  Within the proper-time renormalization group approach, the chiral phase
diagram of a two-flavor quark-meson model is studied. In the chiral limit, the
location of the tricritical point which is linked to a Gaussian fixed point, is
determined. For quark chemical potentials smaller than the tricritical one the
second-order phase transition belongs to the O(4) universality class. For
temperatures below the tricritical one we find initially a weak first-order
phase transition which is commonly seen in model studies and also in recent
lattice simulations. In addition, below temperatures of T <~ 17 MeV we find two
phase transitions. The chiral restoration transition is initially also of
first-order but turns into a second-order transition again. This leads to the
possibility that there may be a "second tricritical" point in the QCD phase
diagram in the chiral limit.
