Short Course: U-Pb Geochronology
and Hf Isotope Geochemistry Applied to Detrital Minerals
Funding from NSF (Instrumentation & Facilities)
Help from Thermo Instruments, GV Instruments, Nu Plasma Instruments, New Wave Research & Photon Machines
George GehrelsDepartment of Geosciences
University of ArizonaTucson, AZ 85721
Jeff VervoortSchool of Earth & Environmental Sciences
Washington State UniversityPullman, WA 99164
Outline 1. Basics of U-Th-Pb decay system2. Measurement methods3. Mineral systems & applications4. Complexities for zircon5. Detrital geochronology (plots & analyses) 6. Future opportunities7. Basics of Lu-Hf decay system8. Measurement challenges9. Application to detrital minerals
U-Th-Pb chronometers:
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 1 2 3 4Age (Ga)
206*
/238
0
10
20
30
40
50
60
70
80
90
0 1 2 3 4Age (Ga)
207*
/235
0.0
0.1
0.1
0.2
0.2
0.3
0.3
0 1 2 3 4Age (Ga)
208*
/232
0.0
5.0
10.0
15.0
20.0
25.0
0 1 2 3 4Age (Ga)
206*
/207
*
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
206Pb/238U 207Pb/235U 206Pb/207Pb 208Pb/232Th
Geochronometers &Thermochronometers
Geochronometer:Mineral grows below blocking temp Age records crystallization
Thermochronometer:Mineral grows above blocking temp Age records cooling
Common: Less common:Garnet MonaziteRutile MagnetiteTourmaline IlmeniteZircon Chromite
OlivineDiamond
Heavy minerals in sand:
Why zircon is king….
ZrSiO4
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)] Low common Pb during crystallization (ppt) Grows @ 600-1100 ºC, retains Pb to >800°C Common in felsic & intermediate igneous rocks Chemically and mechanically resistant.
Zircon (ZrSiO4)• Birthstone for December• Protects travelers from harm• Increases one's appetite• Induces deep sleep (no nightmares)• Helps one be at peace• Loss of luster warns of danger• Provides wisdom, honor, and riches• Heals mental disturbances• Feeding the zircon gorilla = team building activity!
Brown: heals headachesColorless: clears the auraYellow: attracts loveRed: heals injuries, soothes painsGreen: draws wealthLight blue: stabilizes mind & emotionsPink: assists in astral travel at nightViolet: money magnet
Zircon should be placed in dry sea salt once a month for discharge and recharge…
Zircon: Provenance of modern sediment
Zircon: Provenance of ancient sediment…
Zircon: Provenance of artifacts…
Mano Temper sands
Zircon: Age of Sediment Accumulation
Detrital grains =
Maximum depositional age
Dating volcanic layers
Zircon: Growth history & Thermal history
“Multi-Dating”
Rutile (TiO2)... Low-mod U content (~10-100 ppm) Moderate common Pb (<10% of total Pb),
206/204 < 1,000 Blocking temp = ~380–450 °C Resists mechanical and chemical weathering
Strength with love
Ease transitions Calm, reason,
order Stabilizes
relationships Off-planet
connector
Low-mod U content (~10-100 ppm) Moderate common Pb (<10% of total Pb) Blocking temp = 400-500 °C Multi-dating
Apatite Ca5(PO4)3
Appetite suppression
Discern truth within
Enhances creativity
Unconditional acceptance
Vibrates to the number 9
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 CeLaThPO4
??
Monazite CeLaThPO4
From: Hietpas et al. (2010)
zircon
monazite
Occurs as inclusions in other minerals (e.g., garnet) many potential applications!
Monazite CeLaThPO4
From: http://webhost.bridgew.edu/mkrol/Research-2.html
Xenotime YPO4
From: McNaughton et al. (1999)
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??
