Tatsuhiko Kawamoto

Hydrous partial melting of lherzolite at 1 GPa: The effect of H2O on the genesis of basaltic magmas

Partial melting of a natural lherzolite (KLB-1) under H2O-undersaturated conditions was experimentally investigated at 1 GPa over the temperature range from 1100 to 1350°C. The effect of H2O added to the dry lherzolite ranged from 0.1 to 0.9 wt% of the melt composition and the extent of melting was quantitatively estimated by comparison with the results of anhydrous melting experiments on the same lherzolite [1]. With the new capsule configuration presented here, partial melt layers of more than 30 μm in width formed along the inner capsule wall. The composition of quenched glass could successfully be obtained by direct microprobe analyses of these melt layers unmodified during quenching. The degree of partial melting was estimated from mass balance calculations using sodium concentrations. Au75Pd25 and Ag70Pd30 tubes were used as sample containers to minimize iron loss. SIMS analyses of hydrogen in quenched glass showed that loss of hydrogen through the Au75Pd25 capsule was minimal during the experiment. A small amount of H2O in the lherzolite lowers the solidus temperature from slightly below 1250°C to less than 1100°C and significantly increases the extent of melting. The experimental results quantitatively show the increase in extent of melting as a linear function of H2O content in the partial melt above 1200°C. Partial melts with less than 2.5 wt% H2O formed at temperatures above 1200°C are close in major element composition to the anhydrous melts formed by the same degree of partial melting. On the other hand, melts with more than 3 wt% H2O formed at 1100°C are clearly enriched in SiO2 and depleted in MgO compared to the anhydrous melts. The larger amounts of H2O in the mantle source in some hotspot regions such as the Azores platform along the Mid-Atlantic Ridge contribute to a higher extent of melting, resulting in relatively thick oceanic crust.

Changes in the structure of water deduced from the pressure dependence of the Raman OH frequency.

We report on the Raman spectra of water under high temperature and pressure conditions and show a discontinuity in the pressure dependence of the OH stretching frequency. As pressure increases, the strength of hydrogen bonding increases rapidly in the pressure ranges up to 0.4+/-0.1 GPa at 25 degrees C, 1.0+/-0.1 GPa at 100 degrees C, and 1.3+/-0.1 GPa at 300 degrees C and slowly above these pressures. This finding clearly demonstrates the existence of discontinuities in the pressure response of the hydrogen bonds of water, which suggests a possible structural change under these conditions.

Slab melting versus slab dehydration in subduction-zone magmatism.

The second critical endpoint in the basalt-H2O system was directly determined by a high-pressure and high-temperature X-ray radiography technique. We found that the second critical endpoint occurs at around 3.4 GPa and 770 °C (corresponding to a depth of approximately 100 km in a subducting slab), which is much shallower than the previously estimated conditions. Our results indicate that the melting temperature of the subducting oceanic crust can no longer be defined beyond this critical condition and that the fluid released from subducting oceanic crust at depths greater than 100 km under volcanic arcs is supercritical fluid rather than aqueous fluid and/or hydrous melts. The position of the second critical endpoint explains why there is a limitation to the slab depth at which adakitic magmas are produced, as well as the origin of across-arc geochemical variations of trace elements in volcanic rocks in subduction zones.

Dusty and honeycomb plagioclase: indicators of processes in the Uchino stratified magma chamber, Izu Peninsula, Japan

Abstract Two types of melt-inclusion-bearing plagioclase coexist in the Uchino basalt, Izu peninsula, Japan: (1) dusty plagioclase with an Ab-rich core mantled by a dusty zone consisting of more An-rich plagioclase and fine melt inclusions; and (2) honeycomb plagioclase with a central core containing larger melt inclusions. The dusty plagioclase probably forms through partial dissolution of Ab-rich plagioclase, as dusty zones truncate the concentric zonal structure of the Ab-rich inner core. The honeycomb plagioclase is formed by skeletal growth under supercooling conditions. This is indicated by the euhedral geometry of the boundary between the inner inclusion-free and the outer inclusion-bearing zones of the honeycomb plagioclase which has a concentric inner core. Bimodal distribution of the chemical compositions of phenocryst cores and assemblages in crystal clots suggest that hybridization of basaltic and andesitic magmas took place before crystallization of phenocryst rims. The core-rim boundaries of phenocrysts derived from the andesitic magma and the basaltic magma have a rounded and a euhedral geometry, respectively. This suggests the andesitic magma was heated and the basaltic magma was cooled prior to hybridization. These features are consistent with the development of a thermal diffusive interface between overlying andesitic magma and underlying basaltic magma in a stratified magma chamber.

