  The rapid acquisition of positions by the upcoming Swift satellite will allow
the monitoring for X-ray lines in GRB afterglows at much earlier epochs than
was previously feasible. We calculate the possible significance levels of iron
line detections as a function of source redshift and observing time after the
trigger, for the Swift XRT, Chandra ACIS and XMM Epic detectors. For bursts
with standard luminosities, decay rates and equivalent widths of 1 keV assumed
constant starting at early source-frame epochs, Swift may be able to detect
lines up to z~1.5 with a significance of better than 3 sigma for times up to
10^4 s. The same lines would be detectable with better than 4 sigma
significance at z up to 6 by Chandra, and up to 8 by XMM, for times of up to
10^5 s. For similar bursts with a variable equivalent width peaking at 1 keV
between 0.5 and 1 days in the source frame, Swift achieves the same
significance level for z~1 at t~1 day, while Chandra reaches the previous
detection significances around t~ 1-2 days for z~ 2-4, i.e. the line is
detectable near the peak equivalent width times, and undetectable at earlier or
later times. For afterglows in the upper range of initial X-ray luminosites
afterglows, which may also be typical of pop. III bursts, similar significance
levels are obtained out to substantially higher redshifts. A distinction
between broad and narrow lines to better than 3 sigma is possible with Chandra
and XMM out to z~2 and ~6.5, respectively, while Swift can do so up to z~1, for
standard burst parameters. A distinction between different energy centroid
lines of 6.4 keV vs. 6.7 KeV (or 6.7 keV vs. Cobalt 7.2 keV) is possible up to
z~0.6, 1.2, and 2 (z~ 1, 5, 7.5), with Swift,Chandra, and XMM respectively.
