Introduction to Petrology Flashcards
In terms of heat, define the LITHOSPHERE and ASTHENOSPHERE
In the lithosphere, heat is lost via conduction, and in the asthenosphere it is by convection.
Explain the three situations where the mantle geotherm is above the solidus
- In MORs, the pressure is decreased, moving the geotherm up past the solidus while temperature is constant
- In mantle hotspots, the areas of higher temperature move them along, past the solidus while pressure is constant
- In subduction zones, there is a high presence of volatiles like water, which change the solidus’ position past the geotherm which stays stationary.
Giving examples, what are CONCORDANT intrusions?
Intrusions that are parallel to the structure of the country rock.
E.g. sills, lopoliths (lens-shaped), batholith
Giving examples, what are DISCORDANT intrusions?
Intrusions that are perpendicular to the structure of the country rock.
E.g. dykes and dyke swarms
How do rhyolitic and andesitic lavas differ from basaltic lavas?
Rhyolitic lavas are very viscous and don’t move fast enough to accommodate gas. This means they get torn into ash and pyro flows. Andesitic lavas are still viscous, but can accommodate the gases in vesicles. These from generally at lower temperatures.
Basaltic lavas are low viscosity and form at generally higher temperatures. gases can be easily degassed.
What is the Gibbs Phase Rule?
f = c + 2 - ø
f - degrees of freedom
c - number of components
ø - number of phases
2 - accounts for pressure and temperature if they can change (P is often constant in binary phase diagrams. If T can vary, this then equals 1)
What is Bowen’s reaction series?
Displays the expected order in which minerals precipitate from a magma. The first minerals to crystallise are olivine and pyroxene, and quartz and sodium rich feldspars crystallise last, affecting rock composition.
Define the EUTECTIC POINT
This is an invariance point on a binary phase diagram where the melt completely solidifies, at a specific temperature and composition. If the original composition is different to the eutectic composition, minerals are crystallised out so that the composition moves towards the eutectic.
Define FRACTIONAL CRYSTALLISATION
As the composition of the melt changes, so does density, so some crystals float or sink, and are lost, causing the melt to become closer to the eutectic composition.
What is a SOLID SOLUTION, giving examples of plagioclase and alkali feldspars
The gradual substitution of one ion for another in a mineral.
For Plagioclase feldspars, Na and Si in Anorthite can substitute for Ca and Al in Albite, since the charges balance and the ion sizes are similar.
for Alkali feldspars, the Na in Albite can substitute for K in K-feldspars since the charges balance and the structure is open enough to accommodate the change in size.
When is the LEVER RULE used?
Used to find the ratio of liquid to crystals from a solid solution diagram. It assumes no fractional crystallisation.
When do lamellae exsolutions occur?
Exsolution occurs in slow cooling when a solid solution unmixes in the solid state. This is often below a solvus, where crystal of one mineral form first, with the second mineral forming within the host. Fast cooling means the high T phase is preserved and not split.
How do binary phase diagrams of ANHYDROUS melts differ?
The melt reaches a minimum rather than a eutectic since the endmembers can form a solid solution instead of two discrete phases. There is also a solvus below, where the temperature is low enough for two phases to form again.
What is INCONGRUENT and CONGRUENT melting?
Congruent melting is when the composition of liquid and solid are the same
Incongruent melting is when the compositions are different, so when the melting of a solid creates a second solid as well as the liquid.
What is a PERITECTIC POINT?
Another type of invariance point. Solids may need to be resorbed in order to continue down the phase diagram as enough of another solid needs to be produced.
