Download Chapter 8: Major Elements

Chapter 8: Major Elements
Major and Minor Elements shown in orange.
Concentrations (wt%) usually given as oxides
In blue, Hydrogen (H2O, H2S, HCl, HF) and Carbon
(CO2, CH4), Nitrogen (N2, NO2, NH3) and Sulfur
(H2S, SO2) , are important gasses dissolved in
magma, and are given off in eruptions.
Element
O
Si
Al
Fe
Ca
Mg
Na
Wt % Oxide Atom %
60.8
59.3
21.2
15.3
6.4
7.5
2.2
6.9
2.6
4.5
2.4
2.8
1.9
Abundance of the elements
in the Earth’s crust
Usually given as Oxides
Major elements: usually greater than 1%
SiO2 Al2O3 ( Iron as FeO, Fe2O3) MgO CaO
Na2O K2O H2O
Minor elements: usually 0.1 - 1%
TiO2 MnO P2O5 CO2
Trace elements: usually < 0.1%
everything else
Weighing Elements in Rocks
Spectroscopy


Recall that the wavelength l (color) of
light is related to the speed v and the
frequency f.
Also as a light wave front changes
velocity while moving into a different
medium, it refracts, that is it changes its
direction θ.
Snell’s Law
white light
If you pass white light through a prism,
the different wavelengths are refracted
at different angles according to Snell’s
Law
A hot gas gives off characteristic colors of
light, corresponding to the photon given off
when an excited electron loses energy while
hot falling back to its normal state
Light from
glowing gas
white
light
The same gas, if cold, absorbs those
characteristic colors of light, letting the rest
Positions of emission and absorption lines same
of the spectrum pass
Chapter 8: Major Elements
Modern Spectroscopic Techniques
ICP samples are
dissolved, then
mixed with Argon
gas as they are
aspirated into a
Radio-frequency
generator. A
plasma is created,
and the emissions
are spread out with
a grating and
compared to
standards.
Energy Source
Inductively Coupled Plasma
Emitted
radiation
Emission
Detector
Absorbed
radiation
Sample
Output with
emission peak
Absorption
Detector
Atomic Absorption
AA: solution
aspirated into a
flame, and a beam
of light of
predetermined
wavelength is
passed through the
flame. The
absorption is
compared to
standards. We
have an old one in
storage.
Output with
absorption trough
Figure 8-1. The geometry of typical spectroscopic instruments. From Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
2nd floor,
Spectroscopy Lab
Mass Spectrometer
Sample is injected, then ionized in
a strong electrical field. The
charged particles move toward
plates of opposite charge, then
pass through a variable
electromagnet. For each magnetic
field strength, only one atomic
mass (green dashed line) will pass
to the detector. Isotopes vary in
mass and so can be counted.
Electron Microprobe

A beam of electrons is focused on
the specimen, and these energetic
electrons produce characteristic Xrays within a small volume of the
specimen. The characteristic X-rays
are detected at particular
wavelengths, and their intensities
are measured to determine
concentrations. All elements (except
H, He, and Li) can be detected
because each element has a specific
set of X-rays that it emits.
Atoms





All atoms with the same number of protons (same
atomic #) are said to be the same element
Atoms belonging to the same element may have
different numbers of neutrons. Each case is
referred to as a different Isotope of that element.
12C vs 14C
16O vs 18O
Charged atoms (called ions) have more or fewer
electrons than the neutral atom
Recall that positive ions (missing electrons) are
called cations. Examples Fe++ Fe+3 Na+ K+ Mg++
Ca++ Al+3 Si+4
And negative ions are called anions. Examples OHO-2, S-- , Cl- F-
Table 8.1 Chemical analysis of a Basalt, Mid-Atlantic Ridge
H2O+ (structural water)
is present as OHbonded as in hydrous
minerals such as
Amphiboles and micas
H2O- is adsorbed
water, or trapped
water along
mineral grain
boundaries
LOI loss on ignition is
weight loss after heated
to 800oC, removes
structural water.
Absorbed/trapped water
lost previously at 100oC
(Col 1/Col 2) x # cations in oxide x 100
My Oxygen
Prop calcs
 (Column 3/ sum of col 3) x 100
Given Analysis Compute Mole percents Pyroxenite
Jadeite is NaAlSi2O6
Diopside is CaMgSi2O6
We are given the following chemical analysis.
Oxide





