Volcanism at destructive plate margins Flashcards
Give three examples of destructive plate margin volcanic settings.
Island arcs, continental arcs, volcanic arcs.
What type of melting occurs at destructive plate margins?
Hydrous (flux) melting.
Describe the position of the geotherm and solidus at destructive plate margins.
The solidus is pushed to the left, crossing over the geotherm.
Describe hydrous melting.
The melting point of mantle peridotite is lowered by the addition of water.
Where does the water needed for hydrous melting come from?
Subducting slab
Describe the subducting slab.
Comprised of ocean crust and lithosphere with minerals that have been altered by hydrothermal circulation and interaction with the seafloor.
Give three examples of the water-bearing minerals contained in the subducting slab.
Serpentine, chlorine, actinolite.
Describe the three phases of the dehydration of the subducting slab.
1: Pore water is released by the scraping off of sediments.
2: The oceanic crust is dehydrated (doesn’t contain much water).
3: Hydrous minerals start to breakdown from heating and pressurisation (common in serpentine).
Where is the water from the slab released into?
The mantle wedge
What is the mantle wedge?
The mantle that overlays the subducting slab.
What happens to the mantle wedge when water is released into it?
The solidus is lowered.
What does the action of subduction result in?
Drags newly hydrated peridotite deeper and rereleasing some water. This is repeated until there is enough water for partial melting.
Where does the subducting slab get heat from?
The mantle wedge. The subducting slab has to be young and warm to be able to heat enough.
What are subduction zoom volcanoes fed by?
Hydrous melted peridotite from the mantle wedge.
What type of melt does wet peridotite produce?
Primary basaltic melts.
Describe the composition of wet peridotite.
Si-rich. Higher Al,K,Ti oxides than MORB.
When do magmas have a more varied composition?
When they erupt through continental crust.
What composition trend is observed at continental arcs?
Calc-alkaline trend.
What composition trend is observed at island arcs?
Tholeiitic suite.
What do the three corners of the AFM ternary diagram represent?
Alkali, iron, and magnesium.
Describe the calc-alkaline trend.
As the magma cools, it precipitates Fe and Mg at a higher rate than alkalis, evolving towards to A corner of the ternary diagram.
Give the order of four rocks in the calc-alkaline trend.
Basalt, andesite, dacite, rhyolite.
Give four explanations for the calc-alkaline trend.
Magma mixing, crustal assimilation, suppression of plagioclase crystallisation, enhanced crystallisation of Fe-Ti oxides due to the oxidising conditions.
What texture do subduction zone basalts tend to have?
Porphyritic texture
What does the tholeiitic suite show that the calc-alkaline trend does not?
Iron enrichment
What can be crystallised from hydrous magmas?
Hydrous minerals
Give two examples of hydrous minerals that can be crystallised from hydrous magmas.
Amphiboles (e.g. hornblende) and sheet silicates (e.g. biotite)
What can be shown in phenocrysts in subduction zone basalts?
Compositional zoning and disequilibrium textures
Give three examples of disequilibrium textures seen in subduction zone basalt phenocrysts.
Rims, embayments, sieving
Describe the process of embayment in phenocrysts.
The crystal is partially dissolved due to chemical disequilibrium.
What phenocryst in basalt rocks is most likely to exhibit embayments?
Olivine.
Describe a sieve texture.
Plagioclase is resorbed in sieve texture.
Describe the formation of rims.
Melts come through until the crystal is stable enough to recrystallise it.
