Charge transport in concentrated magmatic brines from molecular dynamics simulations

Magmatic brines are high-temperature (>600 K) fluids rich in chlorine and metals that likely accumulate above magmatic reservoirs at depths of 1–6 km beneath active volcanoes. Magnetotelluric methods can detect the presence of these charged fluids by probing electrical conductivity in volcanic plumbing systems. However, linking measured conductivity to the presence of magmatic brines requires a detailed understanding of charge-transport processes in these fluids under relevant pressure–temperature–composition conditions.

 

We use molecular dynamics (MD) simulations to investigate supercritical H₂O–NaCl brines at 673–873 K and 1–1.5 kbar across a wide range of salinities (10–40 wt% NaCl). We show that solvent density fluctuations explain the conductivity trends. Indeed, solvent density fluctuations alter local ionic environments, allowing the formation of large ion clusters that constantly break and reform with picosecond lifetimes. Charge transport, therefore, does not depend on a spatially homogeneous and time-independent dielectric constant but instead on the fraction of ions that migrate from fully solvated regions into low-density, water-poor domains, where strongly reduced electrostatic screening enhances ion association.