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Although partial melt in the asthenosphere is important geodynamically, geophysical constraints on its abundance remain ambiguous. We use a database of seamounts detected using satellite altimetry to constrain the temporal history of erupted asthenospheric melt. We find that intraplate volcanism on young seafloor (<60 Ma) equates to a similar to 20 m thick layer spread across the seafloor. If these seamounts tap partial melt within a similar to 20 km thick layer beneath the ridge flanks, they indicate extraction of an average melt fraction of similar to 0.1%. If they source thinner layers or more laterally restricted domains, larger melt fractions are required. Increased seamount volumes for older lithosphere suggest either more active ridge flank volcanism during the Cretaceous or additional recent melt eruption on older seafloor. Pacific basin age constraints suggest that both processes are important. Our results indicate that small volumes of partial melt may be prevalent in the upper asthenosphere across ocean basins.

Plain Language Summary Thousands of volcanic mountains known as "seamounts" lie beneath the world's oceans. These volcanoes are produced by the eruption of melted rock onto the Earth's surface, but it is unknown how much melted rock has been erupted onto the seafloor to produce the seamounts. We can now estimate this using new catalogs of seamounts that have been detected by satellites. We find that the world's seamounts, if spread out, would cover the seafloor with a rocky layer at least 18 m thick. We find that about half of this thickness is produced on newly created seafloor in the middle of the oceans and the other half accumulates after the seafloor ages to more than 60 Myr. These estimates provide constraints on how much melted rock must be present beneath the Earth's surface to feed these volcanoes. In particular, we find that the 20 km immediately beneath the plates should contain about 0.1% melt. This small amount of melt may be important for weakening the rocks beneath the tectonic plates, which may enable their movement.