Short Course on “U-Th-Pb Geochronology by LA-ICPMS: Applications to Detrital Geochronology and Petrochronology” George Gehrels

Andrew Kylander-Clark & Bradley Hacker

Department of Geosciences

Department of Earth Science

University of Arizona

University of California, Santa Barbara

Tucson, AZ 85721

Santa Barbara, CA 93106-9630

Funding from NSF (Instrumentation & Facilities) Help from Thermo Instruments, GV Instruments, Nu Plasma Instruments, New Wave Research & Photon Machines

Short Course on “U-Th-Pb Geochronology by LA-ICPMS: Applications to Detrital Geochronology and Petrochronology” 1.

Systems of interest & Instrumentation Basics of U-Th-Pb decay system Measurement methods Mineral systems & applications Complexities for zircon

2. 3. 4.

DZ: U-Pb methodology & applications DZ: Hf methodology & applications Petrochronology

Outline

Decay of 238U to 206Pb

(T1/2 = 4.468 Ga) 234 U 230 Th

Atomic Number

90

234 Th

226 Ra 222 Rn

218 Rn 218 At

210 Po

214 Po

210 Bi 218 Po

206 Pb 206 Tl

80

234 Pa

238 U

Rate controlling decay

218 Po

214 Bi

214 Pb

210 Pb

206 Hg

130

140

Possible complications from 230Thorium......

Neutron Number

Decay of 235U to 207Pb

(T1/2 = 0.704 Ga)

227 Th

90 223 Ra

Atomic Number

231 Pa 227 Ac

223 Fr

235 U 231 Th

219 Rn

219 At

215 At

211 Po

215 Po 211 Bi

207 Pb

Rate controlling decay

215 Bi

211 Pb

207 Tl

80

130

Neutron Number

Possible complications from 231Protactinium

140

Decay of 232Th to 208Pb

(T1/2 = 14.01 Ga) 228 Th

90

Atomic Number

224 Ra

228 Ac

232 Th 228 Ra

220 Rn 216 Po

212 Po 212 Bi 208 Pb

208 Tl

Rate controlling decay

212 Pb

80

Neutron Number

130

140

U-Th-Pb decay dN/dt = proportional to N (basic process of radioactive decay) -dN/dt = λN  (integrate both sides)  - ln N = λt + c because N = Ni when t = 0, c = - ln Ni - ln N = λt - ln Ni  N = Ni e-λt D* = Ni - N  Ni = D* + N D* = N (eλt - 1)  D*/N = eλt – 1 (=decay equation) t = ln (D*/ N + 1) / λ

(=age equation)

t = time λ = decay constant N = # parent atoms present at t Ni = # parent atoms present at to D* = # daughter atoms

Example of exponential decay….

D dN/dt = proportional to N crashes / hour proportional to # riders

N

D*/N = eλt – 1 #crashes / #riders = eλt – 1 (λ = probability of a crash per unit time) t = ln (D*/ N + 1) / λ duration of race = ln (#crashes / #riders +1) / λ

D* (daughter) & N (parent) as function of time 238U

 206Pb*

T1/2 = 4.468 Ga

235U

 207Pb*

T1/2 = 0.704 Ga

232Th

 208Pb*

T1/2 = 14.01 Ga

D*/N as function of age: 238U

 206Pb*

235U

T1/2 = 4.468 Ga 206Pb*/238U

 207Pb*

232Th

T1/2 = 0.704 Ga

= eλ1t -1

207Pb*/235U

1.2

 208Pb*

T1/2 = 14.01 Ga

= eλ2t – 1

208Pb*/232Th

90

= eλ3t – 1

0.30

80

1.0

0.25

70 60

0.6

0.4

0.20

50

208*/232

207*/235

206*/238

0.8

40 30

0.15

0.10

20

0.2

0.05

10 0.0

0 0

1

2 3 Age (Ga)

4

0.00

0

1

2 3 Age (Ga)

* = radiogenic

4

0

1

2 3 Age (Ga)

