Chapter 8: Major Elements Chapter 8: Major Elements

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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.

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Major elements

Major elements : usually greater than 1% : usually greater than 1%

SiO SiO

22

Al Al

22

O O

33

( Iron as FeO, Fe ( Iron as FeO, Fe

22

O O

33

) MgO CaO ) MgO CaO Na Na

22

O K O K

22

O H O H

22

O O

Minor elements

Minor elements : usually 0.1 - 1% : usually 0.1 - 1%

TiO TiO

22

MnO P MnO P

22

O O

55

CO CO

22

Trace elements

Trace elements : usually < 0.1% : usually < 0.1%

everything else everything else

Element Wt % Oxide Atom %

O 60.8

Si 59.3 21.2

Al 15.3 6.4

Fe 7.5 2.2

Ca 6.9 2.6

Mg 4.5 2.4

Na 2.8 1.9

Abundance of the elements Abundance of the elements

in the Earth’s crust in the Earth’s crust

Usually given as Oxides

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Weighing Elements in Rocks Weighing Elements in Rocks

Recall that the wavelength Recall that the wavelength   (color) of (color) of light is related to the speed

light is related to the speed v v and the and the

frequency f.

frequency f.

Also as a light wave front changes Also as a light wave front changes velocity while moving into a different velocity while moving into a different

medium, it refracts, that is it changes its medium, it refracts, that is it changes its

direction direction θ . .

Spectroscopy

Snell’s Law

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white light

white light

Light from hot glowing gas

Positions of emission and absorption lines same A hot gas gives off characteristic colors of

light, corresponding to the photon given off when an excited electron loses energy while falling back to its normal state

The same gas, if cold, absorbs those

characteristic colors of light, letting the rest of the spectrum pass

If you pass white light through a prism, the different wavelengths are refracted at different angles according to Snell’s Law

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Chapter 8: Major Elements Chapter 8: Major Elements

Modern Spectroscopic Techniques Modern Spectroscopic Techniques

Energy Source Absorption

Detector Sample

Emission Detector

Output with absorption trough Output with

emission peak

Absorbed radiation Emitted

radiation

Figure 8-1. The geometry of typical spectroscopic instruments. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Atomic Absorption Inductively Coupled Plasma

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.

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.

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Mass Spectrometer 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.

2nd floor,

Spectroscopy Lab

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Electron Microprobe Electron Microprobe

A beam of electrons is focused on A beam of electrons is focused on the specimen, and these energetic the specimen, and these energetic electrons produce characteristic X- electrons produce characteristic X- rays within a small volume of the rays within a small volume of the specimen. The characteristic X-rays specimen. The characteristic X-rays are detected at particular

are detected at particular

wavelengths, and their intensities wavelengths, and their intensities are measured to determine

are measured to determine

concentrations. All elements (except concentrations. All elements (except H, He, and Li) can be detected

H, He, and Li) can be detected

because each element has a specific because each element has a specific set of X-rays that it emits.

set of X-rays that it emits.

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Atoms Atoms

All atoms with the same number of protons (same atomic All atoms with the same number of protons (same atomic

#) are said to be the same element

#) are said to be the same element

Atoms belonging to the same element may have different Atoms belonging to the same element may have different numbers of neutrons. Each case is referred to as a different numbers of neutrons. Each case is referred to as a different

Isotope of that element.

Isotope of that element.

1212

C vs C vs

1414

C C

1616

O vs O vs

1818

O O

Charged atoms (called ions) have more or fewer electrons Charged atoms (called ions) have more or fewer electrons than the neutral atom

than the neutral atom

Recall that positive ions (missing electrons) are called Recall that positive ions (missing electrons) are called cations.

cations.

