Measuring U & Th Enrichment of the Silicate Earth

Investigating Earth’s Origin & Thermal History

Steve DyeHawaii Pacific University

p nu du ud d

νe e+

W

Earth Origin and Chemical Composition

Solar nebula to solar systemFormation time 10 -100 Ma

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07

Sola

r pho

tosp

here

(ato

ms

Si =

1E

6)

C1 carbonaceous chondrite(atoms Si = 1E6)

H

CN

LiB

O

6-Oct-2010 2

Seismology and Geophysics

Heat Flow Measurement Sites

Seismology• density profile• internal structure• solid vs liquid phase

Heat flow probe-thermal conductivity,dT/dx

Heat conduction-q

= -k dT/dx

Geophysics

6-Oct-2010 3(Arevalo et al.,

2009)

Thermal Evolution of Earth

Surface heat flow-

Aq

= 46 ±

3 TW (Jaupart et al., 2007)

(alternative view)-

Aq

= 31 ±

1 TW

(Hofmeister and Criss, 2004)

Crust radiogenic power-

(Mh)Cru

=8±2 TW (Rudnick & Gao, 2003)

Mantle radiogenic power-

(Mh)Man

=(4–13)±2 TW (various models)

Planetary Urey ratio -

U = Mh/Aq

=(0.3–0.5)±0.1

MC(∂T/∂t) = Mh –

Aq

Cooling rate: ∂T/∂t = Aq/MC (Mh/Aq

– 1)

Mantle radiogenic power dominates the uncertainty

6-Oct-2010 4

Meteorites-

Primordial Earth Analogs

EH Enstatite ChondriteO isotopic composition Low in oxidized FeHigh Fe contentDepleted in volatile elements

CI Carbonaceous ChondriteEnriched in refractory elementsDepletion trends of moderately volatilesHighest in volatile elements

Similar HPE abundances: U=8±1 ng/g; Th=29±1 ng/g(Wasson & Kallemeyn, 1988)

Chondrites are primitive, undifferentiated meteorites

Which one?

6-Oct-2010 5

U & Th Enrichment-

Core Formation

Major Differentiation EventPrimordial Earth → Primitive mantle + core

Very little U & Th expected in core

Core formation leads toU & Th enrichment of silicate earth (primitive mantle) by factor of ~1.5

Mass of core ~ 1/3 of mass of earth

Enstatite chondrite (EH) earth: Primitive mantle

U & Th enrichment of ~1.5

(Javoy et al., 2010)

Iron meteorite(core analog)

6-Oct-2010 6

U & Th Enrichment-

Volatile Removal

Volatile elements never condense inhot primordial earth, enriching primitive mantle U & Th content

if planet originated from carbonaceous chondrites (CI) (EH have virtually no volatiles)

Carbonaceous chondrite (CI) earth:Primitive mantle U & Th enrichment of 2.2 to 2.8

(McDonough & Sun, 1995; Palme & O’Niell, 2003;Lyubetskaya & Korenaga, 2007)

6-Oct-2010 7

Comparing Silicate Earth ModelsModels predict wide range of possible U & Th enrichment in silicate earth

Resolution of U & Th enrichment of silicate earth investigatesorigin and thermal evolution of planet and constrains models

U (ng/g) Th (ng/g) Th/U K/U 40K (ng/g) P (TW)Chondrites 8±1 29±1 3.5±0.4 ------ 67-95 ------

Javoy et al., 2010 12±1 44±2 3.5±0.4 12,000 18±4 11±1

Lyubetskaya & Korenaga, 2007 17±3 63±11 3.6±0.9 11,000 23±5 16±3

McDonough & Sun, 1995 20±4 80±12 3.9±1.0 11,800 33±7 20±4

Palme & O’Neill, 2003 22±3 83±13 3.8±0.8 11,900 31±5 21±3

6-Oct-2010 8

1.5

(Javoy et al., 2010)2.2±0.4

(Lyubetskaya & Korenaga,

2007)2.74

(McDonough &Sun, 1995)