4

Age as a function of D*/N 238U

 206Pb*

235U

T1/2 = 4.468 Ga 206Pb*/238U

 207Pb*

232Th

T1/2 = 0.704 Ga

= eλ1t -1

207Pb*/235U

T206/238 = ln (206*/238 + 1) / 238λ

T1/2 = 14.01 Ga

= eλ2t – 1

208Pb*/232Th

T207/235 = ln (207*/235 + 1) / 235λ

1.2

 208Pb* = eλ3t – 1

T208/232 = ln (208*/232 + 1) / 232λ

90

0.30

80

1.0

0.25

70 60

0.6

0.4

0.20

50

208*/232

207*/235

206*/238

0.8

40 30

0.15

0.10

20

0.2

0.05

10 0.0

0 0

1

2 3 Age (Ga)

4

0.00

0

1

2 3 Age (Ga)

* = radiogenic

4

0

1

2 3 Age (Ga)

4

206Pb*

/ 207Pb* age 25.0

20.0

206Pb*

= 238U (eλ1t -1)

15.0

207Pb*

= 235U (eλ2t - 1)

206Pb*

206*/207*

Can also determine age from 206Pb/207Pb (238/235 = 137.88 in nearly all rocks)

10.0

/ 207Pb* = [238U (eλ1t – 1)] / [235U (eλ2t – 1)] = 137.88 [(eλ1t – 1) / (eλ2t – 1)]

 don’t need to know 238U or 235U !

5.0

0.0 0

(transcendental function, so iterate or use table)

1

2 3 Age (Ga)

4

Four U-Th-Pb chronometers! 206Pb/238U

207Pb/235U

206Pb/207Pb 0.30

25.0

90

1.2

208Pb/232Th

80

1.0

0.20

50 40

15.0

208*/232

207*/235

0.6

206*/207*

60

0.8 206*/238

0.25

20.0

70

10.0

30

0.4

0.10

20

0.2

5.0

0.05

10 0

0.0 0

1

2 3 Age (Ga)

4

0.15

0.0 0

1

2 3 Age (Ga)

4

0

1

2 3 Age (Ga)

4

0.00 0

1

2 3 Age (Ga)

4

What about Pb in crystal when it formed? Measured Pb = radiogenic Pb + initial Pb 204M = 204I 206M = 206I + 206R

Radiogenic Pb

207M = 207I + 207R 208M = 208I + 208R (206M/204) = (206R/204) + (206I/204)  (206R/204) = (206M/204) - (206I/204)  206* = 206R = 204 [(206M/204) - (206I/204)]  207* = 207R = 204 [(207M/204) - (207I/204)]  208* = 208R = 204 [(208M/204) - (208I/204)] How determine 206/204, 207/204, 208/204 at time of crystallization?

Initial Pb 204

206 207 208

Inital Pb composition  Ore minerals (e.g. galena) have such high Pb content (and low U content) that they do not change over time  Measure Pb in ore minerals of various ages to determine evolution of Pb in the crust = Stacey-Kramers (1975) model for evolution of crustal Pb 20

16

40

19 18

38

15

16 15 14

208/204

207/204

206/204

17

36

34

14

13 12

32

11 10 0

1

2 Age (Ga)

3

4

13

30

0

1

2 Age (Ga)

3

4

0

1

2 Age (Ga)

3

4

Initial Pb composition But there is variation in modern common Pb composition:  Propagate compositional uncertainty thru age calculation

U-Pb concordia diagram (Wetherill, 1956) 3200 3000

0.6 2800

ordia c n o c

2600 2400 2200

0.4

Pb*/238U

2000 1800

206

1600 1400 1200 1000

0.2

Magma crystallizing at Time = 0 0.0 0

4

8

12 207

16

Pb*/ U 235

20

24

U-Pb concordia diagram 3200 3000

0.6 2800

ordia c n o c

2600 2400 2200

0.4

Pb*/238U

2000 1800

206

1600 1400 1200 1000

0.2

1000 Ma later…. 0.0 0

4

8

12 207

16

Pb*/ U 235

20

24

U-Pb concordia diagram 3200 3000

0.6 2800

ordia c n o c

2600 2400 2200

0.4

Pb*/238U

2000 1800

2000 Ma later….

206

1600 1400 1200 1000

0.2

0.0 0

4

8

12 207

16

Pb*/ U 235

20

24

U-Pb concordia diagram 3200

Analysis on concordia = “concordant” 206

0.6

Pb*/238U => 2900 Ma

3000 2800

rdia o c n co

2600 2400

2900 Ma later….

2200

0.4

Pb*/238U

2000 1800

206

1600 1400 1200 1000

0.2

207

0.0 0

4

8

12 207

16

Pb*/ U 235

Pb*/235U => 2900 Ma

20

24

U-Pb concordia diagram  3 ages! 3200 206

0.6

Pb*/238U => 2900 Ma

3000 2800

ordia c n o c

2600 2400 2200

0.4

Pb*/238U

2000 1800

206

1600 1400 1200 1000

0.2

slope = 206Pb*/238U / 207Pb*/235U = 206Pb*/207Pb* / 137.88 => 2900 Ma 0.0

0

4

8

12 207

207

16

Pb*/ U 235

Pb*/235U => 2900 Ma

20

24

U-Pb concordia diagram = 2 ages!

