In–situ high–temperature Raman spectra of tridymite starting from MC modification were measured up to 500 °C. Low frequency Raman spectra (ν = 15–100 cm−1) of the high–temperature modifications (OP, OS, OC, and HP) were reported for the first time. It was noted that there are significant changes in the low frequency region upon the transitions, and those transition temperatures are consistent with those reported in previous studies except OC/HP transition. The OC/HP transition was detected at around 470 °C. This temperature is about 70 °C higher than the transition temperature determined by previous X–ray diffraction studies. The disagreement on the transition temperature is discussed.
The Miocene Kaikomagatake pluton is one of the Neogene granitoid plutons exposed at the northern end of the Akaishi Range of the Izu collision zone, where the Izu–Bonin oceanic arc is colliding against the Honshu arc. The pluton intrudes discordantly into the Shimanto accretionary complex of the Honshu arc along the Itoigawa–Shizuoka Tectonic Line that marks the collision boundary. We applied Al–in–hornblende geobarometers to constrain the emplacement depth of the Kaikomagatake pluton. A recently proposed geobarometer suitable for relatively shallow granitoid magmas yielded 2.4–2.2 kbar at temperatures close to the water–saturated granite solidus, which corresponds to upper to middle crustal depths (~ 9–8 km). Using previously reported thermochronological data, we estimated the post–emplacement cooling rate at ~ 66–156 °C/m.y. for the pluton. The estimated cooling rate is lower than that reported for other granitoid plutons in the accreted Izu–Bonin arc, such as the Tanzawa plutonic complex and the Tsuburai pluton. The early stage of the collision between the Izu–Bonin and the Honshu arcs contributed little to denudation of the Honshu arc crust at the Kaikomagatake pluton area.
We investigated the structure changes and phase transformation from nanocrystalline mackinawite to pyrite using hydrothermal experiments, synchrotron X–ray diffraction (XRD) technique, atomic Pair Distribution Function (PDF) method, Extended X–ray Absorption Fine Structure (EXAFS) analysis, and transmission electron microscopic (TEM) observation. The first hydrothermal ageing experiment was performed by heating the nanocrystalline mackinawite at 120 °C for 12 h. The nanocrystalline mackinawite remained essentially unchanged for 12 h. The d001 and FWHM values of XRD peaks decreased for the first 2 h and subsequently maintained almost constant. There was no linear relationship between lattice parameters and hydrothermal heating time. The crystallite size quickly increased by the heating of 2 h, leading to the increase of crystallinity and appearance of the medium–range order in the nanocrystalline mackinawite. The nanocrystalline mackinawite preferentially grew in the horizontal direction along the sheet structure. The Fe atoms were distributed in the tetrahedral sites with a site occupancy of approximately 80%. The pre–edge peak energy of Fe K–edge suggested that about 10% Fe3+ was included in the nanocrystalline mackinawite to compensate the charge deficiency of Fe2+. The second hydrothermal ageing experiment was performed by heating the nanocrystalline mackinawite at 120 °C under the presence of elemental sulfur for 24 h. The nanocrystalline mackinawite persisted up to 8 h of heating time. Thereafter, pyrite and greigite instead of the nanocrystalline mackinawite appeared. Finally pyrite became dominant. The d001 and lattice parameters of nanocrystalline mackinawite varied significantly compared with those heated under the absence of elemental sulfur. The pre–edge peak energy indicated that the Fe2+ was oxidized to Fe3+ by elemental sulfur acting as the oxidant during the phase transformation from nanocrystalline mackinawite to greigite. In the phase transformation to pyrite, on the other hand, the Fe3+ was reduced to Fe2+ by sulfur in mackinawite and greigite acting as the reductant. The EXAFS analysis revealed that the second peak from the Fe–Fe interaction appeared at the heating time of 2 h, implying the formation of sheet structure consisting of edge–sharing FeS4 tetrahedra. Intensity of the second shell peak from the Fe–Fe interaction reduced after the heating time of 8 h. Instead, new peaks corresponding to the Fe–S and Fe–Fe interaction appeared after the heating time of 12 h. This result was strongly associated with formation of the disulfide bonds (S–S bonds) in pyrite. Consequently, the elemental sulfur can be recognized as one of the most important factors to promote the phase transformation from mackinawite to pyrite in the reducing lake and marine sediments.
We used high–resolution X–ray computed tomography (HRXCT) combined with scanning electron microscopy (SEM) to obtain 3D and 2D images of multiphase solid inclusions within chromite from the Samail ophiolite to investigate post–entrapment modification of the inclusions. Results indicate that the parental melt of the chromitite was supersaturated in chromian spinel. Chromite continued to crystallize on the inner wall of the host chromite after the melt was trapped. Rapid growth caused crystallization of high–Cr#[= Cr/(Cr + Al)] chromite lining around inclusions. The necking–down of originally large melt inclusions probably produced various assemblages of daughter minerals among the inclusions. We report two observations that are consistent with rapid growth of the host chromite: the 3D distribution of inclusions in host chromite and the host chromite showing skeletal morphology. High–temperature homogenization experiment was conducted to obtain the parental melt composition of the inclusions. We found that the homogenized glass does not represent the parental melt trapped in the host chromite because of the remaining of high–Cr# chromite lining and possible residual phases in the experiments.
The genesis of CLIPPIR diamonds (Cullinan–like, large, inclusion–poor, pure, irregular, and resorbed) have attracted much interest due to their possible crystallization from metal melt in deep horizons of the earth’s mantle. These diamonds usually show a pronounced resorption and irregular morphology. The present paper reports new experimental data on the dissolution of diamond crystals at high P–T parameters in Fe–S melt containing large amounts of silicate components (5–20 wt%). The experiments were performed using a split–sphere multi–anvil apparatus (BARS) at a pressure of 4 GPa and a temperature of 1450 °C. The samples consisted of natural diamond crystals placed in mixtures of Fe, S, and kimberlite. Wide variations in dissolution rates of diamond crystals were obtained. The absence of diamond dissolution in a heterogeneous medium indicates that the amount of solid silicate phases present in metal melt plays a role in the preservation of diamonds. This study demonstrated how diamonds can be stored in natural environments due to the heterogeneity of the medium composition which could insulate diamonds from the metal–sulphide melt. The obtained results improve our understanding of processes that lead to preservation of CLIPPIR diamonds in the deep mantle.