichen Differentiationsserie, bestehend aus Biotit-Hornblende-Granodioriten, Hornblende-Biotit-Graniten und Biotit-Graniten, zusammen. Die Texturen und die Mineralchemie der Fe-Ti Oxide belegen zwei unterschiedliche Prozesse bei der Entwicklung des Hobenzan Granitkomplexes: einerseits einen Oxyexsolution-Prozeß, während des magmatischen und hochtemperierten Subsolidus-Stadiums, andererseits einen Reduktionsprozeß während des tiefertemperierten Subsolidus-Stadiums. Gesteine im nördlichen Hobenzan Komplex belegen vor allem den Oxyexsolution Prozeß und zeigen höhere magnetische Suszeptibilität, während jene im südlichen Teil den Reduktionsprozeß widerspiegeln und niedrigere magnetische Suszeptibilität zeigen.Das Magnetit-Ilmenit Geothermometer ergab Temperaturen von ca. 730°C für Oxidpaare des Frühstadiums. Die Sauerstoff Fugazität liegt um eine bis drei Größenordnungen über der univarianten Reaktionskurve Annit-Sanidin-Magnetit (ASM), und dasfSO2/fH2S Verhältnis der wässrigen Schwefelkomplexe bei ca. 1.0. Dieses Frühstadium korrespondiert mit der Kristallisation von Horblende und Plagioklas im Magma in größerer Tiefe. Ab ca. 700°C erfolgt die Kristallisation von Biotit, Alkalifeldspat und Quarz bis etwa 600°C, wobeifO2 durch die ASM Mineralassoziation gepuffert wird. Dieses zweite Stadium wird als Oxyexsolution Prozeß beschrieben. Unter 600°C erfolgte eine Reduktion durch Assimilation von kohlenstoffreichem Material vor allem im südlichen Teil des Komplexes. Oxidpaare dieses Stadiums belegen, daßfO2 um etwa vier Größenordnungen unterhalb des ASM Puffers liegt, undfSO2/fH2S ist ⩽ 10−8 bei 550°C. Die dramatische Änderung in der Zusammensetzung der Schwefelkomplexe in den Lösungen wird als der Hauptfaktor für die Bildung der Erzmineralisationen angesehen.

Elemental behavior during the fractionation of felsic magma at Hobenzan polymetallic province, SW Japan

Abstract We have performed PIXE microprobe analysis of rock-forming minerals in granitoids at the Hobenzan polymetallic region of unique CuBiAsCoWB mineralization. The analysis was carried out at CSIRO using 5 to 30 μm beam of 3 MeV protons, and the following elements were detected: Cr, Mn, Fe, Ni, Cu, Zn, Ga, As, Rb, Sr, Y, Zr, Nb, Mo and Sn. In the case of biotite, incompatible elements such as Rb and Y show positive correlation with the Differentiation Index (DI) of the rocks. Ore forming elements such as Zn and Sn have similar trend to that of the incompatible elements. This fact implies that some of the ore forming elements do not directly enter the magmatic fluid that is evacuated from parental magma but they need to be extracted from minerals through water/rock interaction to be components of the mineralizing fluid. Our result also indicated that biotite is an ideal mineral to monitor the elemental behavior during the evolution of a magmatic-hydrothermal system.

The Cretaceous Ofuku Pluton and Its Relation to Mineralization in the Western Akiyoshi Plateau, Yamaguchi Prefecture, Japan

