University:
Widener University
Course:
PHYS-313 | Classical Mechanics
Academic year:

2023
Views:

495

Pages:

9
Author:

Whitehall752

Fe Partitioning in a Pyrolite Mantle and the Nature of the 410-km Seismic Discontinuity Tetsuo Irifune & Maiko Isshiki Presented by Jav Santillán Introduction Pyrolite mixture is the model applied to the mantle 410 km = α to β transition (13.5 GPa) 660 γ to MgSiO3-rich perovskite +mv Problem: Seismic transition is sharper than α to β Mineral Physics Disagreement What they did . . . Phase equilibria study of pyrolite composition 6-16 GPa or ~180 to 480 km Examined the way Fe substition for Mg changes as a function of Pressure Found that Pyrolite model can work when Fe is taken into account Started with both artificial pyrolite glass and natural Mg0.89/Fe0.11 olivine Squeezed, Heated, then quenched and x-rayed Note: these are quenched samples Are seismic and mineral data at odds? Seismology suggests a transformation interval less than 3-6 km α to β transition from experiments suggest up to ~18 km Some explanations for the differences . . . Metastable transformation of olivine? Melt at the transition zone? Trace amounts of water? But (to this point) these studies only examined one component Mineral Physics Historically, fixed olivine composition used for ρ and velocity Generally in good agreement with seismology, discrepancies arise Mantle olivine estimated as ~30-50 vol.% Or upwards of ~60 vol.% Irifune and Isshiki conclude . . . Olivine is more Mg-rich as P increases (Mg0.92Fe0.8)2SiO4 at 410 km depth Modified-spinel (Mg0.88-0.89) after phase change Zha et al. 1999 Bulk and Shear Moduli similar pure Mg- end member olivine and Mg#=90 olivine True below 300 km But they were assuming a fixed composition For a non-isochemical olivine. .. Density (ρ) is substantially higher This will allow for increased velocity In turn, this requires more olivine They propose an increase of ~4-5 vol.% We now know that if we add more iron to olivine at high-P Vp and Vs data show that for the alpha/beta transition of olivine, velocity jumps are smaller with increasing pressure--this also requires a larger proportion of olivine.