Nature and origin of what s in the deep mantle

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1 Nature and origin of what s in the deep mantle S. Labrosse 1, B. Bourdon 1, R. Nomura 2, K. Hirose 2 1 École Normale Supérieure de Lyon, Universtité Claude Bernard Lyon-1 2 Earth-Life Science Institute, Tokyo Institute of Technology Neutrino Geosciences 2015

2 Large Low S Velocity Provinces (LLSVPs) V P = K + 4µ/3 µ ; V ρ S = ρ P and S waves speed anomalies not strongly correlated.

3 Large Low S Velocity Provinces (LLSVPs) V P = K + 4µ/3 µ ; V ρ S = ρ P and S waves speed anomalies not strongly correlated. Bulk sound velocity V 2 φ = V 2 P 4 3 V 2 S δv φ and δv S anti-correlated temperature and composition variations.

4 Ultra Low Velocity Zones (ULVZs) Rost et al, Nature (2005) Extremely localised anomalies 100 km across, 10 km width. δv S 30%, δv P 10% δρ 10% Most easily explained by the presence of dense partial melt.

5 Partial melt at the bottom of the mantle TCMB (Andrault et al, 2014) Solidus for FeO- and volatile-rich silicate likely lower than the CMB temperature. Magma denser than solid mantle if rich enough in FeO (Thomas et al, 2012; Sanloup et al, 2013)

6 LLSVPs and ULVZ ULVZs at the edges of the LLSVPs which suggests a link between the two features.

7 What is it made of? Large scale structure: lateral variations of temperature and composition. What composition? What mineral phases? Concentration in U and Th? Depends on the way deep Earth structures have formed. partition coefficients between phases. Likely partially molten ultra low velocity zones: possibly enriched in heat producing incompatible elements.

8 Earth budget in heat producing elements Jaupart et al, in Treatise on Geophysics (2015) source U (ppb) Th (ppb) K (ppm) Total Q (TW) chondritic BSE BSE = 20 average MORB mantle = 11 Continental crust CC = 7 depleted MORB mantle = 2.4 Either an enriched reservoir is required at depth to complement the depleted MORB source, or the Earth is not chondritic (e.g. Caro et al 2008). At depth: LLSVPs, ULVZs or the core?

9 Summary cartoon for the structure of D Large scale compositional and temperature variations Small scale ULVZs How do we form these and is there a link between them?

10 The slab graveyard model Christensen & Hofmann (1994) and followers Basalt eclogite and then higher pressure assemblages which are denser than ambiant mantle. Could also remelt and explain ULVZs (Andrault et al, 2014).

11 Evolution from an initially stratified mantle Davaille (1999) etc. Ability of thermal convection to stir a compositional stratification controlled by the buoyancy number: (Le Bars & Davaille, 2004) B = ρ χ ρα T Gradual entrainment decrease of ρ χ and B over time. destabilisation of the interface and regime transition. What is the origin of the initial stratification?

12 The basal magma ocean Labrosse, Hernlund & Coltice (2007) Presence of partially molten regions at the bottom of the mantle (ULVZ). Large core cooling to maintain the geodynamo for the last 3.5 Gyr (at least). A thicker layer of melt in the past.

13 The basal magma ocean Labrosse, Hernlund & Coltice (2007) Presence of partially molten regions at the bottom of the mantle (ULVZ). Large core cooling to maintain the geodynamo for the last 3.5 Gyr (at least). A thicker layer of melt in the past.

14 Requirements to support the geodynamo The geodynamo is thought to be maintained by thermo-compositional convection in the liquid core Global energy and entropy balances constrain the minmum CMB heat flow required to maintain dynamo action with a given total dissipation. The largest contribution to dissipation is from conduction along the isentropic temperature profile.

15 Thermal conductivity Inner core age (Gyr) Initial CMB Temp. (K) Stacey & Loper (2007) (Seagle et al 2013; Zhang et al, 2014) Initial CMB temperature Initial CMB heat flow carbon silicon (de Koker et al, 2012; Pozzo et al, 2012; Gomi et al 2013) oxygen sulfur Present CMB (W/m/K) (Gomi et al, 2013) Initial heat flow (TW) Stacey s conductivity value has often been considered. Recent upward revision by a factor 2 4. Exact value still debated and depends on the composition of the core. Recent estimates push upward demands on the CMB heat flow and, therefore, core cooling rate.

