  We present the measured gas-phase metal column densities in 155 sub-damped
Lyman alpha systems (subDLAs) with the aim to investigate the contribution of
subDLAs to the chemical evolution of the Universe. The sample was identified
within the absorber-blind XQ-100 quasar spectroscopic survey over the redshift
range 2.4<=z<=4.3. Using all available column densities of the ionic species
investigated (mainly CIV, SiII, MgII, SiIV, AlII, FeII, CII, and OI; in order
of decreasing detection frequency), we estimate the ionization-corrected
gas-phase metallicity of each system using Markov Chain Monte Carlo techniques
to explore a large grid of Cloudy ionization models. Without accounting for
ionization and dust depletion effects, we find that the HI-weighted gas-phase
metallicity evolution of subDLAs are consistent with damped Lyman alpha systems
(DLAs). When ionization corrections are included, subDLAs are systematically
more metal-poor than DLAs (between ~0.5 sigma and ~3 sigma significance) by up
to ~1.0 dex over the redshift range 3<=z<=4.3. The correlation of gas-phase
[Si/Fe] with metallicity in subDLAs appears to be consistent with that of DLAs,
suggesting that the two classes of absorbers have a similar relative dust
depletion pattern. As previously seen for Lyman limit systems, the gas-phase
[C/O] in subDLAs remains constantly solar for all metallicities indicating that
both subDLAs and Lyman limit systems could trace carbon-rich ejecta,
potentially in circumgalactic environments.
