Electrical Conductivity of CO₂ and H₂O‐Bearing Nephelinitic Melt

Abstract

Despite previous measurements on electrical conductivity of carbonate or carbonatite melts, the effect of several wt% CO2 on electrical conductivity of silicate melts has never been experimentally examined. This study investigated the CO2 effect on electrical conductivity of nephelinitic melt at 500°C–1,150°C and 0.5–1.0 GPa with 0.1–6.0 wt% CO2 and 0.2–1.6 wt% H2O in piston cylinder apparatus using sweeping frequency impedance analyses. Experimental results show that volatile‐bearing nephelinitic melt is highly conductive. In contrast with the strong enhancing effect of H2O on electrical conductivity of silicate melts, CO2 shows weak and even slightly negative influence on electrical conductivity of nephelinitic melt within the investigated CO2 range. Literature data of electrical conductivity of hydrous carbonated basaltic melts suggested strong positive CO2 effect at CO2 >10 wt%, but their model prediction at <6 wt% CO2 is consistent with our results. A weak CO2 effect implies that CO32− does not contribute to electrical conduction in any significant way at CO2 <6 wt%. Our results suggest that magnetotelluric survey is much less sensitive to CO2 content than to H2O content of crustal magma reservoirs.

A Review of Relationship between the Metallogenic System of Metallic Mineral Deposits and Lithospheric Electrical Structure: Insight from Magnetotelluric Imaging

“…In comparison with the conductivity of the vapor phase, the volatiles in the fluids or melts have a positive effect on the bulk conductivity of rocks. Taking carbon dioxide (CO2), which is the second most abundant volatile in the lithosphere, as an example, (1) CO2 has much higher solubility in alkali-rich melts [16][17][18][19]; (2) the conductivity of the CO2 presented as the vapor phase is approximately 0.01 S/m (resistivity of 100 Ω•m), and 5-8 vol% of CO2 can reduce the conductivity from 0.04 S/m (resistivity of 250 Ω•m) to 0.01 S/m (resistivity of 100 Ω•m) [20];…”

“…(3) when CO2 is in the melt, the carbonate ion (CO2 −3 ) can act as the effective charge carrier, and contribute significantly to electrical conduction [16,21]. Because conductivity is extremely sensitive to the high-conductivity, high-temperature and high-rheology materials (e.g., metallic minerals, melts and aqueous fluids) [23], the electrical structure (the spatial variation of conductivity or resistivity) may be preferred when studying the metallogenic system (especially metallic minerals).…”