  We present the analysis of the X-ray spectra of 18 distant clusters of
galaxies with redshift 0.3 < z < 1.3. Most of them were observed with the
Chandra satellite in long exposures ranging from 36 ks to 180 ks. For two of
the z>1 clusters we also use deep XMM-Newton observations. Overall, these
clusters probe the temperature range 3<kT<8 keV. Our analysis is aimed at
deriving the iron abundance in the Intra Cluster Medium (ICM) out to the
highest redshifts probed to date. Using a combined spectral fit of cluster
subsamples in different redshift bins, we investigate the evolution of the mean
ICM metallicity with cosmic epoch. We find that the mean Fe abundance at <z>
=0.8 is Z =0.25 +0.04 -0.06 Zo, consistent with the local canonical metallicity
value, Z ~0.3 Zo, within 1sigma confidence level. Medium and low temperature
clusters (kT <5 keV) tend to have larger iron abundances than hot clusters. At
redshift <z>~1.2 (4 clusters at z>1) we obtain a statistically significant
detection of the Fe-K line only in one cluster (Z>0.10 Zo at the 90% c.l.).
Combining all the current data set from Chandra and XMM at z>1, the average
metallicity is measured to be <Z> = 0.21 +0.10 -0.05 Zo (1sigma error), thus
suggesting no evolution of the mean iron abundance out to z~1.2.
