Igneous 2

Page 1

Reading:

Chapter 6 and Chapter 9.

Lecture 1 20/02/17

The Life and Death of Magma

Magma production and accumulation..deep magma transport...storage...local shallow transport....eruption.

Magma is caused by partial melting of rock in the mantle. Magma is born below the crust in the UPPER MANTLE and normally in the asthenospheric mantle.

Mantle is made up of peridotite - olivine and pyroxene. Melting takes place along grain boundaries - the surface energies are different there. Then there is some sort of partial melt sitting between crystals. The composition of melt is not peridotite. e.g. melting chocolate - oily first. PARTIAL MELT OF PERIDOTITE = BASALT. Only about 1-10% melting occurs before the melt drains away from between the crystals. Why does the partial melt move? It gets squeezed out from between the unmelted crystals by the weight of overlying rock bearing down on the mush of melt and crystals i.e. the much compacts forcing melt out from between the crystals. Since the melt is less dense than the rock it was formed from...it is buoyant relative to the rock hence it rises through the mantle.

In our source region there are lots and lots of pockets of magma which are getting squeezed out and rising.

Melting of dry i.e. no hydrous minerals mantle.

THE GEOTHERMAL NEVER ENCOUNTERS THE MELTING OF PERIDOTITE = THE MANTLE IS STABLE

If we go to a mid ocean ridge - the asthenosphere has been stretched and hot material rises up towards the surface. By stretching the crust, the temperature stays the same but the pressure is reduced. By decompressing the mantle, the mantle will melt. Its not melting due to heat, it is melting due to the release of pressure. By reducing pressure, a rock which is otherwise stable, will melt = DECOMPRESSIVE PARTIAL MELTING.

Convection (hot spots) - encounter the liquidus. Brings material from depth and carries it towards the earth surface.

Clay and amphibole become dehydrated and turn in to feldspar and pyroxene(?). If we add water to the mantle - then the liquidus relationship shifts and encounters the standard geothermal. First. reduce pressure - basalts. Then muck about with the water content - melting too. 70km or so the mantle starts to accommodate and absorb the water - the solubility of water is a function of pressure is very variable. Going further down, water becomes less soluble. Adding water, liquidus now intercepts the standard geothermal

Underneath island arcs earthquakes are occurring between 15 and 100km which corresponds with the depths at which the wet peridotite intercepts the geothermal.

What about magmas that have more andesite and rhyolite ?

By partial melting of the continental crust rocks typically these include gabbros diorite and granite igneous rocks along with sedimentary rocks e.g. sandstone mudstone limestone and with metamorphic rocks e.g. schist and gneiss.

Heat from rising basalt magmas entering the lower continental crust warms lower crust until the rocks partially melt to create acid and intermediate composition melts.

MAGMATIC DIFFERENTIATION -any process that can modify the composition of basalt magma.

Magma contamination - either a magma incorporates pieces of foreign rock creating a hybrid contaminated magma i.e. one different in composition or one magma mingles with another of different composition to create a hybrid mixed magma.

Fractional crystallisation - the separation of crystals of minerals from a cooling and hence crystallising magma leaving magma that is changed in composition.

Olivine...olivine + plagioclase...olivine + plagioclase + pyroxene.