Igneous Petrology Flashcards
(27 cards)
What is magma and what determines its properties?
A multiphase mixture of melt and volatiles with a liquid-like texture due to structure of network formers. The amount of network formers determine solidity and malleability
What are the most common magmas from high to low viscosity?
Rhyolite, dacite, andesite, basalt, komatiite
What is the isobaric phase rule?
Phases present + degrees of freedom = components present + 1
What is the lever rule?
The proportion of A in a substance = A/A+B
A is measured from solid to bulk for liquid, as the solid line is 0% liquid
How does cooling affect mineral size & texture?
Faster cooling creates more, smaller crystals
Slower cooling creates coarse grains
Rapid cooling creates fine grains
What is a congruent system?
A system involving the melting of a solid into a liquid with the exact same composition
What is an incongruent system?
A system involving the multi-stage transition of solid to liquid, with changes in composition along the way so that compounds don’t reach the liquidus directly
What happens at the peritectic point?
A solid reacts with a liquid to produce a different solid phase and a new liquid
How do crystals separate in a closed incongruent system?
Gravity settles crystals at the bottom of the melt due to density, or buoyancy lifts melt above the minerals
What is the difference between magma mixing and mingling?
In magma mixing, 2 liquids coexist and exchange mass without becoming homogenised
In magma mingling, 2 liquids are present without interacting
What are cumulates and on which part of the liquid line of descent do they form?
Cumulates are crystals formed from excess minerals in a parent melt. They form to the right of the LLD
What does the partition coefficient represent?
The ratio of concentration in coexisting phases for any trace element and 2 phases
How is the partition coefficient interpreted?
When Kd > 1, the crystal phase is preferred
When Kd < 1, the melt phase is preferred
When multiple Kd > 1, the highest value is the most compatible
What causes the flux of basalt dominated magma from the mantle?
Either the total melting of basaltic rock (eclogites) or consistent partial melting of another type (peridotites)
What are lherzolites and hazburgites?
Lherzolites contain high CO2 levels and partially melt to form Mid Ocean Ridge Basalts, while Hazburgites contain low CO2
In what 3 conditions do volcanoes form?
When the geothermal moves to higher temperatures, mass is removed from the mantle, or the composition of the mantle changes the solidus
Where and how does decompression melting occur?
At divergent plate boundaries as the lithosphere fills the place left by the crust
What are Benioff-Wadati zones?
Planar zones of seismic activity corresponding with the down-going slab of a subduction zone
What are granitoid intrusions?
Intrusions with the general composition and texture of granite, forming in abundant terrains, that are built up in pulses between 100 to 100,000 years apart
What is vapour present melting? How is it self-limiting?
Melting that occurs in rocks in equilibrium with a discrete fluid phase; when pressure is high, solubility increases to dehydrate surrounding rocks and lower their chance of melting
What is vapour absent melting? How does it make melt easier to abstract?
Melting that occurs in anhydrous rocks and hydrous rocks with hydrous components that have been released as melt; the produced melt volume is larger than the solid assemblage, forcing open potential pathways and making melt easier to extract
What are the 2 principal magma series of ocean magmatism?
Tholeiitic series: dominant in crust, with low alkali, silica saturated content and a similar composition to MORBs. Formed by submarine volcanism from low-alkaline mafic magma
Alkaline series: subordinate in crust, with a higher alkali content. Undersaturated in silica and formed at areas of continental rifting and subduction from alkaline mafic magma
What are the 2 main controls on ocean magmatism?
Pressure: increasing pressure drives the eutectic upwards, towards undersaturation
Volatiles: higher CO2 content drives eutectic up; high H2O content drives eutectic down