206Pb*/238U

vs 206Pb*/207Pb* 3200

206

0.6

Pb*/238U => 2900 Ma

3000 2800

ordia c n o c

2600 2400 2200

0.4

Pb*/238U

2000 1800

206

1600 1400 1200 1000

0.2

slope = 206Pb*/238U / 207Pb*/235U = 206Pb*/207Pb* / 137.88 => 2900 Ma 0.0

0

4

8

12 207

207

16

Pb*/235U => 2900 Ma

20

Pb*/ U 235

Don’t measure 207Pb*/235U (not much 235, 137.88 well known) 207Pb*/235U = 206Pb*/238U / 206Pb*/207Pb* / 137.88

24

Tera-Wasserburg concordia diagram = 2 ages! 0.24

206Pb*/207Pb*

 2900 Ma

2600

0.16

Pb/

206

Pb

0.20

207

2200

0.12 1800 1400

0.08

1000 206Pb*/238U

600

 2900 Ma

0.04 0

4

8

238

206

U/

12

Pb

16

20

Uncertainties of

206Pb*/238U

& 206Pb*/207Pb* ages 3200

206

0.6

Pb*/238U => 2900 Ma

3000 2800

ordia c n o c

2600 2400 2200

0.4

Pb*/238U

2000 1800

206

1600 1400 1200 1000

0.2

slope = 206Pb*/238U / 207Pb*/235U = 206Pb*/207Pb* / 137.88 => 2900 Ma 0.0

0

4

8

12 207

207

16

Pb*/ U 235

Pb*/235U => 2900 Ma

20

24

Errors on U-Pb concordia diagram 0.104

Isoplot expresses errors as a pdf

630

0.102

U

620 0.100

206

Pb/

238

206/238 error

20 6/ 20 7e rro r

610

0.098

600

0.096 0.79

0.81

0.83 207

235

Pb/ U

207/235 error

0.85

0.87

Errors on U-Pb concordia diagram 3200

Relative magnitude of errors (in m.y.)

0.6

3000 2800

rdia o c n co

2600

206/238 > 207/235 > 206/207

2400 2200

0.4

Pb*/238U

2000 1800

206

1600 1400

206/238 = 207/235 = 206/207 (about 1400 Ma)

1200 1000

0.2

= most precise age 206/238 < 207/235 < 206/207 0.0

0

4

8

12 207

Pb*/235U

16

20

24

Which age to use?? Pb/ 238U age

60

Pb/ 207Pb age

206

~30%

206

(5200 analyses)

50

~1.3% Error (Ma)

40 30 20

~0.3% 10 0 0

200

400

600

800

1000

1200

1400

1600

1800

2000

2200

2400

2600

2800

Age (Ma) 206Pb*/238U

for ages < ~1400 Ma

206Pb*/207Pb*

Precision cutoff = 1400 Ma

for ages > ~1400 Ma

3000

Common Chronometers Mineral

Zircon

Formula

U content (ppm)

Common Pb (ppm)

Zr [SiO4]

1 to >10,000

CaTi[SiO3](O,OH,F)

4 to 500

Monazite

(Ce,La,Th)PO4

282 to >50,000

<2

mp,sg, hv,gp

Xenotime

YPO4

5,000 to 29,000

<5

gp,sg

Thorite

Th[SiO4]

> 50,000

<2

gp,sg

Allanite

(Ca,Ce)2(Fe+2,Fe+3) Al2O•OH[Si2O7] [SiO4]

130 to 600

5 to 30

Perovskite

(Ca,Na,Fe+2,Ce) (Ti,Nb)O3

21 to 348

< 2 to 90

Baddeleyit e Rutile

ZrO2

58 to 3410

<2

TiO2

< 1 to 390

< 2 to 95

gp,gn, hv

Ca5(PO4)3(OH,F,Cl)

8 to 114

< 5 to 30

most

Titanite

Apatite

<2

Rock Type

most

5 to 40 k,c,a,m,ig,mp, gp,hv, gn,sk

ig,gp,sk

k,c k,c,um, m,a

Summarized from Heaman and Parrish (1991): k = kimberlite; c = carbonatite; a = alkaline; m = mafic; ig = I-type granitoids; sg = S-type granitoids; mp = metapelitic rocks; hv = hydrothermal veins; gp = granitic pegmatites and leucogranites; sk = skarns.

