Re -187Os Nuclear Geochronometry - Copernicus.org · Nuclear geochronometry is a new research field, developed to bridge the gap between geochronology and nuclear astrophysics. It

187Re -187Os Nuclear Geochronometry:

A New Dating Method Applied to Old Ores

0.00

2.00

4.00

6.00

8.00

10.00

0 2 4T i m e (Ga)

18

7O

s / 1

86O

s

Stillwater

Sudbury

?

? 0.00

0.05

0.10

0.15

0.20

0.25

0.30

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

T i m e (billion years ago)

18

7O

s /

18

7R

eeλt -1

B2FH (1957): model line of

sudden nucleosynthesis

1 2

© Götz Roller 2015

Goetz Roller


187Re -187Os Nuclear Geochronometry:

A New Dating Method Applied to Old Ores

21 + =

sudden

nucleosynthesis

sudden

nucleo-

synthesis

B465, Blue Posters

Tuesday, 5.30 P.M.

B641, Blue Posters

Wednesday, 5.30 P.M.

0.00

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10.04

12.00

0 2 4 6 8 10 12T i m e (billion years ago)

18

7O

s / 1

86O

s

Stillwater

Sudbury

≈3 Ga ≈13.78 Ga

mixing

Nuclear Geochronometry

Goetz Roller

3

Nuclear geochronometry is a new research field, developed

to bridge the gap between geochronology and nuclear

astrophysics. It is based upon the discovery of terrestrial

signatures from at least two rapid (r-) neutron-capture

processes, which plot on the astrophysical model line of

sudden nucleosynthesis (see ; Burbidge et al 1957). It

aims at dating rocks by means of radioactivity, which makes

it a subfield of nuclear chemistry. The dating method is

embedded in other scientific fields like cosmology,

cosmochemistry and nuclear theory, which impose tight

constraints on nuclear geochronometry. Or, in other words:

in nuclear geochronometry we start with Becquerel and

Curie, move on to Rutherford and Soddy, pass Chadwick and

Fermi, and finally end up with Fowler, Mather and Smoot …

187Re -187Os Nuclear Geochronometry


2

4

Let‘s first talk about some basics: most of the neighbour

nuclides of Os (Re, Ir) are created via the r-process and

subsequent β- decay. Because of its 41.6 Ga half-life,187Re

becomes a powerful cosmic clock to calculate the age of

the heavy elements, which is a lower boundary for the age

of our Universe. Knowing the nuclear production ratio

(PR), it is possible to solve the following equations for t:187Re/188Osnow = 187Re/188OsPR (e-λt)187Os/188Osnow = 187Re/188OsPR (1 – e-λt)

The PR`s may be derived from nuclear theory (which is the

normal case in astronomy) or from observation:

Ir/Os = 0.96 ; C I Chondrites; (Lodders et al. (2009), arXiv: 0901.1149)

Ir/Os = 0.96 ; solar photosphere; (Asplund et al. 2009, ARA&A 47. 481)

See the following chart, which is an excerpt from the

chart of nuclides, modified for nuclear geochronometry ...


5

191941920.79

1900.02Pt

183 184 1861800.13

18226.3

W

19137.3

19362.7Ir

18537.4

18762.6

75 Re

1921901891881861.58

1871840.01

Os

β- decay

ά decay

p- process

r- process

A

Z

N = 108

Look at the chart: 191Ir and 185Re as well as 193Ir and 187Re have similar

relative abundances (37:63), which could point to the same physics

behind the creation of these nuclides! We find Re/Os ≈ Ir/Os ≈ 1.


6

The most powerful nuclear chronometer currently used is 187Re. Five

nuclear 187Re chronometers (Re/Os ≈ 1) have been identified so far.

There are two age clusters: At TNUC ≈ 13.78 Ga and at TNUC ≈ 3 Ga.

[1] Birck, J.-L. & Allegre, C.-J. (1994) Earth Planet. Sci. Lett. 124, 2139 - 148 (NTIMS) [2] Reisberg, L. C.,

Allegre, C.-J. & Luck, J. M. (1991) Earth Planet. Sci. Lett. 105, 196 – 213 (SIMS) [3] Roller, G. (1996) RKP

Natw. + Techn., Munich. (NTIMS) [4] Roy-Barman, M., Luck, J. M. & Allegre, C.-J. (1996) Chem. Geol.

130, 55 - 64 (SIMS) [5] Luck, J. M. & Allegre, C.-J. (1984) Earth Planet. Sci. Lett. 68, 205 -208 (SIMS)

2.9430.05021.02440.000448.00040.62.040[Cape Smith Sulf.]

