Phase transitions of harzburgite and buckled slab under eastern China

Abstract

[1] Phase relations in harzburgite have been determined between 14 and 24 GPa and 1473 and 1673 K. At 1673 K, harzburgite transformed to wadsleyite + garnet + clinopyroxene below 19 GPa and decomposed into an assemblage of ringwoodite + garnet + stishovite above 20 GPa. Certain amounts of akimotoite were produced at still higher pressures (22)(23). Finally, perovskite and magnesiowüstite were found to coexist with garnet at 24.2 GPa. Compositions of all the phases were analyzed and elemental partitioning coefficients were determined among coexisting phases. Combining our experimental data with available thermoelastic properties of major minerals in the earth's mantle, we modeled the velocity and density signatures of the stagnated oceanic slab in the mantle transition zone (MTZ) under eastern China, based on kinematic slab thermal structure analysis. We examined two end-member slab models: a conventional straight slab with deformation thickening and an undulate slab with an oscillating wavelength of 200 km. We found that an undulated (buckled) slab model yields velocity anomalies (about 1-2% for Vp) that are consistent with seismic tomography models, taking into account low-pass filtering effects in seismic tomography studies. On the other hand, straight slab models yield velocity anomalies that are too high compared with seismic tomography models. Our models provide important constraints on the thermal structure, mineralogy, composition, density, and velocities of slab materials in the MTZ.

Experimental constraints on formation of hematite in olivine at high pressures and temperatures

“…An additional experiment was conducted using Mix C (dry harzburgite) at 12 GPa and 1473 K. The run products consisted of olivine, clinopyroxene and garnet, and no hematite was observed. This is contrary to the experimental results of the two hydrous systems (Mix A and B) at similar P-T conditions (runs R216 and R230), but is consistent with the experimental results in a dry harzburgite system reported by Zhang et al (2013).…”

A ubiquitous low‐velocity layer at the base of the mantle transition zone

“…Stagnant slabs, which are widely recognized in most subduction zones by seismic tomography [ Fukao et al ., ], bring significant oceanic crust into the MTZ. Numerical modeling shows that accumulation of oceanic crustal by stagnant slabs can cause a high‐velocity zone in the mid‐MTZ, hence creating a low‐velocity zone below [ Zhang et al ., ]. Significant amount of water can be transported with the subduction of hydrous oceanic crust to the MTZ.…”

Mantle Structure in the Central Mediterranean Region From P and S Receiver Functions

“…Although the real Earth is certainly more complex, with the proposed simple model, obtained by adding a high-velocity layer (suggested by independent evidence) to the IASP91 model, we can reproduce both S450p and S580p signals. It is noteworthy that a velocity profile similar to ours, based on numerical modeling of HPHT data, that included a high-velocity anomaly in the MTZ, was considered to explain the triplication of P and S waves near the 660 discontinuity and the amplitude of stagnant slab anomalies below eastern China (Zhang et al, 2013).…”