The Mid-Atlantic Ridge at 45 ¿N has a ridge crest depth close to normal for slow spreading ridges and a crustal thickness of 6 km. However, the free-air gravity over the region is higher than expected for such axial depths, and erupted basalts are enriched in incompatible trace elements and radiogenic isotopes, which could be taken to indicate the presence of a mantle plume beneath the spreading axis. But the ridge is at normal depths and the crust is of normal thickness. We model free-air gravity across and parallel to the strike of the ridge. A density structure derived from a cooling model provides a good fit for gravity across the ridge axis. A gravity model along the ridge axis indicates that most of the gravity signal over the Azores and Reykjanes Ridge is related to thicker crust, while the gravity signal over 45 ¿N can be modeled by a density anomaly, which extends along the spreading axis from about 43 ¿N to 50 ¿N and from zero age to 35 m.y. Such a mass excess can be explained by an increase in the garnet content of the mantle within the stability field of garnet peridotite. The enriched basalts occur in the same region as the mass anomaly. We interpret them as originating from cool hotspot mantle. This anomalous mantle may derive from a hotspot that existed at the North America-Africa-Eurasia triple junction when it was at 45 ¿N between 59 and 26 m.y. The King's Trough Complex is the remnant of that hotspot. We consider that this cool hotspot mantle still underlies the spreading axis and contributes to crustal construction. ¿ 1999 American Geophysical Union