DEFORMATION OF OLIVINE SINGLE CRYSTALS IN A HYDROUS ENVIRONMENT: INSIGHT INTO THE RHEOLOGICAL BEHAVIOR OF THE LITHOSPHERIC MANTLE OF TERRESTIRAL PLANETS. Jacob A. Tielke, Mark E. Zimmerman, and David L. Kohlstedt, University of Minnesota, Department of Geology and Geophysics, Pillsbury Hall, Minneapolis, MN 55455 (Jake Tielke, tielk003@umn.edu ). Introduction: Modeling of geodynamic processes of terrestrial planets requires rigorous constitutive equations that describe the rheological properties of mantle rocks. Extensive literature is available on data obtained from high-temperature, high-pressure defor- mation of olivine and mantle rocks [1-3]. However, extrapolation of these data to relatively low tempera- ture and high-stress lithospheric conditions results in significant overestimation of the strength of the litho- spheric mantle [4]. Although low-temperature defor- mation of olivine single crystals has been investigated under anhydrous [5] and hydrous [6] conditions, a constitutive equation describing the rheological behav- ior of olivine crystals as a function of water concentra- tion has yet to be established. In order to more fully characterize the rheological properties of the lithospheric mantles of terrestrial planets, an investigation is underway to derive consti- tutive equations that describe the flow behavior of olivine single crystals in a hydrous environment. Oli- vine is the most abundant and weakest mineral in the lithospheric mantle of terrestrial planets and, therefore, largely controls the rheological response of these im- portant regions. Constitutive equations derived from data obtained by carrying out deformation experiments of olivine single crystals in [101] c , [110] c , and [011] c orientations (see [3] for a notation explanation) allow the relative strengths of slip systems to be assessed. Our experiments described here were carried out on [101] c single crystals at temperatures, confining pres- sures, differential stresses, and water concentrations appropriate for lithospheric mantle conditions. Experimental Methods: Single crystals of San Carlos olivine were oriented using electron backscat- tered diffraction and cut into rectangular prisms of dimensions 3.9 × 4.1 × 8.0 mm. Each crystal was placed in a cylindrical Ni capsule with a 7 mm outer diameter and a 5 mm inner diameter. The Ni capsules were constructed from two, telescoping, single-ended Ni cans to provide a water-tight assembly. Shims of talc or compressed brucite powder were inserted to fill the Ni capsule and supply water. The Ni capsules were positioned between alumina and zirconia pistons and jacketed in an iron sleeve. Each assembly was then inserted into a high-resolution gas-medium deforma- tion apparatus, annealed at a temperature of 1250°C and confining pressure of 300 MPa for 1.25 h. Sam- ples were subsequently deformed at 1200°C in a series of constant stress steps. Dislocation microstructures of deformed samples were observed using optical and scanning electron microscopy employing the oxidation/decoration tech- nique [7,8]. Water concentrations were determined using Fourier transform infrared spectroscopy. Experimental Results: Results from triaxial de- formation experiments on [101] c olivine crystals not only indicate a significant water-weakening effect but also reveal a marked dependence of creep rate on silica activity. Strain rate as a function of differential stresss data from creep experiments on four samples deformed under hydrous (wet) conditions are presented in Fig. 1 along with those for one sample deformed under anhy- drous (dry) conditions [9]. The experiments were car- ried out at differential stresses ranging from 20 to 270 MPa and strain rates of 1.7×10 -6 to 2.2×10 -3 s -1 . Bru- cite-buffered and talc-buffered samples contained similar OH concentrations. The stress exponent is similar for all three types of samples, ranging from 3.5 for talc-buffered to 3.6 for brucite-buffered to 3.7 for dry samples. At a differential stress of 100 MPa, talc- buffered samples deform a factor of ~3 faster than the dry sample, and the brucite-buffered samples deform a factor of ~3 faster than the talc-buffered samples, as illustrated in Fig. 1. Figure 1: Stress versus strain rate plot from triaxial compres- sive creep experiments on brucite-buffered, talc-buffered, and dry [101] c olivine crystals. Red = brucite buffered, blue = talc buffered, and black = dry. SES = [9], SJM = [3], and QB = [1]. 2273.pdf 42nd Lunar and Planetary Science Conference (2011)