Lecture Notes: Studying Earth's Crust and Zircon Dating

Importance of Studying Earth's Crust

• Understanding crust formation over time.
• Implications for sedimentary basins, faults, and earthquakes.

Field Sampling Process

• Importance of collecting fresh, unaltered rock samples.
• Documentation:
  • Label samples.
  • Record GPS location.
  • Mark location on the map.

Studying Mineral Composition

• Focus on identifying zircon within rock samples.
• Zircon characteristics:
  • Small mineral with tiny crystals.
  • Contains uranium, which decays into lead over time.

Zircon and Radioactive Decay

• Zircon allows for uranium-lead (U-Pb) dating:
  • Uranium decays to lead at a known rate (half-life).
  • Measurement of uranium and lead allows age determination.
• Crystallization process:
  • Zircon forms when rock crystallizes from magma deep within Earth's crust.

Laboratory Process

• Crushing rock to extract zircon grains.
• Separation technique using FRANS magnetic separator:
  • Separates magnetic minerals from zircon.
• Analysis using microscope to study zircon crystals:
  • Small size, well-formed edges, and pointy tips.

Uranium-Lead Dating Method

• Selection of approximately 100 zircon crystals for analysis.
• Use of laser ablation and mass spectrometry:
  • Measures uranium and lead isotopes.
  • Determines the uranium-lead ratio.
• Principle of radioactive decay:
  • Initial zircon contains 100% uranium.
  • Over time, uranium decays to lead.
  • Ratio indicates age: More uranium signifies younger rock, more lead signifies older rock.

Uranium Isotopes in Dating

• Two isotopes used:
  • Uranium-235 with a half-life of 700 million years.
  • Uranium-238 with a half-life of 4.5 billion years.
• Long half-lives allow for dating of very old rocks using the uranium-lead method.