Making Crust From Mantle

The mantle is composed of ultramafic silicates and oxides.  Remember, mafic means minerals rich in magnesium and iron (ferrum).  After the Earth formed, it segregated into the metallic, iron-dominated core and the rocky, silicate-dominated mantle.  The crust did not exist.  So how do you make crust, composed of mafic, intermediate, and felsic silicate rocks from the ultramafic mantle?

Question:  What do you get if you melt a block of ultramafic mantle peridotite and then cool it so that it crystallizes?
Answer: You get an ultramafic igneous rock (peridotite if you have centuries to wait around for it to cool slowly enough to form large, visible crystals, or komatiite if it cools and crystallizes quickly at Earth surface conditions thereby forming small, microscopic crystals.

So we would make another rock with the same composition as the original.  But how do we make a rock with a different, less magnesium and iron rich composition?

Recall that rocks are assemblages of minerals.  The different minerals in mantle peridotite have different melting temperatures and also different amounts of magnesium and iron vs. silica (silicon plus oxygen).

When the Earth was young it was very hot.  Ultramafic rocks could be largely or completely melted to produce ultramafic magma.  Komatiite lavas were common.  Today, the Earth is too cool to allow ultramafic lava, which crystallizes at very high temperature, to come to the surface.  In fact, the Earth is too cool to produce ultramafic melts at all.

Today, as in much of the past, the temperature in Earth's interior is too cool to completely melt peridotite (to melt all of the minerals in peridotite).  But, under certain conditions, the temperature may be high enough for the given depth and pressure to allow melting of minerals with the lowest melting temperatures.  In peridotite that might be pyroxenes and plagioclase because felsic minerals like quartz and potassium feldspar don't exist in ultramafic rocks.  The magma formed by partial melting is relatively enriched in silica and poorer in magnesium and iron compared to the peridotite.

Today the conditions for partial melting of mantle peridotite are caused by decompression of rising solid mantle rock beneath midocean ridges and in mantle plumes beneath hotspots (decompression melting).  Partial melting is also induced by flux melting in the mantle wedge over subducting slabs as a result of metamorphic dewatering of the subducting crust. The magma thus produced is mafic (basaltic).

As mafic magma rises into and through the crust its compostion may be altered.  The magma will cool as it rises through cooler country rock causing high temperature minerals like olivine to begin to crystallize.  Olivine is dense and slowly settles as the magma rises.  The olivine takes plenty of iron and magnesium from the melt, leaving the melt more felsic.  The rising magma may also incorporate (melt) some of the country rock if its melting temperature is low enough (it is less mafic), thereby adding more silica to the melt.  So, the mafic magma produced by partial melting of ultramafic mantle rocks can yield basalt, andesite, and rhyolite lavas by the time it reaches the surface.