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

2023
Views:

257

Pages:

14
Author:

Kira Landry

Post-Spinel Transition in Mg2SiO4 determined by high P-T in situ X-ray diffractometry Main Points • Reinvestigated transition of Spinel to Pv + Pc • Phase boundary 22 GPa from 1550-2200 K • 1-1.5 GPa lower than 660 • Clapeyron slope -2 to -0.4 MPa/K • Ineffective barrier to mantle convection 660 km discontinuity • (Mg,Fe)2SiO4 ----(Mg,Fe)SiO3 + (Mg,Fe)O • Well studied: but specifically Ito and Takahashi (1989) quench study • 23 GPa at 1900 K • Perfect for 660 • Steep Clapeyron Slop3 -3 MPa/K Irifune et al. • • • • Same slope: -3 MPa/K Problem for 660 Disallows post-spinel May mean a change in bulk mantle chemistry DAC work • e.g. Shim et al. • Generally consistent with the 660 How is this paper different? • In situ X-ray diffraction of materials using the same Kawai type setup as Irifune et al. • If the results match the 2 GPa discrepency the post-spinel may not be relevant to the 660 • Or the multi-anvil press has associated errors Comparison with Previous works • Katsura: Phase boundary 22 GPa from 1550-2200 K • Irifune: 22 GPa, 1500-1700 K, but lower P at higher T • Possible problems with heating and T measurement • Relied on peak instensity Vs. Chudinovskikh and Boehler • • • • Post spinel at 23 GPa at 1800-2400 K Laser heating Analyzed quench samples Some disagreement on boundaries regarding spinel vs. Shim et al. • Laser heated DAC • 23-25 GPa boundary • Some disagreement on certain points eg. -Periclase disappearance at 22.3 GPa, 1942K -Spinel formation at 22.7 GPa, 1756 K 2 GPa wide coexistence of Sp,Pc,Pv vs. Ito and Takahashi • 1-2 GPa higher (23-24 GPa) • -3 MPa/K • Katsura et al. observe slow spinel dissociation • This could account for the large Clapeyron slope X-ray diffraction in the MAP • Without going into details . . . • Thermocouple issues • Gold pressure standard issues Geophysical Implications • Absolute pressure for determining deep Earth structure not advisable from this experiment • What is important is the small Clapeyron slope 20 km depressions of the 660 • Found in arc regions • 0.86 GPa from PREM • 400 K lower than the surrounding mantle based on the -2 MPa/K slope of this study • Water may be the source of this behavior. 660 as a barrier? • -2 to -3 MPa/K Clapeyron slope for this to occur • Based on their results, the slope is not steep enough (-2 MPa/K is the limit) • Some slabs penetrate, some do not • Clapeyron slope may be close to a critical value for causing slabs to stagnate