Wetter mantle leads to deeper — not more — decompression melting

The extent to which water content affects mantle melt production during decompression melting beneath mid-ocean ridges and mantle plumes is still debated. Here, we use idealized 1-D numerical experiments to explore the effects of source water content and melt-wallrock interactions on decompression melting of the mantle. The experiments demonstrate that while a wetter mantle indeed begins to melt at a greater depth than a drier mantle, the latent heat consumed by deep melting leads to lower amounts of shallow decompression melting and a similar overall amount of melt production for damp vs. dry decompression melting. This melting behavior is seen for both mid-ocean ridge-like starting mantle temperatures and higher plume-like temperatures. These results challenge previous expectations that melting of a wetter mantle source should be linked to much more melting, as has been observed for batch melting under laboratory conditions of constant pressure and increasing temperature. In addition, we find that diffusion of water from melt into surrounding wallrock can enhance decompression melting of previously melted wallrock. However, for small wallrock/mantle ratios, the net effect on total mantle melt production is relatively small (<2%).