Igneous Petrology EPSC 423 - Francis 13
Lab 3: Volcanic Textures and Primary Magmas
The identification and classification of an igneous rock is typically a two fold
process that requires the determination of the type of cooling unit that the rock
sample was taken from in the field and an estimation of its mineralogical and
chemical composition. The identification of the cooling unit is best done on the
basis of structures and textures in the field and you are referred to the EPSC-212
PowerPoint Presentations on volcanics in the field for a review of common features
(in Lab 3 on C423 web site).
The first order question you must decide for any igneous rock is whether it is a
volcanic or plutonic rock. Again this is best done in the field, but can also be
determined on the basis of petrographic textures in hand specimen and thin
section.
Volcanic (extrusive) rocks that have cooled quickly and crystallize rapidly on
the surface are typically relatively fine-grained and have compositions that
approximate that of the liquids from which they
formed. They commonly have a bi-modal grainsize, however, consisting of larger early-formed
phenocrysts set in a late fine-grained matrix.
Other common features of volcanic rocks are the
presence of vesicles or amygdules (filled
vesicles) formed by the exsolution of volatiles,
and may exhibit reddening and/or brecciation
along flow boundaries if they are subaerial. The
presence of pillow structures with glassy
margins is indicative of sub-aqueous eruption.
Plutonic (intrusive) rocks that crystallize
more slowly below the Earth’s surface are
typically equigranular and coarser-grained than
volcanic rocks, with the grain size indicative of
the rate of cooling and therefore the depth of
crystallization, as well as other factors such as
volatile content. Plutonic rocks are commonly in
part cumulate, that is their compositions reflect
the mechanical accumulation of crystals rather
than a frozen liquid. We will look at plutonic
rock textures in detail in a future lab.
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Igneous Petrology EPSC 423 - Francis 13
Volcanic Structures and Textures
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Igneous Petrology EPSC 423 - Francis 13
Textural Criteria for Determining the Sequence of Crystallization of an
Igneous Rock (modified from Williams, Turner, and Gibert, 1954):
1) If grains of one mineral typically occur enclosed in another, then the former
crystallized before the latter. A good example is the occurrence of early
olivine chadocrysts in later pyroxene or feldspar oikocrysts. Exceptions
include phenomena such as exsolution lamellae, cotectic intergrowths, latestage alteration minerals, and accessory phases.
2) Phenocrysts have crystallized before their fine-grained matrices. In general,
larger crystals have crystallized earlier than smaller crystals. This criteria
best applied to volcanic rocks, and is commonly problematic in plutonic
rocks.
3) Free-forming early crystals tend to be more euhedral (show crystal faces)
than later crystals. Again this criteria is best applied in volcanic rocks. In
plutonic rocks it is compromised by the decreasing tendency of the silicates
to be euhedral, in the order orthosilicate > chain silicate> sheet silicate>
framework silicate. In plutonic rocks, late crystallizing phases such as
zircon, titanite and even apatite are often euhedral.
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Igneous Petrology EPSC 423 - Francis 13
Station A – Mafic Lava Flows
The 2 sets of 4 rocks at station B are samples taken across two single lava flows:
A1 - the darker set was taken across an alkaline olivine basalt flow from the
Canadian cordillera that is 4 metres in thickness.
Specimens A, B, C, D (thin sections AOB-4(A), 1(B), 2(C), 3(D))
and
A2 - the lighter set was taken across a tholeiitic
olivine basalt flow from Baffin Island that is 5 metres
in thickness. This is a lavqa flow interbedded with the
PI pillow lavas that you examined in Lab 2.
Specimens Pd 17(48), 41, 44, 46 or 46.
The four samples from each flow have approximately
the same chemical composition, and mineralogy, but
differ texturally because of their different cooling
histories.
Station A Tasks:
For each of the two sets of samples, determine the location of each of the 4
samples in their respective flows by examining their textures in hand specimen
and thin section. Describe the variation in the textures of the minerals between
samples across the same flow and briefly rationalize this variation in terms of
cooling rate within the lava flow. Determine the order of mineral crystallization
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Igneous Petrology EPSC 423 - Francis 13
in the alkaline and tholeiitic magmas by examining the phenocrysts in their
most rapidly cooled samples.
