08/09/2007VLab-workshop1 Geophysical Anomalies in the Central Pacific Upper Mantle Implications for Water Transport by a Plume Shun-ichiro Karato Yale University New Haven, CT, USA

08/09/2007VLab-workshop2 Outline Anomalies in the central Pacific upper mantle –Seismic anisotropy –Electrical conductivity Some mineral physics background –Seismic anisotropy and water –Electrical conductivity and water Conclusions –Plume-asthenosphere interaction

08/09/2007VLab-workshop3 Ekström and Dziewonski (1998) Upper mantle in the central Pacific has unusually strong V SH >V SV anisotropy.

08/09/2007VLab-workshop4 Upper mantle in the central Pacific has unusually low conductivity. Karato (2007) Electrical conductivity, S/m

08/09/2007VLab-workshop5 Cause for anomalies How do various laterally varying parameters affect seismic anisotropy and electrical conductivity? Mineral physics of anisotropy and conductivity TemperatureX Major element chemistryX Partial meltingX O Water(hydrogen)O

08/09/2007VLab-workshop6 Temperature or major element chemistry (e.g., Fe/Mg) does not change the elastic anisotropy so much. --> A change in LPO should be a cause. Seismic anisotropy = elastic anisotropy of minerals + LPO (lattice-preferred orientation)

08/09/2007VLab-workshop7 LPO changes with physical/chemical conditions.  LPO is determined by the dominant slip system(s).  Conventional interpretation of anisotropy assumes one type of slip system ([100](010)).  If the dominant slip system changes, LPO will change (fabric transition), then the nature of seismic anisotropy will change.  How could the dominant slip system change with physical/chemical conditions? olivine

08/09/2007VLab-workshop8 LPO is controlled by the relative strength of slip systems. Deformation with b = [001] slip systems is more enhanced by water than deformation with b = [100] slip systems. Could fabric transition occur at higher water content? [Karato (1995)] -->simple shear deformation experiments (change in the slip direction) at higher water fugacity (0.3 GPa-->15 GPa)

08/09/2007VLab-workshop9 Olivine LPO Conventional interpretation of seismic anisotropy is based on the limited observations and assumed A-type fabric. But lab studies in my group have shown that other LPOs dominate under different conditions. Karato et al. (2007)

08/09/2007VLab-workshop10 Influence of water content and stress on deformation fabrics of olivine (at T~ K (asthenospheric temperature)) Seismic anisotropy in the asthenosphere, plume roots is likely caused by A-, or E- or C-type olivine fabrics depending on the water content. E- or C-type fabric. In the typical asthenosphere, dominant fabric is likely E-type.

08/09/2007VLab-workshop11 E-type A-type a a b b c c A-type olivine fabric causes strong V SH >V SV anisotropy. E-type olivine fabric causes weak V SH >V SV anisotropy. Karato (2007)

08/09/2007VLab-workshop12 Influence of water (hydrogen) on electrical conductivity in olivine Wang et al. (2006) 1.A large amount of hydrogen can be dissolved in olivine. 2.Hydrogen diffusion is fast.  Hydrogen may enhance conductivity?(Karato, 1990) Electrical conductivity, S/m Water content

08/09/2007VLab-workshop13 Conductivity in “normal” asthenosphere can be explained by a typical water content (~0.01 wt%). Conductivity in the central Pacific corresponds to “dry” olivine.

08/09/2007VLab-workshop14  Anomalies can be attributed to the dry asthenosphere in the central Pacific.  Why is the asthenosphere of the central Pacific dry?  What is the role of the Hawaii plume on modifying the properties of the central Pacific asthenosphere?

08/09/2007VLab-workshop15 Roles of a plume to modify the composition of the asthenosphere Plume: enriched (undepleted) = more water –direct mixing does not explain “dry” asthenosphere. –plume=wet + hot-> deep melting-> depleted materials

08/09/2007VLab-workshop16 deep melting in a plume (from Hirschmann (2006))

08/09/2007VLab-workshop17 In a plume column, melting occurs in the deep asthenosphere, providing “depleted” (dry) materials to the asthenosphere.

08/09/2007VLab-workshop18 A plume will feed “depleted (dry)” materials to the asthenosphere due to deep melting --> cause for geophysical anomalies in the central Pacific? Karato et al. (2007)

08/09/2007VLab-workshop19 Conclusions Both seismic anisotropy and electrical conductivity in the central Pacific are anomalous. These anomalies can be attributed to a low water (hydrogen) content in the asthenosphere in this region. A plume supplies “depleted (dry)” materials to the asthenosphere due to deep melting.

08/09/2007VLab-workshop20 Couvy et al. (2004) A E B C A three-dimensional fabric diagram of olivine E C Karato et al. (2007)

08/09/2007VLab-workshop21 At low stress and high T, A-, E- or C-type olivine fabrics will be important.