Activities of olivine slip systems in the upper mantle Paul Raterron a,⇑ , Jennifer Girard a,b , Jiuhua Chen b a Unité Matériaux et Transformations, CNRS 8207, Bât C6, Université Lille 1, F-59655 Villeneuve d’Ascq Cedex, France b Center for the Study of Matter at Extreme Conditions, Department of Mechanical and Materials Engineering, Florida International University, Blg. VH140, Miami, FL 33199, USA article info Article history: Received 19 December 2011 Received in revised form 13 April 2012 Accepted 18 April 2012 Available online 27 April 2012 Keywords: Upper mantle Rheology High pressure Forsterite San Carlos olivine Dislocation creep Slip systems Activation volume Seismic anisotropy abstract We investigated the effect of pressure (P) on olivine [1 0 0](0 0 1) and [0 0 1](1 0 0) dislocation slip systems by carrying out deformation experiments in the Deformation-DIA apparatus (D-DIA) on single crystals of Mg 2 SiO 4 forsterite (Fo100) and San Carlos (SC) olivine (Fo89), at P ranging from 5.7 to 9.7 GPa, temper- ature T = 1473 and 1673 K, differential stress r in the range 140–1500 MPa, and in water-poor conditions. Specimens were deformed in axisymmetry compression along the so-called [1 0 1] c crystallographic direction, which promotes the dual slip of [1 0 0] dislocations in (0 0 1) plane and [0 0 1] dislocations in (1 0 0) plane. Constant r and specimen strain rates ( _ e) were monitored in situ by synchrotron X-ray dif- fraction and radiography, respectively. Comparison of the obtained high-P rheological data with room- P data, previously reported by Darot and Gueguen (1981) for Fo100 and Bai et al. (1991) for SC olivine, allowed quantifying the activation volume V  in classical creep power laws. We obtain V  = 9.1 ± 1.6 cm 3 /mol for Fo100. For SC olivine, we obtain V  = 10.7 ± 5.0 cm 3 /mol taking into account the oxygen- fugacity uncertainty during the high-P runs. These results, combined with previous reports, provide com- plete sets of parameters for quantifying the activities of olivine dislocation slip systems. Extrapolation of the rheological laws obtained for SC olivine crystals to conditions representative of natural deformations show that [1 0 0](0 1 0) slip largely dominates deformation in the shallow upper mantle. At depths greater than 65 km along a 20-Ma oceanic geotherm or 155 km along a continental geotherm, the dual activ- ity of [1 0 0](0 0 1) and [0 0 1](1 0 0) slips becomes comparable to that of [1 0 0](0 1 0) slip. At depths greater than 240 km, [0 0 1](0 1 0) slip becomes dominant over all other investigated slip systems. Such changes in olivine dislocation-slips relative activity provide a straightforward explanation for the seismic anisot- ropy contrast and attenuation with depth observed in the Earth’s upper mantle. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Earth’s upper mantle plasticity is constrained by the rheology of olivine at extreme conditions: upper-mantle rocks consist of oliv- ine-rich aggregates and pressure (P) and temperature (T) are in ex- cess of 13 GPa and 1700 K at the transition zone boundary (410- km depth). Olivine deforms by dislocation creep in the shallow upper mantle (depth <200 km) as revealed by the strong seismic velocity anisotropy observed there (e.g., Montagner, 1994; Gung et al., 2003). The anisotropy is interpreted as lattice preferred ori- entations (LPO) in aggregates deformed by dislocation creep, which induce the anisotropic propagation of seismic waves owing to the elastic anisotropy of individual olivine grains. Although seismic anisotropy is attenuated in the deep upper mantle (Gung et al., 2003), olivine dislocation creep may also play a significant role there as suggested by several authors (e.g., Mainprice et al., 2005; Jung et al., 2009; Ohuchi et al., 2011; Raterron et al., 2009, 2011). The interpretation of seismological data and the modeling of upper-mantle thermal convection, thus require quantifying the ef- fect of P on olivine dislocation activity. This effect can be investi- gated by carrying out high-P deformation experiments on oriented single crystals, within which slip systems can be individ- ually activated by geometrical considerations. Slip-system sensi- tivity to pressure is then quantified by the activation volume V  – the higher V  , the harder the system at high P – in rheological laws such as the classical power law: _ e ¼ Ar n fO m 2 exp  E  þ PV  RT ð1Þ where _ e is the strain rate, A is a pre-exponential factor, r = r 1  r 3 the differential stress with r 1 the principal stress, n and m are con- stants, fO 2 is the oxygen fugacity, E  the activation energy, and R the gas constant. Eq. (1) can accommodate other terms quantifying crystal chemical environment, such as the activity of a given phase (i.e., pyroxene activity a opx ) or the fugacity of a given fluid 0031-9201/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.pepi.2012.04.006 ⇑ Corresponding author. Tel.: +33 (0) 320 43 46 86; fax: +33 (0) 320 43 65 91. E-mail address: Paul.Raterron@univ-lille1.fr (P. Raterron). Physics of the Earth and Planetary Interiors 200–201 (2012) 105–112 Contents lists available at SciVerse ScienceDirect Physics of the Earth and Planetary Interiors journal homepage: www.elsevier.com/locate/pepi