What is the chemical composition of magma in the mantle and how does
this change into basaltic and granitic lavas?
Anonymous
Dear reader,
Geochemists use several elements to track the history of a batch of
magma. For our example we'll start with magnesium (and think of it as
magnesium oxide, MgO, like the geochemists do). Beneath Hawaii, when
the mantle melts it produces a magma with about 17 weight percent MgO.
Volcanologists call magma with this composition a picrite. As the magma
rises crystals begin to form. The first crystal to form is olivine,
which contains the elements magnesium, silicon, and oxygen in the
proportions 2 to 1 to 4. The crystal is more dense than the surrounding
magma and it begins to settle. It is estimated that an olivine crystal 1
mm in diameter falls through the magma at a rate of 20 meters per hour.
The key point is something, the crystals, is being removed and this
causes the chemical composition of magma to change. As more and more
olivine crystals settle, the magma has less and less magnesium oxide and
more and more silica (silicon and oxygen). For most volcanoes, by the
time the original magma from the mantle is erupted its composition has
changed from a picrite to a basalt.
The same process continues but more and different minerals become
involved. In general, the removal of these crystals drives the
composition away from that of a basalt and towards that of a granite
(remember, granite is not a volcanic rock - it is the same composition of
a rhyolite but never reaches the surface to be erupted by a volcano).
Magnesium oxide and iron oxide will decrease and silica, sodium oxide,
and potassium oxide will increase.
Other processes can become involved. The magma may melt and incorporate
crustal rocks that tend to contain more silica. This drives the
composition towards granite. The magma may catch up with and mix with a
magma that has lower magnesium oxide and higher silica (for example, a
batch of magma sitting in a chamber with crystals settling out of it).
These processes (crystal settling, assimilation, and magma mixing)
influence the composition of the magma. Geochemists must study the
minerals and the chemistry of the lava to determine which processes were
involved and how big of a role each one played.
Steve Mattox, University of North Dakota
Source of Information:
Clague, D.A., and Denlinger, R.P., 1994, Role of olivine cumulates in
destabilizing the flanks of Hawaiian volcanoes: Bulletin of Volcanology,
v. 56, p. 425-434.
P.S. Clague and Denlinger report the following composition for average
Kilauea primary magma:
SiO2 (49%), TiO2 (1.9%), Al2O3 (10.7), FeO (11.2%), MgO (16.5%), CaO
(8.55%), Na2O (1.66%), K2O (0.30%), P2O5 (0.18%).
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