Water in mantle eclogite: Origin, distribution and physical consequences

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The composition, structure and thermal state of the largely inaccessible deep lithosphere are inter-rogated by geophysical means with increasing detail, and its dynamics revealed by increasingly sophisticated numerical modelling. The accuracy of both approaches is enhanced by input from xenoliths entrained in deep-seated magmas. However, the physical properties of eclogites, as a subordinate but non-negligible part of dominantly peridotitic cratonic lithosphere, have not been systematically investigated. Like their peridotitic counterparts, some of these rocks have been metasomatised and possibly cryptically hydrated. As rocks consisting of ~50% clinopyroxene, mantle eclogites are expected to significantly contribute to the lithospheric water budget, but there are few studies on their H2O contents, and the nature and timing of their enrichment, and effects on geophysical observables are virtually unknown. The main aims of this study are thus: (1) To determine the H2O content in - and distribution between - clinopyroxene, garnet ± rutile in eight exceptionally well-characterised mantle eclogite suites from various cratons and tectonic settings by Fourier-Transformed Infrared Spectroscopy. Combined with published com¬po¬sitions for peridotite, this will allow the bulk densities, seismic velocities and electrical conductivities of regional eclogite-bearing continental lithospheres to be constrained with unprecedented detail and accuracy; (2) To determine the timing of the major hydration event represented by phlogopite-bearing eclogite, possibly linked to H2O introduction into nominally anhydrous minerals, by Ar-Ar dating of phlogopite; (3) evaluate implications for the rheology and stability of cratonic lithosphere.