Evidence for Crystal Fractionation, Immiscibility and Crustal Contamination: 18-Geol-A1 Mineralogy and Petrology, Undated paper, Q3

Question 3: Evidence for Crystal Fractionation, Immiscibility and Crustal Contamination (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 more than one.

Diagnostic evidence by differentiation process
Process	Diagnostic evidence
(a) Crystal fractionation	Cumulate textures (orthocumulate/adcumulate crystal piles in layered intrusions); smooth, curved trends on major- and trace-element variation (Harker) diagrams consistent with Rayleigh fractionation of a single parent (systematic decrease of compatible Ni, Cr, MgO with increasing $\text{SiO}_2$); normal (core-to-rim) phenocryst zoning recording progressive liquid evolution; mineral assemblage and crystallization order consistent with Bowen's reaction series.
(b) Immiscibility	Blebby/globular texture — rounded droplets of one melt composition suspended within a second, texturally distinct, quenched-to-glass melt, rather than a single homogeneous liquid; a sharp compositional gap on variation diagrams, with no intermediate compositions between the two immiscible liquids (e.g. a late-stage Fe-rich vs. Si-rich split in residual tholeiitic melt); laboratory-determined miscibility gaps (solvi) in the relevant melt system reproducing the observed split; immiscible sulfide-melt droplets within silicate melt as the ore-deposit expression of the same process.
(c) Crustal contamination (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, because isotope ratios are essentially unaffected by fractional crystallization alone 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 variation-diagram trends + normal zoning + cumulates. Immiscibility → blebby texture + a true compositional gap (no intermediate liquids). Contamination/assimilation → isotopic shift toward crustal values + xenoliths.

Why this works. Immiscibility is diagnosed specifically by the absence of intermediate compositions — a true thermodynamic solvus in the melt system — which is exactly what distinguishes it from magma mixing (a continuum along a straight line) and from fractionation (a continuum along a curve); the shape of the variation-diagram trend, continuous vs. gapped, is the discriminating observation between all these processes, while isotopes specifically isolate crustal input because fractional crystallization alone cannot shift a radiogenic isotope ratio.

Common pitfall. Using major-element trends alone to "prove" crustal contamination; a curved major-element trend is equally consistent with ordinary 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, liquid immiscibility, AFC, isotopic tracers).

Question 3 of 12: Evidence for Crystal Fractionation, Immiscibility and Crustal Contamination

Question 3: Evidence for Crystal Fractionation, Immiscibility and Crustal Contamination (Part 1 – 10 marks)