Origin of geochemical variations at hotspots and mid-ocean ridges
Top: This Cartoon illustrates mantle flow (arrows) and melting (darker colors indicate more extentensive melting) of a heterogeneous mantle to give rise to volcanism at midocean ridges (left) and oceanic hotspot islands (right). The residual mantle column (RMC) represents the volume of mantle that has passed through and out of the zone of magma generation in a given amount of time. The composition of the lavas depends on the relative amounts that each heterogeneity has melted as depicted by the colors and shape of the RMC. Compared to mid-ocean ridges the RMC at hotspots is wider near the base of the melting zone where the most fuseable material is melting. Thus hotspots will tend to generate lavas with compositions more like the deepest melting materials. This factor plus the thicker lithosphere at hotspots is predicted to contribute to many of the important differences between composition of lavas at hotspots compared to mid-ocean ridges.
Bottom:Predictions of models that simulate hotspot mantle flow at various seafloor ages. 90% of the mantle is made of mantle that is compositionally similar to those of mid-ocean ridge basalts (MORB, green histograms on left side of plume). The other materials ("C", "EM", "HIMU") make of the remaining 10% and give rise to large differences in composition beneath old plate ages. The predicted compositions fall within the MORB field at zero-seafloor age where the lithosphere is thinnest. These results show how large distinctions between the compositions of ocean islands and mid-ocean ridges can arise out of the same heterogeneous source material.
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