Short Course on “U-Th-Pb Geochronology by LA-ICPMS: Applications to Detrital Geochronology and Petrochronology” 1.

Systems of interest & Instrumentation Basics of U-Th-Pb decay system Measurement methods Mineral systems & applications Complexities for zircon

2. 3. 4.

DZ: U-Pb methodology & applications DZ: Hf methodology & applications Petrochronology

Outline

1. 2. 3. 4. 5.

Detrital geochronology (plots & analyses) Future opportunities Basics of Lu-Hf decay system Measurement challenges LASS petrochronology

Determining ages: Measurement Methods 4000

3400 2800 2200 1600

1000

400

1. Measure ratios 206Pb/207Pb, 206Pb/238U, 208Pb/232Th, 206Pb/204Pb : most common • TIMS • SIMS • LA-ICPMS 2. Assume concordance: Measure elements (U+Th)/Pb • EPMA

Electron-Induced X-ray Analysis

EPMA

Detector

Incident Electron Beam

X-rays emitted





 Th λ 232t  *Pb   e  1  208   232   U  238t e 0 . 9928 1   206   238  









 U  235t  1  207  235 0.0072 e 

•Incident particle knocks electrons out of the occupied states around the atom leaving empty states (vacancies). Energy

electron

•Electron in occupied state makes transition to unfilled vacancy. X-ray is emitted to conserve energy.

E

 Pb ppm  Age  f    (U  Th) ppm  measured

K X-ray

•Energy or wavelength of X-ray identifies the atom # X-rays detected (cps/na)

Titanium K X-ray = 4.51 keV Zinc

K X-ray = 8.63 keV Energy or wavelength (λ)

Major assumption: No independent U, Th, Pb disturbance, no common Pb!

EPMA



Cameca SX 100 Electron Probe



 Th λ 232t  *Pb   e  1  208    232 U   238t  e 0 . 9928 1  238  206 









U   235t 0 . 0072 e 1   235  207

 Pb ppm  Age  f    (U  Th) ppm  measured Major assumption: No independent U, Th, Pb disturbance, no common Pb!

Measure Isotopic Ratios: Mass Spectrometry Separation & measurement of isotopes based on mass:charge ratio

ion source

mass analyzer

detector

thermal (TI)

magnetic sector

Faraday

or

single focus double focus

or

secondary (SI) or

plasma (ICP)

or

quadrupole

electron multiplier

magnetic sector instrument (TI, SI, ICP) mass analyser (momentum analyzer)

electromagnet

+ 6–10 kV

1 mv 2 2

ion source

high mass

r

low mass

ion source (TI, SI or ICP)

mv 2 F  Q( E  v  B)  r r  mv

detector ground

electrostatic analyzer (ESA): “double focusing” (SI, ICP) 1 mv 2 2

energy filter

ground

mass an. or detector

r V+

V-

1 mv 2  E scatter 2

ion source or

mass an. + 6–10 kV

V F  Q( E  v  B)  r V r QE 1 2 r  mv 2

result: reduced sensitivity, but better accuracy and precision

quadrupole instrument (ICP)

hig h-p (os as s c

low-pass mass filter (defocuses)

illa m tes as wit s f h R ilt F) e r

ground

detector V dc V ac +

+ 6000V

ion source

-V a c

- V dc

mass analyzer

±(U+Vcos(ωt)) good: ↑mass range, rapid jumps;

bad: single collector, no energy filter

ion sources: thermal ionization (TI) mass analyzer + 8000V

detector ground

sample

good: v. stable, predictable ionization; bad: single collector, no energy filter

Isotope Dilution – Thermal Ionization Mass Spectrometry (ID-TIMS)   

