Lecrture 9-melting The Mantle Flashcards
What is the only rock type found in the mantel?
Periodotite
What is perioditire composed of?
Olivine,
ORTHOpyroxene,
CLINOpyroxene and
al rich silicates (plagioclase,spinel,garnet)
What is the difference between solids and liquids temperatures?
The solidus temperature is where partial melting of the rock begins
The liquid it’s temperature is the temperature at which the rock is fully molten
What is the solidus dependant on?
Pressure
What is the meting point of a rock dependant on? (Three factors)
- temperature,
- pressure
- composition (particularly water content)
What is the geotherm in the earth?
-the rate of temperature change with depth
When does mantle (perioditite) melting begin?
When the geotherm intersects the melting point gradient
Why does partial melting of perioditie occur in the upper mantel?
The geotherm is close to the melting point gradient- but they do not intersect. Therefore, only partial melting occurs
What is produced from partial melting in the upper mantel?
The liquid/plastic asthenosphere
Where does decompressional melting occur?
At divergent plate margins
At hotspots
Where does fluid induced melting occur?
-at subduction zones
Explain the process of magma formation at subduction zones
-as plates diverge, the lesser weight (from the plates no longer being ontop) reduces the pressure on the mantle. Mantel rises to fill the gap, and as it rises to shallower depths, pressure is further reduced. Melting occurs, and magma is produced
What happens to the geotherm at MORS?
The magma rises rapidly, at such as rate that temperature remains the same but pressure is reduced. The geotherm and solidus curves intersect, and partial melting occurs
Describe decompressional meting at hotspots
The plume exists in lower density than the surrounding mantle, and subsequently rises through the mantle. As this occurs, reassure decreases and the melting point is reached,
at what depths does decompression melting occur?
-40Km at MORS
6-/70Km at hotspots
How/why does basalt change composition at subduction zones?
-hydrothermal circulation chemically alters oceanic crust from.
Olivine is altered into serpentine and magnesium oxide
Explain how the process of dehydration at subduction zones causes melting
Temperature and pressure increases with subduction, which causes hydrate minerals to become unstable and release water+volatiles. This then perlocate upwards through the lithosphere, and lowers the melting point. This causes the solidus and geotherm to intersect, and the perioditite in the NON SUBDUCTING lithosphere lithosphere is partially melted.
Why doesn’t the subucting slab at subduction zones undergo partial melting?
It’s too cold due to the ocean/after sitting atop of it
Explain the process of fractional crystallisation
—the first minerals to crystallise out of cooling magic magma are the magic ones (olivine and pyroxene). These elements are no longer in the bulk magma composition, which causes an increase in the silica content
What are the four types of magma in terms of silica (ascending order)
Basaltic,andesitic,dacitic,rhyolitic
Why does partial meting occur?
Different minerals within the rocks have differing melting points
Describe decompressional melting in terms of heat
As material rises the pressure on it decreases to a critical point, where the solid rock metals spontaneously without the prescience of extra heat
How does water impact the melting point of silicate minerals?
The meting pint decrease in proportion to the amount of water dissolved in the molten silica
By which two processes does new continental crust form?
Magmatic addition and accretion
