[1] To specify quantitatively the effect of pressure and water weakening on the flow strength of feldspar we performed triaxial creep experiments in a gas deformation apparatus at temperatures of 1000-1150°C, confining pressures of 100-450 MPa, and axial stresses of 10-400 MPa, resulting in strain rates of $6 Â 10 À7 to 3 Â 10 À3 s À1 . Dense samples with a grain size of $3 mm were prepared by hot-isostatic pressing of anorthite glass powder. Hydrous samples contain about 0.33 ± 0.14 wt % H 2 O and dry specimens 0.0005-0.02 wt % H 2 O. The estimated residual glass content of wet samples is <2 vol %. Samples deformed by grain boundary diffusion-controlled creep at low stresses and dislocation creep at stresses^150 MPa. We estimate an activation volume of V % 24 cm 3 mol À1 for anhydrous samples deforming in diffusion creep. For wet samples, deformed in hydrous conditions with varying buffers fixing oxygen fugacity, the activation volume is about 38 cm 3 mol À1 . Creep rate of hydrous anorthite aggregates depends on water fugacity raised to a power of r = 1.0 ± 0.3, suggesting hydrolysis of oxygen bonds. Considering the effect of activation volume and water fugacity on extrapolation of constitutive laws to conditions prevailing in the continental lower crust, viscosities of hydrous feldspar aggregates increase by a factor of <3.Citation: Rybacki, E., M. Gottschalk, R. Wirth, and G. Dresen (2006), Influence of water fugacity and activation volume on the flow properties of fine-grained anorthite aggregates,

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Anhydrous and hydrous wadsleyite were synthesized at 13.3-13.5 GPa and 1150-1200 °C in a multianvil press and investigated by Fourier transform infrared (FTIR) spectroscopy, single-crystal X-ray refinement (SC-XRD), and electron microprobe analyses (EMPA). The FTIR spectra agree with previous data, i.e., the spectra are dominated by a broad band around 3380 cm -1 , resolvable in three bands 3326 (ν 2 ), 3382 (ν 3 ), and 3546 (ν 4 ) cm -1 besides some weaker OH-bands around 3600 cm -1 . We confirm that wadsleyite incorporates water in the wt% range and that the concentration strongly increases with decreasing temperature when using secondary ion mass spectrometry (SIMS) and Raman spectroscopy. The quantifications combined with FTIR spectra led us to develop the first IR calibration for water in wadsleyite, i.e., calculating an ε i,tot of 73 000 ± 7000 (L mol -1 H 2 O cm -2 ). A SC-XRD determination of hydrous wadsleyite FD0718, bearing 8000 ± 1000 wt ppm H 2 O, certifies the presence of Mg vacancies at the M3 sites as previously suggested. Furthermore, we found maxima in the electron density map close to the O atoms O1 and O3 of an M3 octahedron assuming the anhydrous structure. Based on our new data we suggest that the main protonation in wadsleyite occurs along the O1···O4 (3.1 Å) and O3···O4 (3.05 Å) edges of a vacant M3 octahedron. H-incorporation seems to be random leading to protonation of either two O1, two O3, or one O1 and one O3 of the vacant M3 octahedra. With this assignment, the observed ambient and high-pressure IR pattern can now be explained.

The pressure and temperature dependence of antigorite polysomatism was investigated in the P,T-range of 350-710°C, 0.2-5.0 GPa within the system MgO-SiO 2-H 2 O (MSH). TEM-study indicated that increasing temperature and decreasing pressure of MSH-antigorite formation are correlated with a shorter a-cell periodicity, i.e. smaller mvalue (m = number of tetrahedra in a single chain along the wavelength a). For the P,T-conditions investigated, the compositional m-range of antigorite is rather narrow (14-18) corresponding to Mg 2.79 Si 2 O 5 (OH) 3.57-Mg 2.83 Si 2 O 5 (OH) 3.67. The change in the crystal structure of antigorite is combined with a gradual partial dehydration process and loss of MgO. For the formulation of reactions that include antigorite, the P,T-dependence of the chemical composition of antigorite has to be considered. However, our results support the conclusion of Mellini et al. (1987) of the limited applicability of a modulation-dependent geothermobarometer, because equilibrium states of antigorite modulation are rarely observed in natural occurrences and have not been reached in our experiments. The successive partial dehydration of antigorite during ongoing subduction of serpentine-bearing oceanic lithosphere probably influences its rheological properties.

A rock magnetic study was performed on sediment cores from six locations in Lake Baikal. For a comprehensive approach of the processes influencing the rock magnetic signal, additional data are presented such as total organic carbon (TOC), total sulphur (TS), opal, water content and relative variations in iron and titanium measured on selected intervals. In glacial sediments, the magnetic signal is dominated by magnetite, which is considered to be of detrital origin. This predominance of magnetite is interrupted by distinct horizons of authigenic greigite, probably confined to reductive microenvironments. In interglacial stages, besides dilution by biogenic silica and a decreasing detrital input, the weakness of the rock magnetic signal is also due to a reductive dissolution of magnetic particles. The magnetic assemblage is strongly linked to the redox history of interglacial sediment. In the oxidised bottom sediments of Lake Baikal, a biogenic magnetite is observed [Peck, J.A., King, J.W., 1996. Magnetofossils in the sediments of lake Baikal, Siberia. Earth Planet. Sci. Lett. 140 (1-4), 159-172]. After burial under the redox front, the magnetite is preferentially dissolved, and detrital hematite remains dominant when the sedimentation rate is low and when the residence time of the magnetite close to the redox boundary is long. During these low sedimentation rate conditions, the redox front is preserved [Granina, L., Mqller, B. and Wehrli, B., 2004. Origin and dynamics of Fe and Mn sedimentary layers in Lake Baikal. Chem. Geol. 205 (1-2), 55-72]. At constant sedimentation rate and fast burial, the magnetite is preserved or transformed into greigite when sulphate-reducing conditions are reached in the sediment. In interglacial sediments, the magnetic assemblages depict changes in the sedimentation rate, which are traced using the ratio of magnetite over hematite (S-ratio). At the beginning of interglacials, the sedimentation rate is constant with an assemblage magnetite+greigite (high S-ratio), and at the end of some interglacials, the sedimentation rate decreases with a predominance of hematite (low S-ratio). D

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