CAPE SMITH5

2.5490.04341.042.5002.60041.51.800[LA 14]

LANZO(4)

2.9010.04950.95110610139.11.9360[Mo369]

IVREA3

3.0340.05180.959740710381.97[R85–62]

RONDA2

13.780.258090.849245208 38.910.04[5085 BasKom]

BARBERTON1

TNUC

[Ga]

187Os/187ReRe/OsOs

[ppt]

Re

[ppt]

187Re/186Os187Os/186OsSample /

CHRONOMETER

Ref.

≈ Ir/Os


7

By means of two-point-isochrone (TPI) ages it is possible to

question the significance of model ages.

Model ages are based on the assumption, that a sample has

a defined initial ratio at a specific time. For an r-process,

this initial ratio is assumed to be ZERO.

TPI ages constrain the initial ratio using the BARBERTON

or IVREA etc. chronometer always as the second data

point in a TPI diagram.

Model Ages and TPI Ages


8

TPI Age (i = derived)

age equation:

t = 1/λ ● ln (tan ά + 1)tan ά = ∆ 7/8Os / ∆7Re/8Os

Model Age (i = assumed)

7/8Os

7Re/8Os

ά∆ 7/8Os

∆7Re/8Os

i

7/8Os

7Re/8Os

∆7Re/8Os

∆ 7/8Os

ά

i

mutual control

Look at the next charts for an example. You will see these

two diagrams attached to the diagrams of the example …


9

Example: Model age diagram for

The initial ratio (i) is assumed to be ZERO, as expected in case of a

sudden nucleosynthesis event. The model age is well constrained by

the cosmological Planck-WP-highL age of the Universe.

Age = 13781±63 MaInitial

187Os/

188Os = 0.00 (assumed)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 1 2 3 4 5 6

data-point error ellipses are 2σ

+ ZERO

13.781 ± 0.063 Ga

TPI ZERO (model) age

Planck+WP+highL age:

13.817 ± 0.048 Ga

7/8Os

7Re/8Os

ά ∆ 7/8Os

∆7Re/8Os

18

7O

s / 1

88O

s

Isoplot/Ex.3.75

Ludwig (2012)

Sp.Publ.Nr. 5

BGC,UCB, Berkeley

187Re / 188Os


10

Example: TPI age diagram to control the model age

The two data points are the BARBERTON chronometer and the

Thielmountain Pallasite . The model age is constrained by this TPI

age, which is also a lower boundary for the age of the Universe.

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6

data-point error ellipses are 2s

Thielmountain (Pallasite)

Data for Thielmountain: Shen et al. (1998), GCA 62,2715; Calculation: Roller (2012), RKP N+T, Munich, mod.

Age = 13775±71 Ma

Initial187

Os/188

Os =0.00059±0.00087

7/8Os

7Re/8Os

∆7Re/8Os

ά+ :

13.775 ± 0.071 Ga

Isoplot/Ex.3.75

Ludwig (2012)

Sp.Publ.Nr. 5

BGC,UCB, Berkeley

Planck+WP+highL age:

13.817 ± 0.048 Ga1.2

1.4

+ ZERO

13.781 ± 0.063 Ga

187Re / 188Os

18

7O

s / 1

88O

s


11

Now, let’s begin with the PGE ores. Below you see a conventional

isochrone calculated with the data of Walker et al. (1991, EPSL 105, 416

- 429) for the Strathcona ores of the Sudbury Igneous Complex:

Dating the Sudbury ores with the BARBERTON chronometer

5

15

25

35

45

55

65

0 400 800 1200 1600 2000

data-point error ellipses are 2 s

Age = 1689 ± 160 Ma187Os/186Osi = 8.8 ± 2.3

MSWD = 5.0

Isoplot/Ex.3.75

Ludwig (2012)

Sp.Publ.Nr. 5

BGC,UCB, Berkeley

187Re / 186Os

18

7O

s / 1

86O

s


12

The nucleogeochronometric TPI ages for these Strathcona ores

are in good agreement with the isochrone age of 1689 ± 160 Ma:

+ PGM-79-126:1715 ± 62 Ma, 187Os/186Osi = 8.913 ± 0.044

+ PGM-79-128:1649 ± 62 Ma, 187Os/186Osi = 8.957 ± 0.044

+ PGM-79-138:1733 ± 84 Ma, 187Os/186Osi = 8.901 ± 0.059

+ PGM-79-139:1586 ± 63 Ma, 187Os/186Osi = 8.998 ± 0.045

Look at the improved 2 σ values!

