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Now showing items 1 - 16 of 35

  • Electrical conductivity of NaCl-bearing aqueous fluids to 900 °C and 5 GPa

    Guo, Haihao   Keppler, Hans  

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  • The speciation of carbon monoxide in silicate melts and glasses

    Yoshioka, Takahiro   McCammon, Catherine   Shcheka, Svyatoslav   Keppler, Hans  

    We have studied the speciation of carbon monoxide in both Fe-bearing and Fe-free basaltic glasses using Raman, FTIR, and Mossbauer spectroscopy. We show that a band at 2110 cm(-1) in the Raman spectrum and another band at 2210 cm(-1) in the FTIR spectrum occur both in the Fe-bearing and Fe-free samples, implying that they cannot be due to any Fe-bearing species. This observation is consistent with Fe-57 Mossbauer spectra, which do not show any evidence for Fe species with zero isomer shift, as expected for carbonyls. Thermodynamic calculations show that iron carbonyl in basaltic melts under crustal and upper mantle conditions may only be a trace species. Rather than being due to distinct chemical species, the range of vibrational frequencies observed for carbon monoxide in silicate glasses appears to be due to rather subtle interactions of the CO molecule with the matrix. Similar effects are known from the extensive literature on carbon monoxide adsorption on oxides and other surfaces. In the melt at high temperature, there is likely little interaction of the CO molecule with the silicate matrix and solubility may be largely controlled by pressure, temperature, and the overall polymerization or ionic porosity of the melt.
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  • GEOLOGY Earth's deep water reservoir

    Keppler, Hans  

    A tiny sample of a mineral included in a diamond confirms predictions from high-pressure laboratory experiments that a water reservoir comparable in size to all the oceans combined is hidden deep in Earth's mantle.
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  • Electrical conductivity of NaCl-bearing aqueous fluids to 600 degrees C and 1 GPa

    Sinmyo, Ryosuke   Keppler, Hans  

    The electrical conductivity of aqueous fluids containing 0.01, 0.1, and 1 M NaCl was measured in an externally heated diamond cell to 600 degrees C and 1 GPa. These measurements therefore more than double the pressure range of previous data and extend it to higher NaCl concentrations relevant for crustal and mantle fluids. Electrical conductivity was generally found to increase with pressure and fluid salinity. The conductivity increase observed upon variation of NaCl concentration from 0.1 to 1 M was smaller than from 0.01 to 0.1 M, which reflects the reduced degree of dissociation at high NaCl concentration. Measured conductivities can be reproduced (R-2 =3D 0.96) by a numerical model with log sigma =3D -1.7060-93.78/ T + 0.8075 log c + 3.0781 log rho + log + triangle(0)(T, rho), where sigma is the conductivity in Sm-1, T is temperature in K, c is NaCl concentration in wt%, rho is the density of pure water (in g/cm(3)) at given pressure and temperature, and triangle(0) (T, rho) is the molar conductivity of NaCl in water at infinite dilution (in S cm(2) mol(-1)), triangle(0) =3D 1573-1212 rho + 537 062/ T-208 122 721/T-2. This model allows accurate predictions of the conductivity of saline fluids throughout most of the crust and upper mantle; it should not be used at temperatures below 100 degrees C. In general, the data show that already a very small fraction of NaCl-bearing aqueous fluid in the deep crust is sufficient to enhance bulk conductivities to values that would be expected for a high degree of partial melting. Accordingly, aqueous fluids may be distinguished from hydrous melts by comparing magnetotelluric and seismic data. H2O-NaCl fluids may enhance electrical conductivities in the deep crust with little disturbance of v(p) or v(p)/v(s) ratios. However, at the high temperatures in the mantle wedge above subduction zones, the conductivity of hydrous basaltic melts and saline aqueous fluids is rather similar, so that distinguishing these two phases from conductivity data alone is difficult. Observed conductivities in forearc regions, where temperatures are too low to allow melting, may be accounted for by not more than 1 wt% of an aqueous fluid with 5 wt% NaCl, if this fluid forms a continuous film or fills interconnected tubes.
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  • Zircon solubility in aqueous fluids at high temperatures and pressures

