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