Geophysical and mineralogical constraints on the post-spinel transformation for the Tonga slab

The depth of the post-spinel transformation is investigated for the Tonga slab, by using data from short period seismic networks at western United States and Japan for about 100 deep and intermediate-depth earthquakes within a small (∼200 km by 200 km) area near 20°S. Later phases in a time window ∼3 to 20 s after direct P waves are analyzed to search for S-to-P converted waves at the 660 km discontinuity, which represents the post-spinel transformation. We find that immediately beneath the foci of the deepest earthquakes the discontinuity is depressed down to the depths of 685 ± 5 km on average, and that it dips towards WNW by 10 ± 3 km within 70 km laterally. We constrain the thermal structure near the S to P conversion points based on a plausible assumption that the deepest earthquakes occur around the coldest core of the Tonga slab. The distribution of the hypocenters relocated in this study as well as previously published tomographic images of the same region suggest that the Tonga slab bends upward when approaching the 660 km discontinuity and transiently stagnates around the discontinuity. With these observations as the constraints, we numerically model the thermal structure of the Tonga slab, and estimate the temperature around the conversion points as 1200 ± 100 °C, which is 300 ± 100 K colder than the surrounding mantle. As the average depression of the discontinuity (down to 685 ± 5 km) corresponds to an pressure excess over the global average (660 km) by 1.0 ± 0.2 GPa, the assumption of equilibrium post-spinel transformation results in an estimate of the Clapeyron slope (C1) of -3.3-2.7+1.3 MPa/K. We also obtain an independent estimate of the Clapeyron slope (C2) of −2.0 ± 1.0 MPa/K, based on the observation of the dip of the discontinuity and the computed temperature variation (by about 200 K). The discrepancy between C1 and C2 is marginally significant and can be diminished by considering that the slab materials at the conversion points are currently descending across the phase boundary fast enough and thus the depth of the post-spinel transformation is controlled by nucleation kinetics as well as by the temperature.

► The depth of the post-spinel transformation is 685 ± 5 km on average in the Tonga slab near 20S.
► The Clapeyron slope is −3.3 (+1.3  − 2.7) MPa/K.
► The slab materials are currently descending across the post-spinel phase boundary.