Magma Ocean Solidification and Formation of a Candidate Terrestrial - PowerPoint PPT Presentation

Magma Ocean Solidification and Formation of a Candidate Terrestrial D” Layer Alessondra Springmann April 20, 2011 1

Roadmap • Background • Methods • Results • Discussion 2

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µ 142 Neodymium � 142 Nd �   144 Nd sample µ 142 Nd = − 1  × 1 , 000 , 000   � 142 Nd �  144 Nd standard 5

EER prediction: chondrite terrestrial µ 142 Nd = -54 µ 142 Nd = -18 µ 142 Nd = 0 Carlson & Boyet (2008) 6

6,378 km 3,488 km D” Layer 0 km Adapted from Beatty et al. (1999), p. 114 7

Measuring the D” Lay et al. (1998) 8

perovskite structure post-perovskite structure 9

Roadmap • Background • Methods • Results • Discussion 10

Model • Elkins-Tanton 2008 Model • Assume a whole-mantle magma ocean • Mineral stability • Tracks solidified assemblage, co-evolving liquid composition 11

Mineral Stability by Depth !"#$#%&'()* +"#%!,-.!/&%&'0(* D !.12!,-.!/&%&'0)* B 6 < C 7'E3. ,"34#!+"35&'(* B6<C) 7 !"#$#%&'()*6'+"#%!,-.!/&%&'0)* 0A( B6<<) !"#$#%&'()* !.12!,-.!/&%&'0)* 43.%&1'7)* +"#%!,-.!/&%&'0(* !.12!,-.!/&%&'0)* B67)) 7( 5,#%&"'(* B6))) 7D (6>)) 00 ! 8!"#$#%&'9)* :3;!.#1&'<(* +"#%!,-.!/&%&'0(* ,&.!$5=#1&'>(* " 8!"#$#%&'9(* :34%&5#!?@51#1&'(* :3;!.#1&'((* ,!518,&.!$5=#1&'7))* 96<0) 7)) <69>) 7C) +!.& ' radius F=:G p .&55@.&'FHI3G ) 12

Mineral Stability by Depth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

Solidification • Magnesium fractionates preferentially out of the melt into solid assemblages • Melt becomes enriched in iron • Happens rapidly • ~10 4 years 14

6500 pre-overturn density high density 6000 5500 radius (km) 5000 4500 4000 low density 3500 3000 2600 2800 3000 3200 3400 3600 3800 4000 density (kg/m3) 15

Overturn • Unstable density profile • Overturn via Rayleigh-Taylor instabilities • ~ 2-4 x 10 6 years 16

6500 pre-overturn density 6000 5500 post-overturn density radius (km) 5000 4500 4000 3500 3000 2600 2800 3000 3200 3400 3600 3800 4000 density (kg/m3) Overturn results in material sorted by density 17

Roadmap • Background • Methods • Results • Discussion 18

6500 pre-overturn density 6000 5500 post-overturn density radius (km) 5000 4500 4000 depth of the deep dense layer 3500 3000 2600 2800 3000 3200 3400 3600 3800 4000 density (kg/m3) Deep dense layer: 250 km thick 19

180 � T = 5000 K 160 change in density (kg/m 3 ) 140 120 100 � T = 2500 K 80 60 � T = 1000 K 40 20 � T = 100 K 0 3300 3310 3320 3330 3340 3350 3360 3370 3380 3390 3400 initial density (kg/m 3 ) Deep dense layer is stable against thermal expansion 20

EER prediction: chondrite terrestrial µ 142 Nd = -54 µ 142 Nd = -18 µ 142 Nd = 0 Carlson & Boyet (2008) 21

� 142 Nd �   144 Nd sample µ 142 Nd = − 1  × 1 , 000 , 000   � 142 Nd �  144 Nd standard 80 0.45 Sm partition coefficient in MgFe-perovskite 60 deep dense layer � 142 Nd (ppm) 0.4 40 0.35 0.3 20 0.25 0 0.2 − 20 0.15 0.1 − 40 0.05 − 60 − 200 0 200 400 600 800 1000 1200 mantle � 142 Nd (ppm) 22

Roadmap • Background • Methods • Results • Discussion 23

Candidate D” Layer • Composition • Initial composition: Hart & Zindler (1986); Anders & Grevesse (1989) • Does not match Carlson & Boyet (2008) predictions • Non-chondritic initial composition? 24

Candidate D” Layer • Physical Properties • 3.24% of the mantle by mass • Dense • 250 km thick • Thermally stable • Melting in this region likely negatively buoyant (Knittle 1998) 25

Future Investigations • Varying of magma ocean depths • Different mineral assemblages by depth • Varying initial composition • Non-chondritic abundances for rare- Earth elements 26

Acknowledgements • L. T. Elkins-Tanton • NSF Astronomy grant to L. T. Elkins-Tanton • R. P. Binzel • R. W. Carlson 27

Acknowledgements II • J. L. Elliot, J. A. Connor, S. D. Benecchi, F. E. DeMeo, S. J. Vance, A. S. Rivkin, F. Vilas, M. E. Skolnik • A. D. Wickert, M. C. Perignon, Z. J. Bailey, E. B. Holmes, M. F. Lockhart, B. A. Black, S. M. Brown, G. T. Farmer, J. Suckale 28

Questions? 29

t = 4.567 Ga Sm Nd ... 142 143 144 ... ... 145 146 147 ... years 8 Sm = 1.03 x 10 6 4 1 t = 0 Ga 2 t / 1 Nd Sm ... 142 143 144 ... ... 145 146 147 ... 30