Telling Time in the Geologic Record and Tracing Crustal Process: An Introdcution to

Radiogenic Isotopes

Jeremy Hourigan

EART205

10/27/2010

North America Stratigraphic

Code

Stratigraphic Correlation

• Lithostratigraphic

• Biostratigraphic

• Chronostratigraphic

• Magnetostratigraphic

• Chemostratigraphic (e.g. iridium anomaly at K-T boundary; stable isotope excursion at PETM)

Decay and Production • Decay Modes

– Beta (Negatron) (Rb-Sr, Lu-Hf, Re-Os) – Beta (Positron) – Alpha (Sm-Nd, U-Pb, (U-Th)/He) – Electron Capture (K-Ar) – Fission (Fission Track)

• Production

– COSMOGENIC

• (N,P) reaction (14C dating) • Spallation reaction (10Be,26Al)

– REACTOR-INDUCED

• Important for a variety geoanalytical techniques • Production of non-naturally occurring isotopes for “spikes” • Induced fission for Fission Track dating • Production of 39Ar from 39K for 40Ar/39Ar dating

Beta Decay

• Negatron (N P + e-)

– Neutron becomes a proton; b- particle (electron) expelled from nucleus

– Nucleus of daughter remains in excited state

– De-excites by emitting two gamma rays

EMgRb - b 224

12

24

11

Beta Decay

• Positron (P N + e+)

– Proton becomes a neutron; b particle expelled from nucleus + neutrino

– Nucleus of daughter remains in excited state

– De-excites by emitting gamma rays

EvOF b18

8

18

9

Electron Capture

• Electron from an extranuclear electron shell (usually from K shell) captured by nucleus

– e-+P N

EArKe

-

40

18

40

19

Frequency Reaction

89.52%

10.32%

0.16%

0.001%

Branched Decay

• 40K actually has 4 decay modes

EArK 40

18

40

19

EArK 40

18

40

19

EArK b40

18

40

19

ECaK - b40

20

40

19

Alpha Decay

• Alpha Particle

EHeThU 4

2

234

90

238

92

238U Decay Chain

EXHePbU - b6)(8 4

2

206

82

238

92

Nuclear Fission 252Cf Fission

nCdSnCf 1

0

117

48

132

50

252

98 3

•Daughter Products of fission decay are variable

•Crystal lattice damage caused by flight of massive fission

fragments through the crystal is what is measured in fission

track analysis, not a specific daughter isotope

Cosmogenic Production

• Foundation: “irradiation” of materials at or near the earths surface produces a suite of unstable radioisotopes with short half-lives. Exploited for chronometric purposed for near-surface processes

• Cosmic Rays consist of energetic H and He nuclei (protons and a-particles)

• Interaction with N2 and 02 in the outer atmosphere

neutrons, protons and muons

Radiocarbon

pCnN 1

1

14

6

1

0

14

7

• Assumption is that 14C achieves a steady-state equilibrium value

• Organisms equilibrate with the atmosphere and achieve steady-steady state

values; at death exchange ceases and 14C begins to decay

QNC - b14

7

14

6

Derivation Decay Equations

Ndt

dN-

Ndt

dN-

- dtN

dN

CtN - ln

0ln NC -

0lnln NtN --

tNN -- 0lnln

tN

N-

0

ln

teN

N -0

teNN - 0

Half-Life Equations

te -21

t-2

1ln

t2ln

2ln21 t

teNN - 0

Homework Question 1

• Solve the equation below for age in terms of parent / daughter ratio. Assume no initial daughter atoms.

teNN - 0

Mass Spectrometry

• Methods and Instrumentation • Instrumentation

– TIMS – SIMS – ICP-MS

History

• 1899-1911 J.J. Thomson (Cambridge) – Development of 1st Mass spectrometer

• 1918 Dempster Electron Ionization and Magnetic Focusing

• 1919 Aston – Atomic Weights using mass spectrometry

• 1946 Stephens – Time-of-flight Instruments

• 1953 Johnson and Nier – Reverse Geometry Double Focusing Instruments

• 1953 Paul and Steinwedel – Quadrupole Analyzers

• 1980 Houk et al. – ICP-MS matures based on work by Fassel and others (1960s)

