Tatsuhiko Kawamoto

Interlayered peridotites and mafic rocks in orogenic peridotite bodies provide evidence for melt–mantle interactions. A 1.2 m-thick interlayered spinel peridotite (SP), plagioclase lherzolite (PL), and mafic rocks is studied in the Horoman peridotite complex, Japan. Major and trace element compositions of whole-rock and minerals provide constraints on the mechanisms responsible for such interlayered structure. The SP and the mafic rocks have the highest and lowest MgO contents, and have the lowest and highest incompatible element contents, respectively. The PL is characterized by millimeter-scale alternating laminae of plagioclase-free and plagioclase-clinopyroxene (Cpx)-bearing peridotite. Its composition is intermediate between that of SP and mafic rocks. The whole-rock and mineral compositions of the mafic rocks indicate that they formed from N-MORB-like melts. A linear compositional trend of major elements except for FeO*, MnO, P2O5and Cr on MgO variation diagrams indicate that various proportions of N-MORB-like melts were added to a massive peridotite to form each layer. The FeO* versus MgO diagram shows that the composition varies along iso-Mg# (100 × molar Mg/(Mg + Fe)) lines, which shows olivine (Ol) crystallization from the injected melts in the open system. This Ol accumulation has been suggested to elucidate whole-rock FeO*–MgO variations in abyssal peridotite. At boundaries between SP and mafic rocks, coarse Cpx crystals exhibit Ti-enriched rims, which provides evidence of chemical disequilibrium during initial melt–mantle interaction. As mantle diapirs upwell beneath mid-oceanic ridges, they experience repeated injections of melts with variable volumes and react with them. Chemical gradation in mafic rock layers from boundaries against SP is observed in Mg# of mafic minerals in a few centimeter-scale. Diffusive equilibration between the mafic rocks layer and the SP with respect to Mg–Fe equilibrium can be achieved only in several millimeter-scale. The PL layers formed through such millimeter-scale equilibrium between the peridotite and the melts with respect to Mg–Fe exchange; in contrast, plagioclase and Cpx components were not smeared out and the interlayered structure remained. The observed chemically interlayered structure in the Horoman peridotite records melt injection and subsequent Ol accumulation processes underneath a mid-oceanic ridge.

Samples of the carbonaceous asteroid Ryugu were brought to Earth by the Hayabusa2 spacecraft. We analyzed 17 Ryugu samples measuring 1 to 8 millimeters. Carbon dioxide–bearing water inclusions are present within a pyrrhotite crystal, indicating that Ryugu’s parent asteroid formed in the outer Solar System. The samples contain low abundances of materials that formed at high temperatures, such as chondrules and calcium- and aluminum-rich inclusions. The samples are rich in phyllosilicates and carbonates, which formed through aqueous alteration reactions at low temperature, high pH, and water/rock ratios of <1 (by mass). Less altered fragments contain olivine, pyroxene, amorphous silicates, calcite, and phosphide. Numerical simulations, based on the mineralogical and physical properties of the samples, indicate that Ryugu’s parent body formed ~2 million years after the beginning of Solar System formation.

Deep tectonic tremors occur in the Nankai subduction zone, defining a belt-like zone with a width of a few ten km located at depths between 30 and 55 km along upper surface of the subducting Philippine sea slab. We interpret the geometry of the tremor belt based on temperature calculations. Time evolution of temperature is calculated using a 3-D heat conduction assuming constant geometry of the slab and the present-day plate convergence velocity. The results show that the tremors occur along the 450°C isotherm along the slab surface where the mantle wedge can be non-convective and already well-serpentinized. Two gaps of the tremor belts are explained by different mechanisms. The Ise gap is where the hanging wall is not mantle wedge but crust. The Kii Gap is not a gap orthogonal to the plate convergence direction, but only appears to be a gap because the isotherm of the plate surface steps in the plate convergence direction. The tremors can be caused by high pore-fluid pressure conditions due to aqueous fluids released by dehydration reactions in blueschist-facies oceanic crust to form eclogite facies. The depth of the tremor belt corresponds to that of decoupled, non-convective mantle wedge. Since the temperature and pressure are within the serpentinite stable condition, the released fluid would normally be absorbed by serpentinization of the mantle wedge peridotite. However, since the non-convective mantle wedge is already well serpentinized from exposure to previously released fluids, the newly released fluid is not absorbed and increases the pore pressure.

I started my research as a petrology student supervised by Shohei Banno and Yoshiyuki Tatsumi at Kyoto University. I described every phenocryst in a single thin section to explain enigmatic plagioclase morphology and obtained PhD by discussion of processes in chemically-zoned magma chamber. I started high-pressure and high-temperature (HPHT) experiments as a postdoc at Ikuo Kushiro's lab at Tokyo, where I conducted partial melting experiments of hydrous mantle peridotite with Kei Hirose and duplicated andesite-dacite-rhyolite magmas by crystal fractionation of hydrous arc basalts. Then I joined the Depths of the Earth lead by John Holloway at Arizona State University (USA) and became the first Japanese who learned how to use multi-anvil type HPHT apparatus in the States. I proposed a choke point of subducting hydrous minerals, hydrous mantle transition and generation of komatiite and kimberlite magmas. After I learned Bassett-type diamond anvil cell from Helene Bureau, Nikolay Zotov, and Hans Keppler at Bayerisches Geoinstitut (Germany), I moved back to Kyoto University. With Kenji Mibe, Masami Kanzaki, Shigeaki Ono, and Kyoko Matsukage, I determined critical endpoints between various magmas and aqueous fluids by use of X-ray radiography and suggested new hypothesis for subduction zone magmatism. I have found seawater-like saline fluid inclusions in mantle xenoliths beneath Pinatubo and others, proposing the importance of being salty in subduction zone fluids.

Jadeite–quartz rocks from the Yorii area, the Kanto Mountains, central Japan occur as tectonic blocks within serpentinite mélange. Primary two–phase (liquid + vapor) aqueous fluid inclusions are observed in jadeite, quartz, albite and zircon. The fluids trapped in jadeite and quartz are H2O with and without CH4 and show a wide range of salinity (4.7 ± 1.1 wt% NaCleqv in jadeite; 3.4 ± 0.7 and 13.6 ± 2.0 wt% NaCleqv in quartz). In this study, we examined the chemical composition of fluid inclusions in quartz from jadeite–quartz rock to understand the characteristics of fluid, which related to the formation of the rocks in the subduction zone. The trace element concentrations of fluid inclusions determined with LA–ICP–MS are enriched in LILE, Li, B, HFSE and transition metals with LILE enriched and relatively HFSE depleted characteristics. These have elemental patterns similar to the HFSE–depleted fluid released from antigorite during subduction, though the orders of magnitude are different. It can be common chemical characteristics of fluid in a subduction zone. There are two possibilities of the chemical composition of the evolved fluid after the formation of jadeite–quartz rocks. HFSE from the fluid can be sequestered into jadeite–quartz rock prior to fluids moving up into a mantle wedge. This process lead the evolved fluids relatively depleted in HFSE and enriched in LILE. Other possibility is that the fluid has same chemical composition of fluid inclusion as the fluid is equilibrated with jadeite–quartz rocks during the jadeite–quartz formation. In both cases, fluid after the metasomatic formation of jadeite–quartz rocks can be relatively enriched in LILE and depleted in HFSE. Magmas generated in subduction zones exhibit distinctive geochemical features of LILE enrichment and HFSE depletion and this characteristic of arc magma can be explained by the behavior of HFSE during the formation of jadeite–quartz rocks.