Clean & weigh crystals (few µg) Dissolve crystals (Teflon with HF-HNO3 in ultra-clean lab) Add 205Pb+233U spike (“Isotope Dilution”)   

 

Separate Pb & U (ion exchange column chemistry) Analyze isotope ratios with TIMS:    

  

Teflon in ultra-clean lab Well calibrated spike Weigh amount of spike added

206Pb/205Pb

(to determine 206Pb) 206Pb/204Pb (for common Pb correction) 206Pb/207Pb (for age) 238U/233U (to determine 238U)

Common Pb correction Calculate 206Pb*/238U age Calculate 206Pb*/207Pb* age

SPIKE

ID-TIMS

1 analysis per hour $100–$300 per analysis analyze entire dissolved fragment very accurate Pb/U ratios (by ID) ± 0.1–0.3% accuracy

ion sources: secondary ionization (SI)

+ 6000V

mass analyzer

detector

Secondary ions

Primary Beam of O ions (~35 micron diameter)

good: high efficiency;

bad: large energy dispersion & matrix effects

Secondary ions SHRIMP

Primary Beam of O- ions (~35 micron diameter)

CAMECA 1270/1280

Secondary Ion Mass Spectrometry (SIMS, Ion Probe, “SHRIMP”)  

No way to add spike because grain not dissolved Comparison with Standards:     

   

Homogeneous in U concentration All fragments have same age Age well-determined by ID-TIMS Abundant Same composition & crystal structure

Mount standards & unknowns together Sample–Standard bracketing (~5:1) Determine factor to set measured value = known value Apply same factor to unknowns for:   

206Pb/238U

(for age) 206Pb/207Pb (for age) 206Pb/204Pb (for common Pb correction) STANDARD

UNKNOWN

Secondary Ion Mass Spectrometry (SIMS, Ion Probe, or “SHRIMP”)    

But!!! 206Pb/238U of standard is variable during session (charging on surface due to O– primary ion beam) UO/U varies with Pb/U, so also measure UO/U Generate calibration line for standard Apply slope to unknowns & project values

4 Analyses per hour $25 per analysis 10-35 micron spot ~1 micron depth ± 1–2% accuracy

ion sources: inductively coupled plasma (ICP)

+ 6000V

mass analyser

good: v. high efficiency;

detector

bad: large energy dispersion

laser

laser ablation (LA) systems

Ar in (mix gas) He in (carrier gas)

ion source Sample Cell

mass analyzer

detector

laser

LA systems: UV Lasers

Ar in (mix gas)

He in (carrier gas)

ion source

mass analyzer

detector

Sample Cell

193 nm excimer (ArF) 5 ns

200 nm Ti:sapphire <110 fs

213 nm Nd:YAG 5 ns

266 nm Nd:YAG <10 ns

Wavelength & Pulse duration absorption

melting

condensation

particle size distribution

ionization efficiency

laser-induced elemental fractionation 238U

volume/time

plasma-induced elemental fractionation

mass bias

206Pb 206Pb 238U

Ar in (mix gas)

He in (carrier gas)

ion source

mass analyser

detector

LA systems: UV Lasers

less heating (more effective/reproducible ablation)

193 nm excimer (ArF) 5 ns

200 nm Ti:sapphire <110 fs

213 nm Nd:YAG 5 ns

266 nm Nd:YAG <10 ns

lower pulse duration = more efficient & reproducible ablation & ionization

LA systems: cell geometry

GOOD

BETTER

Nu Instruments HR MC-ICPMS (multicollector)

Ex-H

H2

238U

232Th

238U

232Th

H1

Ax

L1

Faraday detectors L2 L3

L4

L5

discrete dynode ion counters IC1 IC2 IC3

L6

L7

L8

IC0

208Pb

207Pb

206Pb

204Pb 208Pb

180Hf

179Hf

178Hf

177Hf

176Hf 176Lu 176Yb

174Hf 175Lu 173Yb

172Yb

171Yb

207Pb

202Hg

200Hg

206Pb

204Pb

Nu Instruments AttoM (single collector)

Laser-Ablation Inductively-Coupled-Plasma Mass Spectrometer (LA-ICPMS) Same procedure as SIMS:  Sample–Standard bracketing (5:1)  Comparison with Standards:     

  

Mount standards & unknowns together Determine factor to set measured value = known value Apply same factor to unknowns for:   



Homogeneous in composition All fragments have same age Age well-determined by ID-TIMS Abundant Same composition & crystal structure

