[S14-1] Upper mantle and transition zone dynamics and structure I

[S14-1-04] A three-dimensional electrical conductivity image of the mantle plume of the Society hotspot in French Polynesia

^○Noriko Tada^{1, 3}, Pascal Tarits², Kiyoshi Baba³, Hisashi Utada³, Takafumi Kasaya¹, Daisuke Suetsugu¹ (1.JAMSTEC, Yokosuka, Japan, 2.IUEM, Plouzane, France, 3.The University of Tokyo, Tokyo, Japan)

Electrical conductivity is a physical property, which is independent from seismic velocity, and is sensitive to temperature, melt fraction, and volatiles (hydrogen and carbon). By combining electrical conductivity and seismic velocity, we could constrain the physical properties of the upper mantle. The previous seismic studies found a low-velocity anomaly possible corresponding to the mantle plume beneath the Society hotspot (Suetsugu et al., 2009; Isse et al., 2006). However, because of the sparse observation array, the resolution of these images is insufficient to discuss the thermal and chemical properties of the plume. Besides, a previous marine magnetotelluric (MT) data (Nolasco et al., 1998) was obtained from too narrow region in the vicinity of the Society Islands to get a three-dimensional (3-D) image of the plume. Therefore, we conducted Tomographic Investigation by seafloor ARray Experiment for the Society hotspot Project (TIARES project) to image the possible plume with reasonable resolution using both MT and seismological observations (Suetsugu et al., 2012). In this presentation, we will focus on the results from MT data, both obtained by the previous study and the TIARES project.
We recently obtained a 3-D electrical conductivity image of the upper mantle beneath the Society hotspot (Tada et al., 2016). One of the most remarkable features is the presence of a distinct high-conductivity anomaly that extends from the mantle transition zone to a depth of approximately 50 km below the sea level. The amplitude of electrical conductivity of the anomaly is one to two orders of magnitude higher than the surrounding mantle. Assuming that the bulk mantle can be represented by a mixture of olivine and melt, the anomaly is explained by the existence of melt fraction up to 2.2 vol.% regardless of assumption of temperature. Furthermore, the existence of carbonated silicate melt is confirmed because of CO2 in the melt is well constrained to be at least 8 wt.%.