Download Rock Composition

What igneous rocks
are made of
Minerals
Texture
• Phaneritic (plutonic rocks)
• Aphanitic (volcanic rocks)
• Fragmental (volcanic ashes)
Minerals
• Primary
– Main minerals
• Felsic
• Mafics
– Accessory minerals
• Secondary minerals
(alteration/metamorphism)
Figure 3-7. Euhedral early pyroxene with late interstitial plagioclase (horizontal
twins). Stillwater complex, Montana. Field width 5 mm. © John Winter and
Prentice Hall.
2 micas granites
Tourmaline granite
Bt
Kfs
Ms
Pl
Figure 3-20. a. Pyroxene
largely replaced by hornblende.
Some pyroxene remains as light
areas (Pyx) in the hornblende
core. Width 1 mm. b. Chlorite
(green) replaces biotite (dark
brown) at the rim and along
cleavages. Tonalite. San Diego,
CA. Width 0.3 mm. © John
Winter and Prentice Hall.
Pyx
Hbl
Chl
Bt
IUGS classification
• Based on main minerals (accessory &
secondary minerals not used)
• Quartz, Alkali Fsp, Plagioclase,
Feldspathoids and Mafics – normalized to
100%
• If M < 90: Re-normalize QAPF to 100%
and use QAP or QAF
• If M > 90: Normalize Ol+Px+Pg to 100%
and use Ol-Px-Pg diagram
(a)
Classification
of Igneous
Rocks
The rock must contain a total of
at least 10% of the minerals below.
Renormalize to 100%
Q
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
Granodiorite
Granite
Alkali Fs.
Quartz Syenite
Alkali Fs.
Syenite
20
20
Quartz
Monzonite
Quartz
Syenite
5
10
A
Syenite
(Foid)-bearing
Syenite
35
Monzonite
(Foid)-bearing
Monzonite
Quartz
Monzodiorite
65
Monzodiorite
(Foid)-bearing
Monzodiorite
10
(Foid)-bearing
Alkali Fs. Syenite
(Foid)
Monzosyenite
Figure 2-2. A classification of the phaneritic igneous
rocks. a. Phaneritic rocks with more than 10% (quartz +
feldspar + feldspathoids). After IUGS.
(Foid)
Monzodiorite
60
60
(Foid)olites
F
Qtz. Diorite/
Qtz. Gabbro
5 Diorite/Gabbro/
90
Anorthosite
P
10 (Foid)-bearing
Diorite/Gabbro
Classification of Igneous Rocks
Plagioclase
Anorthosite
Figure 2-2. A classification of the phaneritic
igneous rocks. b. Gabbroic rocks. c. Ultramafic
rocks. After IUGS.
lite
cto
Tro
Ga
bb
ro
90
Olivine
gabbro
Olivine
Dunite
90
Peridotites
Plagioclase-bearing ultramafic rocks
Pyroxene
Lherzolite
Olivine
(b)
40
Pyroxenites
Olivine Websterite
Orthopyroxenite
10
(c)
10
Orthopyroxene
Websterite
Clinopyroxenite
Clinopyroxene
Q
Classification of
Igneous Rocks
60
60
Rhyolite
Dacite
20
20
Trachyte
Latite
35
A
10
(foid)-bearing
Trachyte
Andesite/Basalt
65
(foid)-bearing
Latite
Phonolite
(foid)-bearing
Andesite/Basalt
10
Tephrite
Figure 2-3. A classification and nomenclature
of volcanic rocks. After IUGS.
60
60
(Foid)ites
F
P
Chemical composition
Analytical tools
• Using electromagnetic waves
– Excitation of the sample
• X Rays, Electron beam, etc.
