187Re -187Os Nuclear Geochronometry: A New Dating Method

Transcription

187Re -187Os
Nuclear Geochronometry:
A New Dating Method Applied to Old Ores
Goetz Roller
1
2
10.00
0.30
Sudbury
0.25
6.00
Stillwater
/ 187Re
?
187Os
187Os
/ 186Os
8.00
4.00
0.20
0.15
0.10
0.05
2.00
?
0.00
0
eλt -1
B2FH (1957): model line of
sudden nucleosynthesis
0.00
2
4
T i m e (Ga)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
T i m e (billion years ago)
© Götz Roller 2015
187Re -187Os
Nuclear Geochronometry:
A New Dating Method Applied to Old Ores
Goetz Roller
12.00
1 + 2 =
Nuclear Geochronometry
187Os
/ 186Os
10.04
8.00 Sudbury
Stillwater
6.00
mixing
4.00
sudden
nucleosynthesis
2.00
0.00
sudden
nucleosynthesis
0
B465, Blue Posters
Tuesday, 5.30 P.M.
2 ≈3 Ga 4
6
B641, Blue Posters T i m e
Wednesday, 5.30 P.M.
8
10
12 ≈13.78 Ga
(billion years ago)
187Re -187Os
Nuclear Geochronometry
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 2 ; 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 …
3
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/188Os
187Re/188Os
-λt
now =
PR (e )
187Os/188Os
187Re/188Os
-λt
now =
PR (1 – e )
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 ...
4
A
Z
Pt
190
192
0.02
0.79
Ir
Os
184
186
0.01
1.58
75 Re
180
0.13
W
N=
182
26.3
108
183
187
188
185
187
37.4
62.6
184
186
189
194
191
193
37.3
62.7
190
192
19
r- process
p- process
ά decay
β- decay
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.
5
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.
≈ Ir/Os
Ref.
Sample /
CHRONOMETER
187Os/186Os187Re/186Os
Re
[ppt]
Os
[ppt]
Re/Os
187Os/187Re
TNUC
[Ga]
1
[5085 BasKom]
BARBERTON
10.04
38.9
208
245
0.849
0.25809
13.78
2
[R85–62]
RONDA
1.97
38
710
740
0.959
0.0518
3.034
3
[Mo369]
IVREA
1.9360
39.1
101
106
0.951
0.0495
2.901
1.800
41.5
2.600
2.500
1.04
0.0434
2.549
2.040
40.6
448.000
440.000
1.02
0.0502
2.943
(4)
5
[LA 14]
LANZO
[Cape Smith Sulf.]
CAPE SMITH
[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)
6
Model Ages and TPI Ages
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.
7
Model Age (i = assumed)
7/8Os
TPI Age (i = derived)
7/8Os
mutual control
∆ 7/8Os
ά
∆ 7/8Os
i
∆7Re/8Os
age equation:
i
t = 1/λ ● ln (tan ά + 1)
tan ά = ∆ 7/8Os / ∆7Re/8Os
7Re/8Os
ά
∆7Re/8Os
7Re/8Os
Look at the next charts for an example. You will see these
two diagrams attached to the diagrams of the example …
8
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.
 2σ
data-point error ellipses are
1.4 Planck+WP+highL age:
187Os
/ 188Os
1.2
13.817 ± 0.048 Ga
1.0
7/8Os
+ ZERO
13.781 ± 0.063 Ga
0.8
∆ 7/8Os
ά
∆7Re/8Os
0.6
7Re/8Os
TPI ZERO (model) age
0.4
Age = 13781±63 Ma
188
Os/ Os = 0.00 (assumed)
187
Initial
0.2
0.0
0
1
2
187Re
3
/
4
188Os
5
Isoplot/Ex.3.75
Ludwig (2012)
Sp.Publ.Nr. 5
BGC,UCB, Berkeley
6
9
Example: TPI age diagram to control the model age
The two data points are the BARBERTON
Thielmountain Pallasite
chronometer and the
. The model age is constrained by this TPI
age, which is also a lower boundary for the age of the Universe.
187Os
/ 188Os
s 2
1.4 Planck+WP+highL age:
13.817 ± 0.048 Ga
1.2
+ ZERO
1.0 13.781 ± 0.063 Ga
7/8Os
+
0.8
:
13.775 ± 0.071 Ga
0.6
ά
∆7Re/8Os
Thielmountain (Pallasite)
7Re/8Os
0.4
Age = 13775±71 Ma
Initial Os/188Os =0.00059±0.00087
0.2
187
0.0
0
1
2
3
187Re
4
5
Isoplot/Ex.3.75
Ludwig (2012)
Sp.Publ.Nr. 5
BGC,UCB, Berkeley
6
/ 188Os
Data for Thielmountain: Shen et al. (1998), GCA 62,2715; Calculation: Roller (2012), RKP N+T, Munich, mod.
10
Dating the Sudbury ores with the BARBERTON
chronometer
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:
data-point error ellipses are 2 s
65
Age = 1689 ± 160 Ma
187Os/186Os = 8.8 ± 2.3
i
187Os
/ 186Os
55
MSWD = 5.0
45
35
25
15
Isoplot/Ex.3.75
Ludwig (2012)
Sp.Publ.Nr. 5
BGC,UCB, Berkeley
5
0
400
800
187Re
1200
1600
2000
/ 186Os
11
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
…
12
Below, the TPI ages are attatched to the isochrone data points.
Remember the
and see the next chart ….
 2s
65
Age = 1689 ± 160 Ma
187Os/186Os = 8.8 ± 2.3
i
MSWD = 5.0
187Os
/ 186Os
55
TPI ages
+ PGM-79-126:
1715 ± 62 Ma,
187Os/186Os = 8.913 ± 0.044
i
45
+ PGM-79-128:
1649 ± 62 Ma,