Chemical Methods

• Isotope Dilution

• Column Chemistry

– Concentration (U/Pb)

– Mitigate for isobaric interference (Rb/Sr) that are irresolvable

Isotope Dilution

Column Chemistry

Requires Substantial Calibration

Example:

Trace Rb ionizes well before Sr

But, presence of Ca reduces Rb ionization

efficiency

Ca must be removed by column chemistry to

mitigate isobaric interference

Instrumentation

– TIMS

– SIMS

– ICP-MS

Single-Focusing Magnetic Sector Mass Spectrometer

Thermal Ionization Mass Spectrometry

• Gel of concentrated isotopic material dried onto filament

• Filament is heated strips electrons (i.e. ionization)

• Ions separated based on m/q

• Different Elements Ionize at different temperatures

SIMS • Secondary Ionization Mass Spectrometry

1. Beam of Primary Ions

focused onto a sample

surface

2. Primary beam sputters

material from the sample

surface

3. Positive Ions are

extracted

• High Spatial Resolution

(~30um spots)

• Fewer Ions counted

relative to TIMS so lower

precision

Inductively Coupled Plasma Mass Spectrometry

U-Th-Pb Decay Equations

)1(

)1(

)1(

232

235

238

204

232

204

208

204

208

204

235

204

207

204

207

204

238

204

206

204

206

-

-

-

t

i

t

i

t

i

ePb

Th

Pb

Pb

Pb

Pb

ePb

U

Pb

Pb

Pb

Pb

ePb

U

Pb

Pb

Pb

Pb

207Pb/206Pb Age

)1(

)1(

)(

)(238

235

238

235

204206204206

204207204207

-

-

-

-t

t

i

i

eU

eU

PbPbPbPb

PbPbPbPb

• Solve iteratively for t

• Requires correction for “Common Pb”

• Get initial ratios from a comagmatic feldspar

• Use a model common Pb composition (e.g. Stacey-Kramers (1975))

Common Pb Correction:

206/204 207/204 208/204

Present 18.703 15.629 38.623

250 Mybp 17.918 15.584 37.704

Concordia Diagrams Wetherill Tera-Wasserburg

Generally for ages >0.5-1.0 Ga Good for Precambrian to Phanerozoic Discordia

Generally for ages <0.5-1.0 Ga Good for showing trace Pbcommon

Parrish and Noble,2004

Homework Question 2

• Using Excel generate a Tera-Wasserburg concordia diagram.

Concordance

• Data fall on concord within uncertainty

• Decay constant errors

• Note correlated errors (sloping error ellipses)

Schmitz and Bowring (2003)

Discordance

• Data fall off concord along a chord

• If the data define a linear array then it is called a discordia line.

Sources of Discordance

• Pb loss – Diffusive or Chemical exchange within “metamict” (glassy, radiation damaged) regions of a crystal

• Multi-component mixture – e.g. Igneous core + metamorphic rim

Mitigating Discordance

• Magnetic Separation (Krogh, 1970s)

• Mechanical Abrasion (Krogh and others, 1980s)

Mitigating Discordance –Chemical Abrasion CA-TIMS

• 48 hour, ~1200 C annealing • Stepwise dissolution in a series of increasingly

aggressive leach steps

CA-TIMS & Microbeam Analytical Techniques (SIMS, LA-ICP-MS)

The isochron

Y =mx +b

Isochron Ages of Granites

Rb-Sr and Meteroites

“Seeing Through” Metamorphic Events

“Seeing Through” Metamorphic Events

Radiogenic Isotope Systems as Tracers

Sm-Nd Geochronometry: Model Ages

TDM Model age indicating timing of separation of rock from a depleted mantle reservoir

