MR32B-01
Dynamics in Hydrous Silicates Studied by High Temperature High Pressure Quasielastic Neutron Scattering
Dissolved water in silicate melts plays an important role in many geological processes, especially in active volcanism. The knowledge of microscopic dynamics of the water species represents a key for the understanding of these processes and to predict macroscopic melt properties like viscosity. We study water dynamics in hydrous silicate melts employing quasielastic neutron scattering technique. Neutron scattering provides simultaneously information on the microscopic structure and dynamics of the sample. At the new time-of-flight spectrometer TOFTOF of FRM II a high energy resolution of about several tens μeVs can be obtained together with a large neutron flux as well as an excellent signal-to-noise ratio, which is ideal for such kind of investigation. In order to investigate the water dynamics in hydrous silicate melts at the temperatures relevant for volcanic processes, a pressure of about 150-200 MPa is needed at the mean time to prevent the degassing and foaming of the sample. A high temperature high pressure cell was constructed as sample environment with a relative large opening angle optimized for the tof-spectrometer. The cell provides a temperature range from RT up to 1500 K with a pressure up to 200 MPa at the sample position with an available sample volume of about 1 cm3, achieved by an internally heated NbZr autoclave. Applying the cell, we successfully performed quasielastic neutron scattering experiments on sodium trisilicate (Na2O·3SiO2), sodium aluminosilicate (Al2O3·Na2O·6SiO2, Albite: haplogranitic rock composition) and pure silica (SiO2) samples with 10 mol% water content in the temperature range from 850 K to 1250 K. Taking advantage of the large difference in neutron scattering cross-sections of H and D, a contrast variation via H2O/D2O substitution gives access to the pure incoherent proton dynamics. An unusual behaviour of the density correlation functions in hydrous sodium trisilicate melt has been observed. The proton dynamics is not so fast as expected, its intermediate scattering functions S(q,t) do not fully decay to zero in the time window accessible by our experiment. In contrast the S(q,t) follows a logarithmic decay to a plateau value f2(q), which is a signature of a precursor of a separate localization transition. By analyzing the q and T dependence of this plateau value, decoupling of the H dynamics from the Si-O matrix dynamics has been found. In comparison with the mode coupling theory (MCT), similar features can be already reproduced with a model system of binary mixture of hard-spheres with large size ratio under certain conditions, without the consideration of the complicated chemical interactions between the ions in the system. Our results are therefore currently modeled by the MCT.
MR32B-02
Sound Velocity of MgSiO3 Glass to 24 GPa
The presence of liquid silicates can strongly affects the physical properties of the regions at the top of the transition zone and at the core-mantle boundary. A mainly silicate magma ocean probably played a crucial role in the evolution of the early Earth. For these reasons physical behavior of silicates melts at high pressures is the subject of intense research in Earth sciences. Due to substantial difficulties in investigating in situ melts at high pressures, glasses are often used as “frozen” proxies of melts for physical investigations. Here we present the pressure dependence of sound velocity of MgSiO3 glass throughout the pressure range of the upper mantle and transition zone by Brillouin spectroscopy in the diamond-anvil cell. In our dense experimental dataset we measured at each pressure both compressional and shear velocity. From the measured velocities we could extract information about the pressure dependence of the density of MgSiO3 glass from the value independently measured at ambient conditions. Fixing the starting density to 2.742 ± 0.003 g/cm3, we determined both bulk modulus, shear modulus, and their pressure derivatives at ambient conditions, KS0 = 76.2 ± 1.0 GPa, (∂KS/∂P)T0 = 3.04 ± 0.23. We also determined the shear modulus and its pressure dependence at ambient conditions G0 = 39.86 ± 0.44 GPa, (∂G/∂P)0 = 0.55 ± 0.07. In our experiments we observe a clear discontinuity of their pressure dependence at 7 ± 1 GPa. A second, more subtle change of the pressure dependence of the compressional velocity is present in the range between 13 and 19 GPa. The discontinuity observed at 7 GPa could be connected to structural transitions observed in crystalline MgSiO3 in the range between 7 and 9 GPa. The subtle transition at higher pressures agrees with the change in the pressure dependence of both Si-O-Si bending and Si-O stretching vibrations of the polymerized SiO4 network observed in the same glass in a high-pressure Raman study (Shim et al., submitted to DI08), and interpreted as a close precursor of progressive structural transformation between 19 and 38 GPa corresponding to the increase of the Si-O coordination number, in parallel with the pressure induced stabilization of the perovskite structure in crystalline MgSiO3.
