MANTLE MELTING AND BASALT EXTRACTION BY EQUILIBRIUM POROUS FLOW

The chemical composition of mid-ocean ridge basalt, the most prevalent magma type on the planet, reflects the melt's continuous reequilibrat ion with the surrounding mantle during porous flow. Models of basalt e xtraction that account for the observed uranium-series disequilibria o n the Juan de Fuca ridge constrain both the abundance of melt beneath ridges (0.1 to 0.2 percent) and the style of mantle melting. Unlike mo dels that incorporate near-fractional melts (dynamic melting), mixing of equilibrium porous flow melts derived from heterogeneous source mat erials quantitatively explains the uranium-series observations.