206Pb/238U

(for age) 206Pb/207Pb (for age) 206Pb/204Pb (for common Pb correction)

No charging, so no UO/U correction

STANDARD STANDARD

UNKNOWN UNKNOWN

Comparison of U-Th-Pb techniques EPMA:

4 analyses per hour ~$25 per analysis >2 % accuracy  no loss of sample

ID-TIMS:

1 analysis per hour $100–$300 per analysis ± 0.1–0.3% accuracy from crystal (or fragments)  Best precision and accuracy

SIMS:

4 Analyses per hour $25 per analysis 10–35 micron beam diameter ~1 micron pit depth ± 1–2% accuracy  Best spatial resolution

LA-ICPMS:

40 analyses per hour $4–$8 per analysis 5–50 micron beam diameter ~10 micron pit depth 1–2% accuracy  Highest efficiency

Short Course on “U-Th-Pb Geochronology by LA-ICPMS: Applications to Detrital Geochronology and Petrochronology” 1.

Systems of interest & Instrumentation Basics of U-Th-Pb decay system Measurement methods Mineral systems & applications Complexities for zircon

2. 3. 4.

DZ: U-Pb methodology & applications DZ: Hf methodology & applications Petrochronology

U-Th-Pb chronometers: 206Pb/238U

207Pb/235U

206Pb/207Pb 0.3

25.0

90

1.2

208Pb/232Th

80

1.0

60 50 40

15.0

208*/232

0.6

0.2 206*/207*

207*/235

0.8 206*/238

0.3

20.0

70

10.0

30

0.4

0.1

20

0.2

5.0

0.1

10 0

0.0 0

1

2 3 Age (Ga)

4

0.2

0.0 0

1

2 3 Age (Ga)

4

0.0 0

1

2 3 Age (Ga)

4

0

1

2 3 Age (Ga)

Any mineral with U and/or Th is a potential chronometer! 

Best if little Pb included during crystallization



Need to know conditions of retention of Pb to understand age

4

Geochronometers & Thermochronometers Thermochronometer: Mineral grows above closure temp

 Age records cooling Geochronometer: Mineral grows below closure temp

 Age records crystallization

Why zircon is king….   

  



High U concentration (100–1000 ppm) Moderate Th concentration (10–100 ppm) Zr, U, Th tightly held [similar in size (0.8–1.0 Å) and charge (+4)] Excludes Pb during crystallization (ppt) Grows @ 600–1100 ºC, retains Pb to >800°C Common in felsic–intermediate igneous/metamorphic rocks Chemically and mechanically resistant.

ZrSiO4 Tulloch et al (2009)

Zircon ZrSiO4 • Birthstone for December • Protects travelers from harm

Brown: heals headaches Colorless: clears the aura

• Increases one's appetite

Yellow: attracts love

• Induces deep sleep (no nightmares)

Red: heals injuries, soothes pains

• Helps one be at peace

Green: draws wealth

• Loss of luster warns of danger • Provides wisdom, honor, and riches

Light blue: stabilizes mind & emotions

• Heals mental disturbances

Pink: assists in astral travel at night

• Feeding the zircon gorilla = team building activity!

Violet: money magnet

Zircon should be placed in dry sea salt once a month for discharge and recharge…

Zircon: Growth history & Thermal history

“Multi‐Dating”

Rutile TiO2    



Low-mod U content (~5–100 ppm) low Pb incorporation during crystallization (ppt) Closure temp = ~380–450 °C common in pelites and high-grade metamorphic rocks Resists mechanical and chemical weathering

    

Strength with love Ease transitions Calm, reason, order Stabilizes relationships Off-planet connector

Apatite Ca5(PO4)3    

Low-mod U content (~10–100 ppm) Moderate common Pb (<10% of total Pb) Blocking temp = 400–500 °C Multi-dating

    

Appetite suppression Discern truth within Enhances creativity Unconditional acceptance Vibrates to the number 9

Monazite (Ce,La,Th)PO4       

Very high Th content (1000–10,000 ppm) Moderate U content (100–1000 ppm) Very low common Pb (ppt) Blocking temperature = ~680–750 °C Can tie age to metamorphic reactions mod- to high-grade metamorphism Multi-dating?

? ?

Monazite (Ce,La,Th)PO4 Occurs as inclusions in other minerals (e.g., garnet)  many potential applications!