Examples Examples

Fe Fe

++++

Fe Fe

+3+3

Na Na

++

K K

++

Mg Mg

++++

Ca Ca

++++

Al Al

+3 +3

Si Si

+4+4

And negative ions are called anions. Examples OH And negative ions are called anions. Examples OH

--

O O

-2-2

, S , S

-- --

, ,

Cl Cl

--

F F

--

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Table 8.1 Chemical analysis of a Basalt,

Table 8.1 Chemical analysis of a Basalt,

Mid-Atlantic RidgeMid-Atlantic Ridge

(Col 1/Col 2) x # cations in oxide x 100 My OxygenProp calcs

(Column 3/ sum of col 3) x 100 H2O+ (structural water) is present as OH-

bonded 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

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Given Analysis Compute Mole percents Pyroxenite Jadeite is NaAlSi2O6 Diopside is CaMgSi2O6 We are given the following chemical analysis.

Oxide Wt% MolWt Moles Moles Moles Prop. Cations to O6 Oxide Oxide Cation Oxygen

SiO2 56.64 60.086 .9426 .9426 1.8852 .9426 x 6/2.8278 2.00

Na2O 4.38 61.99 .0707 .1414 .0707 .30

Al2O3 7.21 101.963 .0707 .1414 x3/2 .2121 .30

MgO 13.30 40.312 .3299 .3299 .3299 .7

CaO 18.46 55.96 .3299 .3299 .3299 .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

This page checked Sept 2 2007 CLS

http://www.science.uwaterloo.ca/~cchieh/cact/c120/formula.html

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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

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

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Classification of Aphanitic Igneous Rocks 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%

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Silica Undersaturation Silica Undersaturation

Incompatible Phases Incompatible Phases

 Under magmatic conditions some minerals Under magmatic conditions some minerals

react with free silica to form other (more silica- react with free silica to form other (more silica-

rich) minerals. These reactant minerals are said rich) minerals. These reactant minerals are said

to be undersaturated with respect to SiO2.

to be undersaturated with respect to SiO2.

Typical reactions are: Typical reactions are:

2SiO2 + NaAlSiO4 ==> NaAlSi3O8 2SiO2 + NaAlSiO4 ==> NaAlSi3O8 quartz + nepheline ===> Albite quartz + nepheline ===> Albite

2SiO2 + KAlSiO4 =======> KAlSi3O82SiO2 + KAlSiO4 =======> KAlSi3O8 quartz + kalsilite =======> Orthoclase quartz + kalsilite =======> Orthoclase

SiO2 + Mg2SiO4 =======> 2MgSiO3SiO2 + Mg2SiO4 =======> 2MgSiO3 quartz + Mg-rich olivine ===> Enstatite quartz + Mg-rich olivine ===> Enstatite

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Silica Saturation-Undersaturation Silica Saturation-Undersaturation

Shand (1927) proposed the following list of minerals, subdivided on the Shand (1927) proposed the following list of minerals, subdivided on the

basis of silica saturation and/or undersaturation, i.e. those that coexist with basis of silica saturation and/or undersaturation, i.e. those that coexist with

quartz (+Q) and those that do not coexist with quartz (-Q).

quartz (+Q) and those that do not coexist with quartz (-Q).

Undersaturated and saturated minerals can coexist stably under magmatic Undersaturated and saturated minerals can coexist stably under magmatic conditions, but quartz, tridymite and christobalite can only coexist stably conditions, but quartz, tridymite and christobalite can only coexist stably

with saturated minerals. For example Q + ne is an impossible igneous 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.

assemblage, as is Q + Fo (Mg – rich Ol) but Q + Fa (Fe- rich Ol) is stable.

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CIPW Norm CIPW Norm

 Mode Mode is the volume % of minerals seen is the volume % of minerals seen

 Norm Norm is a calculated “idealized” is a calculated “idealized”

mineralogy 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.

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CIPW Norm CIPW Norm

The magma crystallizes The magma crystallizes under anhydrous

under anhydrous conditions so that no conditions so that no hydrous minerals hydrous minerals

(Hornblende, Biotite) are (Hornblende, Biotite) are formed.

formed.

The ferromagnesian The ferromagnesian minerals are assumed to minerals are assumed to be free of Al

be free of Al22OO33..

The Fe/Mg ratio for all The Fe/Mg ratio for all ferromagnesian minerals ferromagnesian minerals is assumed to be the is assumed to be the same.

same.

