ERB222 Chap.7 Magma Mixing, Assimilation and Layered Intrusions
Magma Mixing, Assimilation and Layered Intrusions
Layered intrusions as fractionation records
The Week 5 lecture uses layered mafic intrusions, above all Skaergaard in Greenland, as the clearest natural record of crystal fractionation.
Mineral proportions change in rhythmic layers, mineral chemistry shifts upward in cryptic layering, and sedimentary-like structures such as graded beds and trough cross-bedding show that density currents, not simple settling, built much of the floor.
Chambers that gain and lose material
Most magma chambers act as open systems.
Mingling stirs two magmas without producing a hybrid and leaves enclaves or banded pumice; mixing creates hybrids on straight lines between end members; assimilation coupled with fractional crystallisation uses latent heat from crystallising minerals to melt wall rock.
The lecture lists seven lines of evidence for mixing, from enclaves to mixing lines, and adds mush models in which crystals may be autocrysts, antecrysts or xenocrysts.
Plagioclase as the recorder
Workshop 6 turns those ideas into numbers.
A microprobe analysis is recalculated on eight oxygens to give An, a melt composition is converted with partition coefficients into the plagioclase it should grow, and zoning profiles plotted against distance from the rim are matched to melts to reconstruct events before eruption.
What this chapter covers
- 01
Layered mafic intrusions and cumulates
- 02
Rhythmic and cryptic layering
- 03
Magma mingling and enclaves
- 04
Magma mixing and linear arrays
- 05
Assimilation-fractional crystallisation
- 06
Autocrysts, antecrysts and xenocrysts
- 07
Plagioclase-melt equilibrium
- 08
Normal, reverse and oscillatory zoning
Worked example · free
Predict the plagioclase a melt should grow
- 1Apply Eq. 1 to calcium: X(Ca) = 0.070 x 1.3 = 0.0910.
- 1Apply Eq. 1 to the alkalis: X(Na) = 0.068 x 0.4 = 0.0272; X(K) = 0.012 x 0.1 = 0.0012.
- 1Sum the three: 0.0910 + 0.0272 + 0.0012 = 0.1194.
- 1Apply Eq. 2: An = 0.0910/0.1194 = 0.762, so An76.
Key terms
- Magma Mingling
- The physical stirring together of two magmas that stay chemically distinct, so that no hybrid composition forms.
- Magma Mixing
- The blending of two magmas of different composition until they form a single hybrid composition lying between them.
- Enclave
- A fine-grained mafic inclusion of magmatic origin hosted by an intermediate or silicic rock, carrying no implication of size or shape.
- Antecryst
- A crystal that grew during an earlier phase of related magmatism, in a mush or solid rock, before being carried by the host magma.
- Cryptic Layering
- An upward change in the chemical composition of minerals through a layered intrusion that is invisible without mineral analyses.
- Oxygen Renormalisation Factor
- The number of oxygens in a mineral formula divided by the summed moles of oxygen in an analysis, used to convert oxides to cations.
Magma Mixing, Assimilation and Layered Intrusions FAQ
What is the difference between magma mingling and magma mixing?
Mingling keeps the two magmas physically distinct, so you see enclaves with chilled, cuspate margins or streaky pumice but no intermediate chemistry. Mixing produces a hybrid, and its analyses fall on a straight line between the two end members. Enclaves prove at least mingling; hybrids are needed to claim mixing.
How much wall rock can a magma assimilate?
The Week 5 lecture puts the maximum at roughly 10 to 20 percent, because the heat available comes from latent heat released as the magma crystallises. Assimilation is more likely for hotter basalts than for granitic magmas, and it is only detectable when magma and wall rock differ strongly in chemistry or isotopes.
What does reverse zoning in plagioclase mean?
The crystal becomes more calcic toward its rim instead of more sodic. That usually means it met a hotter or more calcium-rich melt late in its growth, such as recharge by mafic magma. Check it against other evidence first, because the crystal could be an antecryst carried in from older mush.
Why can't simple crystal settling explain the layering at Skaergaard?
Plagioclase is lighter than the magma yet alternates in bands with olivine, which is denser, so density alone cannot sort them. Layering also appears on side walls, and graded beds and cross-bedding point to convection-driven currents carrying crystals across the chamber floor.
Exam move
Memorise the seven lines of evidence for mixing as a numbered list and pair each with one texture you could see in thin section. Run the eight-oxygen recalculation on two invented analyses until the cation total of 5 comes out first time. Then practise reading An against distance from the rim, saying aloud which end is the core before you classify the zoning.
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