Strontium, Nd, Pb, and He isotope ratios measured in hotspot basalts (OIB) and mid-ocean ridge basalts (MORB) indicate that the two systems sample different geochemical materials in the mantle. The reason for this is not understood but is important to our understanding of the major physical and chemical processes in the solid Earth. One possibility is the mantle is chemically layered, with mantle plumes delivering deep mantle material to the surface and mid-ocean ridges melting the shallow upper mantle. However, two-layer convection separated at or near the upper-lower mantle transition is precluded by recent geophysical observations, and geodynamic models of other forms of layering have difficulty, particular, in explaining both the high 3He/4He and 206Pb/204Pb isotope ratios in OIB compared to MORB. The other possibility is that all geochemical materials are present in a single layer feeding both OIB and MORB volcanism but the differences in magma composition are caused by the melting process. We test this possibility using simple parameterized models of flow and melting of a heterogeneous mantle, which comprises materials with distinct isotope compositions and melting temperatures. Hotspots tend to voluminously melt material that begins melting deepest owing to the presence of thick lithosphere and rapid mantle flow through deeper portions of the melting zone. Mid-ocean ridge melting, however, tends to melt all materials more uniformly due to the thin lithosphere and the more uniform mantle flow rate with depth. We thus show that the same heterogeneous mantle mixtures that explain OIB compositions, when melted under hotspot conditions, can also produce many of the characteristics of MORB, when melted under normal mid-ocean ridge conditions. This allows for the possibility of a single, heterogeneous mantle layer feeding both OIB and MORB volcanism.
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