– Detection
• Optical, X-rays
• Using mass spectrometry
– Ion generation
• Plasma, filament, laser
– Detection
• Mass spec
The electromagnetic spectrum
All electromagnetic waves travel at the speed of light (3 x 108
ms-1) and are discussed in terms of wavelength and frequency
Spectrometry
Emitted
radiation
Energy Source
Emission
Detector
Absorbed
radiation
Sample
Output with
emission peak
Absorption
Detector
Output with
absorption trough
The K emission spectrum of
copper
X-ray spectrum of an olivine
Major and trace elements
• Major elements
– Form main minerals
– Major elements composition tied to
mineralogy
– Classicaly expressed as wt.% oxydes
– O and Si always the most abundant
– Relatively narrow range of varation (ex. SiO2:
45—75% in most rocks)
A typical rock analysis
Wt. % Oxides to Atom % Conversion
Oxide
Wt. %
M ol Wt.
Atom prop Atom %
SiO2
49.20
60.09
0.82
12.25
TiO2
1.84
95.90
0.02
0.29
Al2O3
15.74
101.96
0.31
4.62
Fe2O3
FeO
MnO
MgO
CaO
Na2O
3.79
7.13
0.20
6.73
9.47
2.91
159.70
0.05
71.85
0.10
70.94
0.00
40.31
0.17
56.08
0.17
61.98
0.09
0.71
1.48
0.04
2.50
2.53
1.40
K 2O
1.10
94.20
0.02
0.35
H2O+
(O)
Total
0.95
18.02
0.11
1.58
72.26
100.00
4.83
99.06
6.69
Common types of magma
Classification of Igneous Rocks
Phonolite
13
Tephriphonolite
Wt.% Na2O+K2O
11
9
Phonotephrite
(Foid)ite
Trachyte
Trachy- Trachydacite
andesite
Rhyolite
Basaltic
trachyTephrite
Basanite Trachy- andesite
7
basalt
5
Dacite
3
Basalt
Basaltic
Andesite
Andesite
Picrobasalt
1
37
41
ULTRABASIC
45
45
49
BASIC
53
57
61
52 INTERMEDIATE
65
63
69
73
77
ACIDIC
wt% SiO2
Figure 2-4. A chemical classification of volcanics based on total alkalis vs. silica. After Le Bas et al.
(1986) J. Petrol., 27, 745-750. Oxford University Press.
Major and trace elements (cont.)
• Trace elements
– Substitute for other ions in minerals
– Loosely tied to mineraogy
– Expressed as ppm (1% = 10000 ppm)
– Abundances vary greatly from < 1 ppm (ex.
PGE) to > 1000 ppm (Sr, Ba…)
– A given element shows variations of several
orders of magnitude between different rocks
Major elements
• « Differenciation trends »
• Magmatic series
22
10
Bivariate
(x-y)
diagrams
Al2O3
MgO
17
5
0
12
15
FeO* 10
10
5
Harker
diagram
for
Crater
Lake
CaO
5
0
0
4
6
3
Na2O
4
2
2
0
45
1
0
50
55
60
SiO2
65
70
75 45
50
55
60
SiO2
65
70
75
K2O
Alkali vs. Silica diagram for Hawaiian volcanics:
Seems to be two distinct groupings: alkaline and subalkaline
12
10
Alkaline
8
6
4
2
Subalkaline
35
40
45
50
%SiO
55
60
65
AFM diagram: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
Figure 8-14. AFM diagram showing the distinction
between selected tholeiitic rocks from Iceland, the MidAtlantic Ridge, the Columbia River Basalts, and Hawaii
(solid circles) plus the calc-alkaline rocks of the Cascade
volcanics (open circles). From Irving and Baragar (1971).
After Irvine and Baragar (1971). Can. J. Earth Sci., 8,
523-548.
Tholeiitic
B-A
A
D
R
Calc-alkaline
biotite
muscovite
cordierite
andalusite
garnet
pyroxene
hornblende
biotite
aegirine
riebeckite
arfvedsonite
CaO
CaO
moles
CaO
K2O
K2O
Al2O3
K2O
Na2O
Peraluminous
Al2O3
Al2O3
Na2O
Metaluminous
Na2O
Peralkaline
Figure 18-2. Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) (“A/CNK”) after
Shand (1927). Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid
Rocks. Chapman Hall.
