  We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies
at z>0.3, which cover a temperature range 3<kT<15 keV. Our analysis is aimed at
measuring the iron abundance in the intra-cluster medium (ICM) out to the
highest redshift probed to date. We made use of combined spectral analysis
performed over five redshift bins at 0.3<z<1.3 to estimate the average emission
weighted iron abundance. We find that the emission weighted iron abundance
measured within (0.15-0.3)R_vir in clusters below 5 keV is, on average, a
factor of ~2 higher than in hotter clusters, following Z(T) = 0.88 T^{-0.47}
Z_o, confirming the trend seen in local samples. We also find a constant
average iron abundance Z_{Fe}~0.25Z_o as a function of redshift, but only for
clusters at z>0.5. The emission weighted iron abundance is significantly higher
(Z_{Fe}~0.4 Z_o) in the redshift range z~0.3-0.5, approaching the value
measured locally in the inner 0.15 R_{vir} radii for a mix of cool-core and non
cool-core clusters at 0.1<z<0.3. The decrease in metallicity with redshift can
be parametrized by a power law of the form ~(1+z)^{-1.25}. The observed
evolution implies that the average iron content of the ICM at present epoch is
a factor of ~2 larger than at z~1.2. We confirm that the ICM is already
significantly enriched (Z_{Fe} ~ 0.25 Z_o) at a look-back time of 9 Gyr. Our
data provide significant constraints on the time scales and physical processes
which drive the chemical enrichment of the ICM. (abridged)
