Geochemical Cycles and Element Redistribution

Core Idea

Planetary geochemical cycles redistribute elements between core, mantle, crust, atmosphere, and hydrosphere through volcanism, weathering, and outgassing. Incompatible elements preferentially concentrate in the crust; siderophile elements partition into the core. Comparing geochemical cycles across planets reveals how planetary size, composition, and thermal history shape element cycling and atmospheric composition.

How It's Best Learned

Compare elemental abundances across terrestrial planets. Use partition coefficients to model core-mantle differentiation.

Common Misconceptions

• Geochemical cycles are independent across planets; they are coupled—volcanic outgassing responds to interior cooling rates.
• Element distributions are static; they change over planetary evolution as cooling rates and outgassing patterns shift.

Explainer

From your study of planetary differentiation, you know that early in a terrestrial planet's history, heat from accretion and radioactive decay melts the interior, allowing dense metallic iron to sink toward the center and lighter silicates to float upward. This one-time separation creates the fundamental layered structure — core, mantle, crust — but it is not the end of the story. Geochemical cycles are the ongoing processes that continue to redistribute elements between these reservoirs over the lifetime of a planet, and comparing these cycles across worlds reveals how planetary size and thermal history control a planet's chemical evolution.

The key concept is element partitioning: different elements have chemical affinities that cause them to preferentially concentrate in certain reservoirs. Siderophile elements (iron-loving, such as nickel, cobalt, and the platinum-group metals) partition strongly into the metallic core during differentiation and are therefore depleted in the crust and mantle. Lithophile elements (rock-loving, such as potassium, uranium, and the rare earth elements) prefer silicate phases and concentrate in the crust. Incompatible elements — those whose ionic radius or charge makes them poor fits in common mantle minerals — are progressively extracted from the mantle into the crust with each episode of partial melting and volcanism. Over billions of years, this one-way transfer enriches the crust in elements like potassium, thorium, and uranium while depleting the mantle.

Volcanism is the primary engine driving geochemical cycles on terrestrial planets. When mantle rock partially melts, the resulting magma carries incompatible and volatile elements upward, delivering them to the crust and atmosphere through eruptions and outgassing. On Earth, this volcanic output is balanced by subduction, which returns crustal and sedimentary material back into the mantle, creating a true cycle. On one-plate planets like Mars and Venus, there is no subduction, so the transfer is largely one-directional: the mantle progressively loses its volatile and incompatible elements to the crust and atmosphere without significant return. This difference has profound consequences — Mars's mantle is thought to have become substantially depleted in water and other volatiles over time, contributing to the decline of volcanic activity and the thinning of its atmosphere.

Weathering adds another dimension on planets with atmospheres and hydrospheres. On Earth, chemical weathering of silicate rocks consumes atmospheric CO₂ and delivers dissolved ions to the ocean, where they are eventually buried as carbonate sediments — closing the long-term carbon cycle and regulating climate over millions of years. Venus, despite its dense CO₂ atmosphere, lacks liquid water and therefore lacks this weathering feedback, which may partly explain its runaway greenhouse state. Comparing geochemical cycles across the terrestrial planets — Earth's active, bidirectional cycling versus Mars's diminishing one-way degassing versus Venus's atmosphere-dominated system — demonstrates that a planet's size (which controls how long the interior stays hot enough for volcanism), its distance from the Sun (which governs surface temperature and the stability of liquid water), and its tectonic style collectively determine how elements are distributed and how atmospheres evolve over geological time.