The relationship between the magmatism of the Cretaceous Ofuku pluton and mineralization in and around the Akiyoshi Plateau, Yamaguchi Prefecture, Japan was investigated using a combination of field observation, petrographic and geochemical analyses, K–Ar geochronology, and fluid inclusion data. The Ofuku pluton has a surface area of 1.5 × 1.0 km, and was intruded into the Paleozoic accretionary complexes of the Akiyoshi Limestone, Ota Group and Tsunemori Formation in the western part of the Akiyoshi Plateau. The pluton belongs to the ilmenite-series and is zoned, consisting mainly of early tonalite and granodiorite that share a gradational contact, and later granite and aplite that intruded the tonalite and granodiorite. Harker diagrams show that the Ofuku pluton has intermediate to silicic compositions ranging from 60.4 to 77.9 wt.% SiO2, but a compositional gap exists between 70.5 to 73.4 wt.% SiO2 (anhydrous basis). Modal and chemical variations indicate that the assumed parental magma is tonalitic. Quantitative models of fractional crystallization based on mass balance calculations and the Rayleigh fractionation model using major and trace element data for all crystalline phases indicate that magmatic fractionation was controlled mainly by crystal fractionation of plagioclase, hornblende, clinopyroxene and orthopyroxene at the early stage, and quartz, plagioclase, biotite, hornblende, apatite, ilmenite and zircon at the later stage. The residual melt extracted from the granodiorite mush was subsequently intruded into the northern and western parts of the Ofuku pluton as melt lens to form the granite and aplite. The age of the pluton was estimated at 99–97 Ma and 101–98 Ma based on K–Ar dating of hornblende and biotite, respectively. Both ages are consistent within analytical error, indicating that the Ofuku pluton and the associated Yamato mine belong to the Tungsten Province of the San-yo Belt, which is genetically related to the ilmenite-series granitoids of the Kanmon to Shunan stages. The aplite contains Cl-rich apatite and REE-rich monazite-(Ce), allanite-(Ce), xenotime and bastnasite-(Ce), indicating that the residual melt was rich in halogens and REEs. The tonalite–granodiorite of the Ofuku pluton contains many three-phase fluid inclusions, along with daughter minerals such as NaCl and KCl, and vapor/liquid (V/L) volume ratios range from 0.2 to 0.9, suggesting that the fluid was boiling. In contrast, the granite and aplite contain low salinity two-phase inclusions with low V/L ratios. The granodiorite occupies a large part of the pluton, and the inclusions with various V/L ratios with chloride daughter minerals suggest the boiling fluids might be related to the mineralization. This fluid could have carried base metals such as Cu and Zn, forming Cu ore deposits in and around the Ofuku pluton. The occurrence and composition of fluid inclusions in the igneous rocks from the Akiyoshi Plateau are directly linked to Cu mineralization in the area, demonstrating that fluid inclusions are useful indicators of mineralization.

Effect of oxidation state on Bi mineral speciation in oxidized and reduced granitoids from the Uetsu region, NE Japan

The relationship between bismuth (Bi) mineral speciation and redox state in three types of granitoids from the Uetsu region, northeast Japan is investigated. Electron microprobe analysis of Bi minerals, sphalerite, Mg-Fe-bearing carbonate minerals, and muscovite, as well as sulfur isotope analysis of sulfide minerals and microthermometric study of fluid inclusions reveal that Bi mineral speciation varies according to the redox state of the granitoids. For example, native bismuth and bismuthinite are abundant and Bi sulfosalts are rare in the lowest fS2 and fO2 mineralized zones of the reduced Iwafune granite (S-type, ilmenite-series) while Bi sulfosalts (Bi3+) are abundant and trace amounts of native bismuth (Bi0) and bismuthinite are found in the highest fS2 and fO2 mineralized zones of the oxidized Wasada granodiorite (I-type, magnetite-series). Bismuthinite is a major Bi mineral, and native bismuth and Bi sulfosalts occur in only minor amounts in the mineralized zones of the Nishitagawa granodiorite (I-type, ilmenite-series), which has intermediate fS2 and fO2 to that of the Iwafune and Wasada samples. Our study indicates that Bi mineral speciation related to granitic intrusive activity is controlled by the redox state of the magmatism, such that native bismuth is typical of reducing conditions, whereas Bi sulfosalts are typical of oxidizing conditions.