We consider closed type II and orientifold backgrounds where supersymmetry is spontaneously broken by asymmetric geometrical fluxes. We show that these can be used to describe thermal ensembles with chemical potentials associated to gravito-magnetic fluxes. The thermal free energy is computed at the one-loop string level, and it is shown to be free of the usual Hagedorn-like instabilities for a certain choice of the chemical potentials. In the closed string gravitational sector, as well as in the open string matter sector of the proposed orientifold construction, the free energy turns out to have Temperature duality symmetry, ${cal F}(T/T_H)={T^2over T_H^2} {cal F}(T_H/T)$, which requires interchanging the space-time spinor representations $Sleftrightarrow C$. For small temperatures, $Tto 0$, the anti-spinor $C$ decouples from the spectrum while for large temperatures, $Tto infty$, the spinor $S$ decouples. In both limits the free energy vanishes, as we recover a conventional type II superstring theory. At the self dual point $T=T_H$, the thermal spectra of $S$ and $C$ are identical. Moreover, there are extra massless scalars in the adjoint representation of an SO(4) non-abelian gauge symmetry in the closed-string sector, and open-string massless states charged simultaneously under both the Chan-Paton and the closed-string SO(4) gauge group.
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