    Bernini, Diego   Audetat, Andreas   Dolejs, David   Keppler, Hans  

    The depletion of high field strength elements such as Zr, Nb and Ta is a characteristic feature of arc magmas and it has been attributed to a low solubility of these elements in slab-derived aqueous fluids. We have determined zircon solubility in aqueous fluids up to 1025 degrees C and 20 kbar by in situ observation of dissolving zircon grains in the hydrothermal diamond anvil cell. Zircon solubilities in H2O with silica activity buffered by quartz are very low, from 1.0 to 3.3 ppm Zr, and weakly increase with temperature and pressure. Experimental results were fitted to the following fluid density model: logc(+/- 0.10) = (3.45 +/- 0.92) - (3803 +/- 1098)T-1 + (1.52 +/- 0: 63) log rho where c is the Zr concentration in the fluid (ppm by weight), T is temperature (K) and rho is the fluid density (g cm(-3)). An additional experiment with a saline fluid (15 wt.% NaCl) revealed an increase in zircon solubility by a factor of 3 (4.8 +/- 1.6 ppm Zr at 890 degrees C and 14 kbar) whereas addition of 4.5 wt.% albite as solute increased solubility by about a factor of 5. The Zr solubility at the forsterite-enstatite silica buffer appears to be slightly higher than that at the quartz buffer and it further increases at baddeleyite saturation (48 +/- 15 ppm Zr at 930 degrees C and 16 kbar). These observations are consistent with the stability of zircon relative to ZrO2 + SiO2 and suggest that Zr-Si complexes are not abundant in the fluid. During slab dehydration, the Zr content in the aqueous fluid is predicted to be 1-4 ppm. Mass balance calculations imply that the high field strength element concentrations in primary arc melts will slightly decrease due to the dilution effect of infiltrating fluid. By contrast, mobile lithophile elements are predicted to increase their abundances in the melt by orders of magnitude. Our results suggest that the high abundance of large ion lithophile elements relative to high field strength elements in arc magmas is related to different solubilities of these elements in aqueous fluids migrating from the slab to the magma source regions. (C) 2013 Elsevier Ltd. All rights reserved.
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  • Carbon in Silicate Melts

    Ni, Huaiwei   Keppler, Hans  

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  • Water and hydrogen are immiscible in Earth's mantle

    Audetat, Andreas   Keppler, Hans  

    In the deep, chemically reducing parts of Earth's mantle(1), hydrous fluids contain significant amounts of molecular hydrogen (H-2). Thermodynamic models of fluids in Earth's mantle so far have always assumed that molecular hydrogen and water are completely miscible. Here we show experimental evidence that water and hydrogen can coexist as two separate, immiscible phases. Immiscibility between water and hydrogen may be the cause of the formation of enigmatic, ultra-reducing domains in the mantle that contain moissanite (SiC) and other phases indicative of extremely reducing conditions(2,3). Moreover, the immiscibility between water and hydrogen may provide a mechanism for the rapid oxidation of Earth's upper mantle immediately following core formation(4).
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  • Electrical conductivity of orthopyroxene and plagioclase in the lower crust

