10:15 〜 10:30
[SIT17-06] Electrical conductivity of CO2-bearing hydrous basaltic melt: implications for the conductivity anomaly beneath the ocean floor
キーワード：玄武岩質メルト、水、電気伝導度、二酸化炭素
The origin of this high electrical conductivity has long been the subject of debate and is generally explained by two hypotheses: a presence of hydrous olivine and partial melting. The partial melt hypothesis is due to the presence of small amounts of hydrous basaltic melt in the uppermost asthenosphere, which coincides with the seismic low velocity zone (LVZ). However, there are few electrical conductivity measurements of realistic basaltic melts at conditions relevant to the LAB. Ni et al. (2011a) measured the conductivity of hydrous Fe-free basaltic melt with 0-6 wt% H2O at 2 GPa in a piston cylinder. The actual liquidus temperatures of Fe-bearing basaltic rocks are considerably lower than those of Fe-free system, and their data were not taken into account in the contribution of CO2 to the conductivity, making it dangerous to use them directly. In addition, we found that the basaltic melt is extensively reactive with capsule materials commonly used for electrical conductivity measurements (e.g., alumina and olivine), resulting in a compositional shift from the starting material. Thus, reliable data which can be directly applied to the asthenospheric P-T conditions remain sparce. In this study, the electrical conductivity of dry and hydrous CO2-bearing basaltic melts has been measured up to 1750 K at 2 GPa, corresponding to pressure around the Earth lithosphere-asthenosphere boundary. Water-melt immiscibility has been found in CO2-rich samples, which precluded massive dissolution of water to the melts. The electrical conductivity of the samples are comparable to those reported by previous studies on the Fe-free basaltic melt. While H2O is the main factor enhancing the conductivity of the melts by depolymerizing the Si-O tetrahedra, CO2 plays a role in controlling the dissolved water, intermediately affecting the conductivity. The activation enthalpy of basaltic melt is markedly higher than that of more evolved silicate melts due to the more enriched alkaline earth elements. Our results suggest that an overall melt fraction of 0.1-7.0 vol% is needed to account for the high electrical conductivity anomalies (10-1.3 to 10-0.3 S/m) beneath the southern East Pacific Rise and Cocos plate, if water is the only volatile in the melt. However, for those regions where the electrical conductivity is extremely high (10-0.3 S/m), the dissolved water content should be no less than 4 wt% to maintain a mechanically stable status. Accordingly, the dissolved CO2 content is strictly limited in such melting zones.