Melt generation at very slow-spreading oceanic ridges: Constraints from geochemical and geophysical data

We show that there is a strong and consistent correlation between geochemic al and geophysical estimates of the amount of melt generated in the mantle beneath oceanic ridges. This correlation holds across all spreading rates a nd on scales down to the size of individual ridge segments. there is an abr upt decrease in the amount of melt generated at full spreading rates below similar to 20 mm/a. Our observations are consistent with the conclusion tha t < 10% of the melt is frozen in the mantle before it reaches the crust and that serpentine probably represents only a small percentage of the materia l above the Moho. The melt is well mixed on a ridge segment scale, probably in high-level magma chambers, but the melts remain distinct between segmen ts. The rare earth element concentrations of basalts from very slow-spreadi ng ridges are higher than those from normal oceanic ridges, which is direct ly indicative of reduced mantle melting, and they shaw characteristic light rare earth element enrichment, interpreted as caused by a deep tail of sma ll percentage wet melting. The decrease in melt production at rates below s imilar to 20 mm/a points to the importance of conductive cooling inhibiting melting of the upwelling mantle at very slow-spreading centres.