Highly magnetised and differentiated basalts at the 18–19° S propagating spreading centre in the North Fiji Basin

Abstract

Geochemical and magnetic properties of basalts dredged along a propagating spreading centre (central part of the North Fiji Basin) have been analysed. Two phenomena appear to be important in controlling variations of natural remanent magnetisation (NRM): 1) an extensive magma differentiation to Fe-Ti basalts increases with the intensity of NRM; and both increase towards the propagator tip, 2) low temperature oxidation (maghemitisation) seems to have already occurred along the ridge axis for the zero age basalts. This paper shows that despite the apparent lack of correlation between NRM and the maghemitisation process (masked by the effect of extensive magma differentiation), the latter involves a change of the domain state of magnetic carriers, from pseudo single-domain to single-domain. We separated the NRM into 4 partial NRMs (pNRM) depending on the coercivity of grains. This analysis showed that the contribution to the remanence of grains with high coercivity increases with maghemitisation, whereas the contribution of grains with weak coercivity decreases.