Electromagnetic evidence for volatile‐rich upwelling beneath the society hotspot, French Polynesia

Abstract

We have conducted a seafloor magnetotelluric survey that images, for the first time, three‐dimensional electrical conductivity structure in the upper mantle beneath the Society hotspot. A striking feature in our model is a high‐conductivity anomaly a few hundred kilometers in diameter, which is continuous from the lowest part of the upper mantle to a depth of approximately 50 km below sea level. Using theoretical and experimental results from mineral physics, we interpret the high‐conductivity anomaly as evidence of the melt fraction up to 2.2 vol.%, which is robust regardless of assumed temperature, and the existence of carbonated silicate melt beneath the hotspot. Our results suggest that the Society hotspot is a pathway for ascending volatiles from the deeper part of the upper mantle to the surface.

The Link between the Physical and Chemical Properties of Carbon-Bearing Melts and Their Application for Geophysical Imaging of Earth’s Mantle

“…We calculate that incipient melting of a peridotite can produce an electrical anomaly (~0.1 S m -1 ) 22,23,27,75 if it contains 0.1 AE 0.04% of carbonatite melts, 0.3 AE 0.15% of carbonated basalts, or 1-6% of hydrated basalts at 1350 C and 3 GPa (Figure 7.10). Note that the calculation is only weakly dependent on temperature given the low temperature dependence of the EC of incipient melts (Figure 7.7).…”

Three‐Dimensional Electrical Resistivity Structure Beneath a Volcanically and Seismically Active Island, Kyushu, Southwest Japan Arc

“…The final 3‐D model can be assumed to be sensitive to the 18 parts of these conductive blocks because the RMS misfits of all test models increase the RMS misfit from the final model value. In addition, we examined the RMS variations of the sensitivity test models using an F test for the (67 × 16 × 12 − 1 =) 12,863 degrees of freedom of the data at the 95% confidence level (e.g., Hata et al, ; Hata et al, ; Tada et al, ; Yamaya et al, ). The upper cumulative distribution of the F test ( F = 1.0352) provides a F RMS misfit of 1.2856 based on the RMS misfit of 1.2636 of the final model: .…”

Validity of the dispersion relations in magnetotellurics. Part II: synthetic and field data

“…However, hoping to reveal at least the main features of the offshore MT observations, we applied a similar search algorithm as used above to the integrated database of projects that Earthquake Research Institute (ERI) of the University of Tokyo and/or Japan Agency for Marine-Earth Science and Technology (JAMSTEC) participated in. This database includes: Normal Oceanic Mantle (NOMAN) project in northwestern Pacific (Baba et al 2013(Baba et al , 2017a, Stagnant Slab Project (SSP) in Philippine Sea (Seama et al 2007;Baba et al 2010), Mantle Electromagnetic and Tomography (MELT) experiment on East Pacific Rise (Evans et al 1999;Baba et al2006a, b), Tomographic Investigation by seafloor ARray Experiment for the Society hotspot (TIARES) project in French Polynesia (Nolasco et al 1998;Tada et al 2016), as well as marine data acquired in the vicinity of the Sado ridge (Toh et al 2006), Mariana trough (Matsuno et al 2010), and Tristan-da-Cunha hotspot (Baba et al 2017b). The obtained results are given in Table 3.…”