    Yang, Xiaozhi   Keppler, Hans   McCammon, Catherine   Ni, Huaiwei  

    The electrical conductivities of lower crustal orthopyroxene and plagioclase, as well as their dependence on water content, were measured at 6-12 kbar and 300-1,000 degrees C on both natural and pre-annealed samples prepared from fresh mafic xenolith granulites. The complex impedance was determined in an end-loaded piston cylinder apparatus by a Solarton-1260 Impedance/Gain Phase analyzer in the frequency range of 0.1-10(6) Hz. The spectra usually show an arc over the whole frequency range at low temperature and an arc plus a tail in the high and low frequency range, respectively, at high temperature. The arc is due to conduction in the sample interior, while the tails are probably due to electrode effects. Different conduction mechanisms have been identified under dry and hydrous conditions. For the dry orthopyroxene, the activation enthalpy is similar to 105 kJ/mol, and the conduction is likely due to small polarons, e. g., electrons hopping between Fe(2+) and Fe(3+). For the dry plagioclase, the activation enthalpy is similar to 161 kJ/mol, and the conduction may be related to the mobility of Na(+). For the hydrous samples, the activation enthalpy is similar to 81 kJ/mol for orthopyroxene and similar to 77 kJ/mol for plagioclase, and the electrical conductivity is markedly enhanced, probably due to proton conduction. For each mineral, the conductivity increases with increasing water content, with an exponent of similar to 1, and the activation enthalpies are nearly independent of water content. Combining these data with our previous work on the conductivity of lower crustal clinopyroxene, the bulk conductivity of lower crustal granulites is modeled, which is usually >similar to 10(-4) S/m in the range of 600-1,000 degrees C. We suggest that the high electrical conductivity in most regions of the lower crust, especially where it consists mostly of granulites, can be explained by the main constitutive minerals, particularly if they contain some water. Contributions from other highly conducting materials such as hydrous fluids, melts, or graphite films are not strictly necessary to explain the observed conductivities.
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  • In-situ infrared spectra of OH in olivine to 1100 degrees C

    Yang, Xiao-zhi   Keppler, Hans  

    The infrared spectra of hydrated San Carlos olivine were measured from room temperature to 1100 degrees C at 1 bar using a heating stage. The spectra show that even at the highest temperatures studied, there are still two well-separated groups of OH bands centered between 3200 and 3300 cm(-1) and between 3500 and 3600 cm(-1), respectively. The distinction between "group I" (v> 3450 cm(-1)) and "group II" bands (v < 3450 cm(-1)) is therefore still meaningful at upper mantle temperatures. However, a prominent band at 3612 cm(-1) already loses intensity by 100 degrees C and nearly disappears by 300 degrees C, suggesting that it corresponds to a particular H environment that only formed during cooling of the sample and that it is not stable at high temperature. A similar band is prominent in many olivines from mantle xenoliths and previously it has sometimes been assigned to OH groups surrounding Si vacancies. We show that structural models of water dissolution in olivine derived from infrared spectra at ambient conditions may not be fully applicable for the upper mantle.
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  • The oxidation state of sulfur in magmatic fluids

    Binder, Bernd   Keppler, Hans  

    Sulfur compounds in volcanic gases are responsible for the global cooling after explosive eruptions and they probably controlled the early evolution of the Earth's atmosphere. We have therefore studied the oxidation state of sulfur in aqueous fluids under the pressure and temperature conditions and oxygen fugacities typical for magma chambers (0.5-3 kbar, 650-950 degrees C, Ni-NiO to Re-ReO(2) buffer conditions). Sulfur speciation was determined by Raman spectroscopy of quenched fluids trapped as inclusions in quartz. Our results show that sulfur in hydrothermal fluids and volcanic gases is much more oxidized than previously thought and in particular, some explosive eruptions may release a significant fraction of sulfur as SO(3) or its hydrated forms. In the pressure range from 500 to 2000 bar, the equilibrium constant K(1) of the reaction 2H(2)S + 3O(2)= 2SO(2) + 2H(2)O in aqueous fluids can be described by InK(1) = -(57.1 +/- 7.1) + (173,480 +/- 7592)T(-1), where T is temperature in Kelvin. The equilibrium constant K(2) for the reaction SO(2)+ 1/2O(2)= SO(3) in aqueous fluids, where SO(3) may include hydrated forms, such as H(2)SO(4), was found to be strongly pressure dependent, with InK(2)= (5.2 +/- 5.7) + (19,243 +/- 5993)T(-1) at 1500 bar; InK(2)= (11.1 +/- 1.3) + (25,383 +/- 1371)T(-1) at 2000 bar and InK(2)= (22.1 +/- 2.2) + (37,082 +/- 2248)T(-1) at 2500 bar. Our data imply that volcanoes may directly inject hexavalent sulfur in the form of H(2)SO(4) into the atmosphere, not only on Earth, but possibly also on Venus and on Mars, when it was still tectonically active. Remote measurements from satellites may have underestimated the sulfur yield of some recent eruptions. Moreover, the mechanisms of the interaction of volcanic gases with the stratosphere need to be reconsidered. (C) 2010 Elsevier BM. All rights reserved.
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  • Bowring and Keppler receive 2010 N. L. Bowen Awards: Response

