theImage.com     Notes on Basic Geology
Notes created & information organization based on the book:
"The Dynamic Earth - an introduction to physical geology"
Brian Skinner & Stephen C. Porter   (further book information here)
also look at www.wiley.com for additional resource information
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Volcanoes & Magma - Page 11

Last to Dissolve -- First to Precipitate

As basalt begins to melt, and olivine is the last mineral to dissolve. As the melt (magma) is subjected to a different temperature/pressure gradient, (Probably due to the magma rising, the olivine begin to precipitate first.) (Last in - first out rule)

At some point the temperature/pressure curves favor the production of an amphibole by taking iron (Fe) form the melt and some calcium (Ca) from the melt. The result is a new mineral precipitating (the amphibole) with the pyroxene. As the melt continues to change the ratio of sodium (Na) to calcium (Ca), the calcium (Ca) is used up in the production of both the pyroxene and amphibole, by the time the feldspar precipitates it is more sodium (Na) rich as much of the calcium was depleted.

Result : The basaltic starting material melted and due to pressure and temperature changes, it produced an andesitic lava. ( Bowen's reaction series )
Premelt Composition: Rock - Basalt (fig. 1)
Original rock created at high pressure and temperature (great depth). Composition includes:

Olivine

(Mg,Fe)2SiO4

Pyroxene

(Ca,Mg)(SiO3)2

Feldspar

( NaAlSi3O8 - CaAl2Si2O8 )
After melt there is mix of molesules. ( fig. 3)

Magma

Mg, Ca, Fe, Na, Al, SiO4

As mixture begins to cool, the pressure is lower and the temperature is lowering, thus different minerals are favored. Olivine still precipitates first, but now both pyroxene and amphibole begin to grow. (fig. 4)

Olivine

(Mg,Fe)2SiO4

Amphibole
(Ca, Fe) Si2O6

Pyroxene

(Ca,Mg)(SiO3)2
The material still in solution is less rich in Ca and Fe because of the formation of the amphibole. (fig. 5)

Magma

Mg, Ca, Fe, Na, Al, SiO4

-Ca, -Fe
After resolidification: Rock - Andesite (fig. 6)
Continued cooling of the less Ca, Fe rich melt now produces more sodium rich feldspars and more silica driving the overall rock composition toward an Andesite.

Olivine

(Mg, Fe)2SiO4

Pyroxene

(Ca, Mg)(SiO3)2

Amphibole

(Ca, Fe) Si2O6

Feldspar

( NaAlSi3O8 - CaAl2Si2O8 )

Fundamentally Bowen's reaction series ideas:

Olivine can react with excess silica to produce some pyroxenes, pyroxenes with excess silica to produce amphiboles, amphiboles can react with excess silica to produce micas ...

Look for "skins" around various minerals that formed from magma recrystallization. This shows the presence of fractional crystallization. For instance, differing chemical composition from the inside of a feldspar to the outside, or a pyroxene coating around an olivine.

One of the important economic factors leading from this "fractional crystallization" is that the majority of nickel, vanadium, chromium and platinum ores are created by the fractional crystallization of magmatic materials. This leads to magmatic mineral deposits of economic significance.
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