Field-dependence of AC susceptibility in titanomagnetites

AC susceptibility measurements as a function of field amplitude Hac and of frequency show a strong field dependence for a set of synthetic titanomagnetites (Fe3−xTixO4) and for certain basalts from the SOH-1 Hawaiian drill hole and from Iceland. In-phase susceptibility is constant below fields of about 10–100 A/m, and then increases by as much as a factor of two as Hac is increased to 2000 A/m. Both the initial field-independent susceptibilities and the field-dependence of susceptibility are systematically related to composition: initial susceptibility is 3 SI for a single-crystal sphere of TM0 (x=0) and decreases with increasing titanium content; field-dependence is nearly zero for TM0 and increases systematically to a maximum near TM60 (x=0.6). This field dependence can in some cases be mistaken for frequency dependence, and lead to incorrect interpretations of magnetic grain size and composition when titanomagnetite is present.