Several minerals are Several minerals are assumed to be

assumed to be incompatible, thus incompatible, thus

nepheline and/or olivine nepheline and/or olivine never appear with quartz never appear with quartz in the norm.

in the norm.

This is, of course, an This is, of course, an artificial set of

artificial set of

constraints, and means constraints, and means that the results of the that the results of the CIPW norm do not reflect CIPW norm do not reflect the true course of

the true course of

igneous differentiation in igneous differentiation in nature.

nature.

CIPW norms are so complicated they are best done by a program

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CIPW Norm Cautions CIPW Norm Cautions

A cumulate rock does not represent the melt from which it was extracted. However, if the groundmass of a 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.

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 Oxidation state. If the Fe2+/Fe3+ ratio is known for the sample, the resulting calculation should match the observed mineralogy more closely.

observed mineralogy more closely.

Pressure and temperature. Because the CIPW Norm is based on anhydrous melts and crystallization at fairly low 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 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 normative calculations have been developed that more correctly reflect the particular pressure regimes of the deep crust and mantle.

deep crust and mantle.

Carbon dioxide. The influence of CO2 in some cases, especially Carbonatite, and also certain lamprophyre type 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 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, 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 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 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 calculation toward excess carbonate. Excess elemental C also, in nature, results in reduced oxygen fugacity and alters Fe2+/Fe3+ ratios.

alters Fe2+/Fe3+ ratios.

Mineral disequilibrium. It is improper to calculate normative mineralogy on an igneous breccia, for instance.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 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 silica-undersaturated igneous rocks. In the case of Carbonatite, it is improper to use a CIPW norm upon a melt rich in carbonate.

rich in carbonate.

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Mt. Mazama (Crater Lake)

Mt. Mazama (Crater Lake)

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 By about 30,000 years ago, Mount 30,000 years ago, Mount Mazama began to generate Mazama began to generate increasingly explosive eruptions increasingly explosive eruptions that were followed by thick flows that were followed by thick flows of silica-rich lava, an outward of silica-rich lava, an outward sign of the slow accumulation of sign of the slow accumulation of a large volume of highly

a large volume of highly

explosive magma deep beneath explosive magma deep beneath the volcano.

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.

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

Felsic magmas can result from the fractionation of intermediate magmas.

Dissolved gasses occupy a much smaller volume than free gasses.

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Bivariate Bivariate

diagrams diagrams

Harker Harker diagram diagram

for for Crater Crater

Lake 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).

How do we display How do we display

chemical data in a chemical data in a meaningful way?

meaningful way?

Variation Diagrams Variation Diagrams

Felsic rx have K-spars K+ large, needs low Temps to fit in xtal.

Felsic rx have Albite

Mafic rx have Anorthite Mafic rx have Pyroxenes

Mafic rx have Pyroxenes

CaAl2Si2O8 then(K,Na)AlSi3O8

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Skaergård Skaergård

The Skaergård intrusion is a layered igneous intrusion in East Greenland; it was important to 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, the development of key concepts in igneous petrology, including magma differentiation,

fractional crystallization, and the development of layering. The Skaergård intrusion formed fractional crystallization, and the development of layering. The Skaergård intrusion formed when Tholeiitic magma was emplaced about 55 million years ago (boundary Paleocene and 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 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 well- bottom upward and the top downward. The intrusion is characterized by exceptionally well-

developed cumulate crystal layers of Olivines, Pyroxenes, Plagioclases, and Magnetite.

developed cumulate crystal layers of Olivines, Pyroxenes, Plagioclases, and Magnetite.

http://minerva.union.edu/hollochk/skaer gaard/introduction.htm

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Skaergård

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.

http://minerva.union.edu/hollochk/skaer gaard/introduction.htm

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Models of Magmatic Evolution Models of Magmatic Evolution

hypothetical set of related volcanics.