Station B - Pillow Lavas and Primary Magmas:
Specimens: PI-8, PI-9, P-10, PI-18
The samples at this station are specimens of the margins of pillow lavas that
erupted in the ocean during the Eocene separation of Greenland from Baffin
Is. They come from the same volcanic succession as the PD lava samples at
station A. The thins sections at this station are cut perpendicular to the
outside surface of the pillow so that the textural variation observed along the
thin section records the changing pattern of nucleation in the matrix of the
rock going into the pillow. These volcanic rocks has cooled very rapidly and
'quenched in' a virtual snapshot of the magma as it erupted. Their magma
was an olivine-rich picrite and they represent ideal samples with which to
study the nature of primitive magmas derived directly from the mantle.
A primary magma is one whose composition is controlled by the composition of
the pseudo-invariant point for the Earth's upper mantle, and has not been
affected by crystal fractionation or contamination on its way to the surface. The
Earth's upper mantle is thought to be dominated by the lherzolite mineral
assemblages you saw in the foregoing lab, whose olivine is too Mg-rich (Mg no.=
Mg/(Mg+Fe) = 0.89 to 0.93) to be in equilibrium with all, but the most
magnesian olivine-phyric basalts. Many petrologists argue that lavas that are
candidates for primary magmas are recognized by their relatively high MgO
(>10 wt.%) and olivine phenocrysts contents. Such Mg-rich picrites (rich in
olivine phenocrysts) are thought to approach primary magmas from the Earth’s
mantle, from which less magnesian basalts are derived by crystal fractionation.
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Igneous Petrology EPSC 423 - Francis 13
Station B - Tasks:
Chose one of the specimens and examine its hand specimen and thin section.
Determine the specimen’s phenocryst phases, and the order of appearance of
clinopyroxene, plagioclase, and olivine during crystallization, by examining the
glass transition at one end of the slide?
Plot your picritic basalt composition, and the liquid composition calculated for 15
Kb partial melting at Cpx-out in lab 1 in the “spider” diagram you constructed in
Lab 2 last week for depleted mantle, with the Y axis as Wt% picrite / Wt% Lherz,
and the X axis is the major elements in the sequence: Na, Ti, Ca, Al, Fe, Mg, Cr.
Discuss the relative partitioning of these major elements during partial melting by
explicit comparison of the profiles of fertile lherzolite, depleted harzburgite, and
picritic basalt or calculated partial melts.
Using the following reactions for the crystallisation of olivine components from a
silicate melt, calculate the composition of the first olivine that would crystallise,
and the temperature at which it would appear, in a liquid of the whole rock’s
composition, and in a liquid with the composition of the groundmass glass in the
pillow margin (Table 3-1), assuming the two lattice mixing model of Neilsen and
Dungan (1983) for the silicate melt:
OLIVINE :
#
l MgO(NM) + 1/2SiO2(NF) = MgSi0.5O2
#
2 FeO(NM) + 1/2SiO2(NF) = FeSi0.5O2
NM: Network modifying cations
NF: Network forming cations
K1 = aFo / (aMgOLiq × (aSiO2Liq)1/2)
= XMgoliv / (XMg(NM))×(XSi(NF))1/2 )
K2 = aFa / (aFeOLiq × (aSiO2Liq)1/2) = XFeoliv / (XFe(NM))×(XSi(NF))1/2)
ln K = a + b
T
aFo =
XMgoliv =
a
6,700
b
-3.73
K2 6,874
-4.97
K1
oliv
Mg / (Mg + Fe)
aFa = XFeoliv = Fe / (Mg + Fe)oliv = aFa
NF = Network Formers Σ Si + Na + K
NM = Network Modifiers Σ Mg + Fe + Ni + Ca + Mn + Ti + Cr + Al - (Na + K)
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Igneous Petrology EPSC 423 - Francis 13
Table 1: Baffin Bay Pillow Lavas
Major Elements in wt%
Sample: PI-8
Rock: Margin
PI-9
Margin
PI-10
Margin
PI-18
Glass
Margin__________
SiO2
TiO2
Cr2O3
Al2O3
MgO
FeO
MnO
CaO
Na2O
K2O
45.48
1.00
0.13
13.01
14.24
10.70
0.18
11.27
1.47
0.04
45.40
0.98
0.15
12.88
15.04
10.82
0.18
11.06
1.43
0.04
45.25
0.97
0.15
12.67
15.07
10.78
0.18
10.98
1.41
0.03
45.40
0.82
0.22
11.70
18.95
10.38
0.17
9.84
1.27
0.04
50.36
1.15
14.75
8.68
9.27
0.14
12.85
2.06
0.22
Total
97.53
97.99
97.49
98.80
99.48
Proceed by calculating the activities of forsterite and fayalite at a high
temperature (above the liquidus, say 1600o) for the magma’s chemical composition.
