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IGNOTES
Geol 2033 - Notes on origin of igneous rocks (Lec 15-17)
Igneous rocks originate by the crystallization of magmas either on or below the earth's
surface. These notes briefly discuss the formation of magmas and the rocks they produce,
focussing particularly on the basic processes responsible for the great variety of different igneous
rock compositions.
FORMATION OF MAGMAS
Magmas that form mafic igneous rocks dominantly originate by melting of the upper
mantle
Magmas that form felsic igneous rocks dominantly originate by melting of the crust
Evidence from obducted mantle rocks, xenoliths brought up in magmas, geophysical data
and inferences from meteorites indicate that the mantle has the composition of an ultramafic
rock. One would expect that magma from this source would produce mainly peridotites, but the
average composition of the rocks we see is about that of a basalt or gabbro. Why?
It is not possible to go to the mantle and see magmas form, so in order to understand what
happens when rocks melt, and when they crystallize, geologists have resorted to lab experiments
- taking mixtures of elements or minerals approximately the same as in rocks - then melting them
in a furnace and allowing them to crystallize - easy if no water or other vapour phase is involved.
Temperatures of melting and crystallization and phases present are recorded. Samples that are
partially crystallized can be quenched quickly. In this case any minerals that have solidified
appear as crystals and the liquid forms an amorphous glass. The data can be plotted to yield a
phase diagram, which shows the phases - solid(s), liquid(s) or gas(es) present at various
temperatures and various proportions of components.
Fig 2-5 (page 52 E&B ) ALL EHLERS & BLATT (E&B) FIGURES WILL BE
HANDED OUT IN CLASS shows the phase diagram for various proportions of a pyroxene diopside and a plagioclase - anorthite, which would very roughly approximate the range of
compositions of the mafic rocks we are interested in. Note that compositions from 100%
diopside 0% anorthite on the left to 100% anorthite 0% diopside are plotted (as % Anorthite)
along the bottom and temperature is plotted along the vertical axis. Comparison with the waterethylene glycol diagram shows that these minerals behave essentially in the same way as water
and antifreeze in that mixtures of the two melt at lower temperatures than the pure substances,
and that there is a particular proportion of the two, known as the eutectic composition, at which
the minimum crystallization (or melting) eutectic temperature occurs.
We can see that
a) pure diopside melts at about 1390 C.
b) pure anorthite melts at about 1550 C.
c) the eutectic mixture (42% An and 58% Di) melts at about 1270 C.
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d) an intermediate composition will be completely molten at some intermediate
temperature (e.g. a mixture of 20% An-80%Di would be molten at about 1350 C.
However the diagram also shows us that such a mixture would begin to melt at 1270 C the same temperature as a eutectic mixture. In fact, any mixture other than the pure end members
will begin to melt at 1270 C. To the left of the eutectic, all An solid present would melt at
1270 C and the region above would consist of liquid plus Di solid. To the right the opposite
would occur - there would be only An solid plus liquid above.
The first liquid will not be pure An or Di, but in every case, except that of a single pure
end member, will have the eutectic composition.
You should trace the path of crystallization at various compositions and answer the
following questions;
What is the composition of the last liquid remaining on crystallization?
What is the composition of the first liquid formed on melting?
Does it depend on the proportions of the components?
If we compare this model with the mantle we find that the best approximation for the
mantle would be a composition far to the left of the eutectic. The composition of the average
mafic rock is close to the eutectic composition. This implies that the average mafic magma
represents only a partial melt (about 25%) of the mantle rocks involved. A greater amount of
partial melting will produce a more ultramafic (pyroxene-rich) magma. Other experiments have
shown that pressure (=depth) influences the position of the eutectic and hence the composition of
initial melts.
Mantle melting is caused by
a) temperature increase or
b) pressure decrease.
Fig 6-7 E&B shows that there is a zone of apparent permanent partial melting in the
mantle at depths from 70 to 300 km, which is indicated by lower seismic velocity. Heating of this
material by mantle plumes and/or pressure reduction by faulting could lead to greater melting and
the faults could provide conduits to the surface.
