Abstract

226Ra excesses in arc lavas are inferred to result from fluid addition from the subducting plate and their preservatrion provides an important constraint on the magma extraction rate, indicating rapid transport from source to surface <1000 years. This requires channelled melt ascent but an important question is whether melt can be supplied to veins sufficiently rapidly to preserve short-lived isotope disequilibria. Such high rates require a fluid dynamical explanation, yet are incompatible with a transport history governed by compaction and simple porous flow. We present the results of a 2D axisymmetric numerical model that simulates the lateral flow of viscous melt into low pressure channels in downwelling subarc mantle. Finite element coupling provides a simultaneous solution to the fluid dynamical equations linking the transition between porous and channel flow. The transitioning (Brinkman) local flow field is non-linear and position-dependent, increasing in velocity towards the outer channel boundary. Average maximum lateral flow rates for a reference matrix permeability of 10- 15m2 are c. 10-4m/s, a factor of 104 to 100 times faster than matrix downweling velocites based on plate tectonic rates. Upwards melt flow rates in the channel (r = 0.1 m) are c. 0.03 m/s. Vertical (downwards) melt flow in the porous matrix is c. 104 lower than lateral migration veocities. These physical results are consistent with a downgoing arc mantle wedge source region where melting and extraction are sufficiently rapid to preserve source-derived 238U-230 Th-226Ra and potentially also 226Ra-210Pb disequilibria.