Chapter 8: Major Chapter 8: Major ElementsElements

Major and Minor Elements shown in orange.Concentrations (wt%) usually given as oxidesIn 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.

Major elementsMajor elements: usually greater than 1%: usually greater than 1%SiOSiO22 Al Al22OO33 ( Iron as FeO, Fe ( Iron as FeO, Fe22OO33) MgO CaO ) MgO CaO

NaNa22O KO K22O HO H22OO

Minor elementsMinor elements: usually 0.1 - 1%: usually 0.1 - 1%TiOTiO22 MnO P MnO P22OO55 CO CO22

Trace elementsTrace elements: usually < 0.1%: usually < 0.1%everything elseeverything else

Element Wt % Oxide Atom %O 60.8Si 59.3 21.2Al 15.3 6.4Fe 7.5 2.2Ca 6.9 2.6Mg 4.5 2.4Na 2.8 1.9

Abundance of the elementsAbundance of the elementsin the Earth’s crustin the Earth’s crust

Usually given as Oxides

Weighing Elements in RocksWeighing Elements in Rocks

Recall that the wavelength Recall that the wavelength (color) (color) of light is related to the speed of light is related to the speed vv and the frequency f.and the frequency f.

Also as a light wave front changes Also as a light wave front changes velocity while moving into a velocity while moving into a different medium, it refracts, that is different medium, it refracts, that is it changes its direction it changes its direction θ..

Spectroscopy

Snell’s Law

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

Chapter 8: Major Chapter 8: Major ElementsElements

Modern Spectroscopic Modern Spectroscopic TechniquesTechniques

Energy Source AbsorptionDetectorSample

EmissionDetector

Output withabsorption trough

Output withemission peak

Absorbedradiation

Emittedradiation

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 PlasmaAA: 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.

Mass SpectrometerMass 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

Electron MicroprobeElectron Microprobe

A beam of electrons is focused A beam of electrons is focused on the specimen, and these on the specimen, and these energetic electrons produce energetic electrons produce characteristic X-rays within a characteristic X-rays within a small volume of the specimen. small volume of the specimen. The characteristic X-rays are The characteristic X-rays are detected at particular detected at particular wavelengths, and their wavelengths, and their intensities are measured to intensities are measured to determine concentrations. All determine concentrations. All elements (except H, He, and Li) elements (except H, He, and Li) can be detected because each can be detected because each element has a specific set of X-element has a specific set of X-rays that it emits.rays that it emits.

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

atomic #) are said to be the same elementatomic #) are said to be the same element Atoms belonging to the same element may have Atoms belonging to the same element may have

different numbers of neutrons. Each case is referred to different numbers of neutrons. Each case is referred to as a different Isotope of that element. as a different Isotope of that element. 1212C vs C vs 1414C C 1616O O vs vs 1818OO

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

Recall that positive ions (missing electrons) are called Recall that positive ions (missing electrons) are called cations. cations. Examples Examples FeFe++++ Fe Fe+3+3 Na Na++ K K++ Mg Mg++++ Ca Ca++++ Al Al+3 +3 SiSi+4+4

And negative ions are called anions. Examples OHAnd negative ions are called anions. Examples OH-- O O--

22, S, S-- -- , Cl, Cl-- F F--

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 (Column 3/ sum of col 3) x 100

My OxygenProp calcs

H2O+ (structural water) is present as OH- bonded as in hydrous minerals such as Amphiboles and micasH2O- 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

Given Analysis Compute Mole percents Pyroxenite Jadeite is NaAlSi2O6 Diopside is CaMgSi2O6We 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

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

Classification of Aphanitic Igneous RocksClassification 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 UndersaturationSilica Undersaturation

Incompatible PhasesIncompatible Phases Under magmatic conditions some minerals Under magmatic conditions some minerals

react with free silica to form other (more react with free silica to form other (more silica-rich) minerals. These reactant silica-rich) minerals. These reactant minerals are said to be undersaturated with minerals are said to be undersaturated with respect to SiO2. 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

Silica Saturation-Silica Saturation-UndersaturationUndersaturation Shand (1927) proposed the following list of minerals, subdivided on Shand (1927) proposed the following list of minerals, subdivided on

the basis of silica saturation and/or undersaturation, i.e. those that the basis of silica saturation and/or undersaturation, i.e. those that coexist with quartz (+Q) and those that do not coexist with quartz (-coexist with quartz (+Q) and those that do not coexist with quartz (-Q). Q).

Undersaturated and saturated minerals can coexist stably under Undersaturated and saturated minerals can coexist stably under magmatic conditions, but quartz, tridymite and christobalite can only magmatic conditions, but quartz, tridymite and christobalite can only coexist stably with saturated minerals. For example Q + ne is an coexist stably with saturated minerals. For example Q + ne is an impossible igneous assemblage, as is Q + Fo (Mg – rich Ol) but Q + Fa impossible igneous assemblage, as is Q + Fo (Mg – rich Ol) but Q + Fa (Fe- rich Ol) is stable.(Fe- rich Ol) is stable.

CIPW NormCIPW Norm

ModeMode is the volume % of minerals is the volume % of minerals seenseen

NormNorm is a calculated “idealized” is a calculated “idealized” mineralogymineralogy

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 NormCIPW Norm

The magma The magma crystallizes under crystallizes under anhydrous conditions anhydrous conditions so that no hydrous so that no hydrous minerals (Hornblende, minerals (Hornblende, Biotite) are formed.Biotite) are formed.

The ferromagnesian The ferromagnesian minerals are assumed minerals are assumed to be free of Alto be free of Al22OO33..

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

Several minerals are Several minerals are assumed to be assumed to be incompatible, thus incompatible, thus nepheline and/or nepheline and/or olivine never appear olivine never appear with quartz in the with quartz in the norm.norm.

This is, of course, an This is, of course, an artificial set of artificial set of constraints, and constraints, and means that the results means that the results of the CIPW norm do of the CIPW norm do not reflect the true not reflect the true course of igneous course of igneous differentiation in differentiation in nature.nature.

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

CIPW Norm CautionsCIPW Norm Cautions A cumulate rock does not represent the melt from which it was extracted. However, if the 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 groundmass of a cumulate can be analyzed, it is valid to use a normative calculation to gain information about the parental melt.information about the parental melt.

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

Pressure and temperature. Because the CIPW Norm is based on anhydrous melts and crystallization 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 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 for all rock types. Altered normative calculations have been developed that more correctly reflect the particular pressure regimes of the deep crust and mantle.the particular pressure regimes of the deep crust and mantle.

Carbon dioxide. The influence of CO2 in some cases, especially Carbonatite, and also certain 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 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 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 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 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. 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.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 Mineral disequilibrium. It is improper to calculate normative mineralogy on an igneous breccia, for instance.instance.

For this reason it is not advised to utilize a CIPW norm on Kimberlites, Lamproites, lamprophyres 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 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.CIPW norm upon a melt rich in carbonate.

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

Bivariate Bivariate diagramsdiagrams

HarkerHarkerdiagram diagram

forforCraterCraterLakeLake

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 Variation DiagramsDiagrams

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

Felsic rx have Albite

Mafic rx have AnorthiteMafic rx have Pyroxenes

Mafic rx have Pyroxenes

CaAl2Si2O8 then (K,Na)AlSi3O8

SkaergårdSkaergård

The Skaergård intrusion is a layered igneous intrusion in East Greenland; it was The Skaergård intrusion is a layered igneous intrusion in East Greenland; it was important to the development of key concepts in igneous petrology, including important to the development of key concepts in igneous petrology, including magma differentiation, fractional crystallization, and the development