Diapirs as the source of the sediment signature in arc lavas

Island arc lavas, erupted above subduction zones, commonly contain a geochemical component derived from partial melting of subducted sediment. It is debated whether this sediment melt signature, with enriched trace element concentrations and isotope ratios, forms at relatively low or high temperatures. Here we compile and analyse the geochemistry of metamorphosed sedimentary rocks that have been exposed to pressures between 2.7 and 5 GPa during subduction at a range of locations worldwide. We find that the trace elements that form the sediment melt signature are retained in the sediments until the rocks have experienced temperatures exceeding 1,050 C. According to thermal models, these temperatures are much higher than those at the surface of subducted slabs at similar pressures. This implies that the sediment melt signature cannot form at the slab surface. Using instability calculations, we show that subducted sediments detach from the downgoing slab at temperatures of 500–850 C to form buoyant diapirs. The diapirs rise through the overlying hot mantle wedge, where temperatures exceed 1,050 C, undergo dehydration melting, and release the trace elements that later form the sediment melt signature in the erupted lavas. We conclude that sediment diapirism may reduce the transport of trace elements and volatiles such as CO2 into the deep mantle.