geochem 8

Introduction to Geochemistry and Isotope GeologyLecture Overview

• Topics:

  • Introduction to geochemistry

  • Stable isotopes

  • Mass-dependent fractionation

  • Stable isotopes of natural waters

  • Climate reconstruction using stable isotopes

Learning Objectives

• Differentiate between stable and unstable isotopes

• Utilize δ notation for isotope variations

• Understand stable isotope composition variations in natural waters related to latitude and water type

• Basic geothermometry concepts

• Familiar with terminology: mass-dependent fractionation, stable isotopes, SMOW (Standard Mean Ocean Water)

Isotopes Overview

• Definition: Isotopes are forms of the same chemical element with varying neutron numbers but identical proton counts.

• Characteristics of Isotopes:

  • Identified by atomic number (Z); changes in Z denote changes in element identity

  • Mass number (A) may vary, especially in heavier elements

  • Examples: Potassium isotopes (39K, 40K, 41K) all have Z = 19

Stable Isotopes

• Mass-Dependent Fractionation:

  • Involves changes in isotope ratios due to physical processes

  • Typically subtle differences compared to elemental fractionation

  • Significant fractionation occurs between isotopes with large mass differences

  • Example: Hydrogen (H) and Deuterium (D) show 100% mass difference, leading to significant fractionation

  • Example: Iron isotopes (54Fe and 56Fe) show only slight fractionation due to 4% mass difference

• Defining Stable Isotope Ratios

  • Equation: δ_sample = (R_sample - R_standard) / R_standard * 1000

    • R = isotope ratio (e.g., 13C/12C) - heavier isotope is in the numerator

  • Units: Measured in ‰ (per mil)

  • Commonly Used Ratios:

    • 13C/12C, D/H (Deuterium to Hydrogen), 18O/16O, and other elements like S and N

Stable Isotopes of Water

• Hydrogen Isotopes

  • H (1H) - more abundant

  • D (Deuterium, 2H) - less abundant

• Oxygen Isotopes

  • 16O - most abundant

  • 17O - least abundant

  • 18O - another isotope

• Water Composition

  • H2 16O is light and most prevalent

  • DHO (heavy water) and H2 18O (heavy) also exist

• Mass-Dependent Fractionation of Water

  • Composition:

    • H2 16O: molar mass ~18 g/mol (light)

    • DHO: molar mass ~19 g/mol (heavy)

    • H2 18O: molar mass ~20 g/mol (heavy)

  • Evaporation and Precipitation Factors:

    • Light isotopes evaporate more readily

    • Altered ratios between light and heavy isotopes depend on environmental conditions

Classification of Natural Waters

• Types of Water:

  • Seawater: saline water from oceans

  • Meteoric Water: from precipitation cycles - including rain, snow, and rivers

  • Geothermal Water: heated meteoric water circulating through rocks

  • Formation Water: water in sediment pore spaces

  • Magmatic Water: derived from cooling magma

• Isotopes in Meteoric Water

  • Vapor Properties:

    • Vapor is lighter than liquid water

    • Concentration of light water occurs from repeated precipitation

    • Movement of vapor toward poles amplifies this effect

• Oxygen Isotopes in Geological Materials

  • Mantle δ18O

    • Mantle has a δ18O value of 5.7 ‰

    • Igneous processes can increase δ18O levels

    • Calcite formed from seawater shows high δ18O levels

    • Water interaction with rocks generally increases δ18O but has less effect on δD

Climate Reconstruction Using Stable Isotopes

• Temperature Dependence:

  • Stable isotope fractionation varies with temperature, useful in geothermometry

  • Example: O isotope fractionation between calcite and water inferred with temperature adjustments

Summary and Objectives

• Aspects to Understand:

  • Measurements of isotopes using mass spectrometry

  • Applications of radioactive and radiogenic isotopes in geochronology and petrogenesis

  • Understanding radioactive decay equations, decay constants, and half-lives

  • Impact of trace elemental behavior during magmatic crystallization

  • Interpretation of isotope evolution diagrams

  • Effects of physical processes on stable isotopes and their implications for the water cycle and geothermometry.