Laboratory Electrical Conductivity Measurement of Mantle Minerals

Transcription

1 Laboratory Electrical Conductivity Measurement of Mantle Minerals Takashi Yoshino Institute for Study of the Earth s Interior, Okayama University

2 Outline 1. Brief introduction 2. Conduction mechanisms of silicate minerals 3. Method of electrical conductivity measurement under high pressure 4. A review of EC of main constituent mantle minerals Olivine, Wadsleyite, Ringwoodite, Majorite, Perovskite, ferro-periclase (post-spinel), post-perovskite 5. Applications

3 Electrical conductivity of minerals A useful method to probe the Earth s deep interior Sensitive to a small amount of conductor: hydrogen, graphite, metal, partial melt etc Sensitive to temperature (thermal activation process) Hydrous olivine σ: electrical conductivity σ 0 : pre-exponential factor E: activation energy k: Boltzmann constant T: temperature Wet Dry olivine Karato, Nature (1990) Xu et al., Science (1998)

4 Electrical conductivity structure of mantle Europe: Olsen,GRL (1998), Tarits et al, GRL (2004); Canadian shield: Neal et al., JGR (2000), NE China; Ichiki et al. GRL (2001); Pacific: Utada et al. GRL (2003); Kuvshinov et al. GJI (2005)

5 Mantle minerals Mg 2 SiO 4 Olivine, Wadsleyite, Ringwoodite MgSiO 3 Orthopyroxene, (Ilmenite), Mg-Perovskite Aluminous phase Garnet, Majorite Non-silicate oxide Ferro-periclase (Mg,Fe)O

6 Conduction mechanisms in mantle silicate minerals Mantle minerals: Ionic crystalline materials (Semiconductor) Ferromagnesian silicates Migration mechanism of electric charge in crystal 1. Vacancy (ionic conduction) 2. Fe (hopping conduction) 3. Hydrogen (proton conduction)

7 Ionic conduction Migration of Mg site vacancy High activation energy (> 2 ev) Dominant at very high T

8 Hopping conduction (small polaron) Electron hole hopping between ferric and ferrous iron Migration of electron hole Fe x Me + h = Fe Me Fe 2+ Fe 3+ h: electron hole

9 Small polaron Extra positive charge (Fe 3+ ) Repulsive force for cation Attractive force for anion Local strain (small polaron) Migration of electron holes associates that of the small polaron. For silicate minerals, activation energy is around 1.5 ev.

10 Proton conduction Mg site substituted by H Interstitial H h Hydrogen has high mobility Low activation energy (< 1 ev)

11 Electrical conductivity of ionic crystals The sum of contributions from the different conduction mechanisms Thermally activated Arrhenian behavior σ: electrical conductivity σ 0 : pre-exponential factor E: activation energy k: Boltzmann constant T: temperature

12 High C Identification of three conduction mechanisms High T Ionic conduction Proton conduction Hopping conduction

13 Some problems to determine proton conduction by EC measurement Water escape is unavoidable during measurement at high temperatures 1. Hydrogen diffusion is very fast 2. Forbiddance of usage of metal capsule Methods to improve conventional measurement Measurement at low temperatures (Low hydrogen diffusion)

14 High insulation resistance (up to 10 Gohm) Wang et al., Nature (2006) Yoshino et al., Nature (2006) 10-4 S/m Olivine 10-7 S/m Olivine High insulation resistance makes it possible to measure sample conductivity at low temperatures (<1000K)

15 EC measurement under mantle pressure condition LH-DAC (Laser heating diamond anvil cell) MA SD (Kawai-type (6-8) multianvil with sintered diamond cubes) MA (Kawai-type (6-8) multianvil with tungsten carbide cubes) Piston cylinder DAC 5000 ton KMA press

16 Summary of high P apparatus Press type Max. P Max. T Advantage Disadvantage Research Groups Piston cylinde r 3~4 GPa 2500 K Large volume Accurate T control Limited P Hard to take out many lead wires Universität Frankfurt KMA Kawaitype Multianvil < 30 GPa (< 90 GPa SD) 2500~30 00 K Moderate volume hydrostatic Accurate T control P limited (middle of the lower mantle) BGI (Universität Bayreuth) ISEI (Okayama Univ.) Yale Univ. Easy to take out lead lines etc DAC Diamond anvil < 400 GPa 4000 K (LH) < 1000 K (extern al H) Ultra high P In situ observation by Synchrotron Small volume Large differential stress Large T gradient (LH) Limited T (ext. H) Tokyo Tech

17 EC measurement cell under high P Bayreuth cell (Germany) Metal shield Xu et al., (1998) Misasa cell (Japan)

18 Kawai-type multi anvil cell WC cubes 2nd stage anvil 1st stage anvil

19 EC measurement To avoid electrochemical reaction at electrode We can monitor the electrode reaction by impedance spectroscopy For mantle minerals Impedance analyzer Solartron 1260 Frequency band 32MHz~1mHz

20 Impedance spectroscopy Dry ringwoodite 20 GPa Hydrous ringwoodite Imaginary part Real part Yoshino et al., Nature (2008)

