Anomalous Mantle at 45°N on the Mid-Atlantic Ridge?

Abstract

The Mid-Atlantic Ridge at 45øN 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 45øN can be modeled by a density anomaly, which extends along the spreading axis from about 43øN to 50øN 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 45øN 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.

Evolution of the accretion processes along the Mid‐Atlantic Ridge north of the Azores since 5.5 Ma: An insight into the interactions between the ridge and the plume

“…Low values for mantle densities at the MAR axis near the Azores have also been proposed on the basis of geochemical considerations [e.g., Bourdon et al , 1996] with positive gradients away from the hot spot. Mello et al [1999] derived an along‐axis variation in the density of the uppermost mantle from Pratt isostatic compensation of the topography in order to model the long‐wavelength gravity and topography signals over the north and south of the Azores.…”

“… Goslin and the Triatnord Scientific Party [1999] suggest that the northward limit of the Azores influence would correspond to a major relay zone near 43°40′N although different authors suggest that this limit is located further north, near Charles Gibbs Fracture Zone [ Kurz et al , 1982]. North of the Azores, Mello et al [1999] suggested the presence of an anomalous mantle beneath 45°N, that could explain the flat trend in the free air anomaly as opposed to the trend in the bathymetry. The influence of the Azores could then be limited to this latitude.…”

“…This, together with the long wavelength trend observed in the axial topography and in the Mantle Bouguer anomaly, would reflect the influence of the Azores plume along the ridge axis. This influence would either result in a southward asthenospheric flow away from the hot spot or in the presence of a horizontal density gradient in the mantle under the ridge axial domain [ Vogt , 1976; Thibaud et al , 1998; Mello et al , 1999].…”

High‐resolution Remotely Operated Vehicle (ROV) mapping of a slow‐spreading ridge: Mid‐Atlantic Ridge 45°N

“…Sections, such as the eastern side of the axis between 45 35 0 N and 45 40 0 N may be characterized by detachment faulting [Escartin et al, 2008]. Previous studies [Aumento et al, 1971;Keeton and Searle, 1996;Mello et al, 1999;Searle et al, 2010] identified a number of different volcanic morphologies in the area and noted that the nature of the AVR changes from narrow, focused volcanism south of 45 33 0 N to less focused and more tectonized north of 45 33 0 N [Searle et al, 2010]. The AVR also displays good examples of flat seafloor and volcanic hummocks.…”

Dynamics of plume–triple junction interaction: Results from a series of three‐dimensional numerical models and implications for the formation of oceanic plateaus

“…It should be noted that in addition to the Azores and Shatsky Rise cases, there are other plume‐affected RRR triple junctions that are candidates for future modeling, after varying aspects of the models like spreading rate and ridge geometry. Examples include Tongareva [ Larson et al , ; Viso et al , ], Bouvet [ Ligi et al , ; Georgen and Lin , ], Agulhas [ Hartnady and le Roex , ; Georgen et al , ; Gohl and Uenzelmann‐Neben , ], King's Trough [ Searle and Whitmarsh , ; Mello et al , ], and Afar [ Haase et al , ; Leroy et al , ].…”