Fractionation of Stable isotope, Summaries of Geology

Download Fractionation of Stable isotope and more Summaries Geology in PDF only on Docsity! 0 Mahidol University Wisdom of the Land Fractionation of a Stable Isotope oo 2A. KAGS381 Introduction to Geochemistry 1. or Dr. Patchawee Nualkhao | al Geoscience Division Mahidol University Kanchanaburi Camp. patchawee.nua@mahidol.edu 60) Mahidol University Q ye Wisdom of the Land Contents 1. Isotopes 2. Stable Isotopes 3. Oxygen Isotopes 4. Radioactive Isotopes 5. Radioactive Decay 6. K-Ar System 7. Sr-Rb System 8. lIsochron Technique 9. Sm-Nd System 10. U-Pb-Th System “% Mahidol Universit 0 Y Wisdom of the Land Stable Isotopes e Stable: last ~ forever e Chemical fractionation is impossible e Mass fractionation is the only type possible “% Mahidol Universit 0 Y Wisdom of the Land Example: Oxygen Isotopes 16Q 99.756% of natural oxygen 70 0.039% ‘ 18O 0.205% ‘ Concentrations expressed by reference to a standard International standard for O isotopes = standard mean ocean water (SMOW) “% Mahidol Universit 0 Y Wisdom of the Land 180 and '6O are the commonly used isotopes and their ratio is expressed as d: 187/16 ~ (187/16 d (18QO/16O) — ( O/ ©) sample ( O/ O) ('80/"°O) omerx1000 eq 9.10 SMOW result expressed in per mille (%o) What is d of SMOW?? What is d for meteoric water? Mahidol University Wisdom of the Land -20 -10 0 10 Mean Annual Air Temperature (°C) Figure 9.9. Relationship between d('8O/'®O) and mean annual temperature for meteoric precipitatiogy after Dansgaard (1964). Tellus, 16, 436-468. 6 Mahidol University Q Me L Stable isotopes useful in assessing relative contribution of various reservoirs, each with a distinctive isotopic signature « O and H isotopes - juvenile vs. meteoric vs. brine water « d'8O for mantle rocks + surface-reworked sediments: evaluate contamination of mantle- derived magmas by crustal sediments 11 (4% Mahidol University ys Wisdom of the Land Radioactive Isotopes Unstable isotopes decay to other nuclides The rate of decay is constant, and not affected by P, T, X... Parent nuclide = radioactive nuclide that decays Daughter nuclide(s) are the radiogenic atomic products 12 Isotopic variations between rocks, etc. due to: 1. Mass fractionation (as for stable isotopes) 2. Daughters produced in varying proportions resulting from previous event of chemical fractionation 3. Time The longer *°K — 4°Ar decay takes place, the greater the difference between the basalt and rhyolite will be 15 6: Mahidol University \) J Wisdom of the Land Radioactive Decay The Law of Radioactive Decay dN dN eg.9.11 ~G HN or- =I N 1 ” E £ 2 Ye 6 S + % time > 16 6: Mahidol University \) J Wisdom of the Land D=Nel"l-N=N(elt-1) eq 9.14 — age of a sample (t) if we know: D the amount of the daughter nuclide produced N the amount of the original parent nuclide remaining | the decay constant for the system in question 17 6: Mahidol University Q ¥ Wisdom of the Land e Blocking temperatures for various minerals differ e *°Ar-82Ar technique grew from this discovery 20 “% Mahidol Universit 0 Y Wisdom of the Land Sr-Rb System e ®8’Rb >®’Sr+abeta particle (l= 1.42 x 10° a") e Rb behaves like K + micas and alkali feldspar e Sr behaves like Ca > plagioclase and apatite (but not clinopyroxene) e %8Sr : 8’Sr : 8Sr : 84Sr