2.8 (Palme & O’Niell, 2003)

Enrichment factor

Modeling Mantle U & Th

- =

Primordial Earth (Chondritic U & Th)

Metallic Core(Negligible U&Th) Primitive Mantle

(U & Th (1.5 –

2.8) x chondritic)

- =

Primitive Mantle Crust(Fixed U&Th)

Depleted Mantle(Variable U & Th)

6-Oct-2010 9

Radiogenic Power

Radiogenic power from U, Th, KU & Th from enrichment of

chondritic earth, then K fromK/U of 12000 (±15%)

Earth thermal evolutionMC(∂T/∂t) = Mh –

Aq

∂T/∂t = Aq/MC (Mh/Aq-1)Urey ratio

= Mh/Aq

Mantle power controls convection, earth cooling(Korenaga, 2008)

Aq

is surface heat flowAq

= 46±3 TW

(Jaupart et al., 2007)

6-Oct-2010 10

Terrestrial Antineutrinos

238U232Th40K

νe

+ p+

→ n + e+

1.8 MeV Energy Threshold

212Bi

228Ac

232Th

1α, 1β

4α, 2β

208Pb

1α, 1β

νe

νe

2.3 MeV

2.1 MeV

238U

234Pa

214Bi

1α, 1β

5α, 2β

206Pb

2α, 3β

νe

νe2.3 MeV

3.3 MeV

40K 40Ca1β

6-Oct-2010 11

(Enomoto, Neutrino Sciences 2007)

Electron Antineutrino Detection

PMTs

measure position and amount of deposited energy

γγ

e+e-

γγ

n p+

γγ

Prompt event depositsenergy of

Eν

-0.8 MeV

Delayed event depositsenergy of 2.2 MeV

p+νe

Antineutrino (Eν

>1.8 MeV)interacts with free proton

6-Oct-2010 12

Geo-neutrinos –

Crust Model

Crust thickness & density(Bassin, Laske, and Masters, 2000)

Bulk crust U & Th concentrations(Rudnick and Gao, 2003)

Crust model prediction:radiogenic power: 7.8±1.5 TW

Crust geo-nu rate, c, site dependent

Crust is most accessible geological reservoir of U & Th

6-Oct-2010 13

(Enomoto, Neutrino Sciences 2007)

Reactor Antineutrino Background

Geo Geo ννσ(E)

Φ(E) N(E)

(Enomoto, Neutrino Sciences 2007)

)/27.1(sin)2(sin1 221

212

2

eeeELmP ννν θ Δ−≅→

Expected reactor rate: reactor rate in geo-nu energy

window, r, site dependent6-Oct-2010 14

Geo-neutrinos –

Mantle

Mantle model-

homogeneous, radial-symmetric:Density profile from seismology (Dziewonski & Anderson, 1985)

U & Th from Silicate Earth Enrichment (variable depends on model)

Mantle geo-nu rate, m, site independent but 3–12(±~2) TNU

6-Oct-2010 15

(Enomoto, Neutrino Sciences 2007)

Mantle Geo-neutrinosMaximum mantle signal fromhomogeneous composition

Concentrated layer at base of mantle (D”) is suggested by

Nd isotope studies in earth and chondrites

(Boyet & Carlson, 2005)

Modeling assumes depleted mantle mid-ocean ridge

basalt composition (Workman & Hart, 2005)

for mantle above D”with excess going to D”

D”

reduces mantlegeo-nu rate

6-Oct-2010 16

Observation of Mantle Geo-neutrinosOceanic site (mid-Pacific Ocean is ideal)Resolve enrichment to ±0.3 at 95% CL sys=stat with 34x1032

proton-yr exposure

Measure lateral heterogeneity too!Superswell vs Abyssal Plain

Model directions 30°

resolution

Silicate earth enrichment-

1.5Crust-

Rudnick & Gao, 2003

Data cut at 45°

rejects >90% of crust signal~50% of mantle signal

Continental site (reduce crust uncertainty)1)Measure local crust-

boreholes/heat flow

2)Develop directionalityResolve enrichment to ±0.8 at 95% CL sys=stat with ~22x1032

