  Element abundances in high-redshift quasar absorbers offer excellent probes
of the chemical enrichment of distant galaxies, and can constrain models for
population III and early population II stars. Recent observations indicate that
the sub-damped Lyman-alpha (sub-DLA) absorbers are more metal-rich than DLA
absorbers at redshifts 0$<$$z$$<$3. It has also been suggested that the DLA
metallicity drops suddenly at $z$$>$4.7. However, only 3 DLAs at $z$$>$4.5 and
none at $z$$>$3.5 have "dust-free" metallicity measurements of undepleted
elements. We report the first quasar sub-DLA metallicity measurement at
$z$$>$3.5, from detections of undepleted elements in high-resolution data for a
sub-DLA at $z$=5.0. We obtain fairly robust abundances of C, O, Si, and Fe,
using lines outside the Lyman-alpha forest. This absorber is metal-poor, with
O/H]=-2.00$\pm$0.12, which is $\gtrsim$4$\sigma$ below the level expected from
extrapolation of the trend for $z$$<$3.5 sub-DLAs. The C/O ratio is
1.8$^{+0.4}_{-0.3}$ times lower than in the Sun. More strikingly, Si/O is
3.2$^{+0.6}_{-0.5}$ times lower than in the Sun, while Si/Fe is nearly
(1.2$^{+0.4}_{-0.3}$ times) solar. This absorber does not display a clear
alpha/Fe enhancement. Dust depletion may have removed more Si from the gas
phase than is common in the Milky Way interstellar medium, which may be
expected if high-redshift supernovae form more silicate-rich dust. C/O and Si/O
vary substantially between different velocity components, indicating spatial
variations in dust depletion and/or early stellar nucleosynethesis (e.g.,
population III star initial mass function). The higher velocity gas may trace
an outflow enriched by early stars.
