FIELD-DEPENDENCE OF AC SUSCEPTIBILITY IN TITANOMAGNETITES

AC susceptibility measurements as a function of field amplitude H-ac a nd of frequency show a strong field dependence for a set of synthetic titanomagnetites (Fe3-xTixO4) and for certain basalts from the SOH-1 H awaiian drill hole and from Iceland. In-phase susceptibility is consta nt below fields of about 10-100 A/m, and then increases by as much as a factor of two as H-ac is increased to 2000 A/m. Both the initial fie ld-independent susceptibilities and the field-dependence of susceptibi lity are systematically related to composition: initial susceptibility is 3 SI for a single-crystal sphere of TMO (x = 0) and decreases with increasing titanium content; field-dependence is nearly zero for TMO and increases systematically to a maximum near TM60 (x = 0.6). This fi eld dependence can in some cases be mistaken for frequency dependence, and lead to incorrect interpretations of magnetic grain size and comp osition when titanomagnetite is present. (C) 1998 Elsevier Science B.V . All rights reserved.