Pyrite (FeS2) is a promising reducing agent that can be used to remove atmospheric oxygen in repositories for the geological disposal of high-level radioactive waste. In order to estimate the rate of pyrite oxidation in a repository environment, we conducted experiments using dissolved oxygen (DO) in alkaline solutions at 298 K. Pyrite samples were treated with alkaline solutions (pH adjusted to ∼ 9 and ∼ 10); these solutions were then saturated with air and adjusted to various ionic strengths (IS). The DO, total Fe, and SO42- concentrations in the experimental solutions were measured at regular intervals along with the pH. The rate constant for pyrite oxidation in solution was approximately 1/20 that determined in previous studies on pyrite oxidation in pyrite-containing compacted purified sodium bentonite.
The crystal structure of a unique type of vesuvianite with the space group P4/n [a = 15.576(2), c = 11.835(2) Å] formed by the retrograde alteration of gehlenite in high-temperature skarn at Kushiro (Tojyo, Shobara Hiroshima) southwestern Japan, is determined by single-crystal X-ray diffraction (R1 = 3.63%). Significant vacancies (29%) occur only on one of the two pseudo-equivalent Z1 sites (Z1b). In addition, the Si-O distance at the Z1b site remarkably increases up to 1.707(3) Å, indicating the existence of a hydrogarnet-like substitution, while Z1a, the other pseudo-equivalent Si site, has only 5% vacancies associated with an Si-O distance of 1.636(3) Å. The total refined Si site occupancy of 17.7 atoms per formula unit is in excellent agreement with the results of electron-microprobe analyses.
The Makbal Formation of the Northern Kyrgyz Tien-Shan consists mainly of phengite-quartz schists (meta-quartzite), garnet-chloritoid-talc schists and marbles, all of which contain lenses of amphibolites and eclogites. The garnet-chloritoid-talc schists from the Makbal complex underwent UHP metamorphism. The mineral assemblage of the UHP stage is garnet + chloritoid + talc + phengite + rutile + coesite ± glaucophane ± florencite ± monazite ± apatite. Electron microprobe analyses of monazites from the garnet-chloritoid-talc schists provide two similar CHIME ages. Monazites occurring in the matrix of the schists yield an age of 481 ± 26 Ma, and monazite inclusions in porphyroblastic garnets yield an age of 480 ± 56 Ma. These two ages are essentially the same within error, and suggest the peak UHP metamorphism in the Makbal area of the Northern Kyrgyz Tien-Shan occurred at ∼ 480 Ma.
Hakusan volcanic rocks younger than the major dormant period ∼ 0.3-0.1 Ma are adakitic, while those older than the dormant period have geochemical affinities with common island arc andesite, dacite, and rhyolite that are characterized by low Sr/Y ratios. The younger lavas, not necessarily adakitic in origin, became adakitic by magmatic mixing with magmatic enclaves (MEs), which are richer in Sr to varying extents and less silicic than the host lavas. We suggest that the younger MEs were derived from adakite magmas that interacted with the upper mantle material during their ascent to the surface. The Sr content in MEs in the older lavas is as low as that in the host lavas. The process of magma generation beneath the Hakusan volcano is likely to have changed between 0.3 Ma and 0.1 Ma, from one triggered by the dehydration of the subducted Pacific plate to one caused by the partial melting of the Philippine Sea plate.
In order to understand whether Ce in rare earth (RE) phosphate minerals can be used as a proxy for the estimation of atmospheric oxygen evolution in the Precambrian, we examined the formation mechanisms of RE phosphate minerals in 2.45-Ga paleosol (ancient soil formed by weathering), Pronto, Canada. The RE phosphate minerals were observed and analyzed by scanning electron microscopy with energy-dispersive X-ray spectroscopy and electron back-scattered diffraction analysis, and by transmission electron microscopy. The textures and chemistries of RE phosphate minerals have revealed that monazite-[light RE elements (REE)] and xenotime-(Y) at the rims of apatite and pores are secondary products. Monazite-(light REE) and xenotime-(Y) were found to have been formed by either of two mechanisms: the direct formation during metasomatism occurring after weathering or the formation of rhabdophane containing light REE and Y during weathering and its subsequent decomposition into monazite-(light REE) and xenotime-(Y) during metasomatism. If the latter was the actual reaction pathway, the Ce contents in the secondary monazite suggest a very low atmospheric oxygen level at ∼ 2.45 Ga.
