  We present fluorine abundances in seven red-giant members of the globular
cluster M4 (NGC 6121). These abundances were derived from the HF (1--0) R9 line
at 2.3357 microns in high-resolution infrared spectra obtained with the Phoenix
spectrograph on Gemini-South. Many abundances in the target stars have been
studied previously, so that their overall abundance distributions are
well-mapped. The abundance of fluorine is found to vary by more than a factor
of 6, with the F-19 variations being correlated with the already established
oxygen variations, and anti-correlated with the sodium and aluminium
variations. In this paper we thus add fluorine to the list of elements known to
vary in globular cluster stars, and this provides further evidence that
H-burning is the root cause of the chemical inhomegeneities. The fact that F-19
is found to decrease in the M4 stars, as the signature of H-burning appears,
indicates that the polluting stars must have masses greater than about 3.5
solar masses, as less massive stars than this should produce, not destroy,
fluorine.
