Quantitative analyses of garnet-bearing epidote amphibolite (hereafter garnet amphibolite) from the Funaokayama unit, western Kii Peninsula of the Sanbagawa belt reveal a previously unrecognized high-grade part of the Sanbagawa metamorphism. Petrographic observations suggest that the garnet amphibolite underwent three stages of metamorphism (M1, M2 and M3). Records of M1 are preserved as syn-tectonic prograde-zoned garnet and its inclusions. Pseudosection modeling reproduces the observed M1 assemblage garnet + amphibolite + epidote + phengite + quartz and the growth zoning of garnet records the pressure (P)-temperature (T) evolution from 0.8 GPa, 570 °C to 1.3 GPa, 590 °C. Therefore, this garnet growth and associated deformation (D1) took place during subduction up to eclogite facies conditions. The garnet rim compositions and inclusions of epidote, titanite, rutile and quartz (5titanite + 2clinozoisite = 5rutile + 3grossular + 2quartz + H2O) give consistent peak-P estimates of 1.3-1.9 GPa. The garnet contains quartz inclusions retaining residual pressures of up to ∼ 0.7 GPa, which is as high as the values reported in well-characterized eclogite samples in central Shikoku. The inferred peak-P conditions in the eclogite facies is further supported by the occurrence of aragonite in associated pelitic schist. The assemblage hornblende + epidote + titanite + quartz + albite in the matrix and its equilibrium conditions of ∼ 0.8 GPa, 580 °C suggest decompression to the epidote-amphibolite facies (M2). The M2 minerals locally show replacement by the mineral assemblage actinolite + epidote + chlorite + titanite + calcite + quartz + albite, suggesting a partial re-equilibration in the greenschist facies (M3). M2 and M3 are synchronous with the main phase of ductile deformation during exhumation (D2). Despite the absence of the omphacite + quartz assemblage, it is likely that eclogite facies metamorphism in the Sanbagawa belt can be extended to western Kii Peninsula.
To investigate the chemical composition of possible primitive melts of the Martian mantle, we performed melting experiments of a model Martian mantle derived by Dreibus and Wänke (DWM) at pressures from 1.0 to 4.5 GPa. The chemical compositions of partial melts are systematically related to pressure. The partial melts at pressure of 1.0 GPa in spinel stability field show high Al2O3 and low FeO contents. The partial melts at higher pressure in garnet stability field are, however, characterized by a relatively high FeO content, low Al2O3 content and high CaO/Al2O3 ratio. In garnet stability field, clinopyroxene (= Ca-rich phase) contribute significantly to melt formation near the solidus temperature, although garnet (= Al-rich phase) is stable at temperature above solidus. Therefore Al-poor and Ca-rich partial melt are formed at higher pressure. Comparing the shergottite chemistry with the chemical trends of the partial melts obtained by the experiments, we suggest that one of basaltic shergottite with a high Al2O3 and low CaO/Al2O3 ratio, QUE94201, resembles the composition of the DWM partial melts in major-element chemistry in a low degree (<20%) partial melt of DWM at pressure of 1.0 GPa and temperature of 1360 °C. We conclude that the olivine-poor (or olivine-free) basaltic magma with low CaO/Al2O3 ratio and high Al2O3 could be primitive melts derived from the upper mantle of Mars if the actual Martian mantle is similar in composition to DWM.
Na- and Cr-rich amphibole and clinopyroxene are commonly observed in hydrous mantle xenoliths hosted by Pliocene-Quaternary basalts from Yemen. The amphibole is Cr-rich magnesiokatophorite high in Na2O (3.66-5.65 wt%) and Cr2O3 (2.21-3.09 wt%), relatively low in K2O (0.23-0.66 wt %) and almost free of Cl and F. The clinopyroxene is characterized by high Cr2O3 (up to 5.64 wt%) and Na2O (up to 2.97 wt%), indicating appreciable amounts of kosmochlor. Both minerals show high REE concentrations as well as HFSE depletion, suggesting an involvement of carbonatitic melt in their formation. The magnesiokatophorites and kosmochlor-bearing diopside possibly formed through metasomatism by hydrous Na-rich carbonatite melt. Magnesiokatophorite is an indicator of metasomatism by hydrous Na-rich carbonatite in the upper mantle.