Queen Alexandra Range 97990 (QUE97990) and Yamato 791198 (Y791198) are among the least aqueously altered CM carbonaceous chondrites known to date. Our mineralogical and petrographic study reveals that chondrule mesostases in QUE97990 contain dense arrays of parallel, thin lath-shaped crystallites of diopside. The diopside crystallites are primary quenched products formed during chondrule formation. Thus their presence suggests that QUE97990 preserves a very early state of aqueous alteration. On the other hand, chondrule mesostases in Y791198 consist largely of serpentine without such quenched crystallites. In Y791198, quenched crystallites have probably been replaced by serpentine due to a higher degree of alteration than in QUE97990. Our study also reveals that chondrules in both QUE97990 and Y791198 show topographic depressions on their surfaces. The texture and mineralogy of the depressions suggest that they formed by replacing opaque nodules in their host chondrules during aqueous alteration. These imply that, in spite of the evidence of incipient alteration in the chondrule interiors, there was considerable alteration reaction between the chondrule margins and their surrounding rims in QUE97990. We suggest that the aqueous alteration in both meteorites occurred after the chondrules acquired their own rims on the meteorite parent bodies.
Hornblende peridotite xenolith in the spessartite dike in the Tamba belt, northern Kyoto city, is mainly composed of olivine with high Fo and NiO contents (Fo87−94 and 0.22-0.51 wt%). The olivine encloses chromian spinels with a high Cr/(Cr + Al) atomic ratio, 0.66-0.80, partly replaced and digested by hornblendes with a low Mg/(Mg + total Fe) atomic ratio, 0.68-0.81. Microscopic and geochemical features of the hornblende peridotite xenolith indicate a reaction between a dunitic cumulate from a high-Mg and -Cr melt and an evolved hydrous high-Mg melt to form hornblendes. Olivines are lower in Fo when in contact with hornblendes. Rare earth element (REE) patterns of the hornblendes, common to the xenolith and host spessartite, indicate that all the involved melts are similar but different in the degree of differentiation. The strong hydration of olivine in the xenolith suggests a wide distribution of hornblende-rich rocks, such as hornblendites, formed by the hydration of olivine-rich rocks, such as dunites, around the Moho beneath the arc. This may give rise to a layer with a Vp intermediate between typical lower crustal and mantle values.
Time-resolved, in situ X-ray diffraction analysis was used to determine the dehydration rate and kinetics of serpentine during experimental dehydration at high pressures. The capsule used comprises a diamond sleeve fitted with Au or Pt lids in order to provide high-quality, time-resolved X-ray diffraction data. Antigorite quickly dehydrated to enstatite + forsterite + fluid within 2 h at 650 °C below ∼ 6 GPa. Avrami modeling of the results and SEM observations of the partially dehydrated sample revealed that the nucleation rate was quite high for enstatite but low for forsterite, showing incubation periods of ∼ 10 min before appearing. The crystallization of these minerals is controlled largely by the composition of the fluid generated from serpentine dehydration. The dehydration boundary determined below 6 GPa in the present study is consistent with the results of previous phase equilibrium studies. This study indicates that serpentine in a subducting slab dehydrates rapidly below 6 GPa when the slab intersects the dehydration boundary conditions.
The Heilongjiang Complex in northeastern China is composed mainly of alternating blueschists and pelitic schists which underwent high-P/T type metamorphism of the epidote blueschist facies. Phengites in the pelitic schists from two areas in the Heilongjiang Complex yielded 40Ar/39Ar plateau ages of 165 ± 0.8 Ma (Mudanjiang area) and 171 ± 0.7 Ma (Yilan area). Phengites in blueschists from the Yilan area give 40Ar/39Ar isochron ages of 145 ± 0.9 Ma and 146 ± 1.3 Ma. The previously reported peak metamorphic temperature of 350-450 °C for the Heilongjiang Complex is close to the Ar retention temperatures of white mica, suggesting that the ages reported here closely date the metamorphic culmination in this complex. These ages indicate that a widespread high-P/T metamorphism occurred in the Heilongjiang Complex at 165-171 Ma. Subsequent to this period, a similar type of metamorphism continued at least until 145-146 Ma in the Yilian area.