Models of the Earth: thermal evolution and Geoneutrino studies

Size: px

Start display at page:

Download "Models of the Earth: thermal evolution and Geoneutrino studies"

Shannon Wells
6 years ago
Views:

1 Models of the Earth: thermal evolution and Geoneutrino studies Bill McDonough, Yu Huang and Ondřej Šrámek Geology, U Maryland Steve Dye, Natural Science, Hawaii Pacific U and Physics, U Hawaii Shijie Zhong, Physics, U Colorado Fabio Mantovani, Physics, U Ferrara, Italy

2 Earth Models Update: just the last 6 months! Campbell and O Neill (March , Nature): Evidence against a chondritic Earth Murakami et al (May , Nature): the lower mantle is enriched in silicon consistent with the [CI] chondritic Earth model. Warren (Nov , EPSL): Among known chondrite groups, EH yields a relatively close fit to the stable-isotopic composition of Earth. Zhang et al (March , Nature Geoscience): The Ti isotopic composition of the Earth and Moon overlaps that of enstatite chondrites. Fitoussi and Bourdon (March , Science): Si isotopes support the conclusion that Earth was not built solely from enstatite chondrites. - Compositional models differ widely, implying a factor of two difference in the U & Th content of the Earth

3 Nature & amount of Earth s thermal power radiogenic heating vs secular cooling - abundance of heat producing elements (K, Th, U) in the Earth - clues to planet formation processes - amount of radiogenic power to drive mantle convection & plate tectonics - is the mantle compositionally layered or have large structures? Geoneutrino studies estimates of BSE from 9TW to 36TW constrains chondritic Earth models estimates of mantle 1TW to 28TW layers, LLSVP, superplume piles

4 U content of BSE models Nucelosynthesis: U/Si and Th/Si production probability Solar photosphere: matches C1 carbonaceous chondrites Estimate from Chondrites: ~11ppb planet (16 ppb in BSE) Heat flow: secular cooling vs radiogenic contribution? Modeling composition: which chondrite should we use? A brief (albeit biased) history of U estimates in BSE: Urey (56) 16 ppb Turcotte & Schubert (82; 03) 31 ppb Wasserburg et al (63) 33 ppb Hart & Zindler (86) 20.8 ppb Ganapathy & Anders (74) 18 ppb McDonough & Sun (95) 20 ppb ± 20% Ringwood (75) 20 ppb Allegre et al (95) 21 ppb Jagoutz et al (79) 26 ppb Palme & O Neill (03) 22 ppb ± 15% Schubert et al (80) 31 ppb Lyubetskaya & Korenaga (05) 17 ppb ± 17% Davies (80) ppb O Neill & Palme (08) 10 ppb Wanke (81) 21 ppb Javoy et al (10) 12 ppb

5 What is the composition of the Earth? and where did this stuff come from? Heterogeneous mixtures of components with different formation temperatures and conditions Planet: mix of metal, silicate, volatiles

$Standard Planetary Model Orbital and seismic (if available) constraints Chondrites, primitive meteorites, are key So too, the composition of the solar photosphere Refractory elements (RE) in$

6 Standard Planetary Model Orbital and seismic (if available) constraints Chondrites, primitive meteorites, are key So too, the composition of the solar photosphere Refractory elements (RE) in chondritic proportions Absolute abundances of RE model dependent Mg, Fe, Si & O are non refractory elements Chemical gradient in solar system Non refractory elements: model dependent U& Th are RE, whereas K is moderately volatile

8 Meteorite: Fall statistics (n=1101) (back to ~980 AD) Ordinary Chondrites 80% Iron meteorites Stony Iron meteorites Achondrites ~9% Carbonaceous Chondrites ~4% Enstatite Chondrites ~2% Most studied meteorites fell to the Earth 0.5 Ma ago

9 Mg/Si variation in the SS Forsterite -high temperature -early crystallization -high Mg/Si -fewer volatile elements Enstatite -lower temperature -later crystallization -low Mg/Si -more volatile elements

10 Inner nebular regions of dust to be highly crystallized, Outer region of one star has - equal amounts of pyroxene and olivine - while the inner regions are dominated by olivine. Boekel et al (2004; Nature) Olivine-rich Ol & Pyx

11 Olivine-rich LL Pyrolite-EARTH CO H L CI CM CV Pyroxene-rich EL Enstatite-EARTH EH

12 Olivine-rich 2.5 AU 1 AU Pyroxene-rich LL EL EH H CI L MARS EARTH CO CM CV -thermal -compositional -redox

13 Si Fe Mg weight % elements Moles Fe + Si + Mg + O = ~93% Earth s mass (with Ni, Al and Ca its >98%)

15 Volatiles (alkali metals) in Chondrites CI and Si Normalized Enstatite Chondrites -enriched in volatile elements -High 87 Sr/ 86 Sr [c.f. Earth] - 40 Ar enriched [c.f. Earth]

16 142 Nd Earth What does this Nd data mean for the Earth? Solar S heterogeneous Chondrites are a guide Planets chondrites? Enstatite chondrites Ordinary chondrites Data from: Gannoun et al (2011, PNAS) Carlson et al (Science, 2007) Andreasen & Sharma (Science, 2006) Boyet and Carlson (2005, Science) Jacobsen & Wasserburg (EPSL, 1984) Carbonaceous chondrites