Magnetite is a common accessory mineral in various rocks. Crystal shapes and habits of magnetite show diversity depending on crystallization conditions, especially cooling rate. Characteristic dendritic or skeletal magnetite crystals occur in quench rims of effusive rocks. The dendritic magnetite also occur in micrometeorites undergone quick heating and quenching at atmospheric entry. In this study, we constructed a fine particle free fall apparatus in a high temperature furnace to carry out crystallization experiments with controlled rapid heating and quenching. Experiments were carried out in a high-temperature vertical tube furnace with H2, CO2 and Ar mass flow controllers to control oxygen partial pressure and total gas flow rate. At the top of the furnace, a silica glass tube with an orifice with approximately 0.5 mm in diameter was set to keep falling rate of particles. Particles were retrieved in an alumina crucible at the bottom of the furnace tube. Terminal velocity of silicate particles with 100 μm in diameter in the static Ar gas at 1200 °C is approximately 0.18 m/s. Gas ascent rate at 1200 °C is approximately 0.11 m/s in the furnace tube when gas flow rate is approximately 1 l/min at standard condition. The falling velocity of the particles with 100 μm in diameter, therefore, is reduced to approximately 0.07 m/s. When the highest temperature in the furnace tube set to 1520 °C, the falling particles reach 1400 °C within 2 s, keep above 1400 °C more than 1 s, and are quenched within 1 s. For the fine particles with 100 μm in diameter, time scale of thermal equilibrium by radiation can be achieved within 0.1 s. In the experiments with volcanic ash particles, we found quite characteristic dendritic magnetite crystals in rapid-quenched spherules. From particles with high volume fraction of magnetite, we can see quite characteristic texture in which dendritic magnetite cover almost whole surface of the spherule. Magnetite dendrite crystals with particular crystallographic orientation also occur. The rapid quenching experiments for fine particles can be applied to reproduce atmospheric entry heating processes of micrometeorites.
Tourmaline in a complex type granitic pegmatite of an Li-Cs-Ta enriched (LCT) family from Nagatare pegmatite, Fukuoka Prefecture, Japan, were classified into five types: type-A (black), type-B (indigo), type-C (light blue-pink), type-D (pink), and type-E (pink-light blue), on the basis of localities, associated minerals, color, and texture. Type-C shows color zoning textures of a light blue core and a pink rim. In contrast, type-E has a pink core and a light blue rim. A total of 29 samples were analyzed by an electron microprobe analyzer (EPMA) and an X-ray diffractometer (XRD). Significant substitution of (Li + Al) for Fe is observed at the Y sites. The minor cations at the Y site, such as Mg, Mn, Zn and Ti, were also characteristic on each type. Large amounts of Na were detected at the X site in type-B; the Na contents decreased with increasing (Li + Al) in types-C, D, and E. F contents were well correlated with X-site charge except in type-A. The mineral species from the Nagatare pegmatite were schorl, fluor-schorl, elbaite, fluor-elbaite, and rossmanite. The unit-cell parameters were in the range of rossmanite, elbaite, and schorl. The tourmaline from the Nagatare pegmatite has similar major chemical components but different minor elements such as Zn and Mn as compared to the tourmaline from other LCT pegmatites. The intermediate fractionated tourmaline containing Zn (0.00-0.20 apfu) and Mn (0.11-0.38 apfu) is characteristic of the Nagatare pegmatite. In type-E, the rim showed greater Fe and Mn contents than the core, differing from the trend of the melt development on types-B and C. Furthermore, the parts rich in Al, Li, and OH with considerable site vacancy were selectively replaced by muscovite. These features indicate that several stages of alterations occurred during the late stages of the pegmatite formation.