Phase Transitionsin the Earth’s Mantle

Dan Shim

54-514, 324-0249, sangshim@mit.edu

PREM

Size of Ions in Solids

Close Packed Structures

Close Packed Structures

Inter-atomic Potential

Coordination Number

Other Crystallographic Considerations

http://www.gly.uga.edu/schroeder/geol6550/CM02.html

Changes in Crystal Structures

Olivine (Siiv, Mgvi)

Wadsleyite (Siiv, Mgvi)

Ringwoodite (Siiv, Mgvi)

Perovskite (Sivi, Mgviii-xii)

Akimotoiite (Sivi, Mgvi)

Pyroxene (Siiv, Mgvi or viii)

Post-Perovskite (Sivi, Mgvi-viii)

Garnet (Siiv and vi, Mgvi)

hcp to fcc

Redistribution of Si

Siiv to Sivi

3D network to 2D layer

Periclase (Mgvi)

Comparison of Different Techniques

P

(GPa)

(P)

(GPa)

T

(K)

(T)

(K)

Sample

(mm3)

MA <30 1 <2500 50 1

LHDAC <150 5 1500-4000 100-200 10-4

Pressure Measurement Methods

Calibration curve Equation of State

Shim et al. (2001) EPSL 203, 729+Ito and Takahashi (1989) JGR 94, 10637+

Temperature Measurement Methods

Thermocouple Radiometry

Phase Diagram of (Mg,Fe)2SiO4

Presnall (1995) in Mineral Physics and Crystallography Handbook

410 kmZ = 13 km

520 kmZ = 40 km

660 kmZ = 3 km

Phase Diagram of MgSiO3

Presnall (1995) in Mineral Physics and Crystallography Handbook

660 km

w/ Al

Clapeyron Slopes of Transition Zone Boundaries

Slope (MPa/K)

+2.5-4.0Katsura and Ito (1989) JGR 94, 15663+

Morishima et al. (1994) Science 265, 1202+

Katsura et al. (2004) JGR 109, 002438

+5.0-6.9Katsura and Ito (1989) JGR 94, 15663+

Suzuki et al. (2000) GRL 27, 803+

Post-spinel -2.5-3.0Ito and Takahashi (1989) JGR 94, 10637+

Irifune et al. (1998) Science 297, 1698+

Post-ilmenite -2.5-3.0Ito and Takahashi (1989) JGR 94, 10637+

Kuroda et al. (2000) PCM 27, 523+

Post-garnet +2.4Hirose et al. (2001) EPSL 184, 567+

Implications of the Phase Transitions

Li et al. (1998) Science 281, 675+ Ito and Katsura (1989) GRL 16, 425+

Phase Boundary and Mantle Flow

Gu and Dziewonski (2002) JGR 107, 2135

Thinner 410-Discontinuity?

Presnall (1995) in Mineral Physics and Crystallography Handbook

410-km

Z = 13 km

Seismic observation

< 5 km

Complex System

Irifune et al. (1998) Nature 392, 702+

Smyth (2002) GRL 29, 10.1029

Hydrous

Effect of WaterEffect of Iron

Post-Spinel Boundary at 600 km?

Irifune et al. (1998) Science 279, 1698+

600 km

What about Other Boundaries?

Kuroda et al. (2000) PCM 27, 523+

580 km

Hirose et al. (2001) EPSL 184, 567+

Post-Ilmenite

Post-Garnet

Shim et al. (2001) Nature 411, 571+

Problems in P-T Scales?

0 or (+) Clapeyron Slope of the Post-Spinel Transition?

Katsura et al. (2003) PEPI 136, 11+

Post-Perovskite Transition

MgSiO3 Perovskite (orthorhombic, Pbnm)

MgSiO3 Post-Perovskite (orthorhombic, Cmcm)

Pressure of the Post-Perovskite Transition

Existing experimental data do not provide tight constraints.• 114-125 GPa at 2000-2500 K (Murakami et al., 2004)

• 119-140 GPa at 2000-2500 K (Shim et al., 2004)

• 107-125 GPa at 1500-2500 K (Oganov and Ono, 2004)

Ab-initio calculation• 108-123 GPa at 2500 K (Oganov and Ono 2004)

• 115-125 GPa at 2500 K (Tsuchiya et al., 2004)

○ Seismic constraint (Sidorin et al, 1999): 127 GPa

Clapeyron Slope of the Post-Perovskite Transition

Existing experimental data do not provide constraints.

Ab-initio calculation• 9.56-9.85 MPa/K (Oganov and Ono 2004)

• 7.5±0.3 MPa/K (Tsuchiya et al., 2004)

○ Seismic constraint (Sidorin et al, 1999): 6 MPa/K

Physical Properties of the Post-Perovksite

Density increase

• 1.0-1.2 % (Murakami et al., 2004)

• 1.4 % (Oganov and Ono, 2004)

• 1.5 % (Tsuchiya et al., 2004)

• 0.9-1.3 % (10% Fe, Shieh and Duffy, 2004)

Bulk and shear moduli

• K0 = 232 GPa, K0’ = 4.4, VS = 1.9 %, VP = 0.3 % (Oganov and Ono, 2004)

• K0 = 221 GPa, K0’ = 4.2, VS = 1.5 % (Tsuchiya et al., 2004)

• K0 = 222±9 GPa for K0’ = 4 fixed, V = 2.3-2.6 % (10% Fe, Shieh and Duffy, 2004)

Elastic anisotropy

• VSH > VSV, Anticorrelation between VS and V (Oganov and Ono, 2004)

• VSH > VSV due to vertical alignments of [001], but [010] is possible (Tsuchiya et al., 2004)

Effect of Iron

Mao et al. (2004, PNAS 101, 15867+)

• 12 mol% Fe olivine, 20 and 40 mol% Fe pyroxene

• PPV transition at 100 GPa (700-km above CMB) for 10 %Fe

• Fe partitions predominantly into the post-perovskite phase

Murakami et al. (MR22A-01)

• Pyrolitic gel starting material

• PPV transition at 115 GPa (400-km above CMB)

• Fe partitions predominantly into magnesiowüstite

Shieh and Duffy (MR23A-0182)

• 10 mol% Fe pyroxene

• PPV transition at <105 GPa (600-km above CMB)

Effect of Post-Perovskite Transition for the Mantle Convection

Nakagawa and Tackley (2004) GRL 31, L16611

No Post-Pv Transition

Post-Pv with high (+) slope

Post-Pv with low (+) slope

Depth Variations of the Post-Perovskite Boundary

Due to the large Clapeyron slope, T decrease by 500 K may result in elevation of the transition depth by 100 km.

1% Fe increase may result in elevation of the transition depth by 30-50 km.

Temperature Gradient at CMB Heat from the core?

Experimental constraints?

• Melting curve of iron

• Melting curve of lower-mantle silicates

~10 K/km

Highly non-linear

Locally (laterally) varied

Clapeyron slope of the post-perovskite transition

→ ~5-7 K/km