Using Electrical Conductivity to Map Melt and Fluids at Subduction Zones

Abstract:

Electrical conductivity as measured by the Magnetotelluric (MT) method provides a powerful means of mapping fluid release from subducting slabs and the subsequent melt transport to the volcanic arc.

It is well known that both aqueous fluids and partial melt are electrically conductive, with conductivities that depend on temperature and composition, although many of the details of the relationships between the controlling influences are not well constrained by the existing database of laboratory measurements.

Recent MT images from subduction zones generally highlight two primary areas of fluid release. The first typically occurs at depths around 40km, and is thought to represent the release of fluid as basalt transitions to eclogite. The second, deeper release, appears to relate to the breakdown of serpentinite in the upper mantle of the downgoing slab. In general, the shallower release results in a conductor caused by aqueous fluids while the deeper release is into mantle sufficiently hot that melting occurs. The processes involved in this deeper release and melting make it difficult to untangle the contributions of water and melt to the bulk conductivity and, along with limitations in lab data, make it difficult to constrain melt and fluid fractions.

We look at example sections from a range of subduction zones and highlight some of the controls on where conductors occur, and why in some cases they aren’t seen. Particularly important is the nature of the incoming plate and the kinematics and duration of subduction. We will show through these examples the current limits of our abilities to untangle the signatures of melt from water in the transport processes.