  The geometric phase in the dynamics of a spin qubit driven by transverse
microwave (MW) and longitudinal radiofrequency (RF) fields is studied. The
phase acquired by the qubit during the full period of the "slow" RF field
manifests in the shift of Rabi frequency \omega_{1} of a spin qubit in the MW
field. We find out that, for a linearly polarized RF field, this shift does not
vanish at the second and higher even orders in the adiabaticity parameter
\omega_{rf} / \omega_{1}, where \omega_{rf} is the RF frequency. As a result,
the adiabatic (Berry) phases for the rotating and counter-rotating RF
components compensate each other, and only the higher-order geometric phase is
observed. We experimentally identify that phase in the frequency shift of the
Rabi oscillations detected by a time-resolved electron paramagnetic resonance.