TCHUR Model age indicating timing of separation of rock from a Primitive mantle reservoir

Differentiation enriches the crust in radioactive elements

The Earth’s Reservoirs

Isotopes provide a tool to trace processes occurring both within and between these reservoirs

Sm-Nd Geochronometry: Model Ages

Radiogenic Isotope Systems as Tracers

Igneous Petrogenesis

The Continental Crust

Plenary Lecture

EART 205

Bimodal distribution of crust distinguishes Earth from other planets

in our solar system

Composition of the Crust

• Continental

– Lower Density (~2.65 g/cm3)

– “Andesitic”

– Becomes weak at lower temperature

– Complex, Old

• Oceanic

– Higher Density (~2.85 g/cm3)

– “Basaltic”

– Rigid, Strong

– Young, Simple

Age and Nature of the Oceanic Crust

Young (<180 Ma) and “Simple”

Age and Nature of the Continental Crust

Old and Complex

Crustal Thicknesses

Continental Crust Facts

• 41% of the Earth’s Surface (~25% lies below sea level)

• 200 m isobath is the continent-oceanic crust dividing line

• Thickness – Average 36 km (range 10 - 80 km)

• Volume 7.35 E+9 km^3

• Mass 2.06 E+25g (+/-7%)

– That’s 0.54% of the silicate earth

– And 0.33% of the whole Earth

– But 40% of the K (radiogenic)

The Andesite Paradox (Rudnick, 1995)

• The bulk composition of the average continental crust is that of Andesite.

• However, melting of mantle peridotite produces basalt

Hypotheses for Dealing with the Crustal Composition Paradox

• Intermediate composition melts produced early in the Earth’s history (Archean TTGs) due to slab melting

• Intracrustal differentiation followed by Delamination

• Sedimentary reprocessing

• Complemetary cumulates exist in the mantle

Differing Magma Generation Regimes in the geologic past

Taylor and Mclennan, 1985

How has the crust grown

• dMcc/dt = 0

1.Constant Volume since ~3.6 Ga (e.g. Armstrong et al., 1981)

• dMcc/dt > 0

1.Gradual Growth by Lateral Accretion of Island Arcs (geologically motivated)

2.Episodic Growth (e.g. Condie, 1998)

CRUSTAL GROWTH CURVES

Bowring and Housh, 1995

Is preservation equal to

growth?

Resolution relies on isotopic

tracers of magmatic

processes

And studies on the present

day mass balance

EPISODICITY VS. GRADUAL ACCUMULATION

Hawkesworth

and Kemp,

2007 (Nature)

EPISODICITY VS. GRADUAL

ACCUMULATION

Hawkesworth

and Kemp,

2007 (Nature)

Crustal Mass Balance

Gains

• Magmatic Addition – Mantle-derived melts in

continental arc settings

– Mantle-derived magmatism in island arcs and oceanic plateau (requires preservation at the convergent margin

– Intracontinental underplating

• Sediment Accretion

Losses

• Subduction Erosion

• Sediment Subduction

• Delamination

Accretionary vs. Erosive Margins

Subduction Tectonics

• Er

Clift and Vannucchi

Accretionary Margin

Erosive Margin

“Subduction Erosion”

Accretionary Margin - The Largest – Makran Pakistan

Makran accretionary complex is huge

Cascadia

From Chapter 22, Van der Puijm and

Marshak, 2005d

Cascadia

Nankai Trough – Seismic insight into accretion

Moore et al., 2007, Science

Erosive Margins

Subduction Erosion

Ranero and von Huene 2000

Erosive Margins

• Magmatic Productivity dominates the inputs in the crustal mass balance

• Sediment accretion is minimal

• Subduction Erosion and Sediment Subduction are roughly balanced

Clift and Vannucchi, 2004 Reviews of Geophysics

Delamination - Sierras

Delamination

Zandt et al, 2005 (Science)

Delamination

Zandt et al, 2005 (Science)

Delamination expressed in the Geomorphology