A model of coupled He production/diffusion is used to constrain the question of whether Earth's peridotitic mantle contains ubiquitous mesoscale veins or slabs of other lithologies. The high diffusion rates of helium preclude survival of He isotope heterogeneities on scales smaller than a few tens of meters, especially if they represent long term in-growth of 4 He in the mantle. He variability for these 3 areas is very small, and independent of spreading rate: 0. 13, 0.19 and 0.21 Ra (±1σ). Since these ridges range from ultraslow to very fast-spreading, the variability in size of along-axis magma chambers will lead inevitably to various scales of melt averaging. We conclude that these ridge areas are not sampling mantle that contains enriched veins or recycled oceanic crust slabs of significant size (> tens of meters). It appears difficult to sustain a view of the upper mantle as a ubiquitous mixture of veins and depleted matrix, with MORB always representing an averaging of this mixture.

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