16 Chemical interaction between metal and magma Experiments of Nomura and Hirose Chondrites : Th/U 3.9 ± 0.1 Silicate Earth (as we know it) : Th/U 4.2 ± 0.1 U Th D(Metal/Silicate) Nomura & Hirose GPa DIW [-1.1,-1.8] Low pressure experiments ΔIW [-1,-2] (Wheeler et al, 2006; Malavergne et al, 2007; Bouhifd et al a & b, 2013) Sulfide Difference can be explained by interaction with the core forming metal at a temperature of 5000K [U] = 4.8ppb [Th] = 4.1ppb in the core 10000/T (K -1 )

17 Minimal CMB temperature evolution CMB temperature, K High k, no radioactivity High k, [U]=4.8 ppb [Th]=4.1 ppb Low k, no radioactivity Low k, [U]=4.8 ppb [Th]=4.1 ppb Melting at CMB Age, Ma 1000 Concentrations in heat producing elements considered not large enough to considerably modify the thermal evolution of the core. 0

18 Conclusions Likely existence of a deep reservoir rich in U and Th. Concentration depends on its size: small concentration for the core, larger for the LLSVPs and very large for the ULVZs (i.e. like the continental crust). Putting constraints on these numbers (geoneutrinos) is crucial for our understanding of scenarios of Earth formation and evolution.

19 Conclusions Likely existence of a deep reservoir rich in U and Th. Concentration depends on its size: small concentration for the core, larger for the LLSVPs and very large for the ULVZs (i.e. like the continental crust). Putting constraints on these numbers (geoneutrinos) is crucial for our understanding of scenarios of Earth formation and evolution. Last word: there is no such thing as the geodynamical model! (And probably the same is true for geochemical and cosmochemical models.)

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21 Inner core growth and core cooling rate For a given Ohmic + viscous dissipation, what is the required IC growth rate at present and the cooling rate just before the IC onset? 0.80 IC growth rate and age dr IC /dt, km/ma Age of IC, Ma Total dissipation Φ, TW No radioactivity

22 Inner core growth and core cooling rate For a given Ohmic + viscous dissipation, what is the required IC growth rate at present and the cooling rate just before the IC onset? 0.80 IC growth rate and age dr IC /dt, km/ma Age of IC, Ma Total dissipation Φ, TW No radioactivity [K]=200ppm

23 Inner core growth and core cooling rate For a given Ohmic + viscous dissipation, what is the required IC growth rate at present and the cooling rate just before the IC onset? IC growth rate and age 700 Cooling rate before IC 800 dr IC /dt, km/ma Age of IC, Ma Total dissipation Φ, TW No radioactivity [K]=200ppm No radioactivity [K]=200ppm Total dissipation Φ, TW dt c /dt, K/Ga

24 Inner core growth and core cooling rate For a given Ohmic + viscous dissipation, what is the required IC growth rate at present and the cooling rate just before the IC onset? a IC < 1.5Gyr and T > 1200K in 3 Gyr! IC growth rate and age 700 Cooling rate before IC 800 dr IC /dt, km/ma Age of IC, Ma Total dissipation Φ, TW No radioactivity [K]=200ppm No radioactivity [K]=200ppm Total dissipation Φ, TW dt c /dt, K/Ga

25 Contributions to energy and efficiency equation Contrib. to Q CMB, TW Present time Isentropic heat flow Total 8 Q CMB (0) 7TW Cooling Latent heat Compositional E Contrib. to Φ, TW Lost in conduction Compositional E Total Cooling Latent heat Total dissipation Φ, TW Current dynamo can work with Q CMB < Q S

26 Contributions to energy and efficiency equation Contrib. to Q CMB, TW Contrib. to Φ, TW Present time Just before IC onset Isentropic heat flow Total 8 Q CMB (0) 7TW Lost in conduction Compositional E Total Cooling Latent heat Compositional E Cooling Latent heat Total dissipation Φ, TW Contrib. to Q CMB, TW Contrib. to Φ, TW Cooling Lost in conduction Total Cooling = CMB heat flow Q CMB (-a IC ) 16TW Lost in conduction Total dissipation Φ, TW Current dynamo can work with Q CMB < Q S But Q CMB > 1.15Q S before the IC onset!

27 Age of the inner core Q CMB Q S -a IC Time IC age, Ma Constant Q CMB /Q S Isentropic heat flow CMB heat flow, TW Q CMB < Q S does not allow dynamo action before the IC

28 Age of the inner core Q CMB Q S Q CMB Q S -a IC Time IC age, Ma ~900 =max 800 Constant Q CMB /Q S Decreasing Q CMB Isentropic heat flow -a IC Time ~min CMB heat flow, TW Q CMB < Q S does not allow dynamo action before the IC Minimum Q CMB decreases (here linearly) between Q S at IC onset and a given value for the present time.