Then, see the next two charts and remember the …


13

Below, the TPI ages are attatched to the isochrone data points.

Remember the and see the next chart ….

5

15

25

35

45

55

65

0 400 800 1200 1600

data-point error ellipses are 2 s

Age = 1689 ± 160 Ma187Os/186Osi = 8.8 ± 2.3

MSWD = 5.0

18

7O

s / 1

86O

s

187Re / 186Os

TPI ages

+ PGM-79-126:

1715 ± 62 Ma,187Os/186Osi = 8.913 ± 0.044

+ PGM-79-128:

1649 ± 62 Ma, 187Os/186Osi = 8.957 ± 0.044

+ PGM-79-139:

1586 ± 63 Ma,187Os/186Osi = 8.998 ± 0.045

+ PGM-79-138:

1733 ± 84 Ma,187Os/186Osi = 8.901 ± 0.059


14

BARBERTON chronometerIVREA, RONDA, CAPE SMITH chronometer

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0 2 4 6 8 10 12 14T i m e (billion years ago)

= Sudbury SIC

18

7O

s / 1

86O

s

BARBERTON + IVREA

mingling / mixing field SIC isochrone data

mingling

mixing

7/6

Os

7Re/6Os

7/6

Os

7Re/6Os

Sudbury

TPI age and

SIC isochrone


15

Dating the Stillwater ores by means of the IVREA

RONDA or CAPE SMITH chronometer

For the Stillwater Complex (Montana, USA), Marcantonio et al.

(1993, GCA 57, 4029 – 4037) report a Molybdenite Re/Os age

(ST86-1) from the G-chromitite of 2740 ± 80 Ma, consistent with

the Sm-Nd age of 2701 Ma (DePaolo & Wasserburg 1979).

For this age, the ST86-1 WR1 sample from the G-chromitite

shows an extremely high suprachondritic 187Os/186Osi = 6.55.

(187Os/188Osi ≈ 0.786). [high suprachrondritic = very, very high!]

This is very close to the signature of the BARBERTON

chronometer at the same time.

Remember the and see the next chart …


16


0.00

2.00

4.00

6.00

8.00

10.04

12.00


= Stillwater= Sudbury SIC

18

7O

s / 1

86O

s

BARBERTON + IVREAmingling / mixing field SIC isochrone data

mingling

mixing

7/6

Os

7Re/6Os

7/6

Os

7Re/6Os

ST86-1 WR1


17

Contrary to that, ST86-1 chromite separate 1 from the same

G-chromitite (Ultramafic Series) shows an unusual ultra-

subchondritic 187Os/186Osi = 0.13 (187Os/188Osi ≈ 0.0156). [ultra-

subchondritic = very, very low, much lower than the chondritic

mantle at 2700 Ma, which shows a 187Os/186Osi ≈ 0.92 or

187Os/188Osi ≈ 0.11]

Using the geochronometer, a TPI age of 2717 ± 100 Ma

(187Os/186Osi = 0.125 ± 0.067 or 187Os/188Osi ≈ 0.015) can be

calculated for ST86-1 chromite separate 1 .

Again, keep the in mind and see the next chart …


18


0.00

2.00

4.00

6.00

8.00

10.04

12.00



18

7O

s / 1

86O

sBARBERTON + IVREAmingling / mixing field SIC isochrone data

mingling

mixing

7/6

Os

7Re/6Os

7/6

Os

7Re/6Os

ST86-1

chrom.

sep.


19

From WR2 and WR3 of the Stillwater Complex G-chromitite

reported in Marcantonio et al. (1993, GCA 57, 4029), an isochrone

age of 2671 ± 75 Ma can be calculated, which confirms the TPI age

of 2717 ± 100 Ma and the other ages for the Stillwater Complex

(2740 Ma, 2701 Ma). The initial 187Os/186Os ratio of the isochrone is

1.143 ± 0.089 (187Os/188Os ≈ 0.1372).

This could be explained by mixing of components from the

and geochronometers at ≈ 2671 ± 75 Ma.

Thus, the situation is comparable to the Levack West +

Falconbridge isochrone of the Sudbury Igneous Complex.

See the isochrone diagram for WR2 and WR3 on the next chart, ...

