A new analysis of gravity and topography data over the Mid-Atlantic Ridge: non-compensation of the axial valley

Topographic and associated gravity signals often result from the interference of several phenomena. In the present paper, an admittance function for the superposition of several physical mechanisms acting in the same waveband is derived. If the processes are not phase related, the resulting admittance function is the average of the individual admittance functions weighted by the squared amplitude of the topography for each process.We use this concept of superposition of different processes to understand the compensation of the Atlantic ridge valley. Classical transfer function studies applied to slow-spreading ridges have concluded that the topography was emplaced on an elastic plate about 10 km thick. These analyses of gravity and topography seem to contradict the physical models explaining the median rift valley, which is thought to be due to dynamical effects and thus to be uncompensated. We obtain an “average axial valley” gravity and topography profile by stacking several profiles perpendicular to the Mid-Atlantic Ridge and subtracting a long-wavelength thermal effect. We find that the gravity over topography spectral ratio of the “average axial valley signal” is consistent with an uncompensated process. Our study thus confirms that the mode of formation of the axial valley of a slow-spreading ridge involves an uncompensated mechanism. The presence of an additional process characterized by low admittance values, uncorrelated from one profile to the other, is also suggested in order to explain the observed admittance function. The study of the long wavelength (λ > 300 km) gravity and topography signal leads us to invoke the cooling of the upper portion of the crust by water circulation and to exclude the presence of a large amount of partial melt at depth (more than 5% over a 20 km thick layer at a mean depth of 60 km).     

Carbonatite melt–CO2 fluid inclusions in mantle xenoliths from Tenerife, Canary Islands: a story of trapping, immiscibility and fluid–rock interaction in the upper mantle

Three types of fluid inclusions have been identified in olivine porphyroclasts in the spinel harzburgite and lherzolite xenoliths from Tenerife: pure CO₂ (Type A); carbonate-rich CO₂–SO₂ mixtures (Type B); and polyphase inclusions dominated by silicate glass±fluid±sp±silicate±sulfide±carbonate (Type C). Type A inclusions commonly exhibit a “coating” (a few microns thick) consisting of an aggregate of a platy, hydrous Mg–Fe–Si phase, most likely talc, together with very small amounts of halite, dolomite and other phases. Larger crystals (e.g. (Na,K)Cl, dolomite, spinel, sulfide and phlogopite) may be found on either side of the “coating”, towards the wall of the host mineral or towards the inclusion center. These different fluids were formed through the immiscible separations and fluid–wall-rock reactions from a common, volatile-rich, siliceous, alkaline carbonatite melt infiltrating the upper mantle beneath the Tenerife. First, the original siliceous carbonatite melt is separated from a mixed CO₂–H₂O–NaCl fluid and a silicate/silicocarbonatite melt (preserved in Type A inclusions). The reaction of the carbonaceous silicate melt with the wall-rock minerals gave rise to large poikilitic orthopyroxene and clinopyroxene grains, and smaller neoblasts. During the metasomatic processes, the consumption of the silicate part of the melt produced carbonate-enriched Type B CO₂–SO₂ fluids which were trapped in exsolved orthopyroxene porphyroclasts. At the later stages, the interstitial silicate/silicocarbonatite fluids were trapped as Type C inclusions. At a temperature above 650 °C, the mixed CO₂–H₂O–NaCl fluid inside the Type A inclusions were separated into CO₂-rich fluid and H₂O–NaCl brine. At T<650 °C, the residual silicate melt reacted with the host olivine, forming a reaction rim or “coating” along the inclusion walls consisting of talc (or possibly serpentine) together with minute crystals of NaCl, KCl, carbonates and sulfides, leaving a residual CO₂ fluid. The homogenization temperatures of +2 to +25 °C obtained from the Type A CO₂ inclusions reflect the densities of the residual CO₂ after its reactions with the olivine host, and are unrelated to the initial fluid density or the external pressure at the time of trapping. The latter are restricted by the estimated crystallization temperatures of 1000–1200 °C, and the spinel lherzolite phase assemblage of the xenolith, which is 0.7–1.7 GPa.