The origin of felsic magmas can be explained by another experimental diagram (K&H
Fig. 14.6 page 565), in this case showing the behaviour of the three felsic minerals quartz, Kfeldspar and Na feldspar (albite), which approximate the composition of an average sedimentary
rock. Here the three components are at the corners of a triangle and the complete melting
(liquidus) temperatures are shown as contour lines. There is a minimum melting temperature
region in the centre of the diagram, and the shaded area, which contains the compositions of a
large number of granites, indicates that their parent magmas resulted from partial melting of the
crust.
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EVOLUTION OF MAGMAS
Magmas rise toward or to the surface, cooling as they go. Eventually the temperature of a
magma drops to a point at which crystallization begins.
A series of samples taken in 1965 from a 83m deep lava lake in Hawaii has given us a
real example of crystallization of a magma (For a detailed description see Ehlers and Blatt pp.
86-92)
Samples of molten lava were taken with a small drilling rig set up on the crust of the lake. When
the study began the molten lava was only 9m below the surface, and samples were taken by
pushing ceramic and steel tubes through a drill hole into the melt. More solid lava (up to 1079 C)
was cored.
The slides and Figure 3-9 E&B show thin sections prepared from samples taken at
different temperatures which demonstrate that a minerals crystallized in a sequence as
temperature decreased. Olivine and chrome spinel began to crystallize at about 1200 C.
Clinopyroxene began to crystallize at 1185 C. This was joined by plagioclase at 1175 C,
followed by opaque minerals (ilmenite and magnetite at 1070 C, apatite at 1030 C, with
complete solidification by 1000 C (Fig 3-9 E&B)
Thus we can see that the minerals precipitating from a melt do not all do so at the same
time. One starts, to be followed or joined by another, then a third and so on as the temperature
decreases. Some cease partway through whereas others continue to the end. N.L. Bowen in his
1928 book "The Evolution of the Igneous Rocks" thought that all igneous rocks originated from a
single parent magma of basaltic composition and based on lab experiments and field studies he
summarized the sequence of crystallization in the "reaction series" that bears his name. We now
know that there is not a single source magma, but crystallization typically follows his series.
See Bowen's Reaction Series (Fig. 14.1 Page 559 K&H)
The reaction series works as follows. If we start with a basaltic magma that should yield
a pyroxene and a plagioclase with equilibrium crystallization we find that the first minerals to
form may be an olivine and a Ca-rich plagioclase. Under equilibrium crystallization the
plagioclase will react with the liquid and become more sodic. The olivine will also react,
changing from Mg-rich to Fe-rich. At some point the olivine may be replaced by an Mg pyroxene
joined or followed by a CaMgFe pyroxene (augite). The final rock would be a basalt or gabbro.
If enough water is present in the magma hornblende or biotite will form instead of pyroxene.
Minerals lower in the series form similarly but from more felsic magmas.
The reactions mentioned above are due to the fact that olivine and plagioclase exhibit
solid solution (Mg2SiO4- Fe2SiO4 and CaAl2Si2O8- NaAlSi3O8) respectively, and as they
crystallize from a magma the crystals continuously change composition by reacting with the melt.
The olivine phase diagram is shown and discussed in K&H page 448-9, and the plagioclase
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diagram is shown in K&H Figure 9.5, page 319. Be sure you understand the difference between
equilibrium crystallization, in which the crystals remain and react with the melt, producing final
crystals of the same composition as the melt throughout; and non-equilibrium crystal
fractionation, in which crystals settle out from the melt as they form and do not react. This occurs
because olivine has a higher specific gravity (about 3.4) than the melt (about 2.8) and the olivine
crystals tend to sink as they form. This removal will cause a change in the bulk composition of
the remaining reacting magma, toward the lowest temperature phase (pure fayalite Fe2SiO4 in the
case of olivine), and an upward sequence of differing olivine compositions in the crystals that
settle.
DIFFERENTIATION OF MAGMAS
These processes, fractional crystallization (crystal fractionation), and gravity settling,
which come under the general term differentiation, can yield quite different rocks during
crystallization of a magma. Thus, if we were to examine the rocks from different levels in the
Hawaiian lava lake after it had completely crystallized we would find that not all samples are
exactly alike with respect to mineral percentages or mineral composition. One would find higher
percentages of olivine toward the base than at the top. Volcanic rocks usually crystallize quickly
after they are erupted onto the surface, so the rock typically freezes before differentiation goes
very far. Plutonic rocks, on the other hand crystallize very slowly and the processes of
differentiation can result in very different rocks in different parts of the intrusion.
