Single Crystal Elasticity of Majoritic Garnets: Stagnant Slabs and Thermal Anomalies at the Base of the Transition Zone

Abstract:

The principal method for constraining the thermal and chemical structure of the Earth’s mantle and for tracing the chemical anomalies potentially caused by subduction is the interpretation of seismic observations based on phase equilibria and mineral physics models. In this context, garnet mineral elastic properties are critical since they form major components of both mafic and ultramafic rocks in the upper mantle and transition zone, and are in fact the main mineral host for Al₂O₃ and CaO throughout these regions.

The elastic properties of single crystals of majoritic garnet (Mg_3.24Al_1.53Si_3.23O₁₂ and Mg_3.01Fe_0.17Al_1.68Si_3.15O₁₂) have been experimentally determined as a function of density, temperature and composition under hydrostatic conditions up to ~30 GPa and ~600 K in an externally heated diamond anvil cell. High pressure and temperature structural refinements using single-crystal X-ray diffraction data in combination with measurements of acoustic velocities provided fundamental insights into the interatomic forces and compression mechanisms controlling garnet elasticity.

End-member garnet thermo-elastic properties were fitted using the experimental data collected on the two samples with mixed compositions. The end member properties were then combined to determine the elasticity of complex garnet solid solutions. Acoustic velocities and densities of mafic, harzburgitic and lherzolitic bulk compositions in the Earth’s transition zone and uppermost lower mantle have been then calculated taking into account the phase relations along a typical mantle adiabat. These models are then compared with seismic reference models over the same depth interval to constrain the possible thermal and chemical conditions of these regions. The results indicate that slab units are likely to accumulate and potentially create enduring chemical and thermal anomalies at the base of the transition zone.