Viscosity of silicate melts in CaMgSi2O6–NaAlSi2O6 system at high pressure

In situ X-ray viscometry of the silicate melts was carried out at high pressure and at high temperature. The viscosity of the silicate melts in the diopside(Di)–jadeite(Jd) system was determined in the pressure range from 1.88 GPa to 7.9 GPa and in the temperature range from 2,003 K to 2,173 K. The viscosity of the Di 25%–Jd 75% melt decreases continuously to 5.0 GPa, whereas the viscosity of the Di 50%–Jd 50% melt increases over 3.5 GPa. The viscosity of the Di50%–Jd 50% melt reaches a minimum around 3.5 GPa. Since the amounts of silicon in the two melts are the same, the difference in the pressure dependence of the viscosity may be controlled by another network-forming element, i.e., aluminum. The difference in the pressure dependence of the viscosities in the melts with two intermediate compositions in the Di–Jd system is estimated to be due to the difference in the melt structures at high pressures and high temperatures.     

The equation of state of Al,H-bearing SiO2 stishovite to 58 GPa

We have determined the P-V equation of state of Al-rich H-bearing SiO₂ stishovite by X-ray powder diffraction at pressures up to 58 GPa using synchrotron radiation. The sample contained 1.8 wt% Al₂O₃ and up to 500 ppm H₂O, and had a composition that would coexist with Mg-silicate perovskite in a subducted slab. By fitting a third-order Birch-Murnaghan equation of state to our compression data, we obtained a bulk modulus K _T0=298(7) GPa with K′=4.3(5). With K′ fixed to a value of 4, the bulk modulus K _T0=304(3) GPa. Our results indicate that Al³⁺ and H⁺ have a small effect on the elastic properties of stishovite. Compared with data obtained up to 43.8 GPa, peak intensities changed and we observed a decreased quality of fit to a tetragonal unit cell at pressures of 49 GPa and higher. These changes may be an indication that the rutile↔CaCl₂ transition occurs between these pressures. After laser annealing of the sample at 58.3(10) GPa and subsequent decompression to room conditions, the cell volume is the same as before compression, giving strong evidence that the composition of the recovered sample is also unchanged. This suggests that Al and H are retained in the sample under extreme P-T conditions and that stishovite can be an agent for transporting water to the deepest lower mantle.     

Eolian quartz flux variations in Cheju Island, Korea, during the last 6500 yr and a possible Sun-monsoon linkage

We have obtained a high-resolution sedimentary record covering the last 6500 yr from a maar in Cheju Island, Korea, in order to reconstruct the history of variations in the eolian quartz flux (EQF) and hence Asian dust. The long-term variation of EQF reveals three intervals: a period of high EQF (4000-2000 cal yr B.P.) and two periods of low EQF (6500-4000 cal yr B.P. and 2000 cal yr B.P. to present), which have been affected by the East Asian monsoon due to insolation change and the cold air activity in high latitudes correlated with polar high-pressure systems. This long-term variation is superimposed by millennial- and centennial-scale fluctuations with periodicities of 1137, 739, 214, 162, 137, 127, and 111 yr, implying drier conditions in the source areas in China. The detrended EQF record correlates visually and statistically (cross-spectral analysis) with the atmospheric Δ¹⁴C record (solar proxy). The centennial-scale variability in EQF may be affected by the solar activity through the Sun-East Asian monsoon linkage.     

Phase relations in hydrous MORB at 18-28 GPa: implications for heterogeneity of the lower mantle

The phase relations in hydrous and anhydrous mid-ocean ridge basalt were determined at pressures of 18-28 GPa. Liquidus phase relations in hydrous and anhydrous MORB are different. Garnet is the liquidus phase at pressures below 21 GPa, Ca-Al (CAS) phase and stishovite are the liquidus phases at pressures of 22-27 GPa, and stishovite and Ca-perovskite are the liquidus phases above 27 GPa, whereas Ca-perovskite is a liquidus phase of anhydrous MORB at pressures above 23 GPa. Under subsolidus conditions, we have found that in the hydrous MORB system the stability fields of Al-bearing perovskite and Na-Al (NAL) phase might shift to lower pressure by about 1.5 GPa compared to the dry MORB system. This shift could be explained by oxidation of a garnet-bearing assemblage by hydrous fluid and formation of Fe³⁺-bearing aluminous perovskite at lower pressures relative to the anhydrous system and/or differences in water solubility of the phases existing in perovskite-bearing assemblages. Our data indicate that hydrous basaltic crust remains denser than peridotite along the geotherm of a subducting slab, i.e. there is no density crossover between peridotite and basalt. Therefore, in slabs going through the 660 km discontinuity, basalt would gravitationally sink into the lower mantle under relatively hydrous conditions. The delamination of former basaltic crust near the 660 km discontinuity might be possible under relatively dry conditions of subduction. There are no stable highly hydrous phases in MORB above 10 GPa even at lower temperatures corresponding to subducting slabs. Therefore, MORB cannot be an important carrier of water to the deep Earth interior. However, it can be constantly supplied by water-bearing fluid from the underlying peridotite part of the descending slab. Thus, it is plausible that water can control subduction of the oceanic crust into the lower mantle.     

