The chemical diffusion of ferrous iron has been measured in a peraluminous sodium aluminosilicate melt using a diffusion couple technique. Polythermal measurements (1250¿--1450¿C) were made at 0.1 MPa and at 1.0 GPa and polybaric measurements (0.1 MPa to 2.0 GPa) were made at 1400 ¿C in order to determine the temperature and pressure dependencies of ferrous iron diffusion. The 0.1 MPa polythermal results show that the diffusivity of ferrous iron is non-Arrhenian at temperatures below 1300 ¿C, but is Arrhenian at higher temperatures. The non-Arrhenian behavior is interpreted either as indicating temperatures. The non-Arrhenian behavior is interpreted either as indicating temperature dependent anionic disproportion in the melt or as being due to a change in the diffusive jump frequency from similar to that of SiO-O bond exchange to faster than Si-O bond exchange with increasing temperature. The 0.1 MPa data are adequately described by a Fulcher equation: log10D=7.16¿0.79--280¿43/(T-1366), where the diffusion coefficient is in units of cm2/s and T is the absolute temperature. The 1.0 GPa polythermal results are adequately described by an Arrhenius equation: log10D=1.87¿0.07--2.97,400¿39,000/ 2.303RT, where the diffusive activation energy is 297¿39 kJ/mol. The polybaric results show that there is a significant increase in ferrous iron diffusivity from 0.1 MPa to 0.6 GPa, but that the diffusivity is independent of pressure at higher pressures. The sharp change in the diffusivity is interpreted as indicating changes in the melt structure via anionic disproportionation reactions (e.g., 2Q2≪Q21+Q4) with large (greater than -34 cm3/mol) negative volume changes that occur somewhere in the 0.1 MPa to 0.6 GPa pressure range. ¿American Geophysical Union 1990