Microstructural and fabric development and seismic anisotropy of the low-velocity zone beneath the volcanic front in the mantle wedge: insights from peridotite xenoliths in the Avacha volcano, Kamchatka Peninsula

The aim of this study is to understand the structure of the uppermost mantle wedge beneath the volcanic front. Peridotite xenoliths from the Avacha volcano, derived from the uppermost mantle beneath the volcanic front of the Kamchatka arc, are mainly spinel harzburgites [Ishimaru et al., 2007]. The main constituent minerals are olivine, orthopyroxene, spinel and minor amount of clinopyroxene and amphibole. Fo% of olivine and Cr# of spinel range from 90 to 92 and from 0.5 to 0.8, respectively, indicating that they are mostly depleted peridotites. They are metasomatized to various extents, with the formation of secondary orthopyroxene replacing primary olivine [Ishimaru et al., 2007]. Olivine commonly exhibits a granular texture with partly extended grains (average grain size: 0.57 mm). Olivine CPOs are dominantly (001)[100] patterns. The fabric intensity (J-index) ranges from 3.75 to 21.14. Equilibrium temperatures estimated by Ca in orthopyroxene geothermometer are approximately 800-1050 °C [Ishimaru et al., 2007; Michibayashi et al., 2009]. We found that peridotites having higher J-index and equilibrium temperature tend to show coarser, extended and irregular olivine grains. Moreover, the [100]-axis preferred orientation tends to be more intense than the other two axes as increasing J-index and equilibrium temperature. Microstructures and CPOs of lower J-index types are similar to those of peridotite xenoliths obtained from Ichinomegata volcano, northeastern Japan, located to the back-arc region of the uppermost mantle wedge [Michibayashi et al., 2006]. These indicate that although the peridotites have been previously established as a lithospheric mantle, a higher geothermal gradient in the low velocity zone beneath the Avacha volcanic front could result in reactivation of such lithospheric mantle, where peridotites were inhomogeneously deformed. To compare with seismic anisotropy in the Kamchatka region, average seismic anisotropy of the peridotite xenoliths from the Avacha volcano were calculated from olivine and pyroxenes CPOs and single crystal elastic constants. S-wave splitting (delay time of 0.1-0.3 seconds) observed there could be explained by average seismic anisotropy of the peridotites for approximately 12-38 km thick anisotropic layer. [Michibayashi et al., 2009]. REFERENCES: Ishimaru, S., S. Arai, Y. Ishida, M. Shirasaka, and V. M. Okrugin (2007), Melting and multi-stage metasomatism in the mantle wedge beneath a frontal arc inferred from highly depleted peridotite xenoliths from the Avacha volcano, southern Kamchatka, J. Petrol., 48, 395- 433. Michibayashi, K., N. Abe, A. Okamoto, T. Satsukawa, and K. Michikura (2006), Seismic anisotropy in the uppermost mantle, back-arc region of the northeast Japan arc: Petrophysical analyses of Ichinomegata peridotite xenoliths, Geophys. Res. Lett., 33, L10312, doi:10.1029/2006GL025812. Michibayashi, K., T. Oohara, T. Satsukawa, S. Ishimaru, S. Arai, and V. M. Okrugin (2009), Rock seismic anisotropy of the low-velocity zone beneath the volcanic front in the mantle wedge. Geophys. Res. Lett., 36, L12305, doi:10.1029/2009GL038527.