Earth Materials Lecture 13: Magma Generation and Crystallisation Flashcards
List the main magma generation processes
⚫ Decompression partial melting.
⚫ Dehydration partial melting.
⚫ Within-plate or hotspot related magmatism.
➢ All can include the crust as well as the mantle.
Describe Magma sources
Vary with tectonic setting
Melting of upper mantle (Peridodite):
-Olivine + OPX + CPX +/- plagioclase / spinel /
garnet,
- gives rise to basalts and related rocks
Carbonated peridotite for kimberlites and carbonatites.
Buried sedimentary rocks (or igneous) in orogenic
belts.
Extreme metamorphism in shear zones.
Define the geothermal gradient
Geothermal gradient means that the further down you go into the Earth the hotter it becomes.
25-30 degree increase per km downward in crust
Describe decompression partial melting
Decompression partial melting is when a rock melts due to pressure decreasing and temperature staying constant.
It is easy to visualise with a mantle phase diagram
Define the solidus and liquidus in a mantle phase diagram
The solidus is the line which defines what is solid.
On a phase diagram all to the left of the solidus is solid.
On a phase diagram all to the right of the liquidus
Hydrating reduces the solidus and liquidus temperature
Describe silicate magmas
Silicate magmas are polymerized melts based
on strong Si-O and Al-O bonds.
⚫ Gives viscosity = yield strength.
Describe the relationship between silica content and viscosity
More silica in magma/lava flow = More viscous
Summarise variation in magma generation
Extrusion – surface volcanism
Intrusion – further crystal fractionation - cumulates
Wall-rock assimilation – crustal contamination
Crystal Fractionation - cumulates
Degree and depth of partial melting – silica
(under)saturation
Degree and depth of partial melting – silica
(under)saturation
