- \frac{\mu_{0}V M_{s} H_{K}}{2kT} 
\left( 1- \frac{|H_{0}|}{H_{K}} \right) ^{2} \right] } 
\\[3mm]$$

where $\tau_{0}$ ( 10$^{-9}$ s) is the interval between thermal excitations, $\mu_{0}$ is the permeability of free space, V is grain volume, $M_{s}$ is spontaneous magnetization, $H_{K}$ is the microscopic coercive force, k is Boltzmann’s constant, T is temperature, and $H_{0}$ is the applied field. This relationship is derived from Néel’s [@Neel1949; @Neel1955] theory for single domain TRM; it was used by [@Pullaiah1975] to determine time-temperature relationships that can be in turn used to predict the acquisition of secondary magnetizations:

$$\frac{T_{A} ln(\tau_{A}/\tau_{0})}{M_{s}(T_{A})H_{K}(T_{A})} = 
\frac{ T_{B}ln(\tau_{B}/\tau_{0})  } {M_{s}(T_{B})H_{K}(T_{B})}$$

Where the two relaxation times ($\tau_{A}$, $\tau_{B}$) correspond to temperatures ($T_{A}$, $T_{B}$) respectively, and $H_{K}>>H_{0}$. This relationship describes the tendency for the maximum metamorphic temperature to leak to a higher unblocking temperature range. Hence, for our example of a peak metamorphic temperature of 320 $^{o}$C, and a nominal reheating duration of 1 million years, only SD unblocking temperatures up to $\sim$400 $^{o}$C should be affected [@Dunlop1977].

In summary, the challenges for paleointensity are those of finding samples that have not been chemically altered in nature, will not alter in laboratory, carry small single domain or single-domain like grains (called pseudo-single PSD), 