proton-yr exposure

6-Oct-2010 17

John Learned’stalk for progress

reports

Hanohano5 yrs

Mantle Rate Uncertainty at Continent

δm

= [n/ε

+ (n/ε)2(δε)2

+ (δr)2 + (δc)2]1/2

statistical systematic

n

~ 60 –

100 TNU

Statistical δm

≈

3 TNUrequires ε ≈ (7 –

11)x1032

p-y or 10 –

15 kT-y of LS

Systematic δm

≈

9 TNU totally dominated by crust

Reduce systematic bymeasuring local crust

6-Oct-2010 18

Local crust U & Th content

6-Oct-2010 19

Sudbury and DUSEL siteswith ~500 km radius

geological study circlesCUPP at Pyhasalmi, too!

Heat flow estimates crust geo-nu rate (Perry et al., 2009)Bore hole data & Moho heat flux for crust column heat production

rate

Initial study indicates nearly identical geo-nu rates from homogeneous vs stratified U & Th in crust

Example: Geo-nu Detection at DUSEL

•

~90 gν

events/y•

~60 (16) reactor events/y

•

gν

rate error–

Dominated by statistics

–

~3 TNU after 4y

•

Mantle gν

rate error–

Dominated by crust systematic

–

Reduce to ~4 TNU w/ geo study–

Sys + stat error ~5 TNU possible

•

Resolve U & Th enrichment–

Constrain origin and thermal history of earth

•

Search for geo-reactor•

Monitor for galactic SN–

3x1053

ergs at 10 kpc–

~500 CC p + nue-bar

–

~30 CC carbon + nue-bar–

~50 CC carbon + nue

–

~500 NC p + nu–

~40 NC e + nu

–

~70 NC carbon + nu

•

Solar nu•

ββ

decay w/ loading

•

40K gν

w/ 106Cd loading

6-October-2010 20

2.5 kT of LS (~1.8x1032

protons)

DUSEL Geo-nu Detection Project

Detector specs•

2.5 kT LS active

•

2400 17”

PMTs•

2.3 m oil buffer

•

2.6 m water shielding•

15 m radius cavity–

~60 kT of rock

Cost estimate•

Construction ≈

$32M

–

Excavation ≈

$4M–

Tanks + plant ≈

$10M

–

LS + buffer ≈

$6M–

PMTs + DAQ ≈

$12M

•

5 yr operation ≈

$16M

Total project cost ~$50M

Multidisciplinary science project with data possible before LBNE

6-Oct-2010 21

Summary•

Geo-nu flux from mantle addresses earth–

Origin: EH vs CI by silicate earth enrichment

–

Thermal evolution: radiogenic power•

Geo-nu mantle measurement with Hanohano–

Resolve enrichment of U & Th to ±0.3 (95% CL)

–

Depleted mantle radiogenic heat to +3/-2 TW–

~50 kT-y exposure required

•

Geo-nu mantle measurement from continent–

Crust study and/or directionality

–

Inferior (crust and reactor syst error) to Hanohano•

More studies/collaborative projects

6-Oct-2010 22

backups

6-Oct-2010 23

Deep-Earth Geo-reactor Models

Core-Mantle Boundary P~5TWR.J. de Meijer

& W. van WestrenenS. Afr. J. Sci. 104, 111 (2008)

r=3480 km

r=1222 km Inner Core Boundary P~20-30 TWV.D. Rusov

et al., J. Geophys. Res. 112, B09203 (2007)

Earth Center P~3-10 TWJ.M. Herndon,

Proc. Nat. Acad. Sci. 93, 646 (1996)

Proposed at 3 depths w/ loosely definedpower output sufficient to explain:

• surface heat flow > radiogenic heat33-46 TW > ~20 TW

•3He/4He OIB>MORBtritium fission product-

3H→3He+β-+ν

Deep-earth Geo-reactor:Hypothetical and very speculative

Possible and not ruled out

6-Oct-2010 24