Using Isotopic Methods To Tell Geologic Time

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Using Isotopic Methods To Tell Geologic Time

• Jose Hurtado
• Department of Geological Sciences
• University of Texas at El Paso
• PNACP
• PLU, Tacoma WA
• March 24, 2006

Hourglass Rock by George Mendoza
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Overview

• Why do geologists need time information?
• What chemistry and physics can be used to tell
  time?
• What methodologies have been developed?

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Overview

• Case Study I Thermochronology
• Closure temperature and tectonics
• 40Ar / 39Ar in the Himalaya
• U - Th / He
• Case Study II Cosmogenic nuclide geochronology
• Surfaces and young deposits
• Surface exposure dating and paleoseismology

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Why Time?

• Geology is an historical science.
• Geology is process-oriented.
• Geology is increasingly impacting society.

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Chemical Physical Clocks

• Radioactive decay
• Nuclide production
• Dosimetry
• Calibrated processes

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Chemical Physical Clocks
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Chemical Physical Clocks
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Chemical Physical Clocks
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Chemical Physical Clocks
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Radioactive Decay
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Useful Decay Systems
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Case Study I Thermochronology
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Thermal Structure of the Crust Tectonics
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Thermal Structure of the Crust Tectonics
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Thermal Structure of the Crust Tectonics
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Diffusion Closure Temperature
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Diffusion Closure Temperature
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Diffusion Closure Temperature
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K-Ar and 40Ar/39Ar

• Depends on the decay of 40K to 40Ar in K-rich
  minerals (sanidine, micas, etc.).
• Applicable to 10 ka samples and older. Some
  success in dating historical events!
• Fundamentally dates thermal events related to the
  diffusion of Ar out of the system (closure
  temperature concept).

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Thakkhola Graben, Nepal
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Mustang Granite
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Structural Geology

• Early schistosity and isoclinal folding of
  intruding dikes suggests early N-S shortening
  (D1-D2).

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Structural Geology

• Progressively steeper increasingly non-coaxial
  fabrics indicative of later top-to-east shear
  (D3-D4)

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Structural Geology

• Late-stage brittle deformation on anastamosing
  normal faults of graben-bounding fault (D5)

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40Ar/39Ar Thermochronology

• Samples from Tibetan Sedimentary Sequence, Mugu
  granite, and Mustang granite for cooling ages.
• Age-elevation transect across Dangardzong fault
  within Mustang granite.
• Furnace step-heating of ms, bt, K-feldspar.
  Multiple minerals from the same sample.

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Exhumation History Method 1

• Divide cooling rate curve by an estimated
  paleo-geotherm.
• Based on P-T estimates of Guillot, et al. (1995),
  estimate 60 C/km.
• Resulting exhumation curve is linearly-scaled
  version of cooling history plot.
• Inflection points at 17.5 and 15.25 Ma and fast
  denudation in between.

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Cooling History Plot
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Exhumation History Method 2

• Use age-elevation transects
• Fit a line to T-t data get slope in mm/yr
• Results for Mustang granite samples
• ms 0.15 mm/yr between 18.3-17.4 Ma
• bt v. rapid after ca. 17.5 Ma
• kf (MSD) 0.79 mm/yr at ca. 15.25 Ma
• kf (MID) 0.05 mm/yr between 14.51-12.83 Ma

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Mustang ms 40Ar/39Ar age-elevation
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Mustang bt 40Ar/39Ar age-elevation
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Mustang kf (msa) 40Ar/39Ar age-elevation
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Mustang kf (mia) 40Ar/39Ar age-elevation
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Exhumation History Method 3

• Computational analysis (Moore England, 2001)
• Shape of T-t path with sufficient curvature is
  sensitive primarily to denudation rate (U)
• Calculate synthetic T-t curves by varying U and
  other parameters.
• Minimum misfit selection of best fit curve
• ca. 2.7 mm/yr post 17.5 Ma

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Exhumation History Plot
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U-Th/He

• Depends on decay of U and Th, both of which are
  complex decay chains that release ? particles (He
  nuclei).
• Powerful for young samples since He accumulates
  quickly.
• Powerful for low-temperature, near-surface
  studies due to low closure temperature for He
  diffusion in minerals like apatite (ca. 70 C).

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Case Study II Cosmogenic Nuclide Geochronology
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Cosmogenic Nuclides

• Earth is bombarded by high energy cosmic rays
  from deep space supernovae. In the atmosphere,
  these rays produce secondary cosmic rays
  (neutrons and muons).
• The secondary cosmic rays penetrate several
  meters into geologic materials at Earths
  surface, interacting with atoms in minerals.
• Results in the accumulation of long-lived
  radionuclides (e.g. 10Be, 26Al, 36Cl, 3He, 21Ne)
  in quartz, K-feldspar, calcite and olivine.

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Cosmogenic Nuclides

• Cosmogenic nuclides build up over time in a
  material exposed to the cosmic ray flux (upper
  few meters).
• Simultaneously, the nuclides are decaying.
• There is a steady state that develops after some
  amount of time (the target saturates).

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Cosmogenic Nuclides

• Production of nuclide decreases with depth into
  rock. Also dependent on latitude, Earths
  magnetic field, etc.
• Production rates are very slow (few atoms per
  gram per year)
• Lots of effort goes into quantifying them
• Low rates mean small amounts of cosmogenic
  nuclides produced gt need AMS to detect (few
  atoms per gram)!

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Cosmogenic Nuclides
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Cosmogenic Nuclides

• Build-up through time allows measurement of
  surface exposure ages.
• Competition between nuclide build-up and surface
  processes (e.g. erosion) allows measurement of
  process rates.
• Sequestration of material out of the nuclide flux
  allows measurement of burial ages due to the
  different decay rates of multiple nuclides from
  the same sample (or a depth profile of multiple
  samples).

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Exposure
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Erosion
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Burial
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The Future?

• Younger samples
• Higher precision in time
• Higher spatial precision
• Faster throughput
• Cheaper and more availability
• Geochron on mars and elsewhere? In situ?

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Thank You