Geophysical and geochemical constraints on crustal accretion at the very-slow spreading Mohns Ridge

The composition of upper mantle and lower crustal material at very-slow spr eading centers cannot be reliably determined by seismic studies alone. Sinc e the range of P-wave velocities for serpentinized peridotites and gabbros overlap, additional information provided by the major and rare earth elemen t (REE) content of the basalts is useful to constrain interpretations of se ismic data. Refraction seismic data from the very slow spreading (16 mm/a, full rate) Mohns Ridge in the Norwegian-Greenland Sea yields a highly varia ble thin crust of 4.0 +/- 0.5 km thickness. Analysis of S-waves suggests th at Layer 3 is composed primarily of gabbro containing at most a small perce ntage (< 20%) of mantle material. The Nas content of Mohns Ridge basalts su ggests a magmatic crustal thickness of 4-5 km. Inversion of the REE concent rations yields a melt thickness of similar to 5 km. This agreement between seismic and geochemical data suggests that neither large quantities of mant le material are found in the lower crust nor is a large volume of basaltic magma frozen in the upper mantle.