Alkaline
Calc-alkaline
Tholeitic
Series
Alkaline
Subalkaline
Calcalkaline
Tholeitic
Alkali
content
High
Fe-Mg
Al
Fe-rich
Metaluminous
to peralkaline
Low to
moderate
Mg-rich
Metaluminous
to peraluminous
Low
Fe-rich
Metaluminous
A world-wide survey suggests that there may be
some important differences between the three series
Characteristic
Plate Margin
Series
Convergent Divergent
Alkaline
yes
Tholeiitic
yes
yes
Calc-alkaline
yes
Within Plate
Oceanic Continental
yes
yes
yes
yes
After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer
Trace elements
• Substitutions and Kd
• Spidergrams
Selective affinities
Fe2+
Mg2+
Ni2+
Compatible
(right size & charge)
Au3+
Ag3+
Fe2+
Mg2+
Incompatible
(size/charge does
not match)
• Partition coefficient Kd = Cs/Cl
• Compatible, incompatible (relative to a
mineral)
• Bulk repartition coefficient D =
S Kd X
i
i
Compatibility depends on minerals and melts involved.
Which are incompatible? Why?
Rb
Sr
Ba
Ni
Cr
La
Ce
Nd
Sm
Eu
Dy
Er
Yb
Lu
Rare Earth Elements
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace
Elements in Basaltic and Andesitic Rocks
Olivine
0.010
0.014
0.010
14
0.70
0.007
0.006
0.006
0.007
0.007
0.013
0.026
0.049
0.045
Opx
0.022
0.040
0.013
5
10
0.03
0.02
0.03
0.05
0.05
0.15
0.23
0.34
0.42
Data from Rollinson (1993).
Cpx
Garnet
0.031
0.042
0.060
0.012
0.026
0.023
7
0.955
34
1.345
0.056
0.001
0.092
0.007
0.230
0.026
0.445
0.102
0.474
0.243
0.582
1.940
0.583
4.700
0.542
6.167
0.506
6.950
Plag
Amph Magnetite
0.071
0.29
1.830
0.46
0.23
0.42
0.01
6.8
29
0.01
2.00
7.4
0.148
0.544
2
0.082
0.843
2
0.055
1.340
2
0.039
1.804
1
0.1/1.5*
1.557
1
0.023
2.024
1
0.020
1.740
1.5
0.023
1.642
1.4
0.019
1.563
* Eu3+/Eu2+
Italics are estimated
• Calculate DYb for…
– A lherzolite (80% Ol, 10% Opx, 10%Cpx)
– A Grt-bearing Lherzolite (70% Ol, 10% OpxCpx-Gt)
• Calculate DSr for…
– A Cpx-Plag cumulate (50/50)
– A Cpx-Opx cumulate (50/50)
• How will the residual liquid evolve?
Fingerprinting specific minerals:
• Ni strongly fractionated  olivine > pyroxene
• Cr and Sc  pyroxenes » olivine
• Ni/Cr or Ni/Sc can distinguish the effects of
olivine and augite in a partial melt or a suite of
rocks produced by fractional crystallization
Rb
Sr
Ba
Ni
Cr
La
Ce
nts
Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace
Elements in Basaltic and Andesitic Rocks
Olivine
0.010
0.014
0.010
14
0.70
0.007
0.006
Opx
0.022
0.040
0.013
5
10
0.03
0.02
Cpx
Garnet
0.031
0.042
0.060
0.012
0.026
0.023
7
0.955
34
1.345
0.056
0.001
0.092
0.007
Plag
Amph Magnetite
0.071
0.29
1.830
0.46
0.23
0.42
0.01
6.8
29
0.01
2.00
7.4
0.148
0.544
2
0.082
0.843
2
Concentration of REE in a sample
Chondrites
Contrasted REE patterns
Granites
Basalts
Multi-elements diagrams
Normalized to the PRImitive Mantle (close to chondrites) (Wood version)
A famous “anomaly”: Eu
Granites from the Cape Granite Suite
Darling-Vredenburg area
Kd’s for plagioclase
• REEs are normally 3+ (La3+, etc.)
• Eu can be Eu3+ or Eu2+
• Eu2+ strongly compatible
Reducing (Eu2+)
Oxydizing (Eu3+)