Wt%
MolWt
Oxide
Moles
Oxide
SiO2 56.64 60.086 .9426
Na2O 4.38
61.99
.0707
Al2O3 7.21 101.963 .0707
MgO 13.30
40.312 .3299
CaO 18.46
55.96 .3299
Moles
Cation
Moles
Oxygen
.9426
1.8852
.1414
.0707
.1414 x3/2 .2121
.3299
.3299
.3299
.3299
Prop. Cations to O6
.9426 x 6/2.8278
2.00
.30
.30
.7
.7
2.8278

But pyroxenes here have 6 moles oxygens/mole, not 2.8278. Multiply moles
cation by 6/2.8278

As always, Moles Oxide = weight percentage divided by molec weight

Na
.3
Ca.7 Al.3 Mg
.7
Si2O6
= 30% Jadeite 70% Diopside
http://www.science.uwaterloo.ca/~cchieh/cact/c120/formula.html
This page checked Sept 2 2007 CLS
Volcanics considerable glass, chemical analysis needed. For
example, the Rhyolite had 72.82% SiO2
and total alkalis Na2O + K2O = 3.55 + 4.30 = 7.85% plots in
the Rhyolite field of Figure 2.4 SEE NEXT SLIDE
Phonolites have low to intermediate
silica, but very high Alkali Na2O and
K2O. They form from the partial
melting of highly Aluminous
(feldspar rich) rocks of the lower
crust. Phonolite is the fine-grain
equivalent of Nepheline Syenite
Classification of Aphanitic Igneous Rocks
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. Rhyolite had 72.82% SiO2
and total alkalis Na2O + K2O = 3.55 + 4.30 = 7.85%
Silica Undersaturation
Incompatible Phases
 Under magmatic conditions some minerals
react with free silica to form other (more
silica-rich) minerals. These reactant
minerals are said to be undersaturated
with respect to SiO2.
Typical reactions are:
 2SiO2 + NaAlSiO4 ==> NaAlSi3O8
quartz + nepheline ===> Albite
 2SiO2 + KAlSiO4 =======> KAlSi3O8
quartz + kalsilite =======> Orthoclase
 SiO2 + Mg2SiO4 =======> 2MgSiO3
quartz + Mg-rich olivine ===> Enstatite

Silica Saturation-Undersaturation

Shand (1927) proposed the following list of minerals, subdivided on the
basis of silica saturation and/or undersaturation, i.e. those that coexist with
quartz (+Q) and those that do not coexist with quartz (-Q).

Undersaturated and saturated minerals can coexist stably under magmatic
conditions, but quartz, tridymite and christobalite can only coexist stably
with saturated minerals. For example Q + ne is an impossible igneous
assemblage, as is Q + Fo (Mg – rich Ol) but Q + Fa (Fe- rich Ol) is stable.
CIPW Norm
Mode is the volume % of minerals seen
 Norm is a calculated “idealized”
mineralogy

P135:”Because many volcanic Rocks are too fine-grained to recognize
their mineral components, even microscopically, and many have a glassy
component, a method was devised to calculate an idealized mineralogy
for such rocks…by… Cross, Iddings, Pirsson, and Washington, called the
CIPW norm.” “the step-by-step technique is described in …”. Appendix B.

