  The accretion of rocky material is responsible for the presence of heavy
elements in the atmospheres of a large fraction of white dwarf stars. Those
objects represent a unique opportunity to infer the bulk composition of
exoplanetesimals. This chemical characterization requires the use of detailed
atmosphere models to determine the elemental abundances at the photospheres of
white dwarfs. In this work, we use a state-of-the-art model atmosphere code to
reanalyse the first large survey of metal-polluted white dwarfs for which
abundances are found for multiple elements. We show that the improved
constitutive physics of our models lead to systematically higher Mg abundances
than previous analyses. We find an average log Mg/Ca number abundance ratio of
1.5. This value is significantly above the reference abundance for chondrites,
which is expected as current diffusion models predict that for the cool
helium-atmosphere white dwarfs of our sample Mg should remain in the atmosphere
longer than Ca. This helps resolve a recently identified Mg depletion problem,
where the planetesimals accreted by white dwarfs were reported to be
Mg-deficient compared to the expected composition of their planetary systems.
