Hydration of Olivine and E arth’s Deep Water Cycle

1. Hydration of Olivine and Earth’s Deep Water Cycle IASPEI October 5, 2005

2. Olivine Hydration and Earth’s Deep Water Cycle J. R. Smyth, (University of Colorado) Dan Frost and Fabrizio Nestola (Bayerisches Geoinstitut, Germany) Financial support from Alexander von Humboldt Foundation and US National Science Foundation

3. Oceans cover 71% of the planet’s surface.

4. 71% of the surface But only 0.025% of the mass

5. Earth’s Deep Water Cycle • 0.15 percent H2O by weight in the top 10 km of the descending slab is sufficient to recycle the entire ocean volume once over 4.5 billion years at current subduction rates.

6. H-cycling: Role of Nominally Anhydrous Phases • Synthesis Experiments • Olivine • Wadsleyite (Spinelloid III) • Wadsleyite II (Spinelloid IV) • Ringwoodite (Spinel) • Pyroxene • Structure studies (X-ray, neutron): • Protonation mechanisms • Volume of Hydration

7. H-cycling: Role of Nominally Anhydrous Phases • Synthesis Experiments • Effect of H on volume and density • Effects of H on Transition Depths • Effects of H on elastic properties: • Isothermal Bulk Modulus • P and S velocities • Brillouin • Ultrasonic

8. Nominally Hydrous Brucite Phase A Chondrodite Clinohumite Nominally Anhydrous Periclase Olivine Clinoenstatite Stishovite

9. 3.5 mm Synthesis Experiments: 5000 ton Press

10. Olivine Fo100 : 12 GPa @ 1250°C (With Clinoenstatite) ~8000 ppmw H2O E // a Absorbance (cm-1) E // c E // b

11. Hydration of Olivine • Natural olivine contains less than ~0.03 wt % H2O (300 ppm) • H increases sharply with pressure • 5000 to ~9000 ppm @ 12 GPa • Silica activity has minimal effect on Fe-free forsterite. May have some effect on Fo95 compositions.

12. Hydration of Forsterite@ 12GPa FTIR results (ppmw H2O) 1100º 1250º 1400º 1600º Si-XS 5770 8000 3400 1000 Mg-XS5560 8800 4400

13. Hydration of Olivine @ 12GPa • We observe roughly equivalent amounts in equilibrium with enstatite or clinohumite. • We observe ~20% more H at 1100ºC than previous workers (Kohlstedt et al., 1996). • We observe ~ 2.5 x more at 1250ºC than previous workers at 1100ºC.

14. Hydration of Olivine @ 12GPa • We observe a measurable effect of hydration on unit cell volume. • Structure refinements from X-ray single crystal data and FTIR spectra are consistent with M1 site vacancy as the principal hydration mechanism. • 2H+ <=> Mg2+

15. Cell Volume vs H content

16. Hydration of Olivine @ 12GPa • V (Å3) = 290.107 + 5.5 x 10-5 * H2O • 8000 ppmw H2O in olivine has same effect on density as ~400ºC temperature rise. • Hydration is the third state variable in the equation of state of mantle minerals.

17. 2 Olivines with quartz in DAC @ 4.4 GPa 250mm

18. Mg-Excess 8000 ppmw H2O Ko = 120 GPa K’ = 7.0 Si-Excess 5000 ppmw H2O Ko = 122 GPa K’ = 5.8 Dry Olivine Ko = 129 GPa K’ = ~5

19. Hydration of Olivine Effect of Temperature: • Maximum H at ~1250°C • decreases above 1250º due to melting • Effect of Pressure: • Increases with pressure to 12 GPa (410km) • Effect of Silica Activity: • Minimal effect if any

20. Hydration of Olivine • H becomes compatible at P > 10 GPa • Hydration causes increase in cell volume. • Hydration causes decreased bulk modulus and seismic velocities.

21. Earth’s Deep Water Cycle • Pressure strongly stabilizes H in cation vacancies. • Decreases K and hence • Vp and Vs (no measurements on olivine) • In ringwoodite, the effect of hydration on velocity is larger than effect of temperature!

22. Earth’s Deep Water Cycle • H in nominally anhydrous high pressure silicates may constitute the largest reservoir of water in the planet. • This ‘water’ may have profoundly affected the geology of the planet. • Dehydration melting of olivine may control melt generation in the upper mantle.

23. Earth’s Deep Water Cycle • Below 200km H is captured by olivine in the overlying mantle wedge. • H softens the olivine allowing it to lubricate the descent of the cold slab • H is thus effectively entrained in the descending slab.

25. Interior Reservoir? • Nominally Anhydrous Minerals (NAMs) can incorporate up to 9 times the total Ocean mass if saturated.

26. Water:The Third State Variable in Mantle Dynamics

27. Reprints and preprints at http://ruby.colorado.edu/~smyth/Home.html smyth@colorado.edu

28. Hydration of Olivine

29. Nominally Anhydrous Minerals • Water in the mantle appears to be controlled by H solubility in the nominally anhydrous minerals • Olivine • Wadsleyite • Ringwoodite • Garnet • Pyroxene (Cpx, Opx, Clen) • Perovskite

30. Olivine : 12 GPa @ 1250°C (With Clinohumite) ~8800 ppmw H2O Absorbance (cm-1)

31. Compression ExperimentDiamond Anvil CellSingle-crystal X-ray Diffraction

32. Nominally Anhydrous Periclase Anhydrous B Olivine Wadsleyite Wadsleyite II Ringwoodite Clinoenstatite Stishovite Nominally Hydrous Brucite Phase A Norbergite Chondrodite Humite Clinohumite ShyB Phase B

33. Ringwoodite (spinel) (g-Mg2SiO4):Radiative Heat Transfer • (g-Mg1.63Fe0.22 H0.4 Si0.95O4) • ~10 % of Fe present as ferric (Mössbauer) • Dark blue color

34. Ringwoodite

35. Ringwoodite absorbance vs. Blackbody emission

36. Ringwoodite @ P

37. Radiative Heat Transfer • IR / Visible spectra of ringwoodite show that radiative term cannot be ignored in thermal conduction. • The transparency of ringwoodite in the radiative transfer region increases with pressure.

38. Reprints and preprints at http://ruby.colorado.edu/~smyth/Home.html smyth@colorado.edu

39. Water:The Third State Variable in Mantle Dynamics CU September 19, 2005

40. Hydration of Wadsleyite and Ringwoodite is more consistent with model shear velocity structure Ringwoodite Wet? Wadsleyite Wet ? PREM Olivine

41. Lateral Velocity Variations in TZ May Reflect Hydration • Red means Wet • Blue means Dry

42. A hydrous TZ should be thick and slow

43. Earth’s Deep Water Cycle • There has been running water (implying oceans) as far back as we can see in Earth history. • Water dominates the geology and biology of the planet. • Water also controls the interior processes.

44. Earth’s Deep Water Cycle • Water fluxes melting at ridges. • MORBs contain 0.1 to 0.3 wt % H2O • OIBs contain 0.6 to 1.5 wt % H2O • Plates hydrate on ocean floor. • Plates dehydrate on subduction. • But do they dehydrate completely?

46. Are the oceans just the tip of the iceberg?