3.5. Isotope Separation & Analysis

Transcription

1 3.5. Isotope Separation & Analysis

2 Isotope Distribution, Isoscapes 13 C 2 H 18 O SMOW sample C C C C C SMOW sample H H H H H SMOW sample O O O O O

3 Isotopic ratios Isotope ratios for specific elements can change locally due to geological, climatological, biological, chemical and physical processes during the history of the earth. The analysis of isotope ratios is a unique tool for provenance studies as seen in previous examples. Example: isotope distribution in different Greek quarries Typical isotope ratios used for provenance studies: 13 C/ 12 C 18 O/ 16 O Pb/ 208 Pb Pb Pb Pb Pb Pb sample standard 11000

4 Determination of the Pb isotope ratio Neutron activation technique on Pb or 208 Pb? Pb(n,) 205 Pb; 208 Pb(n,) 209 Pb; Not possible since the cross section is extremely weak and 205 Pb and 209 Pb have no characteristic decay radiation! Alternative method: isotope analysis by mass separation techniques!

5 Separation in magnetic fields m B R q B ; m R q v v particles with velocity v are separated by m/q for a fixed magnetic field B and a spectrometer radius R

6 Example: Separation of Pb and 208 Pb m m R m R R R R R R R v B q v B q m m R v B q m v R B m q m ; 1 for 208 ; R 1 R 2 Separated Pb, 208 Pb beams B

7 Mass spectrum in focal plane detector

8 Isotope Analyzer Small sample of material with mass <g is ionized by sputter technique, accelerated and mass separated by sequence of magnetic and electrical fields

9 Origin of Hittite Mining Industry Tin bronze statue The Hittite empire developed an extensive metallurgy and mining industry during the bronze age, BC that initiated the iron age.

10 Hittite Mining Site determination by lead (Pb) isotope analysis Control over the mining sites was important for Hittite empire trade Origin of metal in artifact is determined by their 207 Pb/ 206 Pb isotope ratio Metal contain tin, copper, nickel and lead! A. Yener, Bibl. Archaeologist 58 (1995)

11 The Geology of Taurus Volcanic origin (earth quake prone region); limestone mountains, lower layers show lava, basalt, soft tuft pillars produced by volcanic eruption and subsequent erosion. Sizable tin, copper, iron, zinc, lead composition in deeper layers; polymetallic composition with abundance variations..

12 The importance of lead isotopes 40% Fe as hematite, magnetite 10-20% Pb with high Zn content 6-8% Zn (~1/2 Pb) 1-2% Cu in some cases 6-7% 3% Co 1.5% Sn in quartz veins Layered sediments in caves and cavities of limestone mass ppm Au ppm Ag A. Yener, J.Archaeol.Sci. 18 (1991) Isotope composition of Pb deposits changes with the environment; this provides a unique tool for the analysis of antique mining and trading patterns!

13 Terrestrial Lead and Radiogenic Lead There are 4 stable lead isotopes: Pb, 206 Pb, 207 Pb, 208 Pb Lead isotope distribution is not necessarily constant but may change if significant Uranium & Plutonium abundance exist in in the ore material. Natural decay of these a unstable isotopes feed the Pb isotopes.

14 Natural decay chains & the feeding of Pb isotopes 208 Pb 207 Pb 206 Pb

15 Pb isotopic ratio Depending on chemical history and water content of ore material significant differences exist in abundances of long-lived mother U, Np, Pt elements in the ore material. This eventually leads to build-up of significant differences in lead daughter isotopic abundances, which often is characteristic for the mine site. Present day lead = primeval lead + radiogenic lead Simple evolution model: t p: time between origin of earth and last geological active period 238 : decay constant of 238 U 235 : decay constant of 235 U 232 : decay constant of 232 Th Pb Pb t Pb Pb t e t e p 207 Pb Pb t Pb Pb t e t ( ) ( e p). 208 Pb Pb t Pb Pb t W e t e p 238 p 238 t ( ) ( ) ( ) 235 p 235 t ( ) 232 p 232 t ( ) ( ) ( )

16 Evolution model applied for US mines Pb/ Pb =9 =7 Certain lead isotope ratios help to identify the original mines, the analysis of the lead isotope ratio reveals ancient trading routes Pb/ Pb Present isotope ratio parameters: W U Pb Th Pb

17 Ancient ship trade ca 1500 BC

18 Egyptian trading vessel at Uluburun Kas G. F. Bass, Amer. J. Archaeology 90 (1986) C. Pulak, Int. J. of Naut. Arch. 27(1998)

19 Difficulty of Underwater Archaeology

20 The Uluburun Wreck

21 Copper and tin ingots Tin ingot Copper ingots

22 Origin of metals? Cyprus, Laurion (mainland Greece), and Cappadocia (Taurus Mountains) were antique mining centers, did the Uluburun transport metals from Laurion eastwards or from Cyprus or Cappadocia westwards? The Pb isotope composition from Cyprus and Laurion metals matches most closely the isotope composition of group 4 from the Taurus mines! But differences show up when comparing different Pb isotope ratios! Taurus 4 Taurus 4 Cyprus Laurion Taurus 4 Taurus 4 Cyprus Laurion

23 Origin and destination? Did the ship sail west or east Origin of copper and tin can be tested on the mine specific lead isotope ratio in the ingots. Typical lead ratio of Taurus mines Typical lead ratio of ship ingots Lead concentration in copper and tin

24 Trading Route