  We investigate the impact on Fe abundance determination of including magnetic
flux in series of 3D radiation-MHD simulations of solar convection which we
used to synthesize spectral intensity profiles corresponding to disc centre. A
differential approach is used to quantify the changes in theoretical equivalent
width of a set of 28 iron spectral lines spanning a wide range in lambda,
excitation potential, oscillator strength, Land\'e factor, and formation
height. The lines were computed in LTE using the spectral synthesis code LILIA.
We used input magnetoconvection snapshots covering 50 minutes of solar
evolution and belonging to series having an average vertical magnetic flux
density of 0, 50, 100 and 200 G. For the relevant calculations we used the
Copenhagen Stagger code. The presence of magnetic fields causes both a direct
(Zeeman-broadening) effect on spectral lines with non-zero Land\'e factor and
an indirect effect on temperature-sensitive lines via a change in the
photospheric T-tau stratification. The corresponding correction in the
estimated atomic abundance ranges from a few hundredths of a dex up to |Delta
log(Fe)| ~ 0.15 dex, depending on the spectral line and on the amount of
average magnetic flux within the range of values we considered. The
Zeeman-broadening effect gains relatively more importance in the IR. The
largest modification to previous solar abundance determinations based on
visible spectral lines is instead due to the indirect effect, i.e., the
line-weakening caused by a warmer stratification on an optical depth scale. Our
results indicate that the average solar iron abundance obtained when using
magnetoconvection models can be 0.03-0.11 dex higher than when using the
simpler HD convection approach. We demonstrate that accounting for magnetic
flux is important in state-of-the-art solar photospheric abundance
determinations based on 3D simulations.
