Re -187Os Nuclear Geochronometry - Copernicus.org · Nuclear geochronometry is a new research...
Transcript of Re -187Os Nuclear Geochronometry - Copernicus.org · Nuclear geochronometry is a new research...
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
© Götz Roller 2015
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
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)
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
© Götz Roller 2015
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 ...
© Götz Roller 2015
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.
© Götz Roller 2015
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
© Götz Roller 2015
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
© Götz Roller 2015
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 …
© Götz Roller 2015
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
© Götz Roller 2015
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
© Götz Roller 2015
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
© Götz Roller 2015
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 …
© Götz Roller 2015
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
© Götz Roller 2015
14
BARBERTON chronometerIVREA, RONDA, CAPE SMITH chronometer
0.00
2.00
4.00
6.00
8.00
10.04
12.00
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
© Götz Roller 2015
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 …
© Götz Roller 2015
16
BARBERTON chronometerIVREA, RONDA, CAPE SMITH chronometer
0.00
2.00
4.00
6.00
8.00
10.04
12.00
0 2 4 6 8 10 12 14T i m e (billion years ago)
= 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
© Götz Roller 2015
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 …
© Götz Roller 2015
18
BARBERTON chronometerIVREA, RONDA, CAPE SMITH chronometer
0.00
2.00
4.00
6.00
8.00
10.04
12.00
0 2 4 6 8 10 12 14T i m e (billion years ago)
= Stillwater= Sudbury SIC
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.
© Götz Roller 2015
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 …
© Götz Roller 2015
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 …
© Götz Roller 2015
21
BARBERTON chronometerIVREA, RONDA, CAPE SMITH chronometer
0.00
2.00
4.00
6.00
8.00
10.04
12.00
0 2 4 6 8 10 12 14T i m e ( billion years ago)
= 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 WR2+WR3
ST86-1 chromite separate
WR1
ST86-1
Levack West
+Falconbridge
isochrone
Sudbury
TPI age and
isochrone
© Götz Roller 2015
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
© Götz Roller 2015
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!
© Götz Roller 2015
24
© Götz Roller 2015
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
© Götz Roller 2015