Evidence for Crystal Fractionation, Liquid Immiscibility and Crustal Assimilation: 18-Geol-A1 Mineralogy and Petrology, December 2019, Q3

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

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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 (e.g. systematic decrease of compatible Ni, Cr, MgO with increasing SiO₂); normal (core-to-rim) phenocryst zoning recording progressive liquid evolution; mineral assemblage/crystallization order consistent with Bowen's reaction series.
(b) Liquid immiscibility	Blebby/globular texture — rounded droplets of one melt composition suspended within a second, texturally distinct melt (quenched to glass), rather than a single homogeneous liquid; a sharp compositional gap on variation diagrams (no intermediate compositions between the two immiscible liquids, e.g. a Fe-rich vs. Si-rich split in late-stage tholeiitic residual melts), unlike the continuous curve of fractionation or straight mixing line of magma mixing; laboratory-determined miscibility gaps (solvi) in the relevant melt system reproducing the observed compositional split; immiscible sulfide-melt droplets within silicate melt (magmatic Ni-Cu-PGE sulfide deposits) as the field/ore-deposit expression of the same process.
(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 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 compositional gap (no intermediate liquids). Assimilation → isotopic shift toward crustal values + xenoliths.

Why this works. Immiscibility is diagnosed by the absence of intermediate compositions (a true miscibility gap in the melt system), which is exactly what distinguishes it from mixing (which produces a continuum along a straight line) and from fractionation (which produces a continuum along a curve) — the shape of the variation-diagram trend, continuous vs. gapped, is therefore the discriminating observation between all three processes.

Common pitfall. Confusing liquid immiscibility with magma mixing; mixing blends two miscible, chemically distinct magmas into intermediate compositions, whereas immiscibility is the spontaneous unmixing of a single originally-homogeneous melt into two liquids that will not remix, governed by a true thermodynamic solvus.

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, Liquid Immiscibility and Crustal Assimilation

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