Olivine will crystallize when the sum of these 2 activities equals or exceeds 1.
Solve for T either by calculating the activities for 3+ different temperatures and
graphing to obtain the temperature for Σ a =1, or by iteratively decreasing the
temperature until the sum of the 2 olivine end-member activities equals 1.
Calculate the temperatures of the liquidus for your whole rock and the glass
compositions using AlphaMelts at 1 bar on the FMQ-1 buffer and compare to the
results of the above calculation.
Go on to calculate the crystallization
temperatures of plagioclase and clinopyroxene at 1 bar. How does the predicted
sequence of crystallization compare with that estimated from your petrographic
observations.
How do the compositions and temperatures of your calculated olivines for the
whole rock and glass compositions compare with the olivine phenocrysts (Fo 89.5)
actually observed in the specimen as determined by microprobe analysis? What
does this comparison imply for whether the whole rock composition represents that
of a liquid or a liquid which has mechanically accumulated olivine?
Proceed as in lab 2 using AlphaMelts to calculate the near liquidus P-T phase
diagram for your picrite lava sample from 1bar to 30 kbs. What would be the
sub-solidus mineralogy of the picritic basalt at 30 kbs? How do the liquidus
phases and liquidus temperature of the picritic lava compare with the solidus
temperature and solidus phases of fertile mantle at 30 kbs.
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Igneous Petrology EPSC 423 - Francis 13
Station C – Garnet Pyroxenites
The three xenoliths in Group C represent a variety garnet pyroxenite xenoliths from the mantle:



Eclogite:
high pressure form of basalt.
Garnet Pyroxenite I: high pressure "cumulate" of a picritic basalt
Garnet Pyroxenite II: high pressure "cumulate" of an alkaline basalt"
Examine each of the xenoliths in Group C. With the aid of thin sections and the
chemical data in Table 2, compare the mineralogy, texture, and bulk compositions
of each the three pyroxenites and decided which of the above it most likely
represents. Plot their compositions in the spider diagram constructed for station A,
and compare them to the composition of the picrite lava and the calculated cpx-out
melt composition at 15 kb from Lab 1. Speculate on which of the above 3 types of
mantle pyroxenites each represents.
Station C:
KA-29/38, EC-1, NK3-1
Table 2: Mantle Pyroxenites
Sample: NK3-1
KA-29/38
EC-1
SiO2
TiO2
Cr2O3
Al2O3
MgO
FeO
MnO
NiO
CaO
Na2O
K2O
P2O5
50.96
0.18
0.54
4.71
19.43
6.04
0.15
0.09
16.45
0.46
0.11
0.02
45.82
1.40
0.03
11.95
13.43
9.38
0.16
0.00
13.83
1.42
0.14
0.06
49.46
0.87
0.00
15.05
10.43
8.10
0.14
0.00
12.03
2.61
0.18
0.10
Total
99.15
97.62
98.97
Cations normalized to
Si
Ti
Cr
Al
Mg
Fe
Mn
Ni
Ca
Na
K
P
O
46.334
0.125
0.387
5.048
26.344
4.593
0.119
0.063
16.030
0.818
0.128
0.013
42.913
0.986
0.022
13.190
18.749
7.346
0.127
0.000
13.876
2.577
0.167
0.048
100 cations
45.564
0.603
0.000
16.339
14.322
6.238
0.109
0.000
11.874
4.661
0.212
0.078
148.723 149.204 152.017
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