Context : We present a Chandra analysis of the X-ray spectra of 56 clusters of galaxies at z \ga 0.3 , which cover a temperature range of 3 \la kT \la 15 keV .

Aims : Our analysis is aimed at measuring the iron abundance in the intra-cluster medium ( ICM ) out to the highest redshift probed to date .

Methods : We made use of combined spectral analysis performed over five redshift bins at 0.3 \la z \la 1.3 to estimate the average emission weighted iron abundance .
We applied non-parametric statistics to assess correlations between temperature , metallicity , and redshift .

Results : 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 \sim 2 higher than in hotter clusters , following Z ( T ) \simeq 0.88 T ^ { -0.47 } Z _ { \odot } , which confirms the trend seen in local samples .
We also find a constant average iron abundance Z _ { Fe } \simeq 0.25 Z _ { \odot } as a function of redshift , but only for clusters at z \ga 0.5 .
The emission-weighted iron abundance is significantly higher ( Z _ { Fe } \simeq 0.4 Z _ { \odot } ) in the redshift range z \simeq 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 in the redshift range 0.1 < z < 0.3 .
The decrease in metallicity with redshift can be parametrized by a power law of the form \sim ( 1 + z ) ^ { -1.25 } .
We tested our results against selection effects and the possible evolution in the occurrence of metallicity and temperature gradients in our sample , and we do not find any evidence of a significant bias associated to these effects .

Conclusions : The observed evolution implies that the average iron content of the ICM at the present epoch is a factor of \sim 2 larger than at z \simeq 1.2 .
We confirm that the ICM is already significantly enriched ( Z _ { Fe } \simeq 0.25 Z _ { \odot } ) at a look-back time of 9 Gyr .
Our data provide significant constraints on the time scales and physical processes that drive the chemical enrichment of the ICM .