Experimental constraints on differentiation and H2O abundance of calc-alkaline magmas

Partial melting experiments were conducted on a natural high-alumina basalt from the Higashi-Izu volcanoes (47.4 wt.% SiO2; 8.3 wt.% MgO) with 1 and 2 wt.% total H2O at 0.5 and 1 GPa, 1125-875°C. The partial melts produced segregate by capillary action into crimped parts of the capsule, preventing chemical modification of the melt during quenching. Liquid compositions in 1 wt.% total H2O runs at 0.5 GPa mimic the chemical variations of calc-alkaline andesites and dacites associated with the basalt. The residual phase assemblages from 1 wt.% total H2O experiments are consistent with the observed phenocrysts in two pyroxene andesite and hornblende (hb)-orthopyroxene (opx) dacite. In contrast, a clinopyroxene (cpx)-hb rhyolite assemblage is produced in the 2 wt.% total H2O experiments. Although no such cpx-hb rhyolite is found in either Northeast Japan or the Izu-Mariana arc, this type of rhyolite is found in Central America, which is also characterized by high H2O contents in the associated basaltic melts ([1]). The abundance of SiO2 in residual liquids increased from 49 to 54 wt.% in the 25°C interval between 1125° and 1100°C in the 1 wt.% total H2O system, and from 63 to 71 wt.% in the 50°C interval between 975° and 925°C. The first gap between basalt and andesite is generated by a sudden change in the amount of partial melt, and the second gap between andesite and dacite is produced by the formation of hornblende by the consumption of pyroxene, because the SiO2 abundance of hornblende is less than that of pyroxene. These two reaction relations result in the existence of two compositional gaps, between basalt and andesite, and between andesite and dacite, which are recognized in the Higashi-Izu volcano group and many other calc-alkaline volcanoes.

Mantle wedge infiltrated with saline fluids from dehydration and decarbonation of subducting slab

Slab-derived fluids play an important role in heat and material transfer in subduction zones. Dehydration and decarbonation reactions of minerals in the subducting slab have been investigated using phase equilibria and modeling of fluid flow. Nevertheless, direct observations of the fluid chemistry and pressure–temperature conditions of fluids are few. This report describes CO2-bearing saline fluid inclusions in spinel-harzburgite xenoliths collected from the 1991 Pinatubo pumice deposits. The fluid inclusions are filled with saline solutions with 5.1 ± 1.0% (wt) NaCl-equivalent magnesite crystals, CO2-bearing vapor bubbles, and a talc and/or chrysotile layer on the walls. The xenoliths contain tremolite amphibole, which is stable in temperatures lower than 830 °C at the uppermost mantle. The Pinatubo volcano is located at the volcanic front of the Luzon arc associated with subduction of warm oceanic plate. The present observation suggests hydration of forearc mantle and the uppermost mantle by slab-derived CO2-bearing saline fluids. Dehydration and decarbonation take place, and seawater-like saline fluids migrate from the subducting slab to the mantle wedge. The presence of saline fluids is important because they can dissolve more metals than pure H2O and affect the chemical evolution of the mantle wedge.