ave. sample = 10 : 0.7: 1 : 0.07 e Sr is a stable isotope, and not created by breakdown of any other parent 3 0 Mahidol University X | - ead lsochron Technique Requires 3 or more cogenetic samples with a range of Rb/Sr __,, 8 b 4 Could be: | \ Re * 3cogenetic rocks derived .,§ 1 from a single source by SO 1 partial melting, FX, etc. 47 | 2 - 4 Figure 9.3. Change in the concentration of Rb and Sr in the melt | Sp derived by progressive batch melting of a basaltic rock consisting 0 : 1 : \ : ' 4 L 4 of plagioclase, augite, and olivine. From Winter (2001) An 0 0.2 0.4 0.6 0.822 1 Introduction to Igneous and Metamorphic Petrology. Prentice Hall. F 6: Mahidol University \) J Wisdom of the Land Begin with 3 rocks plotting ata bc at time t, 87Sr 8°Sr 87Sr M 7 oP Q Ff ot 7A 86Sr 25 6 Mahidol University Q Me L After some time increment (t, >t,) each sample loses some ®’Rb and gains an equivalent amount of ®’Sr 87Sr 86Sr t, Cc a z \ 87Sr) ( ==), a \ Cc t, 87Rb Q Me L 6 Mahidol University At time t, each rock system has evolved —> new line Again still linear and steeper line 87Sr 86Sr 87S/' 86Sr 5 ro 0 Mahidol University Q Wisdom of the Land 01a Me The Sm-Nd System e Both Sm and Nd are LREE ~ Incompatible elements fractionate — melts « Nd has lower Z —> larger — liquids > does sm Haloge Nob Gas Eu* is LIL Lanthanide series Eu® is HFS 60.26] 63 151.9664 157.2565 158.93|66 162.50|67 164.93|68 167.26] 69 168.93/70 173.04/71 174.97] ) Eu | Gd | Tb | Dy | Ho| Er | Tm! Yb} Lu Eu* 1.15| Gd 1.14) Tb* 1.12] Dy 1.11} Ho* 1.10) Er 1.08 | Tm* 1.07] Yb* 1.06/Lu* 1.05 Europium |G LErbiuen | Thuiirn | viterbium 30 oO} Mahidol University 147Sm — '43Nd by alpha decay | = 6.54 x 10°'3 a" (half life 106 Ga) e Decay equation derived by reference to the non-radiogenic '44Nd > 143Nd/144Nd = (143Nd/144Nd), + (147Sm/"44Nd)lt 31 Mahidol University Te Le 0.514. 0.512} Nd 144Nd 0.510}- 0.508}- Partial melting or enrichment event Chondrite 0.506} Meteorite ! | | ! 4 3 2 1 0.5 Time (Ga before present) Figure 9.15. Estimated Nd isotopic evolution of the Earth's upper mantle, assuming a large-scale melting oR D enrichment event at 3.0 Ga b.p. After Wilson (1989). Igneous Petrogenesis. Unwin Hyman/Kluwer. 6 Mahidol University Q Wisdom of the Land The U-Pb-Th System Discordia = \oss of both 206Pb and 2°’Pb Figure 9.16a. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of Pb loss. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York. 1.0 0.8 208 pyr 238 U 0.4 0.2 2 4 6 8 10 12 207Pb*/235|J° The U-Pb-Th System Concordia diagram after 3.5 Ga total evolution 1060S 0~2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 207Pb*/235 Figure 9.16b. Concordia diagram illustrating the Pb isotopic development of a 3.5 Ga old rock with a single episode of PhéSs. After Faure (1986). Principles of Isotope Geology. 2nd, ed. John Wiley & Sons. New York. Mahidol University Wisdom of the Land ot 206 Pb* ‘ 238 U 1 n L 1 L 0 8 16 24 32 40 48 207 Pb*/235y Figure 9.17. Concordia diagram tor three discordant zircons separated trom an Archean gneiss at Morton and Granite Falls, Minnesota. The discordia intersects the concordia at 3.55 Ga, yielding the U-Pb age of the gneiss, and at 1.85 Ga, yielditty ihe U-Pb age of the depletion event. From Faure (1986). Copyright © reprinted by permission of John Wiley & Sons, Inc.