CIPW norms are so
complicated they are
best done by a program
CIPW Norm




The magma crystallizes
under anhydrous
conditions so that no
hydrous minerals
(Hornblende, Biotite) are
formed.
The ferromagnesian
minerals are assumed to
be free of Al2O3.
The Fe/Mg ratio for all
ferromagnesian minerals
is assumed to be the
same.
Several minerals are
assumed to be
incompatible, thus
nepheline and/or olivine
never appear with quartz
in the norm.
This is, of course, an
artificial set of
constraints, and means
that the results of the
CIPW norm do not reflect
the true course of
igneous differentiation in
nature.
CIPW Norm Cautions






A cumulate rock does not represent the melt from which it was extracted. However, if the
groundmass of a cumulate can be analyzed, it is valid to use a normative calculation to gain
information about the parental melt.
Oxidation state. If the Fe2+/Fe3+ ratio is known for the sample, the resulting calculation
should match the observed mineralogy more closely.
Pressure and temperature. Because the CIPW Norm is based on anhydrous melts and
crystallization at fairly low pressures, the resultant normative mineralogy does not reflect
observed mineralogy for all rock types. Altered normative calculations have been developed
that more correctly reflect the particular pressure regimes of the deep crust and mantle.
Carbon dioxide. The influence of CO2 in some cases, especially Carbonatite, and also certain
lamprophyre type rocks, Kimberlite and Lamproite, the presence of carbon dioxide and calcite
in the melt or accessory phases derives erroneous normative mineralogy. This is because if
carbon is not analyzed, there is excess calcium, causing normative silica undersaturation, and
increasing the calcium silicate mineral budget. Similarly, if graphite is present (as is the case
with some Kimberlites) this can produce excess C, and hence skew the calculation toward
excess carbonate. Excess elemental C also, in nature, results in reduced oxygen fugacity and
alters Fe2+/Fe3+ ratios.
Mineral disequilibrium. It is improper to calculate normative mineralogy on an igneous breccia,
for instance.
For this reason it is not advised to utilize a CIPW norm on Kimberlites, Lamproites,
lamprophyres and some silica-undersaturated igneous rocks. In the case of Carbonatite, it is
improper to use a CIPW norm upon a melt rich in carbonate.
Mt. Mazama (Crater Lake)

Felsic magmas can result from the fractionation of
intermediate magmas.
Dissolved gasses occupy a much smaller volume than free
gasses.

Intermediate Silica, and especially Felsic magmas,
have a lot of silica SiO2 and crystallize at low
temperatures. Therefore they are very viscous, and
cannot give up their dissolved volatiles when low
surface pressures cause the volatiles to come out of
solution
Mount Mazama is a destroyed
stratovolcano in the Oregon part
of the Cascade Volcanic Arc and
the Cascade Range located in
the United States. The volcano's
collapsed caldera holds Crater
Lake. It began erupting about
500,000 years ago. By about
30,000 years ago, Mount
Mazama began to generate
increasingly explosive eruptions
that were followed by thick flows
of silica-rich lava, an outward
sign of the slow accumulation of
a large volume of highly
explosive magma deep beneath
the volcano.
The cataclysmic eruption of
Mount Mazama 7,700 years ago
started from a single vent on the
northeast side. So much magma
erupted that the volcano began
to collapse in on itself. As more
magma was erupted, the
collapse progressed until a
caldera formed, 5 miles (8 km)
in diameter and one mile (1.6
km) deep.
How do we display
chemical data in a
meaningful way?
Variation Diagrams
Bivariate
diagrams
Harker
diagram
for
Crater
Lake
Figure 8-2. Harker variation
diagram for 310 analyzed volcanic
rocks from Crater Lake (Mt.
Mazama), Oregon Cascades. Data
compiled by Rick Conrey
(personal communication).
Mafic rx have Pyroxenes
CaAl2Si2O8 then (K,Na)AlSi3O8
Mafic rx have Pyroxenes
Felsic rx have Albite
Felsic rx have K-spars
K+ large, needs low
Temps to fit in xtal.
Mafic rx have Anorthite
http://minerva.union.edu/hollochk/skaer
gaard/introduction.htm
Skaergård

