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Short Course: U-Pb Geochronology and Hf Isotope Geochemistry Applied to Detrital Minerals

Short Course: U-Pb Geochronology and Hf Isotope Geochemistry Applied to Detrital Minerals. George Gehrels Department of Geosciences University of Arizona Tucson, AZ 85721. Jeff Vervoort School of Earth & Environmental Sciences Washington State University Pullman, WA 99164.

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Short Course: U-Pb Geochronology and Hf Isotope Geochemistry Applied to Detrital Minerals

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  1. Short Course: U-Pb Geochronology and Hf Isotope Geochemistry Applied to Detrital Minerals George Gehrels Department of Geosciences University of Arizona Tucson, AZ 85721 Jeff Vervoort School of Earth & Environmental Sciences Washington State University Pullman, WA 99164 Funding from NSF (Instrumentation & Facilities) Help from Thermo Instruments, GV Instruments, Nu Plasma Instruments, New Wave Research & Photon Machines

  2. Outline • Basics of U-Th-Pb decay system • Measurement methods • Mineral systems & applications • Complexities for zircon • Detrital geochronology (plots & analyses) • Future opportunities • Basics of Lu-Hf decay system • Measurement challenges • Application to detrital minerals

  3. U-Th-Pb chronometers: 206Pb/238U 207Pb/235U 206Pb/207Pb 208Pb/232Th 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 blocking temp  Age records cooling Geochronometer: Mineral grows below blocking temp  Age records crystallization

  5. Heavy minerals in sand: Common:Less common: Garnet Monazite Rutile Magnetite Tourmaline Ilmenite Zircon Chromite Olivine Diamond

  6. 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. Why zircon is king…. ZrSiO4

  7. Zircon (ZrSiO4) Brown: heals headaches Colorless: clears the aura Yellow: attracts love Red: heals injuries, soothes pains Green: draws wealth Light blue: stabilizes mind & emotions Pink: assists in astral travel at night Violet: money magnet • 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! Zircon should be placed in dry sea salt once a month for discharge and recharge…

  8. Zircon: Provenance of modern sediment

  9. Zircon: Provenance of ancient sediment…

  10. Zircon: Provenance of artifacts… Temper sands Mano

  11. Zircon: Age of Sediment Accumulation Detrital grains = Maximum depositional age Dating volcanic layers

  12. Zircon: Growth history & Thermal history “Multi-Dating”

  13. 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

  14. 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

  15. Monazite CeLaThPO4 ? ? • 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?

  16. Monazite CeLaThPO4 zircon monazite From: Hietpas et al. (2010)

  17. Monazite CeLaThPO4 Occurs as inclusions in other minerals (e.g., garnet)  many potential applications! From: http://webhost.bridgew.edu/mkrol/Research-2.html

  18. 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)

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