because the measurement is done in the presence of a field, the reference needed for equation 3 ($M_{\rm TRM}$) is compromised. In contrast, heatings for directional measurements are always done in field-free space, and a sample can suffer some alteration and still retain useful information.

Hence, finding samples that will not alter during laboratory experiments is one key requirement. This constraint is also closely related to the metamorphic and weathering history of a rock. In general, the formation of Fe-rich clays spells disaster for paleointensity recording because these can alter during paleointensity experiments to form new iron-oxides [e.g. @Cottrell2000; @Gill2002; @Kars2012]. Ultramafic rocks, including komatiitic lavas, might be thought of as good carriers of Archean fields, but the Fe-rich phyllosilicate serpentine \[(Mg, Fe)$_{3}$Si$_{2}$O$_{5}$(OH)$_{4}$\] and magnetite are commonly formed in these rocks. The magnetite carries a magnetization dating to the alteration age (which is usually uncertain) and the magnetization is not a TRM; instead it is a crystallization or chemical remanent magnetization (CRM), and a straightforward tie to past field strength is lost [@Yoshihara2004; @Smirnov2005].\

Time, Temperature and Domain State
----------------------------------

The principal magnetic mineral recorder of interest for Paleoarchean and older rocks is magnetite having a Curie temperatures of $\sim$580 