    Keppler, Hans  

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  • The origin of the negative niobium tantalum anomaly in subduction zone magmas

    Baier, Johannes   Audetat, Andreas   Keppler, Hans  

    In order to determine the distribution of mobium and tantalum between clinopyroxeres and aqueous fluids, we measured in two separate sets of experiments niobium and tantalum solubility in clinopyroxenes and in aqueous fluids, respectively. The solubility of niobium in clinopyroxenes was experimentally investigated in the system CaMgSi2O6-NaAlSi2O6-Nb2O5-H2O at 1.5 GPa and 700-1100 degrees C using piston-cylinder experiments. In the presence of excess Nb2O5, CaNb2O6 coexists with a clinopyroxene. The solubility of niobium in the pyroxene increases drastically with the aluminum content. While the solubility of mobium, is in the order of 100-300 ppm by weight for Al-poor clinopyroxenes, it reaches 4 wt.% for clinopyroxenes containing 10 wt.% Al2O3. Microprobe analyses suggest that Nb is incorporated in the clinopyroxene as NaNbAl2O6 component. The solubility of CaNb2O6 in aqueous fluid was determined by the direct visual observation of the dissolution of CaNb2O6 crystals in an aqueous fluid using an externally-heated diamond anvil cell. At 1.5 to 1.7 GPa and 800-1000 degrees C, an aqueous fluid saturated with diopside dissolves only 20-100 ppm by weight of CaNb2O6, i.e. mobium solubility in the fluid is orders of magnitude below the solubility in aluminous clinopyroxenes. Experiments on the solubility of CaTa2O6 in clinopyroxenes and in aqueous fluid suggest that Ta behaves generally similar to Nb, but with the notable exception that the solubility of CaTa2O6 in aqueous fluids and aluminous pyroxenes is approximately a factor of five lower than the solubility of the corresponding mobium compound. Our results imply that (1) the fluid/clinopyroxene partition coefficient for Nb and Ta is between 0.1 and 0.001 for aqueous fluids containing little dissolved silicates and for clinopyroxenes of a composition realistic for the subducted slab or the subarc mantle. Only for very Al-poor clinopyroxenes may the partition coefficient approach unity. (2) The depletion of Nb and Ta in subduction zone magmas and fluids is related to the intrinsically low solubility of Nb and Ta in water-rich and solute-poor fluids in the shallow parts of a subduction zone. (3) The presence of rutile in the subducted slab is not a necessary requirement for the development of the negative Nb and Ta anomaly. (4) Fluid transport of niobium and tantalum will affect the Nb/Ta ratio in the subarc mantle only under exceptional circumstances. (5) Previous reports of high fluid/clinopyroxene partition coefficients for Nb and Ta are probably related to the Al-free or Al-poor composition of the systems studied. (C) 2007 Elsevier B.V. All rights reserved.
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  • Carbon solubility in silicate melts in equilibrium with a CO-CO2 gas phase and graphite

    Yoshioka, Takahiro   Nakashima, Daisuke   Nakamura, Tomoki   Shcheka, Svyatoslav   Keppler, Hans  

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  • Crystal chemistry of hydration in aluminous orthopyroxene

    Smyth, Joseph R.   Mierdel, Katrin   Keppler, Hans   Langenhorst, Falko   Dubrovinsky, Leonid   Nestola, Fabrizio  