The Skaergård intrusion is a layered igneous intrusion in East Greenland; it was important to
the development of key concepts in igneous petrology, including magma differentiation,
fractional crystallization, and the development of layering. The Skaergård intrusion formed
when Tholeiitic magma was emplaced about 55 million years ago (boundary Paleocene and
Eocene, PETM). The body is essentially a single pulse of magma, which crystallized from the
bottom upward and the top downward. The intrusion is characterized by exceptionally welldeveloped cumulate crystal layers of Olivines, Pyroxenes, Plagioclases, and Magnetite.
http://minerva.union.edu/hollochk/skaer
gaard/introduction.htm
Skaergård
Model for circulation and
deposition within the
Skaergaard intrusion (from
Irvine et al., 1998).
As the pluton lost heat to
its upper crustal
surroundings, it crystallized
on its roof, floor, and walls.
Accumulation was aided by
the deposition of crystals
from density-driven
(convection!) currents.
These deposits have a wide
range of appearance
depending on the location
within the pluton and the
level within the pluton. In
addition, portions of the
magma chamber roof
periodically collapsed
permitting roof zone
autoliths and xenoliths to
drop into the magma
chamber and impact onto
the floor. Much of our
understanding of the roof
zone comes from the
autolith blocks, as most of
the pluton roof has been
eroded away and access to
the rest is difficult.
Models of Magmatic Evolution
Table 8-5. Chemical analyses (wt. %) of a
hypothetical set of related volcanics.
Dacite is a high
Plagioclase, low
alkali feldspar
aphanitic rock
with lower silica
than Rhyolite
LOI: Loss on ignition,
a measure of
hydration, e.g. OH- in
hornblende
Oxide
SiO2
TiO2
Al2O3
Fe2O3*
MgO
CaO
Na 2O
K2O
LOI
Total
B
50.2
1.1
14.9
10.4
7.4
10.0
2.6
1.0
1.9
99.5
BA
A
D
54.3
60.1
64.9
0.8
0.7
0.6
15.7
16.1
16.4
9.2
6.9
5.1
3.7
2.8
1.7
8.2
5.9
3.6
3.2
3.8
3.6
2.1
2.5
2.5
2.0
1.8
1.6
99.2 100.6 100.0
RD
66.2
0.5
15.3
5.1
0.9
3.5
3.9
3.1
1.2
99.7
R
71.5
0.3
14.1
2.8
0.5
1.1
3.4
4.1
1.4
99.2
B = basalt, BA = basaltic andesite, A = andesite, D = dacite,
RD = rhyo-dacite, R = rhyolite. Data from Ragland (1989)
If large magmas are initially basaltic, how do these differences occur?
Harker diagrams
Oxide vs SiO2
– Smooth trends
– 3 assumptions:
1 Rocks are related by
Fractionation
2 Trends = liquid line of
descent
3 Basalt is the parent magma
from which the others are
derived
Figure 8-7. Stacked Harker diagrams for the
calc-alkaline volcanic series of Table 8.5.
From Ragland (1989). Basic Analytical
Petrology, Oxford Univ. Press.


To get bulk, extrapolate
BA  B and further to
low SiO2
K2O is first element to
zero (at SiO2 = 46.5)
Since the solid basalt
probably had no K, 46.5%
SiO2 is interpreted to be the
concentration in the bulk
SiO2 solid extract and the
vert. blue line  the
concentration of all other
oxides
Figure 8-7. Stacked Harker diagrams for the calcalkaline volcanic series of Table 8-5 (dark circles).
From Ragland (1989). Basic Analytical Petrology,
Oxford Univ. Press.
Cation Norms (Barth – Niggli)
An alternative norm calculation based on
molecular proportions and cations
 Uses the equivalent weights . In the case
of CaO, the Equivalent Weight is the
Molecular weight. In the case of Al2O3 or
Na2O the equivalent weight is half the
molecuar weight

