Thermochronology, cosmogenic isotopes and dating of young sedimentary rocks
Part 8: Usage of U-Pb and U-Th geochronology in
sedimentary environment
István 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 modelling
Detrital chronology (provenance by single-grain ages)
(U-Th)/He thermochronology
K/Ar, Ar/Ar, Luminescence, ESR and Cosmogenic dating of sediments
U/Pb and U/Th dating of sediments
Decay
Two U isotopes: 238U/235U = 137.88
[W. Siebel; M. Williams]
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
Secondary ion mass spectrometry
Chemical analyses
Chemical and isotopic analyses
[Williams, 2008]
Polished surface and "rim piercing" method of laser abblation
[Campbell et al., 2005]
Time-resolved signal of laser ablation
[Frei and Gerdes, 2008]
Comparison of techniques applied at zircon U-Pb dating
[Košler and Tubrett, 2004]
U-Pb data presentation
[Frei and Gerdes, 2008]
Comparison of methods
[G. Gehrels]
Most frequently dated minerals
[Košler and Tubrett, 2004]
Closure temperatures (?)
[W. Siebel]
U-bearing phases and minerals used for U-Pb dating
U-minerals
Uraninite, Pechblende, Brannerite
Carnotite, Autunite, Tobernite
Common accessory minerals that usually do not need
common lead correction
Zircon
Monazite
Xenotime
Baddeleyite
Common lead correction needed
Apatite
Titanite (Sphene)
Allanite (Ortite)
Rutile
Cassiterite
Perovskite
Opal
Calcite
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)
[W. Siebel]
Zoning and heterogeneous metamictization in old zircons
[Paquette et al., 2003]
Provenance
[Veevers et al., 2005]
[Soreghan et al., 2002]
Provenance of
sandstone boulders in
a salt dome
20
Mesozoic sandstone
n=88/98
10
2400
2100
1800
1500
1200
900
600
300
evaporite-hosted
sandstone boulder
n=84/95
20
10
2400
2100
1800
1500
1200
900
600
300
evaporite-hosted
sandstone boulder
n=83/98
20
10
2400
2100
1800
1500
1200
900
600
300
Million years
[Dunkl, unpublished]
Electron microprobe dating of monazite
[Nagy et al., 2003]
Dating of monazite
Monazite petrochronology: age domain maps
(Williams et al. 1999; Goncalves et al. 2005)
extract element
concentrations
from chemical maps
(Th, U, Pb, Y)
solve age (Montel)
equation pixelby-pixel for
selected
domains or
entire grain
link to spot
analyses and
other textural
information
Goncalves et al., Am. Min., 2005
[Dalhousie University]
Diagenetic monazite
[Evans and Zalasiewitz, 1996]
Diagenetic monazite
[Evans and Zalasiewitz, 1996]
Sphene
[Aleinikoff et al., 2007]
Sphene
[Aleinikoff et al., 2007]
Rutile
[Li et al., 2003]
U–Pb dating of MVT ore-stage calcite
(cc ~ 0.x ppm U)
[Grandia et al., 2000]
Regolith
[Robb, 2005]
Opal
[Amelin and Back, 2006]
Opal
[Amelin and Back, 2006]
Opal
[Amelin and Back, 2006]
Opal
[Nemchin et al., 2006]
Speleothem
Facit:
needs lucky composition & a very very clean lab.
Geochronology using the decay of uranium
István Dunkl
http://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
Decay chains - secular equilibrium
[USGS]
238U decay chain halflives
1.7 ka
75.2 ka
248 ka
4.47 Ga
1.E+11
1.E+09
1.E+07
1.E+05
1.E+03
22 y
1.E+01
138 d
24 d
1.E-01
3.8 d
5d
6.7 h
1.E-03
27 m
20 m
7.5 m
4.2 m
3m
1.3 m
1.E-05
1.E-07
238U
234Th
234Pa
234U
230Th
226Ra
222Rn
218Po
214Pb
218At
Bi214
Tl210
218Rn
Po214
Pb210
Hg206
Bi210
Tl206
Po210
206Pb
1.E-09
[from D. Patterson]
Decay series equation
[Cooper and Reid]
[Schmitt, 2009]
Initial isotope ratios are never
ideal - correction needed
Question
[http://ocw.mit.edu]
[Uncertainty of 230Th/234U with time
from Calsteren and Thomas, 2006]
Simplified scheme of Uranium circulation in hypergenic processes
Question
[Hercman and Goslar, 2002]
Recoil effect and leaching process results in non-mass dependent isotope
fractionation where the solid phase is depleted in (234U/238U) and the liquid
phase is enriched.
Scope
environmental science
oceanography
hydrology
science-based archaeology
magma chamber evolution and volcanic hazard prediction
global climatic change through dating of authigenic carbonate deposits
human evolution through dating of bone
groundwater evolution
Materials
Zircon
Plag, Amph, Cpx, Mt, (magmatic Ca-garnet)
corals
molluscan shells
carbonate cement
marine apatite
lacustrine carbonates (marl)
speleotheme, travertine
pedogenic silica and carbonate "caliche", "calcrete"
bones, tooth enamel
(peat)
ferruginous concretions and rinds
opal
ice, water
Secular equilibrium
For a crystal with 100 ppm U, this means:
Ultra-trace element analysis needed
[Schmitt, 2009]
Methods
(1) Bulk separates: partial and total dissolution
Chemical separation (column chemistry)
(2) In situ dating (single grains or rock chips in polished mounts)
Gamma spectrometry
Apha spectrometry
Solution ICP-MS
Laser-ablation ICP-MS
Thermal ionization mass-spectrometry (TIMS)
Synchrotron radiation X-ray microanalysis
SHRIMP
Data presentation
[Robinson et al., 2002]
[Ludwig and Paces, 2002]
Data presentation
[Ludwig and Paces, 2002]
Eruption dating
major pitfall: recycled zircon crystals
[Bacon and Lowenstern, 2005]
Question
Residence time of magma - correlation with the size of eruption
[Reid, 2008]
Marine reservoir for initial isotope ratio
Question
- U (VI) is relatively soluble
- seawater
3.3 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
Corals
(one line = one data)
[Potter et al., 2005]
Usage on lakes
(authigenic
carbonate)
[Calsteren and Thomas, 2006]
U-series dating of speleothemes
climate, earthquakes
Question
[Ford and Hill, 1999]
Speleothems
[Nordhof]
Question
[Eggins et al., 2005]
Recrystallized aragonite–calcite speleothems
[Ortega et al., 2005]
Recrystallized aragonite–
calcite speleothems
(nuclear microprobe
analysis)
[Ortega et al., 2005]
Archaelogy: Dating of calcite laminations covering paintings
(6300 years & also 29000 y. ago?)
[Aubert et al., 2007]
Dunes
(pedogenic carbonate)
Question
[Herczeg and Chapman, 1991]
Pedogenic silica and carbonate
Question
[Ludwig and Paces, 2002]
Pedogenic silica and carbonate
[Ludwig and Paces, 2002]
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
[Heijnis and van der Plicht, 1992]
Chemical sedimentation
[Lemoalle and Dupont]
Ferruginous concretions and rinds
("Bohnenerz", swamp iron pisoids)
[Short et al., 1989]
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
[Fireman; Goldstein et al., 2004]
Groundwater
[Gellermann et al., 1990]
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.
Water
Question
[Plummer et al., 2001]
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
[Gellermann et al., 1990]
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