MR32B-03
Probing and Modeling of Pressure-induced Structural Transformation in Oxide Melts at High Pressure
Pressure-induced bonding transitions in oxide melts give improved prospects for the non-linear pressure dependence of their macroscopic transport properties in the earth's interior. The inherent difficulties of current experimental technologies, however, pose major challenges for probing structural changes of prototypical model oxide melts at high pressure, making it one of the unsolved problems in geophysics. Recent advances in element specific experimental probe of local structures, such as high resolution solid- state NMR and x-ray Raman scattering unveils new structural insights into the pressure-induced changes in the bonding nature (either gradual or abrupt) of the archetypal oxides melts (e.g. Lee SK et al. Proc. Nat. Aca. Sci. 2008, 105, 7925; Lee SK et al. J. Phys. Chem. B. 2008 in press). Here, we report recent progress that we have made using these techniques. Non-random spatial distribution of aluminum in oxide glasses were for the first time revealed via through-space correlation NMR spectroscopy: four, five, six coordinated aluminums have differential proximity among each other but favoring the formation of clusters mainly composed of six coordinated Al. While silicate glasses studied here exhibit a general trend of decreasing non-bridging oxygen fraction with pressure, the details of their pressure dependence is significantly affected by the composition of melts, such as Na/Si, Si/Al ratio as well as types of network modifying cations. We account for these differences with a conceptual model that utilizes pressure flexibility (the resistance to structural changes with increased pressurization). An oxide melts with a large pressure flexibility, thus, may undergo a gradual structural transformation. In contrast, a melts with the opposite behavior undergoes an abrupt coordination transformation. The observed information of connectivity among network forming cations was used to calculate the crystal-melt partitioning coefficient and activity of silica in melts where the fraction of Si-O-Si cluster increases with the activity coefficient of silica.
MR32B-04
Shock compression of liquid silicates to 125 GPa: the anorthite-diopside join
Shock compression of pre-heated liquid silicates provides, at present, the only method for direct measurement of the equation of state of such liquids at lower mantle pressures. At previous AGU meetings we have reported the extension of the pre-heated Hugoniot of the anorthite-diopside eutectic composition Di64An36, initially at 1673 K, to 110 GPa, along with a determination of the density dependence of the Grüneisen parameter of this liquid composition. We have now completed this work by: (1) extending the Di64An36 data to 127 GPa; (2) measuring the diopside (CaMgSi2O6) end-member composition, initially at 1773 K, to 114 GPa; and (3) measuring the anorthite (CaAl2Si2O8) end-member composition, initially at 1923 K, to 125 GPa. For internal consistency we have also re-reduced all of the experiments reported by Rigden et al. (1984, 1988, 1989) as well as some unpublished data from that era, using the latest initial melt densities (Lange, 1997) and hot Mo equation of state (Asimow et al., 2008). We are now in a position to assess the compression behavior of each of these liquids and the model of linear mixing along the compositional join to all pressures relevant to the terrestrial mantle. The total data set for the Di64An36 and diopside compositions can be fit within error by straight line Hugoniots in particle velocity vs. shock velocity. This suggests that a 3rd order equation of state is adequate to describe compression of these liquids over large pressure ranges. Anorthite, on the other hand, clearly requires a more complex model, such as we require for MgSiO3 and Mg2SiO4 liquids (Mosenfelder, Asimow, and Ahrens; this meeting); we