Solubilities of sphalerite were measured in NaHS-NaCI-H2S aqueous solutions at temperatures between 100° and 240°C. The sphalerite solubilities at the experimental condition were also calculated using the thermodynamic data for Zn-sulfide and Zn-chloride complexes. The measured solubilities are systematically higher than the calculated ones by 0.5 m to 1.5 m in log unit. These discrepancies could be reconciled if a mixed sulfide-chloride complex, Zn(HS)2Cl-, is assumed to be present in the solutions. The equilibrium constant (log K) for a reaction ZnS(s)+H2S(aq)+Cl-=Zn(HS)2Cl- is calculated as -3.81±0.49 at 100°C, -3.60±0.62 at 150°C, -3.55±0.66 at 200°C and -4.03±0.51 at 240°C. This result suggests that the concentration of Zn(HS)2Cl-is higher than that of Zn-sulfide complexes at geologically important ranges of ΣS and ΣCl. Besides the Zn-chloride complexes, Zn(HS)2Cl- is likely to be a significant zinc species in natural ore-forming solutions.
Zircon U-Pb isotopic data on the early Cretaceous Dalongshan granitoids in the Yangtze Foldbelt of Southeast China provides evidence for the incorporation of old Archean crustal material. The age of inherited radiogenic lead is determined by the three-dimensional U-Pb discordia plane method to be about 3330±180 Ma, indicating that the early Archean basement probably exists underneath the northwest part of the Yangtze Foldbelt. The nature of the source region of initial lead in the zircon demands that the Archean basement is responsible for a lower crust or a depleted mantle.
Vuggy carbonate crust 10 cm in thickness has been discovered associated with hydrocarbon seepage on the continental shelf of Baffin Island, northeast Canada, at a water depth of about 400 m. The carbonate crust is principally composed of high magnesian calcite (14.1 to 18.1 mol.% MgCO3) with subordinate amounts of aragonite (∼9200 ppm Sr) and calcian dolomite (51.5 to 53.2 mol.% CaCO3). Magnesian calcite occurs as microcrystalline cement, and aragonite forms botryoidal aggregates in cavities and solution pits of the crust. Dolomite occurs as isolated euhedral grains with characteristic corroded surfaces. Calcite and aragonite have the similar isotopic signatures, ranging in δ13C between -26.1 and -33.2‰ PDB and in δ18O between 4.4 and 7.8‰ PDB. Relative to calcite and aragonite, dolomite is enriched in 13C (-8.9 to -11.6‰ PDB δ13C) and depleted in 18O (-1.2 to -1.7‰ PDB δ18O). Combined petrographic and gochemical evidence suggests that dolomite was formed in the sulfate reduction zone prior to the formation of carbonate crust under the strong influence of 18O-depleted freshwater, which presumably was derived from the melting of glacial ice. Then high magnesian calcite precipitated to forma dense carbonate layer in the near surface sediments caused by anaerobic oxidation of hydrate-derived 13C-depleted methane and increased alkalinity. When the sediments on the crust were winnowed and blown away by an expulsion of methane from the sea bed or a strong bottom current, the crust would be exposed to a zone of aerobic methane oxidation at the sediment/water interface. This led to dissolution of the upper surface of the crust and subsequent precipitation of botryoidal aragonite within solution vugs and cavities.
Nine basaltic rocks from the New Georgia Group, the Solomon Islands, have been analyzed for REE, Ba and Sr abundances, Sr and Nd isotopic ratios, and major element compositions. Nd and Sr isotopic ratios of four olivine basalts and two picrite basalts fall in restricted ranges, suggesting that these rocks have a common source. REE patterns of six of these basaltic rocks have the following features: (1) The patterns have no intersection with each other, (2) All of the patterns possess a common fine structure of convex curve around Nd-Gd-Dy, and (3) Inclination of light REE span increases with increase of REE concentration. REE pattern analysis of the basalts leads to inferences that (A) a linear REE partition coefficient function with a sharp break at Ho has operated in the generation of the olivine basalts by the Rayleigh fractional crystallization of a picrite basalt magma, and (B) the absolute magnitudes of the partition coefficients did not change substantially during the magmatic differentiation.
In order to elucidate carbon and oxygen isotopic systematic in metamorphosed limestone and isotopic behavior during metamorphism, microscale carbon and oxygen isotopic compositions of graphite and calcite in crystalline limestone were determined for the contact metamorphic aureole of the polymetamorphosed Hida metamorphic belt, central Japan. In this area granulite facies metamorphic rocks were overprinted by contact metamorphism related to a granitic intrusion. Carbon isotopic fractionation between individual crystalline graphites and the cores of coexisting calcite grains gives a constant value independent of the sampling localities. Graphites armored by silicates have the same δ13C as other graphites in calcite. These indicate that carbon isotopic equilibrium between graphite and calcite was completely attained during granulite facies metamorphism. Values of δ18O in the cores of individual calcite grains vary over about 2‰ near the granite contact, and the range of variation tends to become smaller with increasing distances from the contact. In calcite grains near the granite contact, δ18O values sharply decrease by up to 7‰ within 200∼300 μm wide zones toward grain boundaries, whereas δ13C remain almost constant. This δ18O trough at the grain boundaries is less pronounced with increasing distance from the granite contact and is almost no longer seen at distance about 1.3 km. Oxygen isotopic zonation of calcite at grain boundaries strongly suggests partial isotopic exchange with water-rich metamorphic fluids that migrated along grain boundaries. This fluid-rock interaction occurred preferentially in the grain boundaries with very limited scale at distances up to 1.3 km from the granite contact, although the contact aureole defined by the mineralogical paragenesis is limited to within several tens of meters of the contact. Skeletal graphites growing over the substrata of large hexagonal graphite grains have lower δ13C values than those of the host graphite. It is considered that the fine-grained graphite crystals were formed in isotopic equilibrium with low δ13C carbon in the metamorphic fluids.
The closure temperature for zircon in the fission-track dating has been poorly estimated because of interpolation between minerals with very different closure temperatures. We find that K-Ar ages and fission-track ages of coexisting K-feldspars and zircons in Cretaceous granitic rocks from Japan are concordant within the experimental errors, with the exception of a few cataclastically deformed samples near the Median Tectonic Line. The concordance between the two systems suggests that the closure temperature for zircon in the fission-track system may be lower than 200°C.