17 Enstatite chondrite vs Earth Carbonaceous chondrites diagrams from Warren (2011, EPSL) Carbonaceous chondrites Carbonaceous chondrites

18 Earth is like an Enstatite Chondrite! 1) Mg/Si -- is very different 2) shared isotopic: O, Ti, Ni, Cr, Nd,.. 3) shared origins -- unlikely 4) core composition -- no K, Th, U in core 5) Chondritic Earth -- losing meaning 6) Javoy s model recommend modifications

19 Th & U K from McDonough & Sun, 1995

20 U in the Earth: Differentiation ~13 ng/g U in the Earth Metallic sphere (core) <<<1 ng/g U Silicate sphere 20* ng/g U *Javoy et al (2010) predicts 12 ng/g *Turcotte & Schubert (2002) 31 ng/g Continental Crust 1300 ng/g U Mantle ~12 ng/g U Chromatographic separation Mantle melting & crust formation

21 Parameterized Convection Models Thermal evolution of the mantle Ra = o g (T 1 T 0 )d 3 Ra mantle > Ra critical mantle convects! vigor of convection = viscosity = density g = accel. due gravity = thermal exp. coeff. = thermal diffusivity d = length scale T = boundary layer T o Q Ra Q: heat flux, Ra: Rayleigh number, : an amplifer - balance between viscosity and heat dissipation At what rate does the Earth dissipate its heat? Models with ~ Schubert et al 80; Davies 80; Turcotte et al 01 Models with << Jaupart et al 08; Korenaga 06; Grigne et al 05, 07

22 Convection Urey Ratio and Mantle Models Urey ratio = radioactive heat production heat loss Mantle convection models typically assume: mantle Urey ratio: ~0.7 Geochemical models predict: mantle Urey ratio ~0.3 Factor of 2 discrepancy

23 Earth s surface heat flow 46 ± 3 (47 ± 2) Mantle cooling (18 TW) total R* 20 ± 4 Crust R* (8 ± 1 TW) *R radiogenic heat Mantle R* (12 ± 4 TW) Core (~9 TW) - (4-15 TW) (0.4 TW) Tidal dissipation Chemical differentiation after Jaupart et al 2008 Treatise of Geophysics

24 Plate Tectonics, Convection, Geodynamo Radioactive decay driving the Earth s engine!

25 Detecting Geoneutrinos from the Earth 2010

26 238 U Terrestrial Antineutrinos ν e + p + n + e Th 1.8 MeV Energy Threshold 1α, 1β 1α, 1β 234 Pa ν e 2.3 MeV 31% 238 U 232 Th 40 K ν e 2.1 MeV 1% 228 Ac 5α, 2β 4α, 2β 214 Bi ν e ν e 3.3 MeV 2.3 MeV 46% 20% 212 Bi 2α, 3β 40 K 1β 40 Ca Efforts to detect K geonus underway 1α, 1β 206 Pb Terrestrial antineutrinos from uranium and thorium are detectable 208 Pb

28 Reactor and Earth Signal Geoneutrinos KamLAND Reactor Background with oscillation KamLAND was designed to measure reactor antineutrinos. Reactor antineutrinos are the most significant contributor to the total signal.

29 Latest results KamLAND from 2002 to Nov 2009 Event rates Borexino from May 07 to Dec 09 under construction

Summary of geoneutrino results Constrainting U & Th in the Earth MODELS Cosmochemical: uses meteorites Javoy et al (2010); Warren (2011) Geochemical:

30 Summary of geoneutrino results Constrainting U & Th in the Earth MODELS Cosmochemical: uses meteorites Javoy et al (2010); Warren (2011) Geochemical: uses terrestrial rocks McD & Sun, Palme & O Neil, Allegre et al Geophysical: parameterized convection Schubert et al; Davies; Turcotte et al; Anderson

31 Earth s geoneutrino flux X ( r 0 ) A N X X 2R X R 2 dv a ( X r ) ( r ) r r 2 0 U or Th X (r 0 ) Flux of anti-neutrinos from X at detector position r 0 A X Frequency of radioactive decay of X per unit mass N X R a X (r) (r) Number of anti-neutrinos produced per decay of X Earth radius Concentration of X at position r Density of earth at position r Interrogating thermo-mechanical pile (super-plumes?) in the mantle

32 Present and future LS-detectors SNO+, Canada (1kt) Borexino, Italy (0.6kt) KamLAND, Japan (1kt) Europe LENA, EU (50kt) Hanohano, US ocean-based (10kt)

33 Constructing a 3-D reference model Earth assigning chemical and physical states to Earth voxels

34 Estimating the geoneutrino flux at SNO+ - Geology - Geophysics seismic x-section

35 Global to Regional RRM using only global inputs SNO+ Sudbury Canada improving our flux models adding the regional geology

36 Structures in the mantle

37 Testing Earth Models

38 SUMMARY Earth s radiogenic (Th & U) power 20 ± 9 TW* (23 ± 10) Prediction: models range from 11 to 28 TW Future: -SNO+ online early ?? - Hanohano - LENA - Neutrino Tomography

Neutrino Geoscience a brief history

Neutrino Geoscience a brief history 1930 Pauli invokes the neutrino 1956 Reines & Cowan detect νe 1984 Krauss et al develop the map 2003 KamLAND shows νe oscillate 2005 KamLAND detects ﬁrst geonus 2010