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Thermochronology, cosmogenic isotopes and dating of young sedimentary rocksPart 8: Usage of U-Pb and U-Th geochronology in
sedimentary environmentIstván Dunkl
http://www.sediment.uni-goettingen.de/staff/dunkl/
Intro: Where are the geotherms?Geothermometry in basins by: vitrinite reflectance, bitumen refl.,
graptolite refl., Raman spectroscopy, conodont alteration, spore colour, fluorescence, Rock-Eval, clay mineralogy
Let's start the dating work: research concept, mineral separation Fission track dating: - nuclear physics
- age equation, statistics- track lengths
Volcanic events (= formation ages)Basement exhumation (= cooling ages)Complex T histories of basins & thermal modellingDetrital chronology (provenance by single-grain ages)
(U-Th)/He thermochronologyK/Ar, Ar/Ar, Luminescence, ESR and Cosmogenic dating of sedimentsU/Pb and U/Th dating of sediments
[W. Siebel; M. Williams]
DecayTwo U isotopes: 238U/235U = 137.88
Major methods used for U-Pb geochronology
Isotope Dilution – Thermal Ionization Mass Spectrometry (ID-TIMS)
Single crystal evaporation
Secondary Ion Mass Spectrometry (SIMS, Ion Probe, or “SHRIMP”)
Proton Induced X-ray Emission (PIXE)
Solution ICP-MS
Laser-Abblation ICP-MS
Electron microprobe analysis (EMP)
Applications in sedimentary environment
Dating of volcanic ash layers
Provenance (age distribution, fingerprint method)
Mineralization in diagenetic conditions
Chemical analyses Chemical and isotopic analyses
Secondary ion mass spectrometry
[Williams, 2008]
[Campbell et al., 2005]
Polished surface and "rim piercing" method of laser abblation
Time-resolved signal of laser ablation
[Frei and Gerdes, 2008]
[Košler and Tubrett, 2004]
Comparison of techniques applied at zircon U-Pb dating
U-Pb data presentation
[Frei and Gerdes, 2008]
[G. Gehrels]
Comparison of methods
Most frequently dated minerals
[Košler and Tubrett, 2004]
[W. Siebel]
Closure temperatures (?)
U-bearing phases and minerals used for U-Pb dating
U-mineralsUraninite, Pechblende, BranneriteCarnotite, Autunite, Tobernite
Common accessory minerals that usually do not need common lead correction
ZirconMonaziteXenotimeBaddeleyite
Common lead correction neededApatiteTitanite (Sphene)Allanite (Ortite)RutileCassiteritePerovskite
OpalCalcite
[W. Siebel]
Why zircon?From which rocks?
do not hesitate to sample ugly-looking, even strongly weathered rocks -e.g. bentonites-, but the ideal case if you see quarz (and biotite)
[Paquette et al., 2003]
Zoning and heterogeneous metamictization in old zircons
[Veevers et al., 2005]
Provenance
[Soreghan et al., 2002]
[Dunkl, unpublished]
Provenance of sandstone boulders in a salt dome
20
10
evaporite-hostedsandstone boulder
n=83/98
evaporite-hostedsandstone boulder
n=84/95
Mesozoic sandstonen=88/98
20
20
10
10
300
600
900
1200
1500
1800
2100
2400
300
600
900
1200
1500
1800
2100
2400
300
600
900
1200
1500
1800
2100
2400
Million years
[Nagy et al., 2003]
Electron microprobe dating of monazite
Monazite petrochronology: age domain maps(Williams et al. 1999; Goncalves et al. 2005)
Goncalves et al., Am. Min., 2005
extract elementconcentrations
from chemical maps(Th, U, Pb, Y)
solve age (Montel)equation pixel-
by-pixel forselected
domains orentire grain
link to spotanalyses andother texturalinformation
Dating of monazite
[Dalhousie University]
[Evans and Zalasiewitz, 1996]
Diagenetic monazite
[Evans and Zalasiewitz, 1996]
Diagenetic monazite
[Aleinikoff et al., 2007]
Sphene
[Aleinikoff et al., 2007]
Sphene
[Li et al., 2003]
Rutile
[Grandia et al., 2000]
U–Pb dating of MVT ore-stage calcite(cc ~ 0.x ppm U)
[Robb, 2005]
Regolith
[Amelin and Back, 2006]
Opal
[Amelin and Back, 2006]
Opal
Opal
[Amelin and Back, 2006]
[Nemchin et al., 2006]
Opal
Speleothem
Facit:needs lucky composition & a very very clean lab.
