Assessing the role of thermal disequilibrium in the evolution of the lithosphere-asthenosphere boundary: an idealized model of heat exchange during channelized melt transport

This study explores how the continental lithospheric mantle (CLM) may be heated during channelized melt transport when there is thermal disequilibrium between (melt-rich) channels and surrounding (melt-poor) regions. Specifically, I explore the role of disequilibrium heat exchange in weakening and destabilizing the lithosphere from beneath as melts infiltrate into the lithosphere-asthenosphere boundary (LAB) in intraplate continental settings. During equilibration, hotter-than-ambient melts would be expected to heat the surrounding CLM, but we lack an understanding of the expected spatiotemporal scales and how these depend on channel geometries, infiltration duration, and transport rates. This study assesses the role of heat exchange between migrating material in melt-rich channels and their surroundings in the limit where advective effects are larger than diffusive heat transfer (Peclet numbers > 10). I utilize a 1D advection-diffusion model that includes thermal exchange between melt-rich channels and the surrounding melt-poor region, parameterized by the volume fraction of channels (phi), average relative velocity (nu(channel)) between material inside and outside of channels, channel spacing (d), and timescale of episodic or repeated melt infiltration (tau). The results suggest the following: (1) during episodic infiltration of hotter-than-ambient melt, a steady-state thermal reworking zone (TRZ) associated with spatiotemporally varying disequilibrium heat exchange forms at the LAB. (2) The TRZ grows by the transient migration of a disequilibrium-heating front at a material-dependent velocity, reaching a maximum steady-state width delta proportional to [phi nu(channel) (tau/d)(n)], where n approximate to 2 for periodic thermal perturbations and n approximate to 1 for a single finite-duration thermal pulse. For geologically reasonable model parameters, the spatiotemporal scales associated with establishment of the TRZ are comparable with those inferred for the migration of the LAB based on geologic observations within continental intra-plate settings, such as the western US. The results of this study suggest that, for channelized transport speeds of nu(channel) = 1 m yr(-1), channel spacings d approximate to 10(2)m, and timescales of episodic melt infiltration tau approximate to 10(1) kyr, the steady-state width of the TRZ in the lowermost CLM is approximate to 10 km. (3) Within the TRZ, disequilibrium heat exchange may contribute approximate to 10(-5) W m(-3) to the LAB heat budget.