187Os/186Os = 8.957 ± 0.044
i
35
25
+ PGM-79-139:
1586 ± 63 Ma,
187Os/186Os = 8.998 ± 0.045
i
15
5
0
400
800
187Re
1200
/ 186Os
1600
+ PGM-79-138:
1733 ± 84 Ma,
187Os/186Os = 8.901 ± 0.059
i
13
BARBERTON + IVREA
mingling / mixing field
187Os
mingling
7Re/6Os
Sudbury
TPI age and
SIC isochrone
7/6Os
/ 186Os
10.04
8.00
SIC isochrone data
7/6Os
12.00
6.00
mixing
7Re/6Os
4.00
2.00
0.00
0
2
4
6
8
10
12
BARBERTON chronometer
IVREA, RONDA, CAPE SMITH chronometer
14
= Sudbury SIC
14
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 …
15
BARBERTON + IVREA
mingling
7Re/6Os
8.00
ST86-1 WR1
7/6Os
/ 186Os
10.04
187Os
SIC isochrone data
7/6Os
12.00
6.00
mixing
7Re/6Os
4.00
2.00
0.00
0
2
4
6
8
10
12
14
= Stillwater
= Sudbury SIC
16
Contrary to that, ST86-1 chromite separate 1
from the same
G-chromitite (Ultramafic Series) shows an unusual ultrasubchondritic 187Os/186Osi = 0.13 (187Os/188Osi ≈ 0.0156). [ultrasubchondritic = very, very low, much lower than the chondritic
mantle at 2700 Ma, which shows a 187Os/186Osi ≈ 0.92 or
187Os/188Os
i
Using the
≈ 0.11]
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 …
17
BARBERTON + IVREA
SIC isochrone data
7/6Os
12.00
10.04
mingling
187Os
8.00
7/6Os
/ 186Os
7Re/6Os
6.00
mixing
7Re/6Os
4.00
ST86-1
chrom.
sep.
2.00
0.00
0
2
4
6
8
10
12
14
= Stillwater
= Sudbury SIC
18
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 …
19
data-point error ellipses are 02
187Os
/ 186Os
10
8
6
Age = 2671±75 Ma
186
Initial
Os/ Os =1.143±0.089
ST86-1 G-chrom. WR2+WR3
4
187
Isoplot/Ex.3.75
Ludwig (2012)
Sp.Publ.Nr. 5
BGC,UCB, Berkeley
2
20
60
100
140
187Re
Do you remember all the
,
180
220
/ 186Os
and
you have seen so far?
If not: don’t worry! Just look at the following summary chart …
20
BARBERTON + IVREA
7/6Os
12.00
SIC isochrone data
10.04
mingling
187Os
8.00
6.00
4.00
7/6Os
/ 186Os
7Re/6Os
Sudbury
TPI age and
isochrone
WR1
ST86-1
mixing
7Re/6Os
Levack West
+Falconbridge
isochrone
2.00
ST86-1 WR2+WR3
ST86-1 chromite separate
0.00
0
2
4
6
8
10
12
T i m e ( billion years ago)
14
= Stillwater
= Sudbury SIC
21
Conclusions
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 rprocess 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 rprocess signature from a sudden nucleosynthesis event,
which might have initiated rifting within the Pilbara Craton
and thus plate tectonics on Earth.
22
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!
23
12.00
≈ 3.2 Ga: reservoir interaction, convection
=> onset time of plate tectonics
SIC isochrone data
7/6Os
8.00
6.00
mingling
7Re/6Os
mixing
WC 5-6
WC 1-2 (rifting)
4.00
7/6Os
187Os
/ 186Os
10.04
mixing
7Re/6Os
(Shirey & Richardson 2011,
Science 333, 434)
≈ 3.48 Ga:
core collapse,
=> initiation of plate tectonics
2.00
0.00
0
2 ≈ 3 Ga 4
6
8
10
r-process T i m e (billion years ago)
Earth‘s inner core
Earth‘s outer core
12
≈ 13.78 Ga
r-process
Stillwater Complex G-chromitite (UMS)
Sudbury Complex, Strathcona, Falconbridge etc
WC Wilson Cycle (1-2 Pilbara / 5-6 Kaapvaal)
See the last chart …
24
Nucleogeochronometric Evolution of the Earth‘s Core
13.78 Ga → 3.48 Ga
ancient component
weak magnetic field
Fe, Ni … (?)
Re/Os ≈ 1
BARBERTON
slow cooling
during ≈ 10 Ga
pin ≈ pout
metastable
pin ≈/> pout
≈ 3.48 Ga core collapse
p+ + e- → n + ν, r-process
ignition of nuclear processes
nucleosynthesis etc.
suddenly increasing
magnetic field strength
element element
formation formation
element
formation
Fe, Ni
Re/Os ≈ 0.85
BARBERTON
element
formation
element
formation
element
formation
element
element
formation formation
3 Ga → today
modern Earth,
inner/outer core
convection, pulsation
magnetic field reversals
mantle/crust/plate tectonics
Fe, Ni
Re/Os ≈ .85
BARBERTON
solid
Re/Os ≈ 1
IVREA
T (K)
instable
pin >>>> pout
gravitational collapse
thermal expansion
magnetic pressure
pin ≈ pout
Fe, Ni, … (?).
liquid outer core
T (K)
T (K)
stable
“only“ magmatism,
earthquakes etc.
T = temperature (Kelvin); p = pressure
out = outward, due to thermal expansion etc.
in = inward, due to gravitation etc.
25
= down, = up

187Re -187Os Nuclear Geochronometry: A New Dating Method

Transcription

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