A lower trophic ecosystem model including iron effects in the Okhotsk Sea

We applied a three-dimensional ecosystem-physical coupled model including iron the effect to the Okhotsk Sea. In order to clarify the sources of iron, four dissolved iron compartments, based on the sources of supply, were added to Kawamiya et al.'s [1995, An ecological-physical coupled model applied to Station Papa. Journal of Oceanography, 51, 635-664] model (KKYS) to create our ecosystem model (KKYS-Fe). We hypothesized that four processes supply iron to sea water: atmospheric loadings from Northeastern Asia, input from the Amur River, dissolution from sediments and regeneration by zooplankton and bacteria. We simulated one year, from 1 January 2001 to 31 December 2001, using both KKYS-Fe and KKYS. KKYS could not reproduce the surface nitrate distribution after the spring bloom, whereas KKYS-Fe agreed well with observations in the northwestern Pacific because it includes iron limitation of phytoplankton growth. During the spring bloom, the main source of iron at the sea surface is from the atmosphere. The contribution of riverine iron to the total iron utilized for primary production is small in the Okhotsk Sea. Atmospheric deposition, the iron flux from sediment and regeneration of iron in the water column play important roles in maintaining high primary production in the Okhotsk Sea.     

Mechanical response of the south flank of kilauea volcano, hawaii, to intrusive events along the rift systems

Increased earthquake activity and compression of the south flank of Kilauea volcano, Hawaii, have been recognized by previous investigators to accompany rift intrusions. We further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between December 1971 and January 1981. The seismic response of the south flank to individual rift intrusions is immediate; the increased rate of earthquake activity lasts from 1 to 4 weeks. Horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions. Emplacement of an intrusion at a depth greater than about 4 km, such as the June 1982 southwest rift intrusion, however, results in a slight extension of the subaerial portion of the south flank.Horizontal strain measurements along the south flank are used to locate the January 1983 east-rift intrusion, which resulted in eruptive activity. The intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet. This model suggests that the intrusive body that compressed the south flank in January 1983 extended from the surface to about 2.4 km depth, and was aligned along a strike of N66°E. The intrusion is approximately 11 km in length, extended beyond the January 1983 eruptive fissures, which are 8 km in length and is contained within the 14-km-long region of shallow rift earthquakes.     

Stability field of knorringite Mg3Cr2Si3O12 at high pressure and its implication to the occurrence of Cr-rich pyrope in the upper mantle

The stability field of knorringite (Mg₃Cr₂Si₃O₁₂) is studied experimentally. Knorringite is stable at pressures above 10.5 GPa at 1200°C and 11.8 GPa at 1400°C. Below these pressures, knorringite decomposes to enstatite + eskolaite. A phase diagram of the pyrope-knorringite system is described based on the available experimental data. The solubility of the knorringite molecule in pyrope is essentially dependent only on pressure, and the Cr/Cr+Al value of garnet is considered to be an indicator of the minimum pressure of equilibration. Consideration of the genesis of Cr-rich pyrope and other peridotitic inclusions in diamonds indicates that the fractionation process should have taken place, at least at depths to ca. 240 km, to give rise to the Cr-rich complement of Cr-poor upper mantle materials such as undepleted lherzolite. The knorringite-rich peridotitic suite in diamond will be identified with this complement, which may be the material constituting the deep upper mantle.     

Inequality of eddy transfer coefficients for vertical transport of sensible and latent heats during advective inversions

Similarity of transport of water vapour and sensible heat was investigated within an advective inversion layer by measuring eddy fluxes together with gradients of temperature and humidity. The experimental site was a field of rice, grown under flood irrigation, which was situated in a semi-arid region. The fetch was about 300 m and local stabilities (z/L) over the rice ranged from 0 to 0.1. Results were expressed as the ratio of eddy transfer coefficients for sensible heat (K _H) and water vapour (K _w). Near neutral stability, K _H/K_W approached 1, but the ratio decreased to about 0.65 with increasing stability. Existing theory predicts the result qualitatively but accounts for only about one fifth of the decrease in K _H/K_W with stability.     

