  We measure magnetization as a function of temperature, magnetic field and
time in a BaFe$_2$(As$_{0.68}$P$_{0.32}$)$_2$ single crystal with $T_c$ = 27.6
K. The fish-tail observed on many $M$($H$) curves are used to construct
isofield $M$($T$) curves which show an anomalous peak at some temperature $Tt$
suggesting a possible phase transition in the irreversible regime. A vortex
dynamics study performed along the peaks evidences a minimum in the relaxation
rate occurring at the same position $Tt$ of the minimum value of $M$ in these
peaks. A vortex dynamics study performed on $M$($H$) curves show two distinct
minimum in the relaxation rate: a first minimum ($H1$) for a lower field
correlating with $Tt$ and a second ($H2$) correlating with the peak in the
fish-tail. A phase diagram is constructed and the line corresponding to $Tt$
and $H1$ points obey an expression developed in the literature for a structural
rhombic to square lattice phase transition, further supporting this view.
