The Uranium Series Disequilibrium Dating Method UTh dating
鈾系不平衡定年法 The Uranium Series Disequilibrium Dating Method • 導論 • 定年原理 • 鈾釷定年法(U-Th dating) • 應用 • 洞穴碳酸鹽(Speleothem)
Introduction • Uranium: U 6+ → UO 22+ • Thorium: Th 4+ →Th(OH)4 • Uranium-series dating
Introduction • Daughter-deficiency – 230 Th/234 U • Daughter-excess – 234 U/238 U
Theory 核種圖 234 U 248 ky α 230 Th 232 Th 75 ky 14 By
Sample Criteria • Forms with appreciable U • Forms with negligible Th →initial 230 Th • Remains a closed system
Initial 230 Th • Th 4+ → low solubility, strong • adsorbability Source of 230 Th – Authigenic (from U) – Detrital ➥ colloidal phases ➥ attach to organic molecules ➥ in carbonate complexes • 232 Th • and 230 Th are chemically equivalent The 230 Th/232 Th ratio → isochron technique
Age Equation 簡化 (Cheng et al. , 2000) y x y=ax+b a b λ 230=9. 1577 x 10 -6 λ 234=2. 8263 x 10 -6
230 Th-238 U Isochron Diagram
Application 洞穴岩的形成機制 Ca 2+ + 2 HCO 3 -↔Ca. CO 3 + H 2 O + CO 2
Application 氧同位素平衡 • 平衡方程式 Ca. C 16 O 3 + 3 H 218 O ↔ Ca. C 18 O 3 + 3 H 216 O O 2(air)
Application Experimental Methods • Sub-sampling: Milling • Chemical separation of uranium and thorium – Spectra Gel Ion Exchange • Measurement: – Inductively Coupled Plasma Mass Spectrometry (ICP –MS) • Isochron technique
Application 採樣
Isochron Technique ( Dorale et al. , 1998) Isochron Sub -sample Sample Weight (mg) 238 U conc. (ppm) Middle 40. 5 2. 55 Left 42. 8 Right 37. 7 232 Th conc. (ppb) Uncorrected Age Corrected Agea 5. 4 1, 383 ± 34 1, 344 ± 34 2. 54 22. 8 1, 519 ± 46 1, 349 ± 44 2. 25 291. 6 3, 797 ± 55 1, 348 ± 51
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