Comparison of reductive accumulation of Re and Os in seawater-sediment systems

In order to understand the fractionation of Re and Os in marine environments, their removal from artificial seawater to Tokyo Bay sediments is studied using a multitracer technique. The chemical processes of the removal of Re and Os are also estimated based on their speciation analyses by X-ray absorption fine structure (XAFS) spectroscopy. The partitioning experiments, which use the multitracer technique, provide information on Re and Os regarding (i) their distributions between artificial seawater-sediment systems, (ii) their complexation with humic acid, and (iii) their carriers in sediments. In addition, XAFS spectroscopy provides direct information on the chemical states of Re and Os in the sediments.In an artificial seawater-sediment system containing a multitracer, Re is removed from the artificial seawater only under a reducing environment. The speciation of Re by X-ray absorption near-edge structure (XANES) suggests that the majority of Re remains as in the artificial seawater even under highly reducing conditions, during laboratory time scale (about 2 weeks). Moreover, XANES simulation shows that some Re exists at a lower oxidation state, such as ReO₂, in the reducing sediment. These results can be explained by the slow kinetics of the reaction which is similar to those suggested by previous geochemical studies.In contrast, Os is readily removed from the artificial seawater into sediments under various redox conditions. Even under oxic conditions, a large fraction of Os is removed from the artificial seawater to sediments without organic matter. Based on the Os XANES study, it is confirmed that the oxidation states of Os incorporated in the reducing sediment and oxic sediment are trivalent and tetravalent, respectively. Sequential extraction suggests that the main carrier of Os in the organic-rich sediment is either ferromanganese oxides or organic matter, and that the Os in these two fractions may correspond to hydrolyzed insoluble Os species and Os species interacting with organic matter, at lower valence, respectively. The results of distribution study of Os in the absence and presence of humic acid (HA) also imply that Os assumes more than one chemical species, and a small fraction of Os may interact with HA in the experimental system. Meanwhile, extended X-ray absorption fine structure (EXAFS) confirms that the first neighboring atom of Os in the reducing sediment is oxygen. If Os(VIII) is the main dissolved species in seawater, as is expected thermodynamically, reductive removal may control the enrichment of Os in the sediment. Osmium, which is removed as Os(IV), is reduced further to Os(III) by a diagenetic process and may be complexed with organic matter in the reducing sediment.The results of the removal behaviors of Re and Os obtained in the current study show that Re can be removed from the artificial seawater only under highly reducing conditions within 2 weeks, but Os removal from the artificial seawater can be found under various redox conditions. Thus, a high ¹⁸⁷Re/¹⁸⁸Os ratio can occur only in reducing sediments, such as black shales. The high¹⁸⁷Re/¹⁸⁸Os ratio, in turn, makes black shales suitable for Re-Os dating. In contrast, authigenic sediments (and minerals) under oxic environments can enrich Os, but since Re is not distributed to the sediments under oxic conditions, this will cause a much lower ¹⁸⁷Re/¹⁸⁸Os ratio than that of seawater. The Os isotope system of these materials can be used as a paleo-marine environmental tracer since the ¹⁸⁷Os/¹⁸⁸Os ratio cannot grow significantly due to its extremely low ¹⁸⁷Re/¹⁸⁸Os ratio.     

Magnetic detection of heated soils at paleolithic sites in Japan

We present a methodological approach to detect heated soil on ancient sites, using magnetic measurements. The method is based on changes in magnetic signals of soil by heating. The following three types of soil were used for testing the method: silty soil (SS), weathered volcanic ash (WVA, = loam) and fairly fresh volcanic ash (VA) called Odori tephra. On heating above 250–600°C, the magnetic susceptibility and remanent magnetization intensity increased for the SS and WVA samples, reflecting chemical alteration of magnetic minerals (from goethites to magnetites through hematites). The VA sample showed no susceptibility change suggesting the absence of goethites within it. On heating below 250°C, only the intensities of all the samples increased. This is possibly due to acquisition of thermal remanent magnetization. The largest change of the magnetic signals was identified for the SS sample and the smallest one was seen for the VA sample. Therefore, the in situ susceptibility measurement, which is the nondestructive and indirect method, seems to be effective to detect heated soil for sites of aqueous deposits as the SS. On the other hand, for sites of aeolian deposits as the WVA (loam) and VA, the intensity measurement of collected soils seems to be the most reliable method to detect evidence of heating. The degree of the magnetic stability (coercivity) against progressive alternating-field demagnetization was also an important parameter, indicating whether the investigated soils were heated or unheated. © 1999 John Wiley & Sons, Inc.