From: http://webhost.bridgew.edu/mkrol/Research-2.html

Xenotime YPO4     

Moderate U & Th contents (~100 ppm) Moderate to low common Pb (ppt) Blocking temperature = 600–700 °C (?) Peraluminous igneous rocks Diagenetic overgrowths on zircon  depositional ages??

From: McNaughton et al. (1999)

Titanite (Sphene) CaTiSiO5    

Low–moderate U & Th (~5–100 ppm) Moderate common Pb (ppb) Blocking temperature = 600–700°C felsic–intermediate igneous/metamorphic rocks

Common Chronometers Mineral

Zircon

Formula

U content (ppm)

Common Pb (ppm)

Zr [SiO4]

1 to >10,000

CaTi[SiO3](O,OH,F)

4 to 500

Monazite

(Ce,La,Th)PO4

282 to >50,000

<2

mp,sg, hv,gp

Xenotime

YPO4

5,000 to 29,000

<5

gp,sg

Thorite

Th[SiO4]

> 50,000

<2

gp,sg

Allanite

(Ca,Ce)2(Fe+2,Fe+3) Al2O•OH[Si2O7] [SiO4]

130 to 600

5 to 30

(Ca,Na,Fe+2,Ce) (Ti,Nb)O3

21 to 348

< 2 to 90

Baddeleyite

ZrO2

58 to 3410

<2

Rutile

TiO2

< 1 to 390

< 2 to 95

gp,gn, hv

Ca5(PO4)3(OH,F,Cl)

8 to 114

< 5 to 30

most

Titanite

Perovskite

Apatite

<2

Rock Type

most

5 to 40 k,c,a,m,ig,mp, gp,hv, gn,sk

ig,gp,sk

k,c k,c,um, m,a

from Heaman & Parrish (1991): k = kimberlite; c = carbonatite; a = alkaline; m = mafic; ig = I-type granitoids; sg = S-type granitoids; mp = metapelitic rocks; hv = hydrothermal veins; gp = granitic pegmatites & leucogranites; sk = skarns.

Short Course on “U-Th-Pb Geochronology by LA-ICPMS: Applications to Detrital Geochronology and Petrochronology” 1.

Systems of interest & Instrumentation Basics of U-Th-Pb decay system Measurement methods Mineral systems & applications Complexities for zircon

2. 3. 4.

DZ: U-Pb methodology & applications DZ: Hf methodology & applications Petrochronology

U-Pb concordia diagram 3200 206

0.6

Pb*/238U => 2900 Ma

3000 2800

ordia conc

2600 2400 2200

0.4

Pb*/238U

2000 1800

206

1600 1400 1200 1000

0.2

slope = 206Pb*/238U / 207Pb*/235U = 206Pb*/207Pb* / 137.88 => 2900 Ma 0.0

0

4

8

12 207

207

16

Pb*/ U 235

Pb*/235U => 2900 Ma

20

24

Complexity from Pb loss 206Pb/238U

0.40 0.35 0.30

Hydrothermal alteration of 1400 Ma granite 0%

1600 radiogenic ingrowth

0.25

1200

50%

0.20

800

0.15 0.10

b P 4 0 2

b P 6 0 2

b P 7 0 2

b P 8 0 2

h T 2 3 2

U 5 3 2

75%

400

b P 6 0 2

b P 4 0 2

0.05

b P 7 0 2

b P 8 0 2

h T 2 3 2

U 5 3 2

U 8 3 2

207Pb/235U

0.00

90%

0

1

b P 4 0 2

b P 4 0 2

2

b P 6 0 2

b P 7 0 2

b P 8 0 2

h T 2 3 2

U 5 3 2

U 8 3 2

b P 6 0 2

b

3P702

b P 8 0 2

h T 2 3 2

U 5 3 2

U

4832

5

6

7

U 8 3 2

Complexity from Pb loss 206Pb/238U

0.40 0.35

Hydrothermal alteration of 1400 Ma granite

0.30

1600

0.25 1200

0%

0.20 

0.15 0.10

800 50%



400

0.05





75%

Pb leached out of zircons = “Pb loss” (Pb not charge bound & larger than U & Th) Smaller grains lose more Pb (hi surface/volume) Higher U grains lose more Pb (radiation damage) Happens mainly in hot fluid-rich environments 207Pb/235U

