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
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
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
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
The triple coincidence to double coincidence ratio (TDCR) counting technique PM R SAMPLE PM S PM T
Pulse Length Index (PLI) discrimination with HIDEX SL 300 28 24 20 16 12 8 4 27 38 52 72 99 135 185 252 343 466 634 861 1169 1588 2156 2927 3973 5393 7319 9934 13481 18296 0
α-spectrum of Ra with ingrowing daughters 2 h and 8 h after separation using HIDEX 300 SL LSC 140 Alpha 120 100 80 60 40 20 0 10 100 1000 10000
α-spectrum of Ra with ingrowing daughters obtained 6 days after separation 160 Alpha 140 120 100 80 60 40 20 0 10 100 1000 10000
β-spectrum of 228 Ra with ingrowing 228 Ac 1 h and 8 h after separation using HIDEX 300 SL LSC 50 Beta 45 40 35 30 25 20 15 10 5 0 1 10 100 1000
TDCR vs. Efficiency using high purity radionuclide standard solutions 1.0 210 Pb 0.9 0.8 Ra TDCR 0.7 0.6 228 Ra TDCR = efficiency 0.5 0.4 0.4 0.5 0.6 0.7 0.8 0.9 1.0 efficiency
Comparison of measured Ra and the progeny isotopes 222 Rn, 218 Po and 214 Po with calculated decay/ingrowth curves 4.0 3.5 relative activity 3.0 2.5 2.0 1.5 1.0 0.5 222 Rn+ 218 Po+ 214 Po ingrowth curve Ra: measured data 222 Rn+ 218 Po+ 214 Po: measured data Ra decay curve 0.0 0 5 10 15 20 25 30 time after separation [d]
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 4.196 MeV 234 β Th 1.17min α 4.776 MeV Th 24.1d 7.54x10 4 y α 4.688 MeV Ra 1600y α 4.784 MeV 222 Rn 3.825d α 5.490 MeV 218 Po 214 Po 210 Po α 6.003 MeV 3.11min β 214 Bi 19.9min 214 β Pb 26.8min 22.3y 1.6x10-4 s β 138.4d α 7.687 210 Bi MeV 5.01d 210 β Pb 206 Pb stable 235 U 7.04x10 8 y α 4.395 231 Pa MeV β 3.28x10 4 y 231 Th α 227 5.013 Th 1.06d MeV 18.7d 227 β α Ac 6.038 21.8y MeV α 5.304 MeV 223 Ra 11.4d α 5.716 MeV 219 Rn 3.96s α 6.819 MeV 215 Po 1.8x10-3 s α 7.386 MeV 211 α Pb 207 Pb 36.1min β 232 Th 228 Th 1.41x10 10 y β 1.91y 228 Ac α 4.010 MeV 228 β Ra 5.75y 6.13h α 5.423 MeV 224 Ra 3.66d α 5.686 MeV 220 Rn 55.6s α 6.288 MeV 216 Po 212 Po 211 α Bi 212 Bi 2.14min 6.623 MeV stable 212 Pb 208 Pb 207 β 208 Tl Tl 4.77min 0.15s 66.3% β 6.779 MeV β 1.01h 10.6h 33.7% α 6.051 MeV 3.05min β 3x10-7 s stable α 8.784 MeV
Challange for the isotope geochemist: closing the gap between the established 210 Pb and Th/ 234 U chronometers 238 U 2.0 234 U Th Ra ( 222 ) 210 Rn Pb( 210 Po) Th/ 234 U daughter / parent 1.5 1.0 0.5 210 Po/ 210 Pb 210 Po/ 210 Pb 210 Pb/ Ra 210 Pb/ Ra Ra/ Th Ra/ Th Th/ 234 U 0.0 10-1 10 0 10 1 10 2 10 3 10 4 10 5 10 6 time after formation [y]
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 ) 234 234 λ U( t) = U(0) e 238 t ( t 1 e ) 234 Th( t) = U (0) λ
Principles of U series dating: 1. Th/ 234 U/ 238 U 234 238 238 234 ( λ ) t U ( 0) = U (0) = U Th( t) = U (0) 1 e relative activity (not to scale) 1.0 0.8 0.6 0.4 0.2 238 U, 234 U Th 0.0 0 50 100 150 200 250 300 time after separation [ky]
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 1.0 0.8 0.6 0.4 0.2 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 0.0 0 50 100 150 200 250 300 time after formation [ky]
U-series application with U-Th isochrones in sedimentology Th / 232 Th activity ratio (relative scale) 3.0 2.5 2.0 1.5 1.0 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 0.5 0.5 1.0 1.5 2.0 2.5 3.0 234 U / 232 Th activity ratio (relative scale) T 1 T 0
