Remagnetization in lava flows recording pretransitional directions

Abstract

Basaltic lava flows associated with the last reversal in three distinct volcanic sequences of the island of La Palma are characterized by overlapping components with very high unblocking temperatures. A medium high‐temperature (MHT) component with normal polarity is resistant up to 500–550°C, while in most cases the high‐temperature (HT) primary reverse component cannot be isolated below 520°C. The same situation (with opposite polarities) is observed also in a lava flow associated with the onset of the upper Réunion reversal in the Gamarri sequence in Ethiopia. The presence of both polarities within all these single flows cannot be reconciled with the hypothesis that a full reversal was systematically recorded during their cooling. The direction of MHT is either close or similar to the direction of the HT component of the overlying flows, which indicates that remagnetization occurred shortly after emplacement. Rock magnetic studies, microscopic observations, microprobe analyses and remagnetization experiments have been carried out to investigate the origin of MHT. The rock magnetic parameters do not exhibit specific anomalies. The unblocking temperatures are mostly higher than the simulations involving reheating by the overlying flow combined with thermoviscous overprinting caused by slow cooling. The most plausible scenario is that baking by the overlying flows was accompanied by low‐temperature oxidation of titanomagnetite to cation‐deficient titanomagnetite. This description also includes the presence of cracks, fractures and other heterogeneities of the lava which induce variability between parallel profiles. Similarities with characteristics observed at Steens Mountain could suggest that the hypothesis of rapid geomagnetic changes recorded by a single lava flow should be considered with caution.