Evidence for Crystal Fractionation, Magma Mixing and Crustal Assimilation: 18-Geol-A1 Mineralogy and Petrology, December 2016, Q4

Question 4: 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 distinctive petrographic and/or geochemical fingerprint, because each removes or adds material to the liquid line of descent by a physically different mechanism.

Diagnostic evidence for the three differentiation processes
Process	Petrographic/textural evidence	Geochemical evidence
(a) Crystal fractionation	Cumulate textures (adcumulate/orthocumulate); normal (core-to-rim) phenocryst zoning recording a progressively evolving liquid; modal mineralogy consistent with a removed cumulate assemblage.	Smooth, continuous compositional trends on Harker (variation) diagrams that track a single liquid line of descent; trace-element signatures diagnostic of the removed phase — e.g. a negative Eu anomaly and falling Sr from plagioclase removal, or falling Ni/Cr from olivine/pyroxene removal.
(b) Magma mixing	Disequilibrium textures: resorbed/embayed phenocrysts, reverse (rim-more-mafic) or oscillatory/patchy zoning, mineral assemblages out of equilibrium with each other (e.g. quartz + olivine in the same rock), mafic magmatic enclaves recording incomplete mingling.	Compositions that fall on a straight mixing line between two end-member magmas on bivariate element plots, rather than the curved trend a single liquid line of descent produces; bimodal populations of otherwise-identical phenocrysts with distinct compositions.
(c) Crustal assimilation	Partially digested xenoliths/restite; xenocrysts (e.g. inherited zircons, often with older, discordant ages) out of equilibrium with the host magma; reaction rims on assimilated fragments.	Radiogenic/incompatible-isotope ratios shifted toward crustal values (elevated $^{87}\text{Sr}/^{86}\text{Sr}$, lower $\varepsilon_{\text{Nd}}$) that correlate systematically with the degree of fractionation, tracing a curved AFC (assimilation–fractional-crystallization) trend on an isotope-vs-SiO$_2$ plot rather than the flat trend a closed, uncontaminated fractionating system would show.

Topic. Distinguishing the physical processes of magmatic differentiation from their rock/mineral record.

Key relations. Fractionation → smooth trace-element/Harker trends; mixing → disequilibrium textures + linear mixing arrays; assimilation → isotopic shift toward crustal values, often coupled to fractionation as AFC.

Why this works. Each process changes bulk magma composition by a physically distinct route — removing a solid (fractionation), combining two liquids (mixing), or adding a solid/melt of different isotopic character (assimilation) — and isotopes in particular are powerful discriminators because they are essentially unaffected by fractional crystallization alone (crystal/liquid isotopic fractionation is negligible at magmatic temperatures for Sr/Nd), so any isotopic shift up-section must reflect open-system contamination rather than closed-system fractionation.

Common pitfall. Using a Harker diagram alone to diagnose assimilation — major-element trends from AFC can closely mimic pure fractional crystallization; isotopic or xenolith/xenocryst evidence is required to distinguish the two unambiguously.

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

Question 4 of 13: Evidence for Crystal Fractionation, Magma Mixing and Crustal Assimilation

Question 4: Evidence for Crystal Fractionation, Magma Mixing and Crustal Assimilation (Part 1 – 10 marks)