Geochemical dating of a Swiss freshwater limestone cave using 230 Th/ 234 U ingrow and 226 Ra-excess decay chronometry

Transcription

1 Geochemical dating of a Swiss freshwater limestone cave using Th/ 234 U ingrow and Ra-excess decay chronometry Jost Eikenberg, Maya Jäggi Division for Radiation Protection and Safety Paul Scherrer Institute, CH-5232 Villigen

2 Overview / Topics Presentation of a radiochemical method for simultaneous determination of Ra+ 228 Ra followed by TDCR-measurement and optimized alpha/beta-separation Application of the chronometers Th/ 234 U and Ra for dating sedimentary systems

3 relevant isotopes for quaternary limestone dating radionuclide analytical technique 234 U, ( 235 U), 238 U, Th, 232 Th U/TEVA separation, electrodeposition, α-spectrometry Ra, 228 Ra filtration (RadDisc), OptiPhase Hisafe3 cocktail, LSC 210 Po spontaneous deposition on silver disc, α-spectrometry

4 Method implementation: low level determination of Ra in sediments (limestone) Sample dissolution CaCO 3 in 1 mol/liter HCl, evaporation, dilution with distilled water Filtration of the sample through 3 Empore RadDisc (Mn-oxide impregnated) membrane Elution of Ra with alkaline Na-EDTA solution Measuring via LSC with optimized α/βdiscrimination

5 The triple coincidence to double coincidence ratio (TDCR) counting technique PM R SAMPLE PM S PM T

6 Pulse Length Index (PLI) discrimination with HIDEX SL

7 α-spectrum of Ra with ingrowing daughters 2 h and 8 h after separation using HIDEX 300 SL LSC 140 Alpha

8 α-spectrum of Ra with ingrowing daughters obtained 6 days after separation 160 Alpha

9 β-spectrum of 228 Ra with ingrowing 228 Ac 1 h and 8 h after separation using HIDEX 300 SL LSC 50 Beta

10 TDCR vs. Efficiency using high purity radionuclide standard solutions Pb Ra TDCR Ra TDCR = efficiency efficiency

11 Comparison of measured Ra and the progeny isotopes 222 Rn, 218 Po and 214 Po with calculated decay/ingrowth curves relative activity Rn+ 218 Po+ 214 Po ingrowth curve Ra: measured data 222 Rn+ 218 Po+ 214 Po: measured data Ra decay curve time after separation [d]

12 238 U-Series 235 U-Series 232 Th-Series U Pa Th Ac Ra Fr Rn At Po Bi Pb Tl 238 U 234 U 4.47x10 9 y β 2.45x10 5 y 234 Pa α MeV 234 β Th 1.17min α MeV Th 24.1d 7.54x10 4 y α MeV Ra 1600y α MeV 222 Rn 3.825d α MeV 218 Po 214 Po 210 Po α MeV 3.11min β 214 Bi 19.9min 214 β Pb 26.8min 22.3y 1.6x10-4 s β 138.4d α Bi MeV 5.01d 210 β Pb 206 Pb stable 235 U 7.04x10 8 y α Pa MeV β 3.28x10 4 y 231 Th α Th 1.06d MeV 18.7d 227 β α Ac y MeV α MeV 223 Ra 11.4d α MeV 219 Rn 3.96s α MeV 215 Po 1.8x10-3 s α MeV 211 α Pb 207 Pb 36.1min β 232 Th 228 Th 1.41x10 10 y β 1.91y 228 Ac α MeV 228 β Ra 5.75y 6.13h α MeV 224 Ra 3.66d α MeV 220 Rn 55.6s α MeV 216 Po 212 Po 211 α Bi 212 Bi 2.14min MeV stable 212 Pb 208 Pb 207 β 208 Tl Tl 4.77min 0.15s 66.3% β MeV β 1.01h 10.6h 33.7% α MeV 3.05min β 3x10-7 s stable α MeV

13 Challange for the isotope geochemist: closing the gap between the established 210 Pb and Th/ 234 U chronometers 238 U U Th Ra ( 222 ) 210 Rn Pb( 210 Po) Th/ 234 U daughter / parent Po/ 210 Pb 210 Po/ 210 Pb 210 Pb/ Ra 210 Pb/ Ra Ra/ Th Ra/ Th Th/ 234 U time after formation [y]

14 Principles of U series dating: 1. Th/ 234 U/ 238 U λ t λ t λ t Th( t) = Th(0) e + U(0) ( e 234 e 234 ) 234 λ t λ t Th( t) = U(0) ( e 234 e ) λ U( t) = U(0) e 238 t ( t 1 e ) 234 Th( t) = U (0) λ

15 Principles of U series dating: 1. Th/ 234 U/ 238 U ( λ ) t U ( 0) = U (0) = U Th( t) = U (0) 1 e relative activity (not to scale) U, 234 U Th time after separation [ky]

16 Problem 1: inherited Th, wrong age calculation λ t t ( 1 e ) + Th(0) e 234 Th( t) = U (0) λ Th(t)/ 234 U activity ratio Th/ 234 U = 1 Th ingrowth, without inherited Th Th ingrowth + decay, with initial Th(0)/ 234 U = 0.1 Th ingrowth + decay, with initial Th(0)/ 234 U = 0.2 U-Th-evolution time after formation [ky]

