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