Elasticity of Continental Crust Around the Mantle Transition Zone

Abstract

The fate and amount of granitic materials subducted into the deep mantle are still under debate. The density, elastic property, and phase stability of granitic materials in the mantle pressures are key to clarifying them. Here, we modeled highpressure properties of the granitic assemblage by using ab initio mineral physics data of grossular garnet, K-hollandite, jadeite, stishovite, and calcium ferrite (CF)-type phase. We find that the ongoing subducting granitic assemblage, such as sediments and average upper crust rocks, is much denser than pyrolite in the pressure range from 9 GPa (~270 km), at which coesite undergoes a phase transition to stishovite, to around 27 GPa (~740 km). Above this pressure, granitic material becomes less dense than a pyrolite. This indicates that the granitic assemblage becomes gravitationally stable at the base of the mantle transition zone (MTZ). Results suggest a possibility that the granitic materials could accumulate around the 740 km depth if carried into the depth deeper than 270 km and segregated at some depth. Comparison of the velocities between granitic and pyrolitic materials shows that granitic materials can produce substantial velocity anomalies in the MTZ and the uppermost lower mantle (LM). Seismic observations such as anomalously fast velocities, especially for the shear wave, around the 660-km discontinuity, the complexity of 660-km discontinuity, and the scatterers in the uppermost LM could be associated with the subducted granitic materials.

Relationships Between Seismic Wave-Speed, Density, and Electrical Conductivity Beneath Australia from Seismology, Mineralogy, and Laboratory-Based Conductivity Profiles

“…The decoupling appears uncorrelated with Fe/(Mg + Fe), while thermal anomaliesare smoothed out and less dominant relative to the upper mantle. For additional discussion of transition zone structure see the contribution by Kawai and Tsuchiya (2015).…”

“…Christensen and Hofmann 1994;Xie and Tackley 2004), which are the products of partial melting at mid-ocean ridges. While such models produce a natural metric for understanding the generation and continued renewal of a laterally heterogeneous mantle through subduction of differentiated oceanic lithosphere (e.g., Xu et al 2008), it is not yet clear whether such mixture models can account for the complexity observed across a wide range of geophysical data (e.g., Helffrich and Wood 2001;Schmerr 2015;Kawai and Tsuchiya 2015).…”