The interaction between Fe–metal and magnesite was studied in multianvil experiments at 6 GPa and 1273–1873 K using different capsule materials: Fe, BN, and MgO. It was observed that at subsolidus conditions reaction proceeds with the formation of Fe3C and magnesiowüstite in the stoichiometric proportions according to relation: MgCO3 + 5Fe = 3(Fe0.66Mg0.33)O + Fe3C. At melting conditions (1673–1873 K) magnesite and iron react in nearly equivalent molar proportions with formation of Mg–Fe–carbonatite melt, Fe–C alloy, magnesiowüstite and graphite. The reactions clearly show that free carbon and metallic iron phases cannot coexist in the upper mantle and presumably in transition zone and will always form Fe–carbide. The carbon content in Fe–C alloy and its coexistence with diamond will be strongly dependent on the oxygen fugacity. The studied reactions can be considered as intermediate processes in the reduced mantle domains at the contact with submerging subduction slabs and have further implication to the processes at the core–mantle boundary.
The local structure around Ge in xLi2O–(1 － x)GeO2 (x = 0, 0.17, 0.20, 0.24) glasses was investigated using AXS and EXAFS measurements. The averaged coordination number of the first nearest Ge–O pair increased with increasing Li2O content up to 24 mol% and the amount of six–coordinated Ge was proportional to Li2O content. The introduction of a GeO6 unit is suggested to be one of the most fundamental structural changes accompanying the so–called germanate anomaly detected in density measurements of alkali–germanate glasses.
The trace element and Sr–Nd isotopic compositions of Quaternary magmas from the Pre–Komitake volcano were investigated. The Sr and Nd isotope ratios ranged from 0.703320–0.703476, and 0.512885–0.513087, respectively, which are very similar to those of the lavas from Fuji and Komitake volcanoes that erupted subsequently. Enrichment of large ion lithophile elements, Pb and Sr, can be seen in the primitive mantle–normalized multi–element diagram of the Pre–Komitake, Komitake, and Fuji lavas. These collectively show island arc lava signatures; however, the middle to heavy rare earth elements are more depleted in the Pre–Komitake lavas, compared to those from Fuji. Positive Eu anomalies are observed, although the extents of these anomalies decrease with increasing SiO2 in the Pre–Komitake lavas, whereas this is not observed in Fuji lavas. The Sr/Y ratios of Pre–Komitake lavas increase from basalt to basaltic andesite, but decreases through andesite to dacite. This occurs in combination with a rapid increase in La/Yb ratios, followed by a more gradual increase. A gradual decrease in Dy/Yb ratios is also seen over the entire compositional range. These data suggest deep (>12 kbar) fractionation of garnet and amphibole followed by shallow (i.e., ~ 5 kbar) fractionation of amphibole and plagioclase. Such variations are not observed in the Komitake and Fuji lavas, for which deep fractionation of clinopyroxene and shallow fractionation of plagioclase have been suggested. All three lavas, including those from the Pre–Komitake volcano, show similar isotopic, major, and trace element compositions in the unfractionated basalts. The differing geochemical trends found in the Pre–Komitake lavas are likely to be due to different mineral fractionations occurring in the hydrous Pre–Komitake basalts compared to the dry Fuji and Komitake basalts.
Paragonite–clinozoisite associated within garnets (pyrope2–7 almandine60–64 grossular10–28 spesartine1–10) was newly found in the type locality of the Sanbagawa schist in the Kanto Mountains, Japan. The composite inclusions were confirmed in prograde–zoned garnet porphyroblasts in garnet–zone metapelites that have a typical pelitic whole–rock composition (Mn/Fe = 0.019, Mg/Fe = 0.381). The garnets also contain mineral inclusions of quartz, albite, phengite, chlorite, rutile, calcite, apatite, and zircon. Phengite in the metapelite yielded a K–Ar age of 67.3 ± 1.6 Ma as the younger part of the Sanbagawa schist in this region. This paragonite–clinozoisite association provides a mineralogical index of high pressure (P)–temperature (T) metamorphism of the type locality of the Sanbagawa schist. Moreover, the clinozoisite–paragonite composite inclusions within prograde–zoned garnets suggest a prograde P–T path from the stability of lawsonite and albite.
