The composition of upper mantle and lower crustal material at very-slow spreading centers cannot be reliably determined by seismic studies alone. Since the range of P-wave velocities for serpentinized peridotites and gabbros overlap, additional information provided by the major and rare earth element (REE) content of the basalts is useful to constrain interpretations of seismic data. Refraction seismic data from the very-slow spreading (16 mm/a, full rate) Mohns Ridge in the Norwegian-Greenland Sea yields a highly variable thin crust of 4.0¿0.5 km thickness. Analysis of S-waves suggests that Layer 3 is composed primarily of gabbro containing at most a small percentage (<20%) of mantle material. The Na8 content of Mohns Ridge basalts suggests a magmatic crustal thickness of 4--5 km. Inversion of the REE concentrations yields a melt thickness of ~5 km. This agreement between seismic and geochemical data suggests that neither large quantities of mantle material are found in the lower crust nor is a large volume of basaltic magma frozen in the upper mantle. ¿ 2000 American Geophysical Union