Quaternion Fourier transforms (QFT) provide a powerful tool for the analysis of signals obtained from vector probes. Acoustic particle velocity is one such signal which can be measured with specially designed microphone arrays. This paper presents a time-frequency source separation method based on the short-time quaternion Fourier transform of acoustic particle velocity signals and the agglomerative k-plane clustering of the vector part of the resulting representation. Two example cases, one with a single and one with two interfering sources are presented.