I have noticed many studies which conclude that the observed paleomagnetic signatures are primary, mostly based on the van der Voo criteria, as well as rock magnetic studies and geochronology. However, it appears that some of these rocks which I have studied also, show the complete replacement of magnetite by titanite and ilmenite. How can I account for this discrepancy? Is there something I am not aware of?
Of course, conversion to ilmenite or titanite cannot occur with pure magnetite, but conversion to titanite is commonplace with titanomagnetites. This is sometimes referred to as "titanomagnetite leucoxenization", but it is not a completely correct designation. The term leukoxene was originally introduced for altered ilmenites from the heavy sediment fraction.
To change pure magnetite into titanite or ilmenite needs an introduction of Si and Ti or Ti, respectively, a reaction which can only take place in the hypogene environment. Neither ilmenite nor titanite are magnetic.
If pure magnetite is completely converted into the cited titanite bivalent and trivalent Fe have to be removed, mostly with a small amount of trivalent Fe retained in the lattice of titanite. The resulting titanite no longer shows a ferrimagnetic feature.
The released Fe can be transferred into maghemite [ γ-Fe2O3] under oxidizing conditions and the paleomagnetic character of the rock is preserved because this trivalent Fe bearing oxide in contrast to hematite still shows ferrimagnetic features. Maghemite forms under near-ambient conditions during weathering in duricrusts and oxisols and low-T hydrothermal processes. Maghemite replaces in this case common Fe-bearing spinel s.s.s. such as franklinite or hercynite and ferrite spinels such as magnetite and chromite. There exists also a titanomaghemite derived from titaniferous magnetite.
HGD