Physics of compression of liquids Implication for the evolution of planets

1. Physics of compression of liquidsImplication for the evolution of planets Shun-ichiro Karato Yale University Department of Geology & Geophysics New Haven, CT (in collaboration with Zhicheng Jing) Global Network Symposium

2. Outline • Geological motivation • How does a molten layer in a terrestrial planet evolve? • Physics of compression of melts (bulk modulus, Grüneisen parameter) • How is a liquid compressed? • Compression behavior of non-metallic liquids is totally different from that of solids. [Bottinga-Weill model does not work for compression of silicate liquids.] • Compression behavior of metallic liquids is similar to that of solids. The Birch’s lawis totally violated for non-metallic liquids but is (approximately) satisfied for solids and metallic liquids. --> A new model is developed for non-metallic liquids. Global Network Symposium

3. Motivation-I Melts are more compressible than solids --> density cross-over Why is a melt so compressible? Could a melt compressible even if its density approaches that of solid? density Stolper et al. (1981) Global Network Symposium

4. Motivation - II How does a molten layer in a planet evolve? Global Network Symposium

5. Grüneisen parameter controls dTad/dz and dTm/dz Global Network Symposium

6. Liquid-solid comparison: bulk modulus Global Network Symposium

7. Liquid-solid comparison: Grüneisen parameter solids metallic liquids non-metallic liquids Boehler and Kennedy (1977), Boehler (1983) Global Network Symposium

8. melt (peridotite) solid (perovskite) K~30 GPa K~260 GPa Thermal expansion in a melt is large. Thermal expansion in a melt does not change with pressure (density) so much, although thermal expansion in solids decreases significantly with pressure. Global Network Symposium

9. SiO2: Karki et al. (2007) Densification of a (silicate, oxide) liquid occurs mostly: not by the change in cation-oxygen bond length partly by the change in oxygen-oxygen distance mostly by “something else” Global Network Symposium

10. Liquids versus solids (non-metallic) liquids are more compressible than solids. the bulk moduli of non-metallic liquids do not vary so much among various melts (~30 GPa). the thermal expansion of liquids is larger than solids and does not change with pressure (density) so much. the Grüneisen parameters of (non-metallic) liquids increase with pressure (density) while they decrease with compression in solids. the bulk moduli of glasses are similar to those of solids (at the glass transition), but much larger than those of liquids. bond-length in (silicate) liquids does not change much upon compression. --> compression mechanisms of (non-metallic) liquids are completely different from those of solids. Global Network Symposium

11. Explanation of  relationship Global Network Symposium

12. Entropy elasticity For an ordered solid, the first term dominates (+ small contribution from the second part (vibrational entropy)) -> compression behavior is controlled by inter-atomic bonds, i.e., control by the bond-length: Birch’s law. For a gas, (a complex) liquid the second term dominates. Entropy elasticity --> the Birch’s law does not apply. Global Network Symposium

13. a hard sphere model • Each solid-like element does not change its volume: hard sphere model • These elements (molecules) move only in the space that is not occupied by other molecules: “excluded volume” • Compression is due to the change in molecular configuration, not much due to the change in the bond length Global Network Symposium

14. Consequence of Sconfig model of EOS(scaled particle theory: excluded volumeeffect) (f: packing fraction) Global Network Symposium

15. small KT (10-30 GPa) • small T (large intrinsic T-derivative) • positive density dependence of the Grüneisen parameter Global Network Symposium

16. An extension to a multi-component system (MgO, CaO, SiO2, Al2O3, FeO Na2O, K2O) Bottinga-Weill model A hard sphere model (Stixrude et al., 2005) Global Network Symposium

17. The Bottinga-Weill model (solid mixture model) does not work---> what should we do? a silicate melt = oxygen “sea” + cations (van der Waals model of a complex liquid: Chandler (1983)) assign a hard sphere diameter for each cation determine the hard sphere diameter for each cation from the experimental data on EOS of various melts predict EOS of any melts [modifications 1. Coulombic interaction, 2. Volume dependence of the sphere for Si, 3. T-dependence of a sphere radius] compositional effect is mainly through the mass (m) Global Network Symposium

18. Jing and Karato (2009) Global Network Symposium

19. Jing and Karato (2010) Global Network Symposium

20. Some exceptions • Metals behave differently. • Little difference between solids and liquids <--cohesive energy of a metal is made of free electrons + “screened atomic potential (pseudo-potential)” --> influence of atomic disorder is small Ziman (1961) Global Network Symposium

21. For metals solid~liquid -->solidification from below For silicates solid liquid, liquid becomes large in the deep interior Tad increases more rapidly with P thanTm. --> Solidification from shallow (or middle) mantle. Global Network Symposium

22. Conclusions • Evolution of a molten layer in a planet is controlled largely by the behavior of the bulk modulusand the Grüneisen parameter. • The bulk moduli of silicate liquids are lower than those fo solids ad assume a narrow range. • The dependence of the Grüneisen parameter of liquids on density (pressure) is different from that of solids. • In non-metallic liquids, the Grüneisen parameter increases with compression. • In metallic liquids, the Grüneisen parameter decreases with compression. • Changes in “configuration” (geometrical arrangement, configurational entropy) make an important contribution to the compression of a (complex) liquid such as a silicate melt. • A new equation of state of silicate melts is developed based on the (modified) hard sphere model. • In metallic liquids, the change in free energy upon compression is dominated by that of free electrons, and consequently, the behavior of metallic liquids is similar to that of metallic solids. Global Network Symposium

23. liquid = mixture of solid-like components (Bottinga-Weill model) Stixrude-Karki (2007) Global Network Symposium

24. Problems with a conventional approach • Bond-lengths in liquid do not change with compression as much as expected from the volume change • Bulk moduli for individual oxide components in a liquid are very different from those of corresponding solids, and they take a narrow range of values • Grüneisen parameters of most of liquids increase with compression whereas those for solids decrease with compression. --> fundamental differences in compression mechanisms Global Network Symposium

25. Liquids versus glasses Glasses and solids follow the Birch’s law. Liquids do not follow the Birch’s law. Small K for a liquid is NOT due to small density. Global Network Symposium

26. Global Network Symposium

27. How to formulate an equation of state for a multi-component system? Bottinga-Weill model does not work---> what should we do? majority of silicate melt (MgO, FeO, CaO, Al2O3, SiO2): hard sphere model works, compositional effect is mainly through (mass) m Global Network Symposium

28. (a) (b) solid (or Bottinga-Weill model) (oxide) liquid • Compression of a mineral (solid) can be described by the superposition of compression of individual components (a polyhedra model). • Compression of a silicate melt is mostly attributed to the geometrical rearrangement using a “free volume”. Individual components do not change their volume much. -> compression of a silicate melt cannot be described by the sum of compression of individual components. Global Network Symposium

29. Assign the size of individual hard sphere components: MgO, SiO2, Al2O3 ---- Determine the size based on the existing data Use these sizes to calculate the density at higher P (T) Global Network Symposium

30. Global Network Symposium