SLOW-RIDGE HOTSPOT INTERACTIONS FROM GLOBAL GRAVITY, SEISMIC TOMOGRAPHY AND SR-87/SR-86 ISOTOPE DATA/

Among the mantle hotspots present under oceanic areas, a large number are located on-or close to-active oceanic ridges. This is especially t rue in the slow-spreading Atlantic and Indian oceans. The recent avail ability of worldwide gravity grids and the increasing coverage of geoc hemical data sets along active spreading centres allow a fruitful comp arison of these data with global geoid and seismic tomography models, and allow one to study interactions between mantle plumes and active s low-spreading ridges. The observed correlations allow us to draw preli minary conclusions on the general links between surficial processes, w hich shape the detailed morphology of the ridge axes, and deeper proce sses, active in the upper mantle below the ridge axial domains as a wh ole. The interactions are first studied at the scale of the Atlantic ( the Mid-Atlantic Ridge from Iceland to Bouvet Island) from the correla tion between the zero-age free-air gravity anomaly, which reflects the zero-age depth of the ridge axis, and Sr isotopic ratios of ridge axi s basalts. The study is then extended to a more global scale (the slow ridges from Iceland to the Gulf of Aden) by including geoid and upper mantle tomography models. The interactions appear complex, ranging fro m the effect of large and very productive plumes, almost totally overp rinting the long-wavelength segmentation pattern of the ridge, to that of weaker hotspots, barely marking some of the observables in the rid ge axial domain. Intermediate cases are observed, in which hotspots of medium activity (or whose activity has gradually decreased) located a t some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geo metry of the plume head in the upper mantle. Such observations tend to preclude the use of a single hotspot/ridge interaction model and stre ss the need for additional observations in various plume/ridge configu rations.