  In this work, we study argon abundances in the interstellar medium of
high-redshift galaxies (2<z(abs)<4.2) detected as Damped Lya absorbers (DLA) in
the spectra of background quasars. We use high-resolution quasar spectra
obtained from the ESO-UVES advanced data products (EUADP) database. We present
3 new measurements and 5 upper limits of ArI. We further compiled DLAs/sub-DLA
data from the literature with measurements available of argon and alpha-capture
elements (S or Si), making up a total of 37 systems, i.e. the largest DLA argon
sample investigated so far. We confirm that argon is generally deficient in
DLAs, with a mean value [Ar/alpha]= -0.4+/-0.06dex (standard error of the
mean). The [Ar/alpha] ratios show a weak, positive trend with increasing NHI
and increasing absorption redshift, and a weak, negative trend with dust-free
metallicity, [S/H]. Detailed analysis of the abundance ratios indicates that
ArI ionisation, rather than dust depletion or nucleosynthetic evolution, is
responsible for the argon deficiency. Altogether, the observational evidence is
consistent with a scenario of argon ionisation dominated by quasar metagalactic
radiation modulated by local HI self-shielding inside the DLA host galaxies.
Our measurements and limits of argon abundances suggest that the cosmic
reionisation of HeII is completed above z=3, but more measurements at
z(abs)>3.5 are required to probe the final stages of this process of cosmic
reionisation.
