Emplacement of magma in Eastern Iceland dikes: Insights from magnetic fabric and rock magnetic analyses

The anisotropy of magnetic susceptibility (AMS), coupled with mineral magnetic identification, has been investigated in about 640 samples from 27 basaltic dikes of Tertiary age in Eastern Iceland. The dikes are a few meters thick and they have been sampled all across. Three types of magnetic fabrics with well-defined mean principal axes of anisotropy are recognized. In the first one the maximum axis is vertical and the minimum axis coincides with the pole of the dike plan (“geometrically normal” fabric). The second fabric shows opposite orientation (“geometrically inverse” fabric). The third type is when these two fabrics coexist with the “normal” one at the margins and the “inverse” one in the center of the dike (“mixed” fabric). No relationship is observed between the type of fabric and the width of the dikes. Each fabric is carried by clear magnetic assemblages made of low (x ∼ 0.15) Ti-content fine-grained magnetites for the “normal” one and high Ti-content (around TM60) coarse magnetite grains for the “inverse” fabric. Upon heating, the magnetic properties and the orientation of the anisotropy ellipsoid characterizing the “inverse” fabric in the mixed dikes change and evolve toward the “normal” type. These two geometrically “normal” and “inverse” fabrics are interpreted as primary ones, representing different physico-chemical conditions during the emplacement of the magma. The AMS results indicate that, during the volcanic events occurring about 10 Myr ago, the extrusion of the magma was vertical at the depth of the studied sites in eastern Iceland.