17 U-series application with U-Th isochrones in sedimentology Th / 232 Th activity ratio (relative scale) equipoint = U/Th ratio of detritus component Sedimentary Systems mixed (authigenic + detritus) phases: Th ingrowth dating equiline: T > 0.3 My isochrone U/Th mixing line at T 0 : authigenic phases with detritus contamination U / 232 Th activity ratio (relative scale) T 1 T 0

18 Geological section of the aquifer / recharge area of hell grottoes springs

19 Travertine dating via Ra ex / 234 U and Th/ 234 U

20 PAUL SCHERRER INSTITUT View into the cave system Paul Scherrer Institut 5232 Villigen PSI LSC Int. Conference, Copenhagen,

21 Travertine precipitation: thermodynamic background H2 CO + O2 CO2 + H2O + + H2O H2CO3 H + 3 CO2 HCO H + HCO3 Ca + 3 CaCO3 2HCO 3 2 HCO CO + OH HCO CO + H Ca + CO CaCO HCO + Ca CaCO + CO + H O 3 2 2

22 Ingrowth of Th without an inherited component, why that? 1E-4 1E-6 uraninite solubility (mildly reducing: Eh = pH) concentration [mol/l] 1E-8 1E-10 1E-12 1E-14 detection limit for 232 Th, Th and 228 Th thorianite solubility 1E ph

23 Ra/ Th/ 234 U dating principle λ t 234 λ t 232 Th( t) = Th( 0) e + U ( 1 e ) 234 Th( t) Th( 0) λ e t U λ 1 e t = + ( ) U aut = U m k Thm Th Th Th 40 Th / 232 Th activity ratio Fig. 8-3a equiline 6500 year isochron magnification in Fig. 8-3b 3800 year construction line present day U/Th mixing line U / 232 Th activity ratio

24 Ra/ Th/ 234 U dating principle λ t 234 λ t Th( t) = Th( 0) e + U ( 1 e ) Th( t) Th( 0) λ e t U λ 1 e t = + ( ) Th Th Th U aut = U m k Thm Th / 232 Th activity ratio Fig. 8-3b equiline 6500 year isochron 3800 year construction line present day U/Th mixing line 234 U / 232 Th activity ratio

25 Ra/ Th/ 234 U dating principle λ t 234 λ t Th( t) = Th( 0) e + U ( 1 e ) Let s find an analytical (not numerical) solution for the propagation of the Ra activity with time Th /234 U, Ra /234 U activity ratio Ra(0)/ 234 U = 1.0 Ra(0)/ 234 U = 0.5 Ra(0)/ 234 U = U(t) = 234 U(0) Th(t=0) = time after formation [ky]

26 Ra/ Th/ 234 U dating principle λ Ra( t) = Ra( 0) e + U λt 234 λ 1 e t λ 1 λt ( ) ( e ) λ λ ex aut Ra ( t) = Ra Rasup ( t) 232 Ra = Ra k Th aut m m Ra t Ra Ra t ex ( ) aut sup ( ) e t = Ra( 0) Ra( 0) = λ t age 1 = ln λ Raaut Ra t sup ( ) Ra( 0) F( t) = aut Ra Ra ( t) Ra( 0) sup e λ t

27 Validating the model with the sample data Raex / Ra(0) regression fit: λ = ( )10-3 y -1 T 1/2 = years excess Ra decay curve Ra ex (t)/ Ra(0) = e -λ t time after formation [ky]

28 Calculating Ra ex -ages analytically with the assumption of constant Ra ex / 234 U ratios 0.20 Ra(t)/ 234 U activity ratio Ra(0)/ 234 U initial value 19 Ra(t)/ 234 U decay + ingrowth curve Th supported Ra ingrowth Th ingrowth Ra decay ex time after formation [ky]

29

30 Reasons for a stable aqueous chemistry, i.e. constant initial Ra(0) U/ 238 U activity ratio mean 234 U/ 238 U ratio in travertine = present day 234 U/ 238 U groundwater ratio 0.5 mean Ba/Ca ratio in travertine = ( ) Ba/Ca weight ratio (*10-3 ) Th /234 U sample age [ky]

31 Comparing Ra ex / Ra(0) with Th/ 234 U ages Raex / Ra(0) age [ky] Ra ex / Ra(0) age = Th/ 234 U age Th/ 234 U age [ky]

32 Conclusions Th/ 234 U and Ra ex / 234 U two chronometer dating yields consistent results (agreeing ages) Inherited Th at sample formation is negligible The chemical groundwater composition seems to be highly uniform, obviously there is almost no change of the Ra-initial with time

33 Understanding the dynamics of active volcanic systems: Can we determine melt uplift velocities, melt chamber residence times or eruption events by use of natural tracers??

34 The dynamic earth: sea floor spreading and subduction of oceanic plates

35 Study objects: island arc volcanic rocks from the Sunda-Banda subduction zone

36 Which radio-tracers can be applied? Isotope Half-life [years] Suitable time span Th Pa Ra Pb

37 Ra / Th activity ratio PAUL SCHERRER INSTITUT U-Th-Ra fractionation during melt differentiation Ra > Th > U MORB: (ref 1) alkali basalts: (ref 2) IAB: (this study) Ra > U > Th U / Th activity ratio

38 Thank you for your attention