Temperature and melting of a ridge-centred plume with application to Iceland. Part II: Predictions for electromagnetic and seismic observables

Abstract

The dynamic and melting processes of a ridge-centred plume have been investigated in a companion paper by Ruedas et al. (hereafter referred to as Paper I) in a set of 3-D numerical fluid dynamic models with varying plume excess temperatures and melt extraction thresholds. In Paper I, the modelled thickness of the generated crust has been compared to observations of the Icelandic crust. Using the results of those plume models magnetotelluric (MT) transfer functions and seismic velocity anomalies are predicted in this paper. Together with Paper I, a dynamically consistent set of geophysical observables of a ridge-centred plume is presented and applied to Iceland.

Temperature, partial melting and the connectivity of the melt phase influence the electrical conductivity of crust and mantle rocks. The temperature and melt fraction of our plume models are used to calculate 3-D conductivity models for MT modelling. For the melt geometry ellipsoidal inclusions with appropriate aspect ratios were assumed to control melt connectivity. The resulting transfer functions are compared to each other and to models not including a plume to separate signals from the ridge and the plume. They may be applied to observed MT measurements. If the plume head contains only 1 per cent of melt, the plume signal cannot be distinguished from the ridge signal, at least 3 per cent melt is needed for such distinction. The other predicted observables calculated from the different numerical models are seismic velocity anomalies. The temperature-induced VP and VS anomalies were estimated including anharmonic and anelastic effects as well as the water induced increase of dislocation mobility that lowers seismic velocities. Realistic melt geometries, as observed in laboratory experiments, were used to calculate the effect of partial melts on the seismic velocities. VS anomaly distributions are synthesized from the different plume models and compared to seismic observations. To reconcile seismic anomalies of the plume head and plume stem, a wet plume stem overlain by a partially molten, dehydrated plume head is favoured.

The combined interpretation of available observations, crustal thicknesses (Paper I) and seismic results, with our dynamic plume models (Paper I) leads to a favoured plume model with 135 K excess temperature and a vertical velocity of approximately 13 cm yr−1 at 200 km depth, with 1 per cent melt extraction threshold, and a melting zone of approximately 500 km width and 100 km depth extent.