Trace elements (concentrations <0.1 wt%) obey Henry's Law in dilute solutions within minerals and melts, meaning their behavior is governed by partition coefficients rather than stoichiometric constraints. Elements are classified as compatible (preferentially entering the solid phase during melting, e.g., Ni, Cr, Co) or incompatible (preferentially entering the melt, e.g., Rb, Ba, Th, U, Nb, La). During partial melting, incompatible elements are strongly concentrated in small melt fractions, while compatible elements remain in the residue. This partitioning creates systematic abundance patterns in igneous rocks that encode information about the degree of melting, source composition, crystal fractionation history, and tectonic setting. Normalized multi-element diagrams (spider diagrams) and REE plots are the standard visualization tools.
Trace elements are the fingerprints of geological processes. While major elements (Si, Al, Fe, Mg, Ca, Na, K) define the rock type (basalt vs. granite), trace elements resolve the processes that produced it: how much the mantle melted, what minerals crystallized from the magma, whether subducted sediment or fluids were involved, and what was left behind in the source.
The partition coefficient D (concentration in mineral / concentration in melt) is the fundamental parameter. For olivine, D_Ni ~10-30 (nickel is strongly compatible), while D_La ~0.001 (lanthanum is strongly incompatible). During partial melting, an element with D << 1 concentrates almost entirely in the melt, especially at small melt fractions. An element with D >> 1 remains locked in the residual solid. The batch melting equation, C_melt = C_source / (D + F(1-D)), quantifies this for any element and any melt fraction F.
Multi-element diagrams normalize trace element concentrations to a reference (primitive mantle, chondrite, or N-MORB) and plot them in order of decreasing incompatibility. The resulting pattern contains enormous information. A smooth, downward-sloping pattern indicates derivation from a depleted mantle source by moderate to large degrees of melting (like N-MORB). A steep, enriched pattern indicates small-degree melting of an enriched source (like ocean island basalts). Negative anomalies at specific elements (Nb, Ti depletion in arc basalts; Eu anomalies in plagioclase-fractionated rocks) diagnose specific mineral controls.
The power of trace element geochemistry lies in the fact that different elements respond differently to the same process: a single melting event simultaneously enriches Ba 100-fold while barely affecting Yb. This differential behavior creates patterns that constrain not just whether melting occurred, but the degree, depth, and residual mineralogy of the melting event -- information that major elements alone cannot provide.