  We present an analysis of stellar populations in passive galaxies in seven
massive X-ray clusters at z=0.19-0.89. Based on absorption line strengths
measured from our high signal-to-noise spectra, the data support primarily
passive evolution of the galaxies. We use the scaling relations between
velocity dispersions and the absorption line strengths to determine
representative mean line strengths for the clusters. From the age
determinations based on the line strengths (and stellar population models), we
find a formation redshift of z_form=1.96(-0.19,+0.24). Based on line strength
measurements from high signal-to-noise composite spectra of our data, we
establish the relations between velocity dispersion, ages, metallicities [M/H]
and abundance ratios [alpha/Fe] as a function of redshift. The [M/H]-velocity
dispersion and [alpha/Fe]-velocity dispersion relations are steep and tight.
The age-velocity dispersion relation is flat, with zero point changes
reflecting passive evolution. The scatter in all three parameters are within
0.08-0.15 dex at fixed velocity dispersions, indicating a large degree of
synchronization in the evolution of the galaxies. We find indication of
cluster-to-cluster differences in metallicities and abundance ratios. However,
variations in stellar populations with the cluster environment can only account
for a very small fraction of the intrinsic scatter in the scaling relations.
Thus, within these very massive clusters the main driver of the properties of
the stellar populations in passive galaxies appears to be the galaxy velocity
dispersion.
