Experimental constraints on coesite abundances in eclogite and implications for the X seismic discontinuity

Abstract We have experimentally tested the possibility that the coesite‐stishovite transition in eclogite bodies is responsible for the X discontinuity, a locally observed, low‐impedance jump in seismic wave velocities at 260–330 km depth. We determined phase relations and free SiO2 abundances in three natural‐analog eclogite compositions that simulate different subduction scenarios in terms of pressure‐temperature conditions and whether or not melt extraction occurred. Eclogitic compositions representing residues after either shallow or deep melting contain either no coesite or else too little (<4 wt %) to produce the observed impedance contrast for the X discontinuity. Only an unmodified mid‐ocean ridge basalt (MORB) composition was found to contain just enough coesite (6–8 wt %) to be consistent with the expected impedance contrast when it transforms to stishovite. However, we assert that MORB cannot remain compositionally unmodified during subduction down to ~300 km. Fluid loss due to dehydration reactions during the transformation from basalt to eclogite lowers bulk SiO2 content. In addition, the MORB wet solidus intersects the coesite‐stishovite boundary at ~290 km, implying that at greater depths a melt phase should be present before stishovite stability is reached. Our data indicate that melt generation is an efficient means of lowering the free SiO2 content in the mineral assemblage. This study also confirms previous work indicating that exsolution of SiO2 from the Ca‐Eskola (Ca0.5AlSi2O6) component in clinopyroxene is not a feasible mechanism for producing significant stishovite upon reaching its stability field. We conclude that the coesite‐stishovite transition in eclogite bodies is not a viable petrological explanation for the X discontinuity.