Evidence for Crystal Fractionation, Magma Mixing and Crustal Assimilation: 18-Geol-A1 Mineralogy and Petrology, December 2018, Q3

Question 3: Evidence for Crystal Fractionation, Magma Mixing and Crustal Assimilation (Part 1 – 10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Each process leaves a distinct textural and geochemical fingerprint that lets a petrologist distinguish it from the other two, even though real magma systems often combine all three.

Diagnostic evidence by differentiation process
Process	Diagnostic evidence
(a) Crystal fractionation	Cumulate textures (orthocumulate/adcumulate crystal piles); smooth, curved trends on major- and trace-element variation (Harker) diagrams consistent with Rayleigh fractionation of a single parent (e.g. systematic decrease of compatible elements Ni, Cr, MgO with increasing SiO₂); normal (core-to-rim) phenocryst zoning recording progressive liquid evolution; mineral assemblage and crystallization order consistent with Bowen's reaction series.
(b) Magma mixing	Disequilibrium phenocryst textures — resorbed/embayed crystal margins, reverse (rim more primitive than core) zoning, two chemically distinct populations of the same phenocryst phase; mafic magmatic enclaves within a more felsic host; banded pumice or glass with two coexisting compositions; linear (rather than curved) trends on variation diagrams, since mixing two end-member liquids produces a straight mixing line, not a fractionation curve.
(c) Crustal assimilation	Partially resorbed xenoliths/xenocrysts of country rock; radiogenic-isotope shifts (elevated $^{87}\text{Sr}/^{86}\text{Sr}$, less-radiogenic $\varepsilon_{Nd}$, or anomalous $\delta^{18}\text{O}$) toward crustal values, since isotope ratios are essentially unaffected by fractional crystallization but are strongly shifted by bulk mixing with isotopically distinct crust; incompatible trace-element enrichment (K, Rb, Th) beyond what fractional crystallization alone predicts; curved AFC (assimilation–fractional-crystallization) trajectories on combined trace-element/isotope plots.

Topic. Distinguishing magmatic differentiation mechanisms from field, textural and geochemical evidence.

Key relations. Fractionation → curved trends + normal zoning; mixing → linear trends + disequilibrium/reverse zoning; assimilation → isotopic shift toward crustal values + xenoliths.

Why this works. Isotope ratios are powerful because fractional crystallization cannot fractionate radiogenic isotope ratios (only concentrations), so any isotopic shift in a comagmatic suite requires mixing with an isotopically distinct reservoir — typically crust; this is why isotopes and trace elements are used together (trace elements alone cannot distinguish assimilation from fractionation, since both can produce incompatible-element enrichment).

Common pitfall. Using major-element trends alone to "prove" assimilation — a curved major-element trend is equally consistent with fractional crystallization; isotopic or xenolith evidence is required to confirm crustal input specifically.

Source. Winter, Principles of Igneous and Metamorphic Petrology, 2nd ed., §7–8 (differentiation mechanisms, AFC, isotopic tracers).

Question 3 of 12: Evidence for Crystal Fractionation, Magma Mixing and Crustal Assimilation

Question 3: Evidence for Crystal Fractionation, Magma Mixing and Crustal Assimilation (Part 1 – 10 marks)