Cosmic Abundance and Geochemical Principles

Posted on Jul 29, 2026 in Chemistry

1. Cosmic Abundance of Elements

The cosmic abundance of elements refers to the relative distribution and concentration of chemical elements throughout the universe. Determination of cosmic abundance relies primarily on stellar spectroscopy and the geochemical analysis of primitive meteorites (specifically CI carbonaceous chondrites).

Key Characteristics

  • Dominance of Light Elements: Hydrogen (H) and Helium (He) make up approximately 98% to 99% of all ordinary (baryonic) matter in the universe (H ≈ 74%, He ≈ 24%).
  • Exponential Decrease with Atomic Mass: As atomic number (Z) increases, cosmic abundance drops exponentially up to Z = 30.
  • The Lithium-Beryllium-Boron (Li-Be-B) Dip: Elements with atomic numbers 3, 4, and 5 (Li, Be, B) show remarkably low abundances compared to neighboring light elements due to their destruction inside stellar cores.
  • The Odd-Even Effect (Harkins’ Rule): Elements with even atomic numbers (Z) are systematically more stable and abundant than adjacent elements with odd atomic numbers.
  • The Iron Peak (Fe Peak): A pronounced peak in abundance occurs around Iron (Fe, Z = 26) and Nickel (Ni, Z = 28) because the nuclear binding energy per nucleon reaches its maximum near 56Fe.

Nucleosynthesis Processes

The relative abundances are a direct consequence of nucleosynthesis:

  • Big Bang Nucleosynthesis: Formed the initial mass of H, He, and traces of Li roughly 13.8 billion years ago.
  • Stellar Hydrogen & Helium Burning: Star cores fuse H into He, followed by the Triple-Alpha process (3 4He → 12C) and subsequent alpha-capture reactions producing oxygen (16O), neon (20Ne), and magnesium (24Mg).
  • Advanced Stellar Burning: Heavier elements up to iron are generated during carbon, neon, oxygen, and silicon burning stages in massive stars.
  • Neutron Capture (s-process & r-process): Elements heavier than iron (Z > 26) cannot form via exothermal fusion. They are created via slow (s-process in AGB stars) and rapid (r-process in supernovae/neutron star mergers) neutron capture.

2. Geochemical Cycles

A geochemical cycle describes the continuous pathways through which chemical elements circulate between the major reservoirs of the Earth system: the lithosphere, hydrosphere, atmosphere, and biosphere.

Structure of the Geochemical Cycle

Geochemical cycles are divided into two main components:

Primary (Endogenic / Deep Earth) Cycle

Driven by Earth’s internal heat and tectonic forces:

  • Magmatism & Igneous Crystallization: Mantle melting transfers elements to the crust via magma.
  • Metamorphism: High heat and pressure alter mineral compositions and re-mobilize elements.
  • Subduction: Surface sediments and oceanic crust return back into the mantle.

Secondary (Exogenic / Surface) Cycle

Driven by solar energy, gravity, and fluids at Earth’s surface:

  • Weathering & Erosion: Physical breakdown and chemical alteration of rocks release elements into soils and solutions.
  • Transport & Deposition: Rivers and winds transport solutes and particles to basins.
  • Sedimentation & Diagenesis: Burial and compaction convert loose sediments into sedimentary rocks.

Key Geochemical Parameters

  • Reservoirs & Stocks: Mass or volume of an element stored in a given Earth realm.
  • Flux (J): Rate of mass transfer between reservoirs per unit time.
  • Residence Time (τ): Average duration an element remains within a reservoir.

3. Composition of Planets

Planets in our Solar System are divided compositionally into two main groups based on distance from the Sun and volatile condensation temperatures.

Terrestrial (Rocky) Planets

  • Mercury: High metal-to-silicate ratio. Metallic core makes up ~60–70% of its total mass (Fe-Ni rich).
  • Venus: Dense metallic core (Fe-Ni), silicate mantle, and a basaltic silicate crust covered by a dense CO2 atmosphere.
  • Earth: Differentiated into an iron-nickel core (approx. 32.5% planet mass), silicate mantle (approx. 67.5%), and thin crust (Si-Al-Fe-Mg).
  • Mars: Richer in total iron and volatile sulfur than Earth. Features an Fe-Ni-S core and an iron-enriched mantle.

Jovian (Gas & Ice Giant) Planets

  • Gas Giants (Jupiter & Saturn): Composed primarily of Hydrogen (~75%) and Helium (~24%).
  • Ice Giants (Uranus & Neptune): Composed of heavy volatiles (water, ammonia, and methane) making up 60–70% of their mass, surrounding a small rocky core.

4. Geochemical Classification of Elements

In 1923, Victor Goldschmidt categorized elements into four primary groups based on their bonding behavior:

  • Lithophile “Rock-lovin”): High affinity for oxygen; form silicates and oxides (e.g., Li, Na, K, Mg, Ca, Al, Si).
  • Siderophile “Iron-lovin”): High affinity for metallic iron; concentrated in planetary cores (e.g., Fe, Ni, Co, Pt, Au).
  • Chalcophile “Sulfur-lovin”): High affinity for sulfur; form sulfides (e.g., Cu, Zn, Pb, Ag, S).
  • Atmophile “Gas-lovin”): Prefer volatile gas phases; concentrate in atmospheres (e.g., H, C, N, Noble gases).

5. Meteorite Classification

Meteorites are extraterrestrial bodies that offer insights into the early Solar System.

  • Stony Meteorites: Includes Chondrites (contain chondrules) and Achondrites (igneous textures).
  • Iron Meteorites: Consist of iron-nickel alloys; display Widmanstätten patterns.
  • Stony-Iron Meteorites: Includes Pallasites (olivine in metal) and Mesosiderites (impact breccias).

6. Chemical Bonding

Atoms bond to achieve stable electronic configurations. Types include:

  • Ionic: Electrostatic attraction between oppositely charged ions (e.g., NaCl).
  • Covalent: Sharing of electron pairs (e.g., Diamond).
  • Metallic: Sea of delocalized electrons (e.g., Copper).
  • Hydrogen: Weak attraction between H and electronegative atoms (e.g., Kaolinite).
  • Van der Waals: Weak, short-range secondary forces (e.g., Graphite).

7. Chromatography

A laboratory technique used to separate components from complex mixtures based on partitioning between a Stationary Phase and a Mobile Phase.

Applications

  • Gas Chromatography (GC): Analyzes volatile organic compounds and stable isotopes.
  • Liquid Chromatography (HPLC): Separates trace organic biomarkers and rare earth elements.
  • Ion Chromatography (IC): Used for water quality testing and hydrogeochemical analysis.