… and then the nucleogeochronometric summary chart …


20

Isoplot/Ex.3.75

Ludwig (2012)

Sp.Publ.Nr. 5

BGC,UCB, Berkeley

2

4

6

8

10

20 60 100 140 180 220

Age = 2671±75 Ma

Initial 187

Os/186

Os =1.143±0.089

ST86-1 G-chrom. WR2+WR3

data-point error ellipses are 20

187Re / 186Os

18

7O

s / 1

86O

s

Do you remember all the , and you have seen so far?

If not: don’t worry! Just look at the following summary chart …


21


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8.00

10.04

12.00

0 2 4 6 8 10 12 14T i m e ( billion years ago)


18

7O

s / 1

86O

s

BARBERTON + IVREAmingling / mixing field SIC isochrone data

mingling

mixing

7/6

Os

7Re/6Os

7/6

Os

7Re/6Os

ST86-1 WR2+WR3

ST86-1 chromite separate

WR1

ST86-1

Levack West

+Falconbridge

isochrone

Sudbury

TPI age and

isochrone


22

1. Ultra-suprachrondritic 187Os/186Os or 187Os/188Os initial ratios

in Proterozoic magmatic rocks may be explained as being

derived from a magmatic source with a ≈ 13.78 Ga old r-

process signature, created in a sudden nucleosynthesis

shortly after the Big Bang.

2. Ultra-subchrondritic 187Os/186Os or 187Os/188Os initial ratios in

Archean magmatic rocks may be explained as being

derived from a magmatic source containing a ≈ 3 Ga old r-

process signature from a sudden nucleosynthesis event,

which might have initiated rifting within the Pilbara Craton

and thus plate tectonics on Earth.

Conclusions


23

3. Initial Os isotopic ratios between the and evolution lines

(as in the case of the Stillwater and the Sudbury Complex) may

be explained as a mixture between the two r-process reservoirs

and / or the convecting mantle signature.

4. The interaction of the two reservoirs may be responsible for

convection within the Earth`s core, thus being still today‘s

driving force for plate tectonics and the Earth‘s magnetic field.

5. Nuclear geochronometry favours an endogenic origin for the

Sudbury and Stillwater ores, with at least some of the PGE‘s

being genetically related to the Earth‘s core.

… and since a picture tells more than 1000 words, see the next chart!


24


Earth‘s inner core

Earth‘s outer core

0.00

2.00

4.00

6.00

8.00

10.04

12.00

0 2 4 6 8 10 12T i m e (billion years ago)

Stillwater Complex G-chromitite (UMS)Sudbury Complex, Strathcona, Falconbridge etc

18

7O

s / 1

86O

s

WC 1-2 (rifting)(Shirey & Richardson 2011,

Science 333, 434)

WC 5-6

SIC isochrone data

mingling

7/6

Os

7Re/6Os

WC Wilson Cycle (1-2 Pilbara / 5-6 Kaapvaal)

7/6

Os

7Re/6Os

mixing

≈ 3.2 Ga: reservoir interaction, convection

=> onset time of plate tectonics

≈ 3.48 Ga:

core collapse,

=> initiation of plate tectonics

≈ 13.78 Ga

r-process

≈ 3 Ga

r-process

mixing

See the last chart …

25

13.78 Ga → 3.48 Ga

ancient component

weak magnetic field

≈ 3.48 Ga core collapse

p+ + e- → n + ν, r-process

ignition of nuclear processes

nucleosynthesis etc.

suddenly increasing

magnetic field strength

3 Ga → today

modern Earth,

inner/outer core

convection, pulsation

magnetic field reversals

mantle/crust/plate tectonics

Fe, Ni … (?)

Re/Os ≈ 1

BARBERTON

Fe, Ni

Re/Os ≈ 0.85

BARBERTON

element

formation

element

formation

element

formation element

formation

element

formation

element

formation

element

formation

element

formation

Fe, Ni

Re/Os ≈ .85

BARBERTON

solid

Fe, Ni, … (?).

liquid outer core

Re/Os ≈ 1IVREA

Nucleogeochronometric Evolution of the Earth‘s Core

metastable

instable

pin >>>> pout

gravitational collapse

stable

“only“ magmatism,

earthquakes etc.

slow cooling

during ≈ 10 Ga

T (K)

T (K) pin ≈ pout

pin ≈/> pout

T = temperature (Kelvin); p = pressure

out = outward, due to thermal expansion etc.

in = inward, due to gravitation etc.

= down, = up

T (K) pin ≈ pout

thermal expansion

magnetic pressure


Documents

Re -187Os Nuclear Geochronometry - Copernicus.org · Nuclear geochronometry is a new research field, developed to bridge the gap between geochronology and nuclear astrophysics. It