G314 Advanced Igneous Petrology 2007

Week 1 – Lectures 1 to 3

Some background

See Winter, chap. 1

  1. The life cycle of magmas: from melting to cooling

1.1.Melting

  • Is always partial (5-50 %), there is therefore a solid residuum (95-50 %).
  • Melting of the mantle generates basalt or andesites; melting of the crust generates granites.
  • Melt needs to be extracted from the partially molten system.

1.2.Magma evolution: fractional crystallization

As the system cools down, the magma progressively evolves in the “solid+liquid” field, between solidus and liquidus.

  • Crystals are progressively formed, and can settle down at the bottom of the “magma chamber”.
  • The remaining magma chemically evolves. Its evolution is a “mirror” image of the crystals removed: typically towards SiO2-rich, mafic poor composition.
  • Basalts evolve towards high-SiO2.
  • Granites are already SiO2 rich… and remain so (little or no evolution)

1.3.Magma transfert and emplacement

Magma moves upwards through the crust; mafic magmas can be seen to move through dyke swarms. The equivalent for granitic magma is not seen.

Plutons and batholiths

If the magmas emplace at depth (typically, crustal magmas), they fill small pockets = plutons, with rocks slowly crystallizing at depth. Plutons are typically found as clusters of several to many intrusions (batholiths), corresponding to successive magma inputs, typically during a small time period.

Volcanoes

If the magma reaches surface (more common for mantle-derived rocks), they erupt as volcanoes. The roots of the volcanoes are also dykes (but localized below the actual volcanic vent).

1.4.Cooling and sub-solidus evolution

As the magma finally cools, it evolves from its liquidus at 900° (granites) to 1200° (basalts) down temperature towards cooler conditions: the solid magmatic rocks starts its life in the amphibolite facies (typically) and cools down to greenschist, therefore making (retrograde) metamorphic evolution possible:

  1. Earth’s structure and plate tectonics

2.1.Three main units

From the surface to the interior:

  • The crust (or crusts):
  • Oceanic crust, 7 km thick
  • Continental crust, 35 km thick
  • The mantle (2900 km)
  • The core

The mantle corresponds to the largest volume and mass (> 80%). The crust is negligible (1%).

2.2.Plates and plate tectonics

The upper mantle + crust = lithosphere

Moving on a weak astenosphere

Lithosphere made of different plates.

Plate boundaries:

  • Rifts (divergent)
  • Subduction and collision (convergent)

Most geological activity (deformation, movement, igneous activity, metamorphism…) occurs on plate boundaries. Rare “intraplate” volcanoes.

2.3.P—T conditions in the Earth and the possibility of melting

Pressure

Pressure increases as a function of the weight (density) of the overlying matter. In the upper part (crust+upper mantle), a reasonable approximation is 1 kbar = 3 km. Hence the moho (base of the crust) is at ca. 10 kbar (= 1 GPa). The base of the lithosphere is at ca. 30 kbar (= 3 GPa).

Temperature

Two types of thermal gradients in the Earth:

  • Conductive gradients in the lithosphere
  • Convective gradients in the rest of the mantle.

Conductive gradients are relatively steep ( > 20 C/km), convective gradients are more gentle (< 1 C/km)

Due to in-situ heat production, the gradient is not linear. Close to the surface, 30 C/km is a good approximation, but it decreases further down; the moho is at ca. 600 C and the base of the lithosphere at 1300 C.

Possibility of melting

Combining P and T gradients shows that there is no part in Earth in which melting is a normal situation. Melting requires to move away from normal PT regime (melting occurs in conjunction with tectonics – on plate boundaries).

Three possibilities to reach the solidus:

  • Increase T
  • Decrease P (fast enough to avoid thermal reequilibration)
  • Move the solidus, typically by adding water.
  1. From the suns to the Earth

Structure of atoms: protons, neutrons, electrons

One element = given number of protons = nb of electrons = charge (or atomic number) = chemical properties

Atomic mass = nb protons + nb neutrons

Different isotopes of a same element have the same atomic number (= nb of electrons/protons) but different masses (= nb of neutrons)

3.1.Nucleosynthesis and the origin of elements

  • Nuclear reactions in stars (light nuclei combine to form heavy atoms)
  • Bethe’s cycle (favors even-numbered atoms)
  • Evolution towards the more stable atoms (Fe)

3.2.Earth accretion and differenciation

  • Planetary nebula
  • Condensation
  • Accretion of proto-planets
  • Differentiation in three/four main units: core, mantle/crust, atmosphere s.l., as a function of chemical affinities (Fe/Silicates/lights)

“Primitive” solar system material: the chondrites.

3.3.Composition of Earth’s shells

The silicate Earth (“lithophile” elements)

Crust

  • Can be directly sampled/studied
  • Continental crust : ca. 30 km, mostly granitic (SiO2 = 70 %).
  • Oceanic crust: ca. 5-7 km, basaltic (SiO2 = 50%, some FeO)

Mantle

  • Few samples (in volcanoes)
  • Peridotites (SiO2 = 45 %, FeO, MgO)

The core (“siderophile” elements)

  • Fe+Ni alloy
  • Minor amounts of light elements

Also differences in trace elements!

Departement of Geology, Geography and Environmental Studies