Accumulations of melt at the base of young oceanic crust

Abstract

As hot, solid rock ascends through the mantle, the pressure around it decreases while its temperature decreases approximately adiabatically. During the ascent, the melting temperature decreases faster than the adiabat temperature, and if the initial temperature of the rock is high enough, the rock will begin to melt at the depth where the adiabat crosses the melting-temperature curve. Models of upwelling beneath mid-ocean ridges predict significant melt production over a zone several tens of kilometres wider than the ridge-axis region suggesting that some of the ascending melt must arrive away from the ridge axis. Here I report ocean-bottom seismograph observations of an uncommon and previously unreported marine seismic phase, which provide strong evidence that melt accumulates in sills at the base of young oceanic crust. The phase results from the conversion of compressional (P) waves to shear (S) waves on interaction with the crust-mantle boundary (the Moho). This phase is rare and very difficult to model, but when visible, it is strikingly prominent and usually appears at unexpectedly great source-to-receiver ranges. The observations imply the presence of bodies of extremely low rigidity, presumably melt sills, at the Moho close to the receiver. Melt may be fairly mobile at the base of the crust, allowing ridges to accumulate melt through a lateral plumbing system at the locally uplifted boundary between the crust and the underlying thermally expanded mantle. Escape of this melt to the sea floor may be responsible for the formation of off-axis seamounts.