Noble gas isotopic evolution of the Earth’s mantle contro - PowerPoint Presentation

Slide1

Noble gas isotopic evolution of the Earth’s mantle controlled by U and Th contents(just a review of noble gas reservoirs....)

2013. 10. 30@Workshop on Particle Geophysics, Sendai

Hirochika SUMINOGeochemical Research Center (GCRC)University of TokyoSlide2

Cover a wide mass range.

Insensitive to

chemical processes. – because of chemical inertness.Sensitive to mixing of several reservoirs. – vary by several orders of magnitude depending on the origin. Provide temporal information.– because some isotopes accumulate with time.

Determinable with high sensitivity / precision using special mass spectrometric systems.

Noble gas isotopes

element

isotope

He

3

He

4

HeNe20Ne21Ne22NeAr36Ar38Ar40ArKr78~86KrXe124~136XeSlide3

Noble gas components in the solar system Solar / Primordial: Original composition of material from which the solar system or the Earth formed.Radiogenic: Produced by decay of radioactive nuclides. e.g.,

a-decay of U, Th → 4He 40K (E.C.)

→ 40Ar 129I (β-) → 129XeNucleogenic: Product of nuclear reactions induced by a-particles or neutrons. e.g.,

6Li (n,a

) → 3H (β-) → 3He

18O (a,n) → 21Ne

Fissiogenic Fission products of 238U and 244

Pu.Cosmogenic: Product of spallation induced by cosmic-rays.Slide4

Helium isotope ratios of MORBs and OIBsdegassed

less degassed

high 3He/(U+Th)

low 3He/(

U+Th)

(

Barfod

et al., JGR

1999)

R

A

= atmospheric 3He/4He = 1.4  10-63He/4He (RA)4He/3HeSlide5

Plume source

 50 RA

Hotspot5~50 RA3

He/4He of geochemical reservoirs

Solar

(Primordial) 3He/4He >

120 RA

Radiogenic (from U, Th)

3

He/

4He ~ 0.01 RA +Mid Ocean Ridge Basalts (MORB)8 RAAtmosphereCrustMantleAtmosphere3He/4He = 1 RA (1.410-6)MORB source8 RAUpwelling“Plume”Lower mantle or core-mantle boundary ?Crust~0.01

RASlide6

Neon isotopes of MORBs and OIBs

MORB source

3He/4He ~ 8 RA 40Ar/36Ar ~ 40000 High 21

Ne/22Ne

OIB source (Plume)

3

He/4He > 50 RA

40

Ar/

36

Ar ~ 8000 Low 21Ne/22NeAtmosphere 3He/4He = 1 RA (1.410-6) 40Ar/36Ar = 296PrimordialRadiogenic/Nucleogenic3He 20Ne, 22Ne36Ar4He21Ne40Ar degassedless degassed(Trieloff et al., EPSL 2002)

Nucleogenic

MORB source

Crustal

Primordial

18

O

(

a

,n

)

→

21

Ne

high

22

Ne

/(

U+Th

)

low

22

Ne

/(

U+Th

)Slide7

Where is the less degassed mantle domain?

(Porcelli & Ballentine, Rev. Mineral. Geochem. 2002)

:

high (

3He,

20Ne)/(U+Th) (=more primitive, less degassed)

Convection mode

A, B: two-layeredC, D, E: whole mantle

Less degassed reservoir

A, B: lower mantle

C: heterogeneities or

deeper layers D: D” E: CoreSlide8

He isotope evolution in the convecting mantle

(Porcelli & Elliott, EPSL 2008)

Model inputsInitial 3He/4He

120 or 330

RAPresent 3

He/4He

8 RAInitial 3

He conc.(2.8 or 11)  1010

atoms/g

Present

3

He conc.8.7  108 atoms/gInitial U conc.21 ppbPresent U conc.3 ppbInitial U/Th3.8Present U/Th2.5Model resultsFactional melting rate2.1–3.6  10-9 yr-1Decrease in degassing rate6.0–7.3  10-10 yr-13He output from ridges490 – 2900 mol yr-1obs.) 1000 mol yr-1Slide9

Early separation of 3He-enriched hidden reservoirTo maintain high 3

He/4He as high as 50 RA, the plume source must have been isolated earlier or exhibit high

3He/U. (Porcelli & Elliott, EPSL 2008)– Core with primordial He?

