  We examine the flux density ratio anomaly in the quadruply-imaged strong
gravitational lens, B1422+231, and consider the contribution of
$10-10^3M_{\odot}$ primordial black holes (PBHs) as a potential dark matter
constituent. We describe the first flux density ratio measurement of B1422+231
in the millimeter-wave band using the Atacama Large Millimeter Array (ALMA).
This fills an important multi-wavelength gap in our knowledge of this key
lensed system. The flux density of the quasar at 233 GHz is dominated by
synchrotron emission and the source size is estimated to be 66.9 pc. The
observed flux density ratios at 233 GHz are similar to those measured in radio,
mid-infrared and optical bands, which cannot be explained by a simple smooth
mass model of the lens galaxy. We examine the probability of the flux density
ratio anomaly arising from PBH microlensing using ray tracing simulations. The
simulations consider the cases where 10% and 50% of dark matter are
$10-10^3M_{\odot}$ PBHs with a power law mass function. Our analysis shows that
the anomalous flux density ratio for B1422+231 can be explained by a lens model
with a significant fraction of dark matter being PBHs. This study demonstrates
the potential for new constraints on PBH dark matter using ALMA observations of
multiply imaged strong gravitational lenses.