examine the 4th-order Birch-Murnaghan and Ghiorso equations of state as well as an explicit speciation model that accounts for continuous coordination change of Si and Al. The complexity of anorthite liquid compression causes an apparent failure of linear mixing, suggesting that calibration of a predictive model of the equation of state of any silicate liquid composition will require more than a small number of end-member determinations. We will discuss the implications of our new data and models for melting and crystallization in a whole mantle magma ocean or at the modern core-mantle boundary. Asimow, P.D., Sun, D. and Ahrens, T.J., 2008. Phys. Earth Planet. Int. 10.1016/j.pepi.2008.08.004. Lange, R.A., 1997. Contrib. Mineral. Petrol 130: 1-11. Rigden, S.M., Ahrens, T.J. and Stolper, E.M., 1984. Science 226(4678): 1071-1074. Rigden, S.M., Ahrens, T.J. and Stolper, E.M., 1988. J. Geophys. Res. 93(B1): 367-382. Rigden, S.M., Ahrens, T.J. and Stolper, E.M., 1989. J. Geophys. Res. 94(B7): 9508-9522.
MR32B-05
Techniques for Elastic Properties Measurements of Partial Molten Rocks, Hydrous Minerals and Melts in Gas Pressure Vessels and Multi-Anvil Devices
The interpretation of highly resolved seismic data from Earth's deep interior require measurements of the
physical properties of Earth materials under experimental simulated mantle conditions. For deep crustal to
uppermost mantle conditions high performance gas pressure vessels enable a virtually unrestricted
optimization of the measuring configurations for high p-T-conditions [1]. Exhumed high pressure rocks can be
used as representative samples. The paper presents transient measurements of elastic wave velocities for
granulite facies rocks under partial melting conditions. Despite the compact natural rock samples as a result
of long-term experiments exceeding pressures of 1.5 GPa and temperatures of 1,000°C newly-formed
garnets, orthopyroxenes and potash feldspars could be found in the samples after the experiments.
Discovering the huge water storage capacity of nominally anhydrous minerals (NAMs) under high pressure
conditions dramatically changed our image of state and dynamics of Earth's deep interior [2]. The simulation
of these in situ conditions require using of diamond anvil cells (DAC) and multi-anvil devices (MAD) as well as
mostly synthetical samples. MADs are more limited in pressure, but provide sample volumes 3 to 7 orders of
magnitude bigger. They offer small and even adjustable temperature gradients over the whole sample. The
bigger samples make anisotropy and structural effects in complex systems accessible for measurements in
principle. Using ultrasonic interferometry the measurement of both elastic wave velocities have no limits for
opaque and encapsulated samples. Using the 6 to 8 anvils of a MAD as buffers allow the simultaneous
recording of acoustic emissions from different directions of space and consequently the localization of the
spikes during ongoing phase transitions and dehydration. The recent development of deformation-DIA MADs
(D-DIA) make not only deformation measurements under simulated mantle conditions possible, but also the
measurement of elastic attenuation even in the seismic frequency range [3]. An indispensable condition for
all these MAD techniques is the installation at a synchrotron. Only application of synchrotron radiation allows
in situ pressure measurement, in situ X-ray diffraction (XRD), in situ deformation measurement, as well as in
situ density and viscosity measurements of melts. We review recent techniques and present developments
and results for MADs [4, 5].
(1) Mueller & Massonne, Phys. Chem. Earth (A) 2001, 26, 325-332.
(2) Ohtani & Litasov, Rev. Min. Chem.
2006, 62, 397-420.
(3) Li et al., Am. Min. 2006, 91, 517-527.
(4) Mueller et al., High Press. Res. 2006, 26,
1-9.
(5) Mueller et al., In: E. Ohtani (ed), Advances in High-Pressure Mineralogy 2007, 207-226.
http://www.gfz-potsdam.de