(Porcelli

& Halliday, EPSL 2001;

Bouhifd et al., Nature

Geosci. 2013)– D” layer with high

3He and U? (Tolstikhin

& Hofmann,

PEPI

2005)

(Porcelli & Elliott, EPSL 2008)Slide10

Alternative model

(Gonnermann & Mukhopadhyay,

Nature 2009)Different evolution resulted from different processing rate– several times for UM.– approx. once for LM.explains

present-day 3

He and 40Ar.Slide11

When the two mantle domains separated?(Mukhopadhyay, Nature 2012)

Correction for atmospheric contamination based on relationship with

20Ne/22Ne and primordial (= solar wind) 20Ne/

22

Ne value.Slide12

When the two mantle domains separated?129I (β-)

→ 129Xe (T1/2 = 15.7 Ma)244Pu → 131

Xe, 132Xe, 134Xe, 136Xe (T1/2 = 80.0Ma)238U → 131

Xe, 132Xe,

134Xe, 136Xe (T1/2 = 4.47Ga)

244Pu-derived

136Xe: 1-40% for MORB 70-99% for Iceland

 (Almost) undegassed Iceland mantle source has been isolated since 4.45 Ga.

(

Mukhopadhyay

,

Nature 2012)Slide13

Where is the less degassed mantle domain?

(Porcelli & Ballentine, Rev. Mineral. Geochem. 2002)

:

high (

3He,

20Ne)/(U+Th) (=more primitive, less degassed)

Convection mode

A, B: two-layeredC, D, E: whole mantle

Less degassed reservoi

r

A, B: lower mantle

C: heterogeneities or deeper layers  LLSVPs?D: D”E: CoreSlide14

The undegassed mantle = LLSVPs ?

(Bull et al., EPSL 2009)–

“LLSVPs are features that have existed since the formation of the Earth and cannot exclusively be composed of subducted slabs”. (Mukhopadhyay, Nature

2012).

– Consistent with EM-high 3He/4He (primordial) and HIMU-low

3He/4He (recycle) components in Polynesian

OIBs. (Parai et al.

, EPSL 2009)

If the undegassed mantle domains correspond to LLSVPs,

“

A

low velocity anomaly beneath Iceland is confined to the upper mantle”. (Ritsema et al., Science 1999)Slide15

Possible primordial noble gas reservoirs and their U estimationsLLSVPs – a mixture of undegassed mantle and subducting materials (Mukhopadhyay

, Nature 2012) ~20 ppb

(BSE value) or more U. ~40% or more of total U in the mantle.D” layer – a mixture of early-formed crust and chondritic debris

(

Tolstikhin & Hofmann, PEPI 2005)

 ~70 ppb U

~30% of total U in the mantle.

 Can be discriminated via

g

eoneutrino

?Slide16

Helium in subcontinental lithospheric mantle (SCLM)

N= 154

Lherzolite

, crush only

Mean = 5.9 ± 2.2 RAMed. = 6.5

RA

MORBData: Africa (N=22; Aka

et al., 2004; Barfod et al

.

, 1999;

Hilton

et al., 2011; Hopp et al., 2004; 2007), Europe (N=51; Buikin et al., 2005; Correale et al., 2012; Gautheron et al., 2005; Martelli et al., 2011; Sapienza et al., 2005), Siberia (N=18; Yamamoto et al., 2004; Barry et al., 2007), Eastern Asia (N = 28; Sumino, unpublished data; Kim et al., 2005; Chen et al., 2007; He et al., 2011; Sun, unpublished data), Australia (N = 24; Czuppon et al., 2009; 2010; Matsumoto et al., 1998; 2000; Hoke et al., 2000), South America (N = 11; Jalowitzki, unpublished data)Slide17

Closed system evolution of SCLM

3

He/4He (RA)Time before present (Ma)

150

100

500

Convecting mantle 6.0 R

A4.6 RA

0.2

R

A

U/3He  30U/3He  60U/3He  3000Metasomatic event(U/3He increase)(KIM et al., Geochem. J. 2005)Similar or higher radiogenic 4He/40Ar ratios (proxy for (U+Th)/K) than the MORB source suggest U/3He increase mainly due to U (and Th, K) addition by slab-derived fluids rather than substantial loss of 3He. (Yamamoto et al., Chem. Geol. 2004; Kim et al., Geochem. J. 2005)U in metasomatized SCLM (for 6 RA): 90 ppbcf) 25 ppb (Archean) (Rudbuck et al., Chem. Geol. 1998) 40 ppb (post-Archean) (McDonough, EPSL 1990)Slide18

Neon

in

SCLM

Air

Iceland source

MORB source

SCLM?

22

Ne/(

U+Th

): Iceland > MORB > Patagonian SCLM

undegassed degassed enriched in U?(Jalowitzki et al., in prep.)18O (a,n) → 21NeSlide19

SummaryNoble gas (especially He) isotopic evolution in the mantle is directly related to U and Th contents in their reservoirs.

As the deep mantle plume source associated with primordial noble gases, the strongest candidates are LLSVPs and D” layer possibly enriched in 3

He and U+Th. They contain 30-40% of total U and Th in the mantle, thus would be detectable via future geoneutrino observation.SCLM enriched in U and Th is another reservoir of noble gases in the mantle

. Although it contains 10-30 times as much of U than the convecting mantle, its small volume fraction (ca. 1.5

% ) results in insignificant contribution to global geoneutrino flux. However, it may

be significant for a detector located in continental margin.