The Mineral Lake Intrusion of northern Wisconsin (Figures 9-8 and 9-9 E&B) provides a
striking example of the effect of differentiation. The Mineral Lake Intrusion formed during the
late Precambrian, more than 1 billion years ago. Some time after, the area was tilted about 45 to
60, and eroded, so that we now see a cross-section through the 4.5 km thick intrusion.
Figure 9.9A shows that the intrusion is differentiated into rocks ranging from ultramafic,
through anorthositic olivine gabbro, anorthosite, ferrodiorite to granite. The rocks are named on
the basis of mineral percentages, which are shown in the figure. The early fraction of mafic
minerals and Ca plagioclases caused the remaining magma to be enriched in felsic minerals such
as K-feldspar and quartz. The presence of the hydrous minerals hornblende and biotite in the
upper part of the intrusion indicates a concentration of water in the later (residual) magma due to
crystallization of the earlier anhydrous phases (olivine, pyroxene, plagioclase). The sequence fits
Bowen’s reaction series well.
Figure 9.9B shows the changes in composition of the minerals, which become more iron
rich in the cases of olivine, and pyroxene; and sodium rich in the case of plagioclase toward the
top as one would predict from the experimental studies (phase diagrams).
Mafic intrusions, and flows to a lesser extent, are commonly layered in this fashion.
Felsic intrusions more commonly have concentric or adjacent bodies of increasingly felsic
composition formed by pulses of melt from deeper differentiating magma chambers. The
fractionating minerals are hornblende, plagioclase and/or biotite. Age relations are defined by
cross-cutting relationships, chilled margins and the presence of xenoliths of older phases. See
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the Gillis Mountain, Cape Breton example in class. The rocks here are in a sequence quartz
monzodiorite - porphyritic granite - fine grained granite - aplite. Plots of some elements indicate
that the alkalis (Na and K) increase as the mafics (Fe and Mg) decrease in the sequence, and that
aluminum decreases as silica increases. See if you can explain these in terms of the changes in
mineralogy implied by the names.
Summary - Differentiation - some points to bear in mind regarding crystallization of a magma.
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The various minerals precipitating from a melt do so in a sequence, not all at once.
Usually one mineral precipitates first, to be joined by a second, third.... as cooling occurs.
Some minerals, such as the plagioclases or olivines react with the melt, changing
composition as crystallization proceeds. If crystals settle (essentially leave the system) the
remaining melt changes as well.
If the crystals stay in contact with the melt throughout crystallization and they are
permitted to react with it this is called equilibrium crystallization and the resulting rock
will be homogeneous and the same composition as the original magma.
If early-formed crystals are prevented from reacting (non-equilibrium crystallization) then
a variety of rock types can result from a single magma.
There are a variety of processes that lead to a single originally homogeneous magma
producing a variety of chemically and mineralogically different rocks. These processes
are given the general name differentiation.
The most important method of differentiation is the process of removal of crystallizing
phases from the melt called fractional crystallization (or crystal fractionation).
Other processes include:
liquid immiscibility: the production of two or more immiscible magmas such as
silicate and sulphide or silicate and carbonate, and
gaseous transfer: the transfer of elements, particularly volatile ones such as
alkalis, CO2 and halogens in vesiculating fluids.
When fractional crystallization occurs the early crystals are segregated, most often by
gravity. Crystals are usually denser than the melt and sink toward the bottom of the
magma chamber. In some circumstances crystals are lighter and they rise to the top of the
magma chamber. Plagioclase is sometimes less dense than the magma depending on the
composition of both.
Even a closed magma chamber will display different composition rocks from one place to
another. If the chamber is open (conduit to the surface) the magma will evolve (change
composition with time) and the layers of erupted volcanic rock will show a variation from
first to last (from bottom to top).
Removal of early crystals allows the magma to evolve further than it would if it
continuously reacted with crystals.
Because they cool more slowly, intrusive rocks show more differentiation than extrusive
An important result of differentiation is that the removal of high temperature anhydrous
phases from a melt with even a small amount of volatiles (usually water) results in the
concentration of water in the remaining magma. Because many rare elements are
preferentially held by the water (they don't like to enter into high temperature silicate
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phases) the later magmas are enriched in these rare elements, such as metals. These often
escape with the water, to be deposited in veins outward from the intrusion.