Geological section of the aquifer / recharge area of hell grottoes springs
Travertine dating via Ra ex / 234 U and Th/ 234 U
PAUL SCHERRER INSTITUT View into the cave system Paul Scherrer Institut 5232 Villigen PSI LSC Int. Conference, Copenhagen, 01-05.05.2017
Travertine precipitation: thermodynamic background H2 CO + O2 CO2 + H2O + + H2O H2CO3 H + 3 CO2 HCO + 2+ + H + HCO3 Ca + 3 CaCO3 2HCO 3 2 HCO CO + OH HCO CO + H 3 3 2 + 2+ 3 2 3 Ca + CO CaCO 2+ 2 3 HCO + Ca CaCO + CO + H O 3 2 2
Ingrowth of Th without an inherited component, why that? 1E-4 1E-6 uraninite solubility (mildly reducing: Eh = 0.5-0.059pH) concentration [mol/l] 1E-8 1E-10 1E-12 1E-14 detection limit for 232 Th, Th and 228 Th thorianite solubility 1E-16 3 4 5 6 7 8 9 ph
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 234 234 232 = + ( ) U 232 232 aut = U m k Thm Th Th Th 40 Th / 232 Th activity ratio 30 20 10 Fig. 8-3a equiline 6500 year isochron magnification in Fig. 8-3b 3800 year construction line present day U/Th mixing line 0 0 100 200 300 400 500 234 U / 232 Th activity ratio
Ra/ Th/ 234 U dating principle λ t 234 λ t Th( t) = Th( 0) e + U ( 1 e ) 232 234 Th( t) Th( 0) λ e t U λ 1 e t = + ( ) 232 232 Th Th Th 234 234 232 U aut = U m k Thm Th / 232 Th activity ratio 4 3 2 1 0 0 10 20 30 40 50 Fig. 8-3b equiline 6500 year isochron 3800 year construction line present day U/Th mixing line 234 U / 232 Th activity ratio
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 1.0 0.8 0.6 0.4 0.2 0.0 Ra(0)/ 234 U = 1.0 Ra(0)/ 234 U = 0.5 Ra(0)/ 234 U = 0.2 234 U(t) = 234 U(0) Th(t=0) = 0 0 5 10 15 20 time after formation [ky]
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
Validating the model with the sample data Raex / Ra(0) 10 0 10-1 10-2 10 19 18 17 2 4 15 regression fit: λ = (0.444 + 0.041)10-3 y -1 T 1/2 = 1560 + 150 years excess Ra decay curve Ra ex (t)/ Ra(0) = e -λ t 8 5 11 12 0 2 4 6 8 10 9 6 14 time after formation [ky] 16 13 1
Calculating Ra ex -ages analytically with the assumption of constant Ra ex / 234 U ratios 0.20 Ra(t)/ 234 U activity ratio 0.15 0.10 0.05 0.00 10 Ra(0)/ 234 U initial value 19 Ra(t)/ 234 U decay + ingrowth curve Th supported Ra ingrowth 18 15 17 14 8 13 4 12 6 2 1 Th ingrowth 5 11 16 9 Ra decay ex 0 2 4 6 8 10 time after formation [ky]
Reasons for a stable aqueous chemistry, i.e. constant initial Ra(0) 2.0 2.0 234 U/ 238 U activity ratio 1.5 1.0 0.5 0.0 mean 234 U/ 238 U ratio in travertine = 1.28 + 0.06 1.5 1.0 present day 234 U/ 238 U groundwater ratio 0.5 mean Ba/Ca ratio in travertine = (0.49 + 0.02)10-3 0.0 0 2 4 6 8 10 Ba/Ca weight ratio (*10-3 ) Th /234 U sample age [ky]
Comparing Ra ex / Ra(0) with Th/ 234 U ages Raex / Ra(0) age [ky] 8 6 4 2 0 Ra ex / Ra(0) age = Th/ 234 U age 19 10 12 15 9 6 11 5 17 8 2 4 18 0 2 4 6 8 Th/ 234 U age [ky]
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
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Which radio-tracers can be applied? Isotope Half-life [years] Suitable time span Th 76000 2000 200000 231 Pa 33000 1000-100000 Ra 1600 100-6000 210 Pb 22 3-100
Ra / Th activity ratio PAUL SCHERRER INSTITUT U-Th-Ra fractionation during melt differentiation 3.0 2.5 2.0 1.5 1.0 0.5 Ra > Th > U MORB: (ref 1) alkali basalts: (ref 2) IAB: (this study) Ra > U > Th 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 238 U / Th activity ratio
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