Separation of supercritical slab-fluids to form aqueous fluid and melt components in subduction zone magmatism

Subduction-zone magmatism is triggered by the addition of H2O-rich slab-derived components: aqueous fluid, hydrous partial melts, or supercritical fluids from the subducting slab. Geochemical analyses of island arc basalts suggest two slab-derived signatures of a melt and a fluid. These two liquids unite to a supercritical fluid under pressure and temperature conditions beyond a critical endpoint. We ascertain critical endpoints between aqueous fluids and sediment or high-Mg andesite (HMA) melts located, respectively, at 83-km and 92-km depths by using an in situ observation technique. These depths are within the mantle wedge underlying volcanic fronts, which are formed 90 to 200 km above subducting slabs. These data suggest that sediment-derived supercritical fluids, which are fed to the mantle wedge from the subducting slab, react with mantle peridotite to form HMA supercritical fluids. Such HMA supercritical fluids separate into aqueous fluids and HMA melts at 92 km depth during ascent. The aqueous fluids are fluxed into the asthenospheric mantle to form arc basalts, which are locally associated with HMAs in hot subduction zones. The separated HMA melts retain their composition in limited equilibrium with the surrounding mantle. Alternatively, they equilibrate with the surrounding mantle and change the major element chemistry to basaltic composition. However, trace element signatures of sediment-derived supercritical fluids remain more in the melt-derived magma than in the fluid-induced magma, which inherits only fluid-mobile elements from the sediment-derived supercritical fluids. Separation of slab-derived supercritical fluids into melts and aqueous fluids can elucidate the two slab-derived components observed in subduction zone magma chemistry.

Mg/Si ratios of aqueous fluids coexisting with forsterite and enstatite based on the phase relations in the Mg2SiO4-SiO2-H2O system

Abstract Direct observation of aqueous fluids coexisting with MgSiO3 (enstatite) and/or Mg2SiO4 (forsterite) was performed at 0.5-5.8 GPa and 800-1000 °C with an externally heated diamond-anvil cell and synchrotron X-rays. At 1000 °C in the MgSiO3 -H2O system, forsterite crystallizes below 3 GPa but not above that pressure. At 1000 °C in the Mg2SiO4 -H2O system, forsterite congruently dissolves into the aqueous fluids up to 5 GPa. These observations suggest that the aqueous fluids coexisting with enstatite and forsterite have Mg/Si < 1 below 3 GPa and 1 < Mg/Si < 2 above that pressure. Comparison with the previous studies reporting Mg/Si ratios of the aqueous fluid coexisting with enstatite and forsterite indicates that the Mg/Si ratios change rapidly from SiO2-rich to MgO-rich at around 3 GPa and 1000 °C. This change can be related to possible structural changes of liquid water under these conditions. The aqueous fluids coexisting with enstatite and forsterite do have Mg/Si ratios similar to those found in the partial melts of H2O-saturated peridotite. Somewhere within the upper mantle, these two fluids unite to form a single regime and cannot be distinguished from each other.

Raman spectroscopy of cubic boron nitride under high temperature and pressure conditions: A new optical pressure marker

The pressure dependence of Raman peaks of cubic boron nitride (cBN) is determined at 100, 200 and 300 °C using pressure scales of ruby and gold. At pressures lower than 6 GPa, the pressure dependences of cBN Raman determined with the ruby pressure scale for transverse-optical (TO) and longitudinal-optical modes are 3.45±0.02 and 3.36±0.02 cm−1/GPa at 100 °C and 3.43±0.02 and 3.44±0.07 cm−1/GPa at 300 °C, respectively. These values are consistent with those in a previous study conducted at room temperature using the ruby pressure scale. Synchrotron x-ray diffraction experiments using a gold pressure marker also yield 3.45±0.03 cm−1/GPa for TO mode at 200 °C in a range of pressure up to 32 GPa. Under the present pressure and temperature conditions, the pressure dependence of Raman peaks of cBN seems to be independent of the temperature conditions. cBN can be used as an optical pressure marker under high temperature conditions.

Oxygen K-edge fine structures of water by x-ray Raman scattering spectroscopy under pressure conditions

Fine structure of the oxygen K edge was investigated for water at ambient pressure, 0.16, 0.21, 0.27, 0.47, and 0.60 GPa using x-ray Raman scattering spectroscopy (XRS). Similarity in near-edge structures at 0.16 and 0.60 GPa suggests little difference in the electronic state of oxygen in the low-pressure and high-pressure forms of water. Yet, we observed significant variation of preedge structure of the XRS spectra with compression. The intensity of the preedge peak at 535.7 eV has a minimal value at around 0.3 GPa, indicating that the number of hydrogen bonding increases first and then decreases as a function of pressure.