Melting the mantle 4-7 Flashcards
Geothermal gradient at the surface (Geotherm)
30*C/km
% of radiogenic heat generated in the crust
50%
Inner core temperature
4700*C
% of inner heat from primordial sources
36%
4 major sources of radiogenic heating
Potassium-40 - 58% (1/3 of total heat) - 1.3Ga
Thorium-232 - 21% - 14Ga
Uranium-238 - 20% - 4.5Ga
Uranium-235 - 1% - 710Ma
Geotherm distribution
Most heat generated in the crust as potassium is incompatible with mantle rocks
Convection in mantle and outer core is more efficient than conduction at transferring heat away
Mantle composition before partial melting
Lherzolite
Depleted mantle composition after mantle melting
Harzburgite
Lherzolite
- ultramafic igneous rock (a type of peridotite)
- mantle composition
- 40 to 90% olivine along orthopyroxene and a little clinopyroxene
Harzburgite
- ultramafic igneous rock (a type of peridotite)
- DEPLETED mantle composition
- 60 to 90% olivine along little orthopyroxene
3 types of mantle melting
Mid Ocean Ridges - Decompression melting
Ocean Island Hotspot - Hot Mantle Plume
Island Arc (Subduction) - Hydration melting
Adiabatic rise
Change in pressure without change in temperature
Mid Ocean Ridge melting
- Decompression melting: adiabatic rise and a drop in pressure
- Occurring as the oceanic crust thins as tensional pressure is stretching it and the mantle rises to fill the gap faster than it can lose heat
- Vertical geotherm that passes the solidus
- 15% partial melting of the Lherzolite mantle forming MORB (Mid Ocean Ridge Basalt)
- 25-40 km depth (not deep)
Obduction
- Denser oceanic crust scraped off a descending ocean plate in a subduction zone
- Thrust onto an adjacent plate, even if it consists of lighter continental crust.
Fractional crystallisation
- Partial melts pond into a magma chamber, the denser materials sink to the bottom and cool allowing crystals with the highest melting points to form
- Removal of early formed crystals from an originally homogeneous magma (for example, by gravity settling) so that these crystals are prevented from further reaction with the residual melt.
Fractional crystallization order (from bottom)
Olivines - Dunite (Olivine), Wehrlite (Olivine and CPx), Troctolite (anorthite and olivine)
Gabbros - (Plagioclase and CPx) Massive and isotropic
Sheeted dykes - form as magma pushes up into the crust cooling rapidly into fine-grained basalt
Pillow lavas - result from magma being pushed up into the cool sea where it is quenched forming very fine-grained basalt
Triple junction
- The point where the boundaries of three tectonic plates meet
- Each of the three boundaries will be one of 3 types – a ridge, trench or transform fault
- Can be described according to the types of plate margin that meet at them.
Failed rifts
- Result of continental rifting that failed to continue to the point of break-up
- Typically the transition from rifting to spreading develops at a triple junction where three converging rifts meet over a hotspot.
Island arc (subduction) partial melts
- Wet solidus means melting at a much lower temperature
- 80-100km (relatively deep)
- Different compositions with more water-rich minerals e.g. micas and amphiboles. Less large plagioclase compared to clinopyroxene as due to hydration clinopyroxene forms first.
Back-arc basin
Submarine basin that forms behind an island arc
Slab rollback
- As the old, subducting slab collapses into the asthenosphere, it can “roll back” through the mantle
- Slab rollback can pull the upper plate with it, causing an extension in the overlying plate, and possibly resulting in backarc spreading
Assimilation
- Ascending magmas evolve chemically by absorbing easily melted/dissolved components from the walls of their conduits
- This creates an andesitic to granitic composition and happens in ocean-continent subduction.
Extrusive igneous rocks in order of mafic to felsic
Basalt
Andesite
Dacite
Rhyolite
Intrusive igneous rocks from mafic to felsic
Peridotite Gabbro Diorite Grano-diorite Granite
Bowen’s recation series
Olivine
Pyroxene Ca Plagioclase
Amphibole Ca/Na Plagioclase
Biotite Na Plagioclase
Orthoclase
Muscovite
Quartz
Hotspot melt composition
- Deeper -> less silica than other melts (depth of 100-150 km)
- Higher temperatures -> more magnesium than MORB (21% vs 9%)
- Smaller melt fraction -> more Potassium (0.8-3%)
TAS Diagrams
- Total Alkali-Silica diagram
- shows that OIB are alkaline while IAB and MORB are subalkaline
- OIB are silica undersaturated
OIB
- Ocean Island basalts - hotspot volcanism
- OIB are silica undersaturated
- Much less viscious as there is less silica
- Forms shield volcanoes
Large Igneous provinces
- Massive accumulations of magma over more than 10,000km2
- More than 10,000km3 of magma pored over 75% in the span of 5Ma in intraplate settings.
Plutons
- Deep-seated intrusion of igneous rock
- From a body of magma that made its way into pre-existing rocks underground in the Earth’s crust and then solidified
Batholith
- Large mass of intrusive igneous rock, larger than 100 km² in area
- Several plutons joined together
Pegmatites
- Igneous rock with very coarse grains
- Formed by slow crystallization at high temperature and pressure at depth
- Crystals usually greater in size than 25 mm