90%

0.00 0

1

2

3

4

5

6

7

Complexity from Pb loss 600 Myr later…

206Pb/238U

0.40 0.35 0.30

d r o c s i D

1600

0.25 1200

0.20 0.15 0.10

Concordant analysis

Discordant analyses

800 400

0.05 207Pb/235U

0.00 0

1

2

3

4

5

6

7

Complexity from inheritance 206Pb/238U

0.40 0.35

metamorphic or igneous recrystallization of 1400 Ma granite

0.30

1600

0.25 1200

Country rock with 1400 Ma zircons

0.20 0.15 0.10

800 400

0.05

new zircon growing over inherited zircon

207Pb/235U

0.00 0

1

2

3

4

5

6

7

Complexity from inheritance 206Pb/238U

0.40 0.35

600 Myr later… Country rock with 2000 Ma zircons

0.30

1600

d r o c s i D

0.25 0.20

Concordant analysis

0.15

800

0.10

1200

Concordant analysis

Discordant analyses

400

0.05

400 Ma zircon grown over inherited zircon

207Pb/235U

0.00 0

1

2

3

4

5

6

7

Unravel complexity through wise spot choices 206Pb/238U

0.40 0.35 0.30

1600 Concordant analysis

0.25 1200

0.20 0.15 0.10

800 400

Concordant analyses

0.05

207Pb/235U

0.00 0

1

2

3

4

5

6

7

inheritance and cathodoluminescence 206Pb/238U

0.40 0.35

Discordant analyses

0.30

1600 Concordant analysis

0.25 1200

0.20 0.15 0.10

800 400

Concordant analyses

0.05

207Pb/235U

0.00 0

1

2

3

4

5

Dark CL = High U; Light CL = Low U

6

7

Complexity from Metamorphic Overgrowths Hi U/Th = metamorphic Lo U/Th = igneous

206Pb/238U

0.40 0.35 0.30

1600

0.25

If CL pattern is same from growth of metamorphic zircon rims

1200

0.20 0.15 0.10

U/Th may distinguish igneous from metamorphic zircon…

800 400

0.05 207Pb/235U

0.00 0

1

2

3

4

5

6

7

Complexity from initial Pb 1.2

Tera–Wasserburg diagram 5200

1.0

207Pb/206Pb

0.8

inc rea s

0.6 4000

0.4

ing

Pb c

om mo n /U

2800

0.2

1600

400

238U/238Pb

0.0 0

5

10

15

20

Pbcommon is most likely an issue in titanite, allanite, apatite, rutile

More complexities: initial Pb + inheritance 1.2

Tera–Wasserburg 5200

1.0

207Pb/206Pb

0.8 0.6 4000

0.4 2800

0.2 0.0

400

1600

0 original discord 5

10

15

238U/238Pb

20

Can correct for Pbcommon using 204Pb and assuming 206Pb/ 204Pbi

initial Pb + inheritance Th/U

Pbi

1.0

5200

Th/U

2.6

2.6

0

0

207Pb/206Pb

0.8

0.6

4400

0.4

3600 0.2

2800 2000 1200

0.0 0

2

4

400 Ma

900 Ma 6

8

238U/238Pb

Can correct for Pbcommon using 204Pb and assuming 206Pb/ 204Pbi

inheritance + Pb loss 206Pb/238U

0.40 0.35

600 Ma zircon grown over 2000 Ma zircon with recent Pb loss

0.30

1600

0.25 0.20

original discord

0.15

800

0.10

1200 No discordia regression! Intercepts meaningless….

400

0.05 207Pb/235U

0.00 0

1

2

3

4

5

6

7

Summary of discordance…. 206Pb/238U

1.0 0.9 0.8

Pb loss

0.7 0.6 0.5 Inheritance & Pb loss

0.4 0.3

Inheritance

0.2

Initial Pb correction (analytical)

0.1

207Pb/235U

0.0 0

10

20

30

40

50

60

70

80

Discordance on Tera–Wasserburg diagram inheritance & Pb loss 0.10

12.5

13.5

14.5

15.5 0.061

inheritance

1700

0.059

initial Pb

1500

0.057

Pb loss

480 0 440 400 4 00 0 0

0.5

207Pb/206Pb

0.08

1300

0.055

0.053

1100 900

Th/U

700

0.06

500

0

initial Pb 4

8

228U/238Pb

12

16