  X-ray radiation from black hole binary (BHB) systems regularly displays
quasi-periodic oscillations (QPOs). In principle, a number of suggested
physical mechanisms can reproduce their power spectral properties, thus more
powerful diagnostics which preserve phase are required to discern between
different models. In this paper, we first find for two Rossi X-ray Timing
Explorer (RXTE) observations of the BHB GRS 1915+105 that the QPO has a well
defined average waveform. That is, the phase difference and amplitude ratios
between the first two harmonics vary tightly around a well defined mean. This
enables us to reconstruct QPO waveforms in each energy channel, in order to
constrain QPO phase-resolved spectra. We fit these phase resolved spectra
across 16 phases with a model including Comptonisation and reflection (Gaussian
and smeared edge components) to find strong spectral pivoting and a modulation
in the iron line equivalent width. The latter indicates the observed reflection
fraction is changing throughout the QPO cycle. This points to a geometric QPO
origin, although we note that the data presented here do not entirely rule out
an alternative interpretation of variable disc ionisation state. We also see
tentative hints of modulations in the iron line centroid and width which,
although not statistically significant, could result from a non-azimuthally
symmetric QPO mechanism.
