  Spectropolarimetric measurements of gamma-ray burst (GRB) optical afterglows
contain polarization information for both continuum and absorption lines. Based
on the Zeeman effect, an absorption line in a strong magnetic field is
polarized and split into a triplet. In this paper, we solve the polarization
radiative transfer equations of the absorption lines, and obtain the degree of
linear polarization of the absorption lines as a function of the optical depth.
In order to effectively measure the degree of linear polarization for the
absorption lines, a magnetic field strength of at least $10^3$ G is required.
The metal elements that produce the polarized absorption lines should be
sufficiently abundant and have large oscillation strengths or Einstein
absorption coefficients. We encourage both polarization measurements and
high-dispersion observations of the absorption lines in order to detect the
triplet structure in early GRB optical afterglows.
