Extrinsic Elastic Anisotropy in a Compositionally Heterogeneous Earth's Mantle.

Area of Science:

• Geophysics
• Seismology
• Mineral Physics

Background:

• Seismic anisotropy is often attributed to strain-induced intrinsic fabric.
• Extrinsic anisotropy from compositional layering is a potential alternative explanation.
• The relative contributions of intrinsic versus extrinsic anisotropy remain debated.

Purpose of the Study:

• To quantify the contribution of grain-scale and rock-scale compositional anisotropy to observed seismic anisotropy.
• To differentiate between extrinsic anisotropy from layering and intrinsic anisotropy from strain.
• To assess the detectability of compositional anisotropy using seismic methods.

Main Methods:

• Effective medium theories were combined with mineral elastic properties.
• Synthetic seismic wave velocities were measured through modeled microstructures.
• Calculations focused on mantle and oceanic crust compositions and subducting slab structures.

Main Results:

• Rock-scale compositional layering generates minimal extrinsic anisotropy (<1%) in typical mantle and crustal rocks.
• Quasi-laminated structures in subducting slabs produce minor seismic anisotropy (<0.1-0.2%).
• Grain-scale layering can generate significant extrinsic anisotropy (2-6% Vs) in specific mantle regions.

Conclusions:

• Rock-scale compositional layering is not a primary source of detectable seismic anisotropy but may be observed via wave scattering.
• Grain-scale layering, particularly shape-preferred orientation, is a significant contributor to seismic anisotropy in certain mantle zones.
• Extrinsic anisotropy can also arise from velocity variations due to phase transitions in compositionally homogeneous mantle.