Geochronology using the decay of uranium
István Dunklhttp://www.sediment.uni-goettingen.de/staff/dunkl
Part 2: U-series dating(230Th/U dating, U/Th disequilibrium method)
Focus:
principles, methodology, statistics
applicability, limitations
case studies
[USGS]
Decay chains - secular equilibrium
[from D. Patterson]
238U decay chain halflives
1.E-09
1.E-07
1.E-05
1.E-03
1.E-01
1.E+01
1.E+03
1.E+05
1.E+07
1.E+09
1.E+11
238U
234T
h
234P
a
234U
230T
h
226R
a
222R
n
218P
o
214P
b
218A
t
Bi2
14
Tl21
0
218R
n
Po2
14
Pb2
10
Hg2
06
Bi2
10
Tl20
6
Po2
10
206P
b4.47 Ga
24 d
6.7 h
248 ka75.2 ka1.7 ka
3.8 d
3 m27 m20 m
1.3 m
22 y
7.5 m
5 d
4.2 m
138 d
[Cooper and Reid]
Decay series equation
[Schmitt, 2009]
Initial isotope ratios are never ideal - correction needed
[http://ocw.mit.edu]
[Uncertainty of 230Th/234U with timefrom Calsteren and Thomas, 2006]
Question
[Hercman and Goslar, 2002]
Simplified scheme of Uranium circulation in hypergenic processes
Question
Recoil effect and leaching process results in non-mass dependent isotope fractionation where the solid phase is depleted in (234U/238U) and the liquidphase is enriched.
Scope
environmental scienceoceanographyhydrologyscience-based archaeology
magma chamber evolution and volcanic hazard predictionglobal climatic change through dating of authigenic carbonate deposits human evolution through dating of bonegroundwater evolution
ZirconPlag, Amph, Cpx, Mt, (magmatic Ca-garnet)
coralsmolluscan shellscarbonate cement marine apatitelacustrine carbonates (marl)speleotheme, travertine pedogenic silica and carbonate "caliche", "calcrete"bones, tooth enamel(peat)ferruginous concretions and rindsopalice, water
Materials
[Schmitt, 2009]
Secular equilibrium
For a crystal with 100 ppm U, this means:
Ultra-trace element analysis needed
Methods
(1) Bulk separates: partial and total dissolutionChemical separation (column chemistry)
(2) In situ dating (single grains or rock chips in polished mounts)
Gamma spectrometryApha spectrometrySolution ICP-MSLaser-ablation ICP-MSThermal ionization mass-spectrometry (TIMS)Synchrotron radiation X-ray microanalysisSHRIMP
Data presentation
[Robinson et al., 2002]
[Ludwig and Paces, 2002]
Data presentation
[Ludwig and Paces, 2002]
[Bacon and Lowenstern, 2005]
Eruption datingmajor pitfall: recycled zircon crystals
Question
[Reid, 2008]
Residence time of magma - correlation with the size of eruption
Marine reservoir for initial isotope ratio
- U (VI) is relatively soluble
- seawater3.3 ppb U (appears to be conservative in the ocean)0.5*10-4 ppb 232Th
- 230Th and 231Pa are particle-reactive; i.e. it tends to attach to surfaces rapidly, and so it is removed from seawater on a time scale of ~30 years.
- occurs at low concentrations in seawater 0.7*10-8 ppb 230Th(<0.1 dpm/100kg at the surface; ~1 dpm/100kg in deep waters)
- corals incorporate uranium (~2ppm) but very little 230Th
Question
[Potter et al., 2005]
Corals
(one line = one data)
Usage on lakes(authigenic carbonate)
[Calsteren and Thomas, 2006]
[Ford and Hill, 1999]
U-series dating of speleothemesclimate, earthquakes Question
[Nordhof]
Speleothems
[Eggins et al., 2005]
Question
[Ortega et al., 2005]
Recrystallized aragonite–calcite speleothems
[Ortega et al., 2005]
Recrystallized aragonite–calcite speleothems
(nuclear microprobeanalysis)
Archaelogy: Dating of calcite laminations covering paintings(6300 years & also 29000 y. ago?)
[Aubert et al., 2007]
[Herczeg and Chapman, 1991]
Dunes(pedogenic carbonate)
Question
[Ludwig and Paces, 2002]
Pedogenic silica and carbonate Question
[Ludwig and Paces, 2002]
Pedogenic silica and carbonate
[Heijnis and van der Plicht, 1992]
Peat
- organic material adsorbs U
- siliciclastic contribution (mainly clay) carries Th
- top and base of the sequence are open systems (forget geochronology there!)
- pro: can agree well with TL dating
- contra: not always applicable
Chemical sedimentation
[Lemoalle and Dupont]
[Short et al., 1989]
Ferruginous concretions and rinds
("Bohnenerz", swamp iron pisoids)
[Fireman; Goldstein et al., 2004]
U-series dating of ice
hardest task
- with or without grain contamination (where are the nuclides?)
- pro: no matrix
- contra: extremely high risk of contamination
Groundwater
Dabous et al.(2002): Uranium/Thorium isotope evidence for ground-water history in the Eastern Desert of Egypt. Journal of Arid Environments, 50, 343-357.
[Gellermann et al., 1990]
[Plummer et al., 2001]
WaterQuestion
[Gellermann et al., 1990]
Groundwater
transport of radionuclides in the water by advection (v)
leaching of radionuclides from the rock (e)
removal of radionuclides from the water by precipitation (r)
reversible exchange between water and rock surface (R)
radioactive decay and production