Cation Norm Example



http://www.amazon.com/Using-Geochemical-Data-Presentation-
Wt% oxide values
(col1) are divided by
their equivalent weights
(divide by col 2 and
multiply by col 4),
converted into cation
proportions (col 5) and
then converted into
cation%.
Then CIPW rules except
cations are allocated
differently. In the case
of CIPW norm the
proportion of
components allocated
to Albite is Na/Al/Si =
1:1:6 on the basis of
combined oxygen,
whereas in Cation Norm
the Albite allocation is
1:1:3 on the basis of
cation proportions.
The cation norm is not
recalculated on a wt%
basis, rather the result
is recalculated as a
molecular percentage.
Extrapolate the other curves
back BA  B  blue line and
read off X of oxide
Then calculate a CIPW norm, or a cation
norm, to give amts. plagioclases,
pyroxenes, olivine, Fe-Ti oxides, etc.
Symbols: an Albite, an Anorthite, di Diopside, hy
Hypersthene (old name Opx, ss Enstatite to
Ferrosilite) Olivine ol magnetite mt Ilmenite il
(FeTiO3)
Oxide
Wt%
Cation Norm
SiO2
TiO2
Al2O3
Fe2O3*
MgO
CaO
Na2O
K2O
Total
46.5
1.4
14.2
11.5
10.8
11.5
2.1
0
98.1
ab
an
di
hy
ol
mt
il
18.3
30.1
23.2
4.7
19.3
1.7
2.7
100
Magma Series
Can chemistry be used to distinguish
families of magma types?
Early on it was recognized that some
chemical parameters were very useful
in regard to distinguishing magmatic
groups
– Total Alkalis (Na2O + K2O)
– Silica (SiO2) and silica saturation
– Alumina (Al2O3)
Alkali vs. Silica diagram for Hawaiian volcanics:
Seem to be two distinct groupings: alkaline and subalkaline
Tholeiites
and CalcAlkaline
Figure 8-11. Total
alkalis vs. silica
diagram for the alkaline
and sub-alkaline rocks
of Hawaii. After
MacDonald (1968).
GSA Memoir 116
Recall from last time, we plotted Tholeiitic versus Alkaline Basalts
Ne
Volatile-free
3GPa
2GPa
1GPa
Ab
Highly undersaturated
(nepheline-bearing)
1atm
alkali olivine
basalts
Oversaturated
(quartz-bearing)
tholeiitic basalts
Fo
En
SiO2
The Basalt Tetrahedron and the Ne-Ol-Q base
Alkaline and Subalkaline fields are again distinct
Down here on the bottom plane
Figure 8-12. Left: the basalt tetrahedron (after Yoder and Tilley, 1962). J. Pet., 3, 342-532. Right: the base of the
basalt tetrahedron using cation normative minerals, with the compositions of subalkaline rocks (black) and alkaline
rocks (yellow) from Figure 8-11, projected from the Cpx Diopside. After Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548.
A Thermal divide separates the silica-saturated
(subalkaline) from the silica-undersaturated (alkaline) fields
at low pressure
Cannot cross this divide, cooling liquids move away from the
divide, so can’t derive one series from the other with
fractionation. At high pressures the phase diagram is different, but that’s another
topic, these are eruptions at the surface.
1070
Figure 8-13
1713
Liquid
Thermal
Divide
Ne + L
Ab + LAb + L
Alkaline Field
Ne + Ab
Ne
Tr + L
SubAlkaline Field
1060
Ab + Tr
Ab
Q
AFM diagram: Tilley: can further subdivide the subalkaline
magma series into a tholeiitic and a calc-alkaline series
MORs and Plumes
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.
Cascades above
subduction zone
AFM diagram showing “typical” areas for
various extents of evolution from primitive
magma types. Tholeites go through a
Ferro-Basalt stage before continuing
towards Rhyolite.
A world-wide survey suggests that there may be
some important differences between the three series
*
* http://petrology.oxfordjournals.org/content/39/6/1197.full.pdf
Modified after Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer