Kyoko N. Matsukage

Density of hydrous silicate melt at the conditions of Earth's deep upper mantle

The chemical evolution of the Earth and the terrestrial planets is largely controlled by the density of silicate melts. If melt density is higher than that of the surrounding solid, incompatible elements dissolved in the melt will be sequestered in the deep mantle. Previous studies on dry (water-free) melts showed that the density of silicate melts can be higher than that of surrounding solids under deep mantle conditions. However, melts formed under deep mantle conditions are also likely to contain some water, which will reduce the melt density. Here we present data constraining the density of hydrous silicate melt at the conditions of ∼410 km depth. We show that the water in the silicate melt is more compressible than the other components, and therefore the effect of water in reducing melt density is markedly diminished under high-pressure conditions. Our study indicates that there is a range of conditions under which a (hydrous) melt could be trapped at the 410-km boundary and hence incompatible elements could be sequestered in the deep mantle, although these conditions are sensitive to melt composition as well as the composition of the surrounding mantle.

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.

Petrology of a chromitite micropod from Hess Deep, equatorial Pacific: a comparison between abyssal and alpine-type podiform chromitites

Abstract This is the first report of podiform chromitite from the ocean floor. A micropod of chromitite, about 2 cm across and more than 10 cm long, was found in dunite drilled from Hess Deep, equatorial Pacific, during ODP Leg 147. The pod is composed of coarse, up to 5 mm across, chromian spinel with the Cr# (=Cr/[Cr+Al] atomic ratio) of 0.5 to 0.6, often full of minute mineral inclusions, such as hydrous minerals relatively enriched with TiO2 (

Concentration of incompatible elements in oceanic mantle: Effect of melt/wall interaction in stagnant or failed melt conduits within peridotite

Abstract Primary hydrous and other minerals enriched with incompatible components were found in ocean-floor ultramafic-mafic plutonic rock suites recovered from two contrasting ridge systems, i.e., the East Pacific Rise (Hess Deep, equatorial Pacific), a typical fast-spreading system, and Mid-Cayman Trough, a typical slow-spreading system. They are characteristically associated with chromian spinel, enriched with Cr, one of compatible elements. The hydrous minerals can be formed through interaction between depleted mid-ocean ridge basalts (MORBs) and oceanic peridotite. Primary MORB produced in the deeper part inevitably react with shallower mantle peridotite; the magma selectively dissolves orthopyroxene with simultaneous olivine precipitation. Chromium is supplied to the melt from orthopyroxene, which is enriched with Cr over Al relative to ordinary basaltic melts. The effects of zone refining are also important for concentrations of the incompatible components, especially H20, Na, and Ti, in the modified magma, which in the extreme case is able to precipitate hydrous minerals. This mechanism is common to both fast- and slow-spreading ridges, and is more effective in stagnant or failed melt conduits. Some ultramafic rocks from upper mantle or transition-zone members of ophiolites have primary hydrous minerals, usually included by chromian spinel. Despite the often a priori assumption that slab-derived components are necessary for the formation of the chromitite with inclusions of primary hydrous minerals, this is clearly not necessary.

In situ X-ray observation of the reaction dolomite = aragonite + magnesite at 900–1300 K

Abstract To determine the reaction boundary dolomite = aragonite + magnesite, in situ X-ray experiments on dolomite decomposition and synthesis were carried out in the temperature range 900 to 1300 K. Dolomite decomposition experiments were conducted with increasing pressure at constant temperature, and the boundary was determined to be 5.3-5.9 GPa in the temperature range 800-1200 K and 5.9-6.3 GPa at 1300 K. Dolomite synthesis experiments were carried out with decreasing pressure at constant temperature or with increasing temperature at constant press load. The dolomite synthesis boundary was determined to be 6.7-6.9 GPa at 1300 K, 3.7-4.4 GPa at 1100 K, and 2-3 GPa at 800 K. Except at 1300K, the synthesis boundary is much lower in pressure and has a steeper dP/dT slope than the decomposition boundary. The difference in the reaction boundary reflects the different kinetics between decomposition and synthesis reactions, and the former may be closer to the equilibrium phase boundary. The experimental results show that the phase boundary between dolomite and aragonite + magnesite is located at ◻6.4 GPa at 1300 K and has a dP/dT slope of 0.001 ± 0.001 GPa/ K in the temperature range 900 to 1200 K.

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.

Model of layering formation in a mantle peridotite (Horoman, Hokkaido, Japan)

Abstract The Horoman peridotite complex exhibits a conspicuous layered structure. It is found, from the geological and petrological survey, that the pattern of layering has three characteristics: symmetry, asymmetry (subtly collapsed symmetry) and scale invariance. Especially, symmetric and asymmetric patterns clearly recognized in the sequence of mafic layers at the Northern ridge of Apoi-dake peak, and at the Western ridge of Bozu-yama peak are noticeable. We present a simple mathematical model describing stretching (thinning) and folding during deformation that accounts for the three characteristics. The model quantitatively reproduces the slope in cumulative frequency distribution of the width of mafic layers and indicates that the frequency distribution is strongly influenced by the spatial strain contrast. Applying the model result to the observational data for mafic layers, it is found that the strain contrast approximately ranges several to 10 times between regions with the highest and the lowest strain rates.

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.

Towards Mapping the Three‐Dimensional Distribution of Water in the Upper Mantle from Velocity and Attenuation Tomography

A new method is developed to determine the three-dimensional variation in water content, temperature, and other parameters such as major element chemistry or the melt fraction from anomalies in seismic wave velocities and attenuation. The key to this method is mineral physics observations indicating different sensitivity of seismic wave velocities and attenuation to temperature, water content and other parameters such as major element chemistry, melt fraction or grain-size. Our analysis shows that among these parameters, temperature and water content generally have a more important influence on seismic wave velocities and attenuation than other factors such as major element chemistry, which are important only in limited regions. The method is applied to the upper mantle beneath northern Philippine Sea including the Izu-Bonin subduction zone, where high-resolution velocity and attenuation tomographic models are available down to a depth of ∼400 km. We show that the tomographic images of this region can be explained by lateral variations in temperature and water content, with only little influence of major element chemistry. A broad region of high attenuation with modestly low velocities at 300-400 km depth away from the slab in this region is interpreted as region of high water contents. We speculate that this water-rich region may have been formed by the efficient transport of water to deeper mantle by a fast (and cold) subducting slab in this region or water may come from the transition zone.