Publication Date:
2015-06-18
Description:
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 SiO 2 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 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 SiO 2 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 SiO 2 content in the mineral assemblage. This study also confirms previous work indicating that exsolution of SiO 2 from the Ca-Eskola (Ca 0.5 AlSi 2 O 6 ) 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.
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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