$^{o}$C. Hematite, with a Curie temperature of 675 $^{o}$C is also a potential recorder, but its occurrence as a primary phase recording a TRM is rare relative to magnetite. Even if we have a pristine rock that has undergone no chemical change, magnetite within the rock can be remagnetized by the aforementioned 320 $^{o}$C metamorphic event even though the peak temperature is less than the Curie temperature of magnetite. To understand this phenomena, we start with the recognition that metamorphic heating will affect magnetic grains differently, according to their size and related domain state.

Large magnetic grains (greater than one micron, for example), are composed of multiple magnetic domains (called the multidomain, or MD, state). Even under low grade metamorphic conditions, we expect that new magnetizations will be acquired due to movement of domain walls. These new magnetizations will be recorded by some MD grains up to the Curie point of magnetite.

Small magnetite grains containing only a single domain (SD) have the potential to withstand low grade metamorphic conditions. The parameter of interest for these grains in the thermal relaxation time ($\tau$), which can be expressed in terms of rock magnetic parameters as follows [@Dunlop1997]:

$$\frac{1}{\tau}=\frac{1}{\tau_{0}} exp { \left[  