Seismic models of magma chambers at fast-spreading oceanic ridges predict small perched magma chambers above a wide low-velocity zone. Seismic attenuation in a low-velocity zone allows for ∼10% melt in hot gabbros. In order to reconcile these data with a structural model of magma chambers that is supported by compelling field data in the Oman ophiolite, we propose that the low-velocity zone belongs to the magma chamber. In such a dynamic magma chamber, gabbros develop strong shape fabrics during magmatic flow in a concentrated suspension. This process results in a drastic reduction of the melt fraction that is compatible with magmatic flow, as confirmed by preliminary experiments. We propose here that dynamic magma chambers below ridges may contain as little as 10% melt.

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