The History of Metal Enrichment Traced by X-ray Observations of High
  Redshift Galaxy Clusters
  We present the analysis of deep X-ray observations of 10 massive galaxy
clusters at redshifts $1.05 < z < 1.71$, with the primary goal of measuring the
metallicity of the intracluster medium (ICM) at intermediate radii, to better
constrain models of the metal enrichment of the intergalactic medium. The
targets were selected from X-ray and Sunyaev-Zel'dovich (SZ) effect surveys,
and observed with both the \textit{XMM-Newton} and \textit{Chandra} satellites.
For each cluster, a precise gas mass profile was extracted, from which the
value of $r_{500}$ could be estimated. This allows us to define consistent
radial ranges over which the metallicity measurements can be compared. In
general, the data are of sufficient quality to extract meaningful metallicity
measurements in two radial bins, $r<0.3r_{500}$ and $0.3<r/r_{500}<1.0$. For
the outer bin, the combined measurement for all ten clusters, $Z/Z_{\odot} =
0.21 \pm 0.09$, represents a substantial improvement in precision over previous
results. This measurement is consistent with, but slightly lower than, the
average metallicity of 0.315 Solar measured at intermediate-to-large radii in
low-redshift clusters. Combining our new high-redshift data with the previous
low-redshift results allows us to place the tightest constraints to date on
models of the evolution of cluster metallicity at intermediate radii. Adopting
a power law model of the form $Z \propto \left(1+z\right)^\gamma$, we measure a
slope $\gamma = -0.5^{+0.4}_{-0.3}$, consistent with the majority of the
enrichment of the ICM having occurred at very early times and before massive
clusters formed, but leaving open the possibility that some additional
enrichment in these regions may have occurred since a redshift of 2.
