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Interior Geology > Melting the mantle 4-7 > Flashcards

Melting the mantle 4-7 Flashcards

1
Q

Geothermal gradient at the surface (Geotherm)

A

30*C/km

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2
Q

% of radiogenic heat generated in the crust

A

50%

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3
Q

Inner core temperature

A

4700*C

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4
Q

% of inner heat from primordial sources

A

36%

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5
Q

4 major sources of radiogenic heating

A

Potassium-40 - 58% (1/3 of total heat) - 1.3Ga
Thorium-232 - 21% - 14Ga
Uranium-238 - 20% - 4.5Ga
Uranium-235 - 1% - 710Ma

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6
Q

Geotherm distribution

A

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

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7
Q

Mantle composition before partial melting

A

Lherzolite

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8
Q

Depleted mantle composition after mantle melting

A

Harzburgite

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9
Q

Lherzolite

A
  • ultramafic igneous rock (a type of peridotite)
  • mantle composition
  • 40 to 90% olivine along orthopyroxene and a little clinopyroxene
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10
Q

Harzburgite

A
  • ultramafic igneous rock (a type of peridotite)
  • DEPLETED mantle composition
  • 60 to 90% olivine along little orthopyroxene
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11
Q

3 types of mantle melting

A

Mid Ocean Ridges - Decompression melting
Ocean Island Hotspot - Hot Mantle Plume
Island Arc (Subduction) - Hydration melting

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12
Q

Adiabatic rise

A

Change in pressure without change in temperature

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13
Q

Mid Ocean Ridge melting

A
  • 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)
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14
Q

Obduction

A
  • 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.
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15
Q

Fractional crystallisation

A
  • 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.
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16
Q

Fractional crystallization order (from bottom)

A

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

17
Q

Triple junction

A
  • 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.
18
Q

Failed rifts

A
  • 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.
19
Q

Island arc (subduction) partial melts

A
  • 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.
20
Q

Back-arc basin

A

Submarine basin that forms behind an island arc

21
Q

Slab rollback

A
  • 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
22
Q

Assimilation

A
  • 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.
23
Q

Extrusive igneous rocks in order of mafic to felsic

A

Basalt
Andesite
Dacite
Rhyolite

24
Q

Intrusive igneous rocks from mafic to felsic

A 
Peridotite
Gabbro
Diorite
Grano-diorite
Granite
25
Q

Bowen’s recation series

A

Olivine
Pyroxene Ca Plagioclase
Amphibole Ca/Na Plagioclase
Biotite Na Plagioclase
Orthoclase
Muscovite
Quartz

26
Q

Hotspot melt composition

A
  • 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%)
27
Q

TAS Diagrams

A
  • Total Alkali-Silica diagram
  • shows that OIB are alkaline while IAB and MORB are subalkaline
  • OIB are silica undersaturated
27
Q

OIB

A
  • Ocean Island basalts - hotspot volcanism
  • OIB are silica undersaturated
  • Much less viscious as there is less silica
  • Forms shield volcanoes
28
Q

Large Igneous provinces

A
  • 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.

29
Q

Plutons

A
  • 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

30
Q

Batholith

A
  • Large mass of intrusive igneous rock, larger than 100 km² in area
  • Several plutons joined together
31
Q

Pegmatites

A
  • 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

Interior Geology (5 decks)

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