Crystallization of ruby requires excess Al and appreciable amounts of Cr in the system. A ruby–bearing feldspathic dike crosscuts dunite in the Ray–Iz massif, the Polar Urals, and the dominant mineral of the dike changes from plagioclase at the center to amphibole outward. Ruby has been observed in between, and the zone is composed of plagioclase and phlogopite with minor chromian spinel and ruby as primary phases, and paragonite as a secondary phase. The Cr2O3 content of the ruby is <7.5 wt% and close to the values of those found in serpentinite and chromitite from other localities. The petrographical and highly LREE–enriched and HREE–depleted features of the ruby–bearing rock imply a metasomatic origin for the dike through the interaction between feldspathic component–rich aqueous fluid and wall rock dunite with chromitite. Based on the primary occurrence of plagioclase, it is inferred that the fluid infiltration possibly occurred at 1.0–1.5 GPa, and the fluid interacts with peridotite. The lithological change of the dike indicates effective consumption of Si, Ca, and K and assimilation of Cr and Mg in the fluid at the contact with the wall–rock dunite, and the fluid composition could have evolved to be peraluminous through the interaction. Chromium is effectively transported by aqueous fluid with some anions, e.g., Cl−, CO32−, and SO32−, and the interaction of peridotite as a source of Cr with such fluids is one of the important formation processes of ruby within the mantle wedge where fluids are available from the downgoing slab.
This study examines the effect of jadeite content (Xjd) in omphacite on Fe–Mg distribution coefficient between garnet and clinopyroxene (KD) by using natural data obtained from ultrahigh–pressure eclogites of the Sulu region, China. A previous study has already pointed out that a negative correlation between lnKD and Xjd and a positive correlation between Fe# [= Fe2+/(Fe2+ + Mg)] of omphacite and Xjd are present. On the other hand, this study newly recognized that KD decreases with decreasing Ca + Na + K contents in M2 site of omphacite, indicating that excess Fe2+ may be incorporated into M2 site of omphacite with decreasing Ca + Na + K contents and hence total Fe# values of omphacite may become excessively high. Thus, application of garnet–clinopyroxene geothermometer for omphacite with significant amount of jadeite and/or ferrosilite (and enstatite) components may overestimate the peak metamorphic temperatures in some cases. As a first approximation, it is better to avoid the use of omphacite with Ca + Na + K contents significantly lower than 1.0 a.p.f.u. for the application of garnet–clinopyroxene geothermometer.
The local structure of K–T boundary clays was studied by Zr K–edge X–ray absorption fine structure (XAFS) in order to provide quantitative data on Zr–O bonding distance, coordination number and oxidation state. The Zr XANES spectra in K–T boundary clays are similar to those from rhyolitic volcanic glass samples (obsidian and pitch stone) and tektite. Since tektite was formed at meteorite impact, those observations suggest that the thermal quenching history at meteorite impact is kept preserved as the local structure of Zr in K–T clays. The Zr–O distance in K–T boundary clays is 2.164 Å (4) and the average coordination number is 6.3. The threshold energy on Zr XANES spectra from the K–T clays is lower than those found on natural glasses, and total effective charge of zirconium ion (formally 4+) estimated from the shift of the spectrum is 3.8+ – 3.9+. The partial reduction of zirconium ion in K–T clays is ascribed to its formation environment, high temperature under reductive environment. Preservation of the partial reduction state also indicates quick quench of K–T clays at their formation, and this observation give as a clue to distinguish sedimentary rocks deposited at periods of so–called extinction events by investigating Zr local structure.
Stratlingite is one of the constituent minerals of the bonding matrix of low–cement castables. In this study, the crystallization processes of stratlingite in hydrates of high alumina cement at 60 °C and 10 °C were analyzed using X–ray diffraction (XRD) and electron probe microanalyzer (EPMA). The stratlingite has crystallized in large quantities in the samples cured at 60 °C, while in the samples cured at 10 °C, stratlingite was not detected beyond the period of fifteen months.