Poisson’s ratio of eclogite: the role of retrogression

Available laboratory measurements of combined P- and S-wave velocities of eclogites from the Dabie–Sulu ultrahigh-pressure metamorphic belt and data from Manghnani et al. [J. Geophys. Res. 79 (1974) 5427–5446] show clear positive correlations between Poisson’s ratio and the intrinsic water content H2O+ for samples with SiO2<56% with the correlation coefficient r of 0.63–0.72. There are no significant correlations (|r|≤0.43) between Poisson’s ratio and other major oxides. The positive correlations are interpreted to result from hydration of eclogites during their exhumation and exposure at the surface. Our best estimates of the average Poisson’s ratios for unaltered quartz-bearing eclogites with H2O+<1.0% are 0.24–0.25 (±0.01), which is identical to that calculated from single crystal elastic properties, but lower than the averages (0.27±0.02) of measurements compiled by Rudnick and Fountain [Rev. Geophys. 33 (1995) 267–309] and Christensen [J. Geophys. Res. 101 (1996) 3139–3156]. Accordingly, unaltered quartz-bearing eclogites have Poisson’s ratios distinct from mafic granulites (0.28±0.01) and marginally different from ultramafic rocks (0.26±0.02).