ERB222 Chap.6 Fractional Crystallisation Modelling
Fractional Crystallisation Modelling
Crystals that leave change the liquid
Fractional crystallisation modifies a magma by growing crystals and then separating them from the melt, by settling or by compaction of a crystal mush that squeezes interstitial liquid out. The Week 4 lecture follows a liquid through the An-Di-Fo system: olivine first, then olivine with diopside along a cotectic, then anorthite at the eutectic.
Each removal pushes the remaining liquid directly away from the composition of the crystals being taken out.
Reading the record on variation diagrams
The path an evolving liquid follows is its liquid line of descent, and Harker diagrams plot it as oxides against SiO2. MgO falls as olivine and pyroxene remove magnesium, while elements that no crystallising phase accepts climb.
When a new mineral joins, an oxide that was rising can turn over, because compatibility depends on the assemblage crystallising at that moment.
A suite's parental magma sits at the high-MgO, low-SiO2 end.
Naming the series with AFM
Workshop 1 sorts sub-alkaline suites with the AFM triangle, built from alkalis, iron and magnesium recast to a total of 100. A suite whose array bulges toward the F apex above the dividing line shows iron enrichment and is tholeiitic; one that stays mostly below the line is calc-alkaline.
Problem-Solving Task 2 then asks for two magmatic processes explained in one page, so these plots are your evidence.
What this chapter covers
- 01
Crystal removal by settling and mush compaction
- 02
Liquid line of descent
- 03
Harker variation diagrams
- 04
Compatible and incompatible trends
- 05
Inflections where a new phase joins
- 06
Parental and derivative magmas
- 07
AFM end-member calculation
- 08
Tholeiitic versus calc-alkaline arrays
Worked example · free
Recast one andesite onto the AFM triangle
- 1Sum the four oxides: 3.2 + 1.1 + 7.4 + 3.3 = 15.0 wt%.
- 1A = (3.2 + 1.1)/15.0 = 28.7%, using both alkalis.
- 1F = 7.4/15.0 = 49.3% and M = 3.3/15.0 = 22.0%; the three total 100.0%.
- 1A tholeiitic suite needs an array that is concave downward and rises above the dividing line toward F; one point cannot decide the series.
Key terms
- Liquid Line of Descent
- The route an evolving magmatic liquid follows through composition and pressure-temperature space as crystals form and are separated from it.
- Parental Magma
- The magma in a suite from which the others descend, with the highest liquidus temperature and the most primitive composition of the group.
- Harker Diagram
- A variation diagram that plots oxides of a related rock suite against SiO2 to display changes between parental and derivative magmas.
- AFM Diagram
- A ternary plot of alkalis, total iron as FeO and MgO, recast to 100, used to separate tholeiitic from calc-alkaline sub-alkaline suites.
- Iron Enrichment Trend
- An early rise in iron relative to magnesium in an evolving liquid, the defining signature of a tholeiitic suite such as many island arc series.
Fractional Crystallisation Modelling FAQ
How do you tell a tholeiitic suite from a calc-alkaline suite on an AFM diagram?
Plot every sample from the suite rather than one rock. If the array climbs toward the iron apex, bending concave downward above the dividing line before swinging toward the alkali corner, it is tholeiitic. If it runs from the magnesium side toward alkalis and stays mostly below the line, it is calc-alkaline.
Why does CaO rise and then fall on a Harker diagram?
While olivine crystallises alone it takes almost no calcium, so CaO is concentrated in the shrinking liquid. Once clinopyroxene or calcic plagioclase starts crystallising, calcium becomes compatible and CaO declines. The turning point marks the composition at which that calcium-bearing mineral joined the assemblage.
Can fractional crystallisation of basalt make large volumes of granite?
Not on its own. Bowen's model yields only about one twentieth as much granite as the starting basalt volume, and it would leave huge ultramafic cumulate masses at the base of the crust. Lower crustal xenoliths are granulites instead, and seismic data do not show those masses, so crustal melting is also needed.
Exam move
Redraw the six-sample Harker plot from memory, then explain each line in one sentence that names the mineral responsible. Practise AFM arithmetic until you can recast four oxides in under two minutes, always checking the total of 100. For Problem-Solving Task 2, pick two processes, attach one figure to each, and test yourself on spotting inflections in TiO2, P2O5 and CaO plots.
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