Lecture Notes: Studying Earth's Crust through Rock Analysis

Purpose of Studying Rocks

• Understanding how Earth's crust formed over time.
• Implications for sedimentary basins, faults, and earthquakes.

Fieldwork and Sample Collection

• Collect rock samples that are unaltered and fresh.
• Label samples with GPS location and mark on the map for reference.
• Goal: Identify minerals, especially zircon.

Importance of Zircon

• Zircon contains uranium, useful for dating rocks.
• Uranium in zircon decays to lead over time.
• Measuring uranium and lead can reveal the age of the rock.

Formation of Zircon

• Zircon crystals crystallize when rock was molten magma.
• Provides information on the time of magma crystallization in Earth's crust.

Extraction Process

• Crush rock to extract zircon grains.
• Use FRANS magnetic separator to separate zircons from other minerals.
• Non-magnetic zircons separated from magnetic minerals.

Analysis of Zircon Crystals

• Use a microscope to view zircon crystals.
• Zircon crystals are small with sharp crystal edges and pointy tips.

Uranium-Lead Dating Method

• Select around 100 zircon crystals for analysis.
• Use laser ablation and mass spectrometer to measure uranium and lead isotopes.
• Determine the age of crystal crystallization based on uranium-lead ratio.

Principles of Radioactive Decay

• Zircon initially contains 100% uranium, no lead.
• Uranium decays to lead isotopes over time.
• High uranium, low lead = young rock; low uranium, high lead = old rock.

Uranium Isotopes for Dating

• Uranium-235: Half-life of 700 million years.
• Uranium-238: Half-life of 4.5 billion years.
• Long half-life of uranium allows dating of very old rocks.