measuring u & th enrichment of the silicate...
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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
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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?
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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)
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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)
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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
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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)
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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)
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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
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(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
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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
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(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
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(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
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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
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Local crust U & Th content
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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
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