21 Olivine The main constituent mineral of the upper mantle (< 410 km depth)

22 Effect of oxygen fugacity on EC of olivine oxidation 1atm DuFrane et al, GRL 2005

23 Conductivity anisotropy of olivine Mantle Temperature 1atm DuFrane et al, GRL (2005)

24 Three conduction mechanisms > 2 ev ~1.5 ev 10 GPa Polycrystalline San Carlos olivine < 1eV Yoshino et al., in prep

25 Electrical conductivity of hydrous olivine 4 GPa NNO buffer Wang et al. Nature (2006) EC of hydrous olivine is higher than those of dry one 0.1 wt.% 0.03 wt.% 0.01 wt.% Activation energies of EC for hydrous olivine are lower than those for dry one Effect of the proton conductivity at higher T is masked by different conductive mechanism (small polaron). Wang et al. reported higher conductivity values (Grain boundary water?) 3 GPa NNO buffer Yoshino et al. Nature (2006)

26 Wadsleyite and Ringwoodite The main constituent minerals of the mantle transition zone (410~660 km depth)

27 Electrical conductivity of wadsleyite and ringwoodite by Xu et al wadsleyite ringwoodite olivine 1. EC of WD and RW are similar 2. EC of WD and RW are two orders of magnitude higher than that of OL. Xu et al., Science (1998) Their samples (WD and RW) contain a significant amount of water!!

28 Conductivity-depth profile across the transition zone Xu et al. (1998) Continent Electrical conductivity based on lab data is too high!!!

29 Effect of water on EC of wadsleyite and ringwoodite 1 wt. % 0.1 wt. % Huang et al. (2005) 1 wt.% 0.1 wt.% 0.01 wt.% Huang et al. (2005) 0.01 wt. % 1. EC (RW) > EC (WD) 2. Two conduction mechanisms (small polaron and proton) 3. EC increases with increasing water content 4. H for proton conduction decreases with water content Yoshino et al. Nature, (2008) 5. EC of dry sample are much lower than those of Xu et al., Science (1998) and Huang et al., Nature (2005).

30 Formulation Electrical conductivity of iron-bearing hydrous silicate minerals Fe H 2 O Concentration dependence of activation energy on proton conduction (Debye and Conwell, 1954) n-type semiconductor Concentration dependence of pre-exponential factor on proton conduction (Nernst-Einstein relation) N: Number of charge carriers Final formulae Fe H 2 O C w : water content

31 Fitting results At High T, proton conduction is masked by small polaron conduction.

32 Effect of iron content on EC of ringwoodite Fe 9% Fe 30% Fe 20% MMO buffer 20 GPa Fe content increases Increase of EC Yoshino et al. PEPI in press

33 Garnet The 2nd abundant mineral of the mantle transition zone (410~660 km depth)

34 Electrical conductivity of majorite garnet low Fe (7%) high Fe (29%) Subducted oceanic crust (MORB) MJ > WD MJ > RW Transition zone (Pyrolite-olivine) MJ > WD MJ < RW Increase Fe content Increase of EC Yoshino et al. PEPI in press

35 Lower mantle minerals Perovskite, ferropericlase Post-perovskite (660~2900 km depth)

36 Perovskite Ferro-periclase Multi anvil 25 GPa 5 GPa 30 GPa DAC Xu et al. Science (1998) EC(Al-free PV) < EC (Al-PV) Dobson et al. Science (1997) EC(PV) < EC (FP)

37 EC of post-spinel (PV+FP) At 660 km discontinuity Ringwoodite post-spinel (PV+FP aggregate) EC(PS) = EC (PV) No interconnection of FP in post-spinel Yoshino et al. GRL, (2008)

38 EC measurement in DAC PPV Post-perovskite (D layer) PV (high spin) Low P: high spin Iron spin transition High P: low spin PV (low spin) Ohta et al. Science (2008)

39 Summary of electrical conductivity of mantle minerals High P mineral

40 Conductivity-depth profile model across the transition zone based on our laboratory data Assumptions Oxygen fugacity: Mo/MoO 2 buffer close to IW McCammon, Science (2003), Hirschmann, EPSL (2006) Normal geotherm (Mantle adiabat) from Katsura et al., JGR (2004) Single phase composed of olivine (Xu et al., PEPI, 2000), wadsleyite (Yoshino et al., Nature, 2008), ringwoodite (Yoshino et al., Nature, 2008), perovskite (Katsura et al., Nature, 1998) Activation volume: constant Grain size effect: no consideration

41 EC structure of mantle Xu et al. (1998) Garnet 3 conductivity jumps at 410, 520 and 660 km depth. Less than 0.1 wt.% water is difficult to identify Water effect on EC RW>WD

42 EC structure in the transition zone beneath Pacific 0 wt.% 0.1 wt.% 0.5 wt.% 1.0 wt.% Conductivity-depth profile of Pacific (Kuvshinov et al., 2005)

43 EC structure in the transition zone beneath continent (Europe, Canadian shield) -300K Olsen, GRL (1998); Neal et al., JGR (2000), Tarits et al., GRL (2004)

44 EC structure in the transition zone of wedge mantle (NE. China) Ichiki et al., GRL (2000)

45 Concluding remarks EC of the main mantle constituent minerals increases in order of olivine, wadsleyite, ringwoodite, perovskite and post-perovskite. Electrical conductivity increases with increasing water and iron contents. Activation energy for conduction decreases with increasing impurity (hydrogen and iron) concentration. At least three conductivity jumps should be present at 410, 520 and 660 km discontinuities due to the phase transformation. Our conductivity-depth model can explain well those of the mantle transition zone obtained from the electromagnetic studies, even if the mantle is dry.