Oxide B BA A D RD R

SiO2 50.2 54.3 60.1 64.9 66.2 71.5

TiO2 1.1 0.8 0.7 0.6 0.5 0.3

Al2O3 14.9 15.7 16.1 16.4 15.3 14.1 Fe2O3* 10.4 9.2 6.9 5.1 5.1 2.8

MgO 7.4 3.7 2.8 1.7 0.9 0.5

CaO 10.0 8.2 5.9 3.6 3.5 1.1

Na2O 2.6 3.2 3.8 3.6 3.9 3.4

K2O 1.0 2.1 2.5 2.5 3.1 4.1

LOI 1.9 2.0 1.8 1.6 1.2 1.4

Total 99.5 99.2 100.6 100.0 99.7 99.2 B = basalt, BA = basaltic andesite, A = andesite, D = dacite, RD = rhyo-dacite, R = rhyolite. Data from Ragland (1989)

Table 8-5. Chemical analyses (wt. %) of a

If large magmas are initially basaltic, how do these differences occur?

LOI: Loss on ignition, a measure of

hydration, e.g. OH- in hornblende

Dacite is a high Plagioclase, low alkali feldspar aphanitic rock with lower silica than Rhyolite

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Harker diagrams

Harker diagrams

Oxide vs SiOOxide vs SiO22

– Smooth trends Smooth trends – 3 assumptions: 3 assumptions:

1 Rocks are related by 1 Rocks are related by

Fractionation Fractionation

2 Trends = liquid line of 2 Trends = liquid line of

descent descent

3 Basalt is the parent 3 Basalt is the parent

magma from which the magma from which the

others are derived 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.

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To get bulk, extrapolate BA  B and further to low SiO

2

K

2

O is first element to zero (at SiO

2

= 46.5)

Since the solid basalt Since the solid basalt

probably had no K, 46.5%

probably had no K, 46.5%

SiO SiO

22

is interpreted to be the is interpreted to be the concentration in the bulk concentration in the bulk SiO SiO

2 2

solid extract and the solid extract and the

vert. blue line

vert. blue line   the the

concentration of all other concentration of all other

oxides oxides

Figure 8-7. Stacked Harker diagrams for the calc- alkaline volcanic series of Table 8-5 (dark circles).

From Ragland (1989). Basic Analytical Petrology, Oxford Univ. Press.

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Cation Norms (Barth – Niggli) Cation Norms (Barth – Niggli)

 An alternative norm calculation based on An alternative norm calculation based on molecular proportions and cations

molecular proportions and cations

 Uses the equivalent weights . In the case Uses the equivalent weights . In the case of CaO, the Equivalent Weight is the

of CaO, the Equivalent Weight is the

Molecular weight. In the case of Al2O3 or Molecular weight. In the case of Al2O3 or

Na2O the equivalent weight is half the Na2O the equivalent weight is half the

molecuar weight

molecuar weight

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Cation Norm Example Cation Norm Example

Wt% oxide values Wt% oxide values (col1) are divided by (col1) are divided by their equivalent weights their equivalent weights (divide by col 2 and (divide by col 2 and multiply by col 4), multiply by col 4), converted into cation converted into cation proportions (col 5) and proportions (col 5) and then converted into then converted into cation%.

cation%.

Then CIPW rules except Then CIPW rules except cations are allocated cations are allocated differently. In the case differently. In the case of CIPW norm the of CIPW norm the proportion of proportion of

components allocated components allocated to Albite is Na/Al/Si = to Albite is Na/Al/Si = 1:1:6 on the basis of 1:1:6 on the basis of combined oxygen, combined oxygen,

whereas in Cation Norm whereas in Cation Norm the Albite allocation is the Albite allocation is 1:1:3 on the basis of 1:1:3 on the basis of cation proportions.

cation proportions.

The cation norm is not The cation norm is not recalculated on a wt%

recalculated on a wt%

basis, rather the result basis, rather the result is recalculated as a is recalculated as a molecular percentage.

molecular percentage.

http://www.amazon.com/Using-Geochemical-Data-Presentation- Interpretation/dp/0582067014/ref=sr_1_1?

s=books&ie=UTF8&qid=1395072806&sr=1- 1&keywords=Rollinson+Geochemical

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Extrapolate the other curves Extrapolate the other curves back BA

back BA  B  B  blue line and blue line and read off X of oxide

read off X of oxide

Oxide Wt% Cation Norm

SiO2 46.5 ab 18.3

TiO2 1.4 an 30.1

Al2O3 14.2 di 23.2 Fe2O3* 11.5 hy 4.7

MgO 10.8 ol 19.3

CaO 11.5 mt 1.7

Na2O 2.1 il 2.7

K2O 0

Total 98.1 100

Then calculate a CIPW norm, or a cation Then calculate a CIPW norm, or a cation norm, to give amts. plagioclases,

norm, to give amts. plagioclases, pyroxenes, olivine, Fe-Ti oxides, etc.

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)

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Magma Series Magma Series

Can chemistry be used to distinguish

Can chemistry be used to distinguish families families of of magma types?

magma types?

Early on it was recognized that some chemical Early on it was recognized that some chemical

parameters were very useful in regard to parameters were very useful in regard to

distinguishing magmatic groups distinguishing magmatic groups

– Total Alkalis (Na Total Alkalis (Na

22

O + K O + K

22

O) O)

– Silica (SiO Silica (SiO

22

) and silica saturation ) and silica saturation

– Alumina (Al Alumina (Al

22

O O

33

) )

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Alkali vs. Silica diagram for Hawaiian volcanics:

Alkali vs. Silica diagram for Hawaiian volcanics:

Seem to be two distinct groupings:

Seem to be two distinct groupings: alkaline alkaline and and subalkaline subalkaline

Figure 8-11. Total alkalis vs. silica diagram for the alkaline and sub-alkaline rocks of Hawaii. After MacDonald (1968).

GSA Memoir 116

Tholeiites and Calc- Alkaline

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Ne

Fo En

Ab

SiO2

Oversaturated (quartz-bearing) tholeiitic basalts

Highly undersaturated (nepheline-bearing)

alkali olivine basalts

Undersaturated tholeiitic basalts

3GPa

2GPa 1GPa

1atm

Volatile-free

Recall from last time, we plotted Tholeiitic versus Alkaline Basalts

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The Basalt Tetrahedron and the Ne-Ol-Q base The Basalt Tetrahedron and the Ne-Ol-Q base

Alkaline and Subalkaline fields are again distinct Alkaline and Subalkaline fields are again distinct

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.

Down here on the bottom plane

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Ne Ab Q

1070 1060

1713

Ab + Tr

Tr + L Ab + L

Ne + L

Liquid

Ab + L Ne + Ab

Thermal Divide

A Thermal divide

A Thermal divide separates the silica-saturated separates the silica-saturated

(subalkaline) from the silica-undersaturated (alkaline) fields (subalkaline) from the silica-undersaturated (alkaline) fields at low pressure

at low pressure

Cannot cross this divide, cooling liquids move away from Cannot cross this divide, cooling liquids move away from the divide, so can’t derive one series from the other with the divide, so can’t derive one series from the other with fractionation

fractionation

. At high pressures the phase diagram is different, but that’s . At high pressures the phase diagram is different, but that’s another topic, these are eruptions at the surface.

another topic, these are eruptions at the surface.

SubAlkaline Field

Alkaline Field

Figure 8-13

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F

A M

Calc-alkaline Tho leiitic

AFM diagram:

AFM diagram: Tilley: Tilley: can further subdivide the subalkaline can further subdivide the subalkaline magma series into a

magma series into a tholeiitic tholeiitic and a and a calc-alkaline calc-alkaline series series

Figure 8-14. AFM diagram showing the distinction between selected tholeiitic rocks from Iceland, the Mid- Atlantic 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.

MORs and Plumes

Cascades above subduction zone

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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.

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A world-wide survey suggests that there may be A world-wide survey suggests that there may be

some important differences between the three series some important differences between the three series

Modified after Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer

* http://petrology.oxfordjournals.org/content/39/6/1197.full.pdf

*

Figure

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