    Hydrogen incorporation in aluminous orthopyroxene may control the generation of melt and dominate the seismic properties at the base of the Earth's lithosphere. To clarify the substitution mechanism of H, we have synthesized, characterized, and refined the crystal structure of this potentially significant variant of orthopyroxene. The experimentally produced crystals are small needles up to approximately 20 x 20 x 100 mu m in size. Electron microprobe chemical analysis indicates about 11.7 wt% Al2O3. FTIR spectra indicate 7500 ppmw H2O with absorbance features qualitatively similar to natural mantle orthopyroxenes. TEM imagery indicates that the phase is pure orthopyroxene with low concentrations of defects and inclusions. Cell-parameter refinement from single-crystal X-ray diffraction gives alpha = 18.1876(7) angstrom; b = 8.7352(7) angstrom; c = 5.1789 (5) angstrom, V = 822.79(11) angstrom(3), which is 1.2% smaller than pure Mg anhydrous orthoenstatite. The crystal structure has been refined from single-crystal X-ray intensity data measured using a rotating anode X-ray generator, micro-focused X-ray beam, and CCD detector system. The refined structure indicates about 5% vacancy in M2 and significant Al occupancy in both M1 and T2, consistent with its composition, (Mg-0.95,Mg-0.05)(M2), (Mg0.79Al0.21)(M1), (Al0.25Si0.75)(T2) Si-T1 O-6. The existence of hydrous orthopyroxene in the mantle could absorb water released from olivine on decompression to delay the onset of melting in the spinel stability region in mantle peridotite compositions.
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  • Water solubility in aluminous orthopyroxene and the origin of Earth's asthenosphere

    Mierdel, Katrin   Keppler, Hans   Smyth, Joseph R.   Langenhorst, Falko  

    Plate tectonics is based on the concept of rigid lithosphere plates sliding on a mechanically weak asthenosphere. Many models assume that the weakness of the asthenosphere is related to the presence of small amounts of hydrous melts. However, the mechanism that may cause melting in the asthenosphere is not well understood. We show that the asthenosphere coincides with a zone where the water solubility in mantle minerals has a pronounced minimum. The minimum is due to a sharp decrease of water solubility in aluminous orthopyroxene with depth, whereas the water solubility in olivine continuously increases with pressure. Melting in the asthenosphere may therefore be related not to volatile enrichment but to a minimum in water solubility, which causes excess water to form a hydrous silicate melt.
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  • Effect of water on the electrical conductivity of lower crustal clinopyroxene RID G-9873-2011

    Yang, Xiaozhi   Keppler, Hans   McCammon, Catherine   Ni, Huaiwei   Xia, Qunke   Fan, Qicheng  

    The electrical conductivity of lower crustal clinopyroxene was measured at 6-12 kbar, 250-1000 degrees C, and Ni-NiO buffer conditions. The dependence of electrical conductivity on water content was studied using both natural and preannealed samples separated from a fresh xenolith granulite, with water contents from 0 to 375 ppm. An end-loaded piston cylinder apparatus and a Solarton-1260 Impedance/Gain Phase Analyzer were used in the study over a frequency range of 0.01-10(6) Hz to obtain the complex impedance spectra. The results show that the influence of pressure is very weak relative to temperature and water content and that two distinct mechanisms with different activation enthalpies dominate electrical conduction under dry and wet conditions. For the dry sample, the activation enthalpy is similar to 102 kJ/mol and the main charge carriers are small polarons, i.e., hopping of electron holes between ferrous and ferric irons. For wet samples, the electrical conductivity is significantly enhanced with an activation enthalpy of similar to 70 kJ/mol and the charge carriers are likely to be protons. Under hydrous conditions, the activation enthalpies are nearly independent of water content and the conductivity is a function of water content with an exponent of similar to 1. As a major constituent of granulites with > 60% modal volume in some regions, clinopyroxene containing minor amounts of water may contribute significantly to the high electrical conductivity in the lower crust, especially under stable continental regions.
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