Igneous Petrology Flashcards
This assumes you already know 1st Year Igneous Rocks
What are the technical names for the two branches in Bowen’s reaction series?
Continuous series (right)
Discontinuous series (left)
What does the continuous series represent (Bowen’s)?
The crystallisation of plagioclase feldspar from Ca-rich to Na-rich when temperatures decrease
When do the continuous and discontinuous series occur (in relation to each other)?
They both happen at the same time
What minerals are present on the discontinuous series of Bowen’s reaction series?
Olivine, Pyroxene, Amphibole, Biotite
What is the mnemonic to remember the discontinuous series?
Old People Are Boring
In the discontinuous series, what type of pyroxene is typically crystallised?
Augite
In the discontinuous series, what type of Amphibole is typically crystallised?
Hornblende
What does the discontinuous series represent? (Bowen’s)
The crystallisation if minerals rich in iron & magnesium (mafic).
What temperatures does olivine crystallise?
> 1500C
Why does the discontinuous series happen during cooling?
Slow cooling allows the minerals to react with the magma to form lower temperature minerals (e.g. olivine+magma+time=Pyroxenes)
How could olivine be preserved and why?
Fast cooling (eruption) could allow for preserve. There is not enough time for the olivine to react with the magma
What forms when a reaction is incomplete in the discontinuous series?
A reaction rim
What is a reaction rim?
One minerals surrounding another. Results from a reaction between the inner minerals with the melt, to form the outer mineral
What type of olivine forms first at higher temperatures?
Magnesium-rich (Forsterite)
At what temperatures does forsterite form?
1500C
What type of olivine forms last (out of the olivines) at lower temperatures?
Iron-rich (Fayalite)
What temperature does fayalite form?
1400C
Why does olivine crystallisation progress from Mg-rich to Fe-rich?
As the temperatures temperatures decrease, iron substitutes for magnesium
What type of plagioclase feldspar crystallisation first at higher temperatures?
Calcium-rich (Anorthite)
What type of plagioclase feldspar crystallisation first at lower temperatures?
Sodium-rich (Albite)
What temperatures does Anorthite to crystallise? (Bowen’s)
<1500C
What temperatures does Albite crystallise? (Bowen’s)
> 700C
In what type of rocks can you find Anorthite (Ca-rich)?
Ultramafic and mafic
In what type of rocks can you find Albite (Na-rich)?
Intermediate and sometimes silicic
How do zoned crystals form?
At high temperatures, crystallisation is mafic. As the magma cools, more layers are added to the crystal which may be more intermediate or silicic
What example of zoned crystals do we use?
Plagioclase feldspar. Ca-rich feldspar found in the centre and Na-rich feldspar found at the outer layers
When do the lower temperature minerals form in Bowen’s reaction series?
When the discontinuous and continuous branches merge (the bottom)
At what temperatures do the low temperature minerals start to form? (Bowen’s)
Around 700C (and lower)
What are the low temperature minerals? (Bowen’s)
Orthoclase (k-feldspar)
Muscovite
Quartz
What is the mnemonic used to remember the lower temperature minerals? (Bowen’s)
Oiled Male Queers
What is different about the minerals that remain below 700C? (Bowen’s)
They will not react with the remaining liquid
What type of minerals are remaining at temperatures below 700C? (Bowen’s)
Those rich in silica. Silicic/felsic
When does a eutectic system occur (melt conditions)?
When a melt is homogenous
When happens when crystals form in a homogenous melt?
The crystals are immiscible (don’t mix), and become non-homogenous to form two solid phases
How can a eutectic system be shown/illustrated?
On a eutectic phase diagram
What does the liquidus line tell us?
Everything above the line is a melt. Everything below the line is solid or mush
What is a mush?
Solid+melt. or crystals+melt
What is the liquidus?
A phase boundary showing the temperature that the last solid particle melt
What is the solidus?
A phase boundary showing the temperature a rock first begins to melt when heated
What is the eutectic point?
The point where all three phases could exist.
Solid, mush, or melt
What is the end composition like in a eutectic phase diagram?
The same as the original composition of the melt
What is the technical name for a ‘two-blob’ diagram?
Binary phase diagram
What is the technical name for a ‘one-blob’ diagram?
Solid solution phase diagram
Where do solid solution phase diagrams get their nickname from?
One-blob diagrams.
There is only one region for crystals+mush because there is only one type of mineral
How do you record the original composition using the new composition in a solid solution phase diagram?
Move down from the new composition until you reach the mush section.
Move horizontally to the other end of this section.
Then continue to drop down and record.
What is worth remembering about the X-axis in eutectic diagrams?
It measures from B
This is for solid solution and binary phase diagrams
What are batholiths?
Batholiths are bigger than plutons.
Batholiths are aggregates of plutons
What is a large igneous province?
A large accumulation of igneous rock, e.g. Batholith
What does LIP stand for?
Large igneous province
What reasons may a large igneous intrusion display different rock types?
Non-homogenous magma.
Different cooling rates.
Magma differentiation.
Intrusions of new magma.
Why would a non-homogenous magma result in a large igneous intrusion displaying different rock types?
There are different compositions within the magmas which crystallise into different rock types
Magmas are usually immiscible, so in what situations may they mix?
If there is a strong mixing mechanism. For example if there are strong convection currents
How does magmatic contamination occur?
Stoping and assimilation. Rocks entering magma through stoping are xenoliths until they are assimilated to then change the bulk composition
What are the three types of magmatic differentiation?
Fractional crystallisation, gravity settling, filter pressing
What is magma differentiation?
When there are different compositions and rock types found across one magma
What is fractional crystallisation?
When the composition changes over time. Different minerals form at different temperatures.
For example, Olivine and pyroxene crystallisation takes the mafic material out of the magma to form more silicic rocks.
What is gravity settling?
The solid version of any substance is denser than its liquid form. The denser material sinks to the bottom off the . chamber (gravity). It forms cumulate layers with the densest material at the bottom
What is filter pressing?
When the mass of overlying crystals squeezes the liquid out and forming a layer above. The more mass on top, the more pressure (less pore space), the more liquid layers
What is the composition of the liquids in filter pressing?
Silicic/felsic
What are the two real world examples of magmatic differentiation we study?
The Palisades sill.
The Skaergaard intrusion
Where is the palisades sill?
New Jersey, NE USA
What is different about the palisades sill from other sills?
Sills are usually known as small igneous intrusions, but the Palisades sill is 300m deep
How is gravity settling present in the Palisades sill?
There is an olivine-rich layer at the bottom
What rock is the chilled margin of the Palisades sill?
A fine basalt. It is the same composition as the original intrusion
Why does the basalt in the Palisades sill represent the composition of the original intrusion?
Because the crystals cooled too quickly for magma differentiation to occur here
What was the country rock around the Palisades sill?
Sandstone
What rock type is the baked margin of the Palisades sill?
Metaquartzite
How is fractional crystallisation present in the Palisades sill?
The olivine cooled first, which removed the ultramafic material from the magma, allowing for mafic minerals to form instead
What rocks are found in the middle of the Palisades sill?
Gabbro in the centre. Dolerite around this. These have the same composition but cooled at different types
When was the Skaergaard intrusion intruded?
The tertiary period
Where is the Skaergaard intrusion?
Greenland
What is the marginal border zone of the Skaergaard intrusion?
The chilled margin.
Finer grained basalt and has similar composition to the original melt.
Why is the chilled margin of the Skaergaard intrusion not exactly the same as the original composition?
It has been contaminated by assimilation of the country rock and alteration of hot liquids
What is the country rock of the Skaergaard intrusion?
Gneiss
What is the bottom layer of the Skaergaard intrusion?
The lower zone layered series
What is the lower zone layered series?
Here there are rhythmic layers of olivine underlying less dense layers of plagioclase. Cumulate layers
What magmatic differentiation processes are involved in the lower zone layered series?
Gravity settling (olivine)
Filter pressing
Fractional crystallisation (olivine + pyroxenes first)
Why are there alternating cumulate layers?
Density driven convection within the magma chamber
What is the upper border series?
Mirrors the lower zone layered series, but thinner
How did the upper border series form?
Early formed crystals are carried by convection currents and stick to the top of the chamber
Why is there a section of silicic rock withing the upper border series?
After the mafic material has crystallised, only the silicic magma remains. The last material to crystallise is the most evolved, most silicic and the lowest melting point materials.
What is special about the Bushveld igneous complex?
It contains some of the richest ore deposits on Earth and the largest reserves of platinum group elements
How big is the Bushveld igneous complex?
400 km x 800 km with a total volume of over 1,000,000 km^3.
In some areas, it is up to 9km thick
What is the Bushveld igneous complex?
A large layered igneous intrusion in South Africa
What are the platinum group elements?
Platinum, palladium, osmium, iridium, ruthenium, rhodium
What is the mnemonic for remembering the platinum group elements?
Rude People Really Irritate Old People
Why are platinum group elements rare in the crust?
Due to their iron-loving nature, they’re usually found in the core
What happens to platinum group elements in the absence of other minerals?
They react with sulphides and act like chalcophiles
Quartz
Hardness of 7. Framework tetrahedra. All oxygens are bridging. Vitreous. White/grey. No streak
Muscovite Mica
Light in colour. Breaks into sheets with a perfect cleavage in one directions. Sheet silica tetrahedra. 2-2.5 hardness
Biotite mica
Dark in colour. Perfect cleavage in one directions. Sheet silica tetrahedra. 2-3.5 hardness
Pyroxene
In igneous and metamorphic rocks.
Vitreous and dark. White streaks. 5-7 hardness. Mafic. Single chain silicate
Augite
Most common type of pyroxene. Near 90 cleavage. Brittle with a 5.5-6 hardness. Dark blue or green. White streak. Vitreous. Single chain silicate
Diopside
Type of pyroxene.
5-6 hardness. Green, black, yellow, brown. Two perfect cleavage. Vitreous. Single chain silicate
Amphibole
Dark coloured. Colours range. Chain silicate. Hornblende is a type of amphibole
Hornblende
Most common type of amphibole.
Black or dark green. Cleavage of 60 and 120. 5-6 hardness. Vitreous or dull
Orthoclase feldspar (K-feldspar)
Light in colour. Green, yellow, pink. White streak. Vitreous. Hardness 6-6.5. Two cleavages at 90
Plagioclase feldspar
White, grey. Hardness 6-6.5. White streak. Dominate mafic igneous rocks. Framework silicate
Olivine
Include forsterite and fayalite. No cleavage. Green, pale yellow. Ultramafic
Forsterite
Type of olivine. Magnesium rich. Green, yellow
Fayalite
Type of olivine. Iron rich. Pale yellow, amber. Cleavage at 10 and 100
