The Rb-Sr method

Based on the decay reaction: with a half-life

T1/2 = 48.8 Ga geochronometry equation written in terms of the

ratio 87Sr/86Sr because ratios are more accurately determined by mass spectrometry

−+→ βSrRb 8738

8737

)1(86

87

086

87

86

87

−+

= te

SrRb

SrSr

SrSr λ

measured measured always some initial Sr present in a rock

The Rb-Sr method is commonly used to date Rb-rich minerals such as muscovite, biotite and K-feldspar; these minerals usually do not incorporate much Sr at the time of their formation (Goldschmidt’s rules).

During the last decades also cogenetic whole

rock samples were analysed by this method.

The Rb-Sr method

87Rb=27.83% 85Rb=72.17% 88Sr=82.53% 87Sr=7.04% 86Sr=9.87% 84Sr=0.56%

ALL STABLE

What accounts for huge range in Rb/Sr ratios of rocks? 1. Rb subsitutes for K in K-bearing minerals while Sr substitutes for Ca in Ca-bearing minerals 2. Rb and Sr are fractionated by igneous processes: Rb tends to prefer melt (more incompatible than Sr)

Rb/Sr ratios for various rocks: Ultrabasic 0.2 Basaltic 0.06 Granites 0.25-10 Shale 0.46 Sandstone 3

High Rb/Sr rocks contain more 87Sr Low Rb/Sr rocks contain less 87Sr

The Rb-Sr method

Igneous Processes and 87Sr/86Sr ratios

87Sr/86Sr ratios of igneous rocks: MORB 0.7025 Ocean Islands >0.704 Continents 0.7119

87Rb goes into the melt

MORB

igneous rocks are heterogeneous, different mineral phases will have different Rb/Sr ratios, even though they have the same crystallization age and the same 87Sr/86Sr initial

MANTLE 87Sr/86Sr = 0.702

ROCK (87Sr/86Sr) i= 0.702

Rb/Sr=0.6

Rb/Sr=1.2 Rb/Sr= 0.8

t=Ti

me

of

crys

talli

zatio

n

how to get the initial 87Sr/86Sr ratio?

The Rb-Sr method

usually the isochron method is employed to determine the age and initial 87Sr/86Sr ratio of a suite of rock samples

87Rb/86Sr

y = 0.7114

0 5 10 15 20 25 30 350.70

0.75

0.80

0.85

0.90

0.95

87Sr

/86S

r

Rb-Sr isochron diagram for a series of cogenetic rock samples formed at the same time

( 87 Sr/ 86 Sr) 0 = 0.7114 slope = (e λ t - 1) = 0.04855

t = 3.34 Ga

r 2 = 0.9980 if x=(87Rb/86Sr)m

and y=(87Sr/86Sr)m

y=b+mx intercept b=(87Sr/86Sr)i

slope m=(eλt-1)

Isochron (part II): regression treatment with pocket calculator

Exercise 11

sample 87Rb/86Sr 87Sr/86Sr L14 446.6 2.76164 L12 600.4 3.4311 L16 820.6 4.4054 L15 999.1 5.1927

BABI - Basaltic Achondrite Best Initial = Bulk Earth, undifferentiated

Rb-Sr isochron from meteorites 87Sr/86Sr ratios of igneous rocks:

MORB 0.7025 Continents 0.7119 Ocean Islands >0.704 vs. Meteorites 0.699

T=4.5Ga

The Rb-Sr method

87Rb/86Sr

87Sr

/86Sr

t = 0

t = t2

t = t1

M1 R2M2R1

M1

R2

M2R1

Rb-Sr isochron diagram illustrating how the isochron evolves as a function of time. M1 and M2 are cogenetic minerals and R1 and R2 are cogenetic rocks, all with different initial Rb/Sr ratios

less thana few cm

several km

batholith

TimeTime

feldsparfeldspar

feldspar

whole rock

whole rock

whole rock

biotite

biotite

biotite

tm tm tp

to to

ri ri

rm

87Sr

/86S

r

87Sr

/86S

r

diffusion through crystal lattice and along grain boundaries during metamorphism

Response of Rb/Sr-system during metamorphism

Response of Rb/Sr-system during metamorphism

Response of Rb/Sr-system during metamorphism

Rb/Sr Rb/Sr

slope = e -1λti

slope = e -1λtm

fd fd

fd

wr wr

wr

bt bt

bt

rm rm

ri ri

87Sr

/86S

r

87Sr

/86S

r

TimeTime

feldsparfeldspar

feldspar

whole rock

whole rock

whole rock

biotite

biotite

biotite

tm tm tp

to to

ri ri

rm

87Sr

/86S

r

87Sr

/86S

r

Response of Rb/Sr-system during metamorphism

Sr isotopic evolution of the Earth

time

crustextracted

crust

mantledepletedmantle87

Sr/8

6Sr

Rb

Sr

Rb Sr 87Sr/86Sr ratio of the crust is higher than that of the mantle due to the preferential partitioning of Rb into the crust relative to Sr.

Continental crust: 32-78 ppm Rb, 260-333 ppm Sr Depleted Mantle: 0.6 ppm Rb, 19.9 ppm Sr

Age in Ga0 1 2 3 4

(87S

r/86S

r)

0.695

0.700

0.705

0.710

0.715

0.720

0.725

Tracking (87Sr/86Sr)i through time

BABI

Average continental crust

MORB

early continental differentiation

continuing continental growth

continuing upper mantle depletion

Ocean islands

(87Sr/86Sr)i ratios indicate how enriched or depleted its mantle source was i.e. (87Sr/86Sr)i = 0.7020 at 1 Ga means a depleted source (87Sr/86Sr)i value of 0.728 at 1 Ga?

The evolution of 87Sr/86Sr with time in the continental crust and mantle

(87Sr/86Sr)0 ratios can be used as a tracer to determine if a magma evolved from the mantle or if crust was involved For mantle-derived rocks: (87Sr/86Sr)0 ≈ 0.700-0.706 For crustal involvement: (87Sr/86Sr)0 ≈ 0.705-0.740

Sr isotopes as tracer of rock origin

U n d i f f e r e n t i a t e d e a r t h C r u s t a t 1 . 0 M a 8 7 8 6 S r / S r = 0 . 7 1 4 0

M a n t l e a t 1 . 0 M a 8 7 8 6 S r / S r = 0 . 7 0 3 4

c r u s t a n d m a n t l e d i f f e r e n t i a t e a t 2 . 7 M a

p r e s e n t d a y r o c k w i t h

( S r / S r ) = 0 . 7 1 4 0 (

8 7 8 6 0

8 7 8 6 S r / S r ) = 0 . 7 2 1 1 c r u s t

p r e s e n t d a y r o c k w i t h

( S r / S r ) = 0 . 7 0 3 4 (

8 7 8 6 0

8 7 8 6 S r / S r ) = 0 . 7 0 4 5 m a n t l e

r o c k f o r m s f r o m c r u s t a t 1 . 0 M a

r o c k f o r m s f r o m m a n t l e a t 1 . 0 M a

U n d i f f e r e n t i a t e d e a r t h

8 7 8 6 S r / S r = 0 . 6 9 9 a t 4 . 5 M a

8 7 8 6 S r / S r = 0 . 7 0 14 a t 2 . 7 M a

As crystals form, Rb enriched in melt, eventually can get ultra-enriched (87Sr/86Sr)

Crystals form in magma chamber, Rb stays in melt

magma melts host rock, which has high 87Sr/86Sr

magma chambers with different histories mix prior to eruption

Sr isotopes as tracer of rock origin

carbonate shells

hydrothermal fluids

sea water

river water87Sr/86Sr = 0.711 87Sr/86Sr = 0.703

87Sr/86Sr = 0.709

87Sr/86Sr = 0.709

Sr in the oceans through time

Sr isotope composition of the oceans is determined by the relative contributions of Sr from river waters and hydrothermal sources

Why is the river Sr isotope

value the highest? Why is the hydrothermal Sr isotope

value the lowest? Why is carbonate recrystallization Sr

isotope value equal to that of seawater?

Controls on seawater Sr isotopic composition

Seawater Sr Isotopic Curve (as measured on old and young carbonates)

mountain- building

hydrothermal activity

Himalayan uplift

Sr flux rate

Sr isotope ratio

Sr in the oceans through time

Elemental- and isotopic mixtures

Binary mixtures

(X)M = (X)AfA + (X)B(1-fA)

fB = 1 - fA

fA = A / (A + B)

−×

+

×

=

M

BA

BM

AA

AM SrSrf

SrSr

SrSrf

SrSr

SrSr )1(86

87

86

87

86

87

Component A Component B

Sr 100 ppm 500 ppm 87Sr/86Sr 0.800 0.700

G. Faure, 1977, 2005

Binary mixtures

−×

+

×

=

M

BA

BM

AA

AM SrSrf

SrSr

SrSrf

SrSr

SrSr )1(86

87

86

87

86

87

Briquet & Lancelot 1979

(X)M = (X)AfA + (X)B(1-fA)

Water mixing in estuaries

Chester Marine Geochemistry

DePaolo & Wasserburg (1979)

Models for crustal contamination

AFC - process

DePaolo (1981)

K- metasomatism?

Sr isotopic fingerprinting

Saldenburg granite

Diorite

1 cm

drill holes

Siebel et al. (2005) Chem Geol 222

Saldenburg granite

Diorite

Sr isotope fingerprinting

1 cm

Sr isotope fingerprinting

Isochron or mixing line?

“Redwitzite”

Si (pfu)8

0.4

0.5

0.6

0.7

0.8

7.5 7.0 6.5

Mg/

Mg+

Fe Magnesio-Hornblende

Ferro-Actinolite

Actinolite

Actin

oliti

cH

ornb

lend

e

Ferro-ActinoliticHornblende

Ferro-Hornblende

Sample R2B

“Redwitzites”: amphibole composition

Siebel et al. (1998) Geology 26

pyx

amp

pyx

bi

“Redwitzites”: 40Ar-39Ar geochronology

37Ar/39Ar ~ Ca/K 40Ar/39Ar ~ age

Siebel et al. (1998) Geology 26

White: Geochemistry Lec. 31

Source contamination vs. crustal contamination

Magaritz et al. (1978) EPSL 40: 220-230 James (1981) J. Geol Soc Lond 141:823-830