Poisson's ratio of eclogite: Implications for lower crustal delamination of orogens

Laboratory measurements of combined P- and S-wave-velocities of eclogite from the Dabie-Sulu ultrahigh pressure metamorphic belt and from literature data show a significant increase of Poisson's ratio of eclogite with its intrinsic water content H(2)O(+) and thus the degree of hydration. Unaltered eclogites with H(2)O(+)<1.0% have an average Poisson's ratio between 0.24 and 0.25, which is identical to that calculated from single crystal elastic properties but lower than the averages (0.27+/-0.02) of measurements compiled by previous studies. Thus, the Poisson's ratio of unaltered eclogites is considerably lower than that of lower crustal mafic granulite and upper mantle peridotite. The lower crust and upper mantle of the Dabie ultrahigh pressure metamorphic belt, the northern and southern parts of the Tibetan Plateau as well as the central Andes, where eclogite may have formed during Mesozoic and Cenozoic tectonism, are characterized by the Poisson's ratio >0.26. This, together with their normal upper mantle P-wave velocities, implies that eclogites are no longer an important component of the present-day lower crust and upper mantle of these subduction-collision belts. Combined with age constraints on eclogite-facies metamorphism and subsequent exhumation, this in turn suggests that the interval from formation to delamination of eclogites is confined to a very short period of <15 Ma.