Short wavelength vertical fluctuations of the melting regime in the suboceanic melting region

Modelling of mantle residua cpx REE patterns allow recognizing short wavelenght vertical variability of the porosity regime of a melting region differing from that deriving after melt focusing processes ultimately leading to dunitic channelling of the mantle section. A trace element detailed study of residual clinopyroxenes from the ultraslow eastern SWIR section shows compositional trends crosscutting the expected partial melting trends at the typical kilometre lenghtscale. In the REE compositional space these trends appear as pattern rotations around a mid-point. Open-system melting modelling reveals the intensity of the rotation and the position of the pivot element depending mainly on the ratio between input/output melt flux and on the enrichment of the percolating melt with respect to the depleted screen. We derived two important indications on the process: first some regions experience near-batch melting, i.e. melt accumulation with very low output melt flux, in regions soon after the grt/sp transition. This observations suggest permeability barriers to occur in the main melting region possibly due to porosity consumption by melt/rock reaction enhanced by grt breakdown energy competition. Consequently melt stagnation processes similar to those described for the plagioclase facies may act in the spinel field portion of the melting region. Second: we attest the presence of enriched melts delivered to the spinel field region. These melts may derive from very low degrees of melting in the garnet field of a DMM source or by consumption of an enriched heterogeneity with a lower melting point than the surrounding mantle.