Geochemistry of Rocks I – Melting and Element Partitioning

These flashcards cover key concepts from the Geochemistry of Rocks lecture, focusing on melting, element partitioning, thermodynamics, and mineral composition.

What does enthalpy (H) describe?

Energy in the form of heat, with positive ΔH as endothermic and negative ΔH as exothermic.

What is entropy (S) in a system?

A measure of the disorder of that system.

What does Gibbs free energy (G) indicate?

The amount of energy available in a system to do work, with a preference for lower G.

What is the significance of ΔG in chemical reactions?

If ΔG = 0, the reaction is spontaneous; ΔG > 0 is not favorable; ΔG < 0 is favorable.

What principle does Hess' Law illustrate?

Changes in Enthalpy/Entropy during a reaction.

What does chemical potential (μ) measure?

The change in G relative to change in system composition.

What do equilibrium conditions tell us about chemical potential?

At equilibrium, the chemical potential of any one component within a phase must be the same in every other phase.

What are the most abundant elements in the Earth by mass?

Fe, O, Si, Mg, S, Ni, Ca, Al, Na.

How does melting affect elements differently?

Compatible elements leave late in melting, while incompatible elements leave early.

What does KD represent in geochemistry?

The partitioning coefficient of elements between minerals and melts.

What happens to REE concentrations during melting?

The steeper the REE gradient, the lower the degrees of melting.

What is a 'spidergram' in geochemistry?

A plot that shows how trace element concentrations vary from a standard, such as primitive mantle.

What is the main difference between batch and fractional melting?

Batch melting is set in closed systems with equilibrium, while fractional melting allows for immediate extraction without equilibration.

What role do REEs play in petrogenesis?

They serve as indicators of depth and melt degree due to partitioning into different minerals.

What is the principle behind Goldschmidt's rules?

Trace elements can substitute into major phases if their ionic radii differ by <15% and have the same charge or a charge difference of ±1.