  We study metal abundances in the z=0.9313 damped Lya system observed in the
two lines-of-sight, A and B, toward the gravitationally-lensed double QSO
HE0512-3329. Spatially resolved STIS spectra constrain the neutral-gas column
density to be LogN(HI)=20.5 in both Aand B. UVES spectra (spectral resolution
FWHM=9.8 km/s) show, in contrast, significant line-of-sight differences in the
column densities of MnII and FeII; these are not due to observational
systematics. We find that [Mn/H]=-1.44 and [Fe/H]=-1.52 in damped Lya system A,
while [Mn/H]=-0.98 and [Fe/H]>-1.32, and possibly as high as [Fe/H] approx. -1
in damped Lya system B. A careful assessment of possible systematic errors
leads us to conclude that these transverse differences are significant at a 5
sigma level or greater. Although nucleosynthesis effects may also be at play,
we favor differential dust-depletion as the main mechanism producing the
observed abundance gradient. The transverse separation is 5 kpc at the redshift
of the absorber, which is also likely to be the lensing galaxy. The derived
abundances therefore probe two opposite sides of a single galaxy hosting both
damped Lya systems. This is the first time firm abundance constraints have been
obtained for a single damped system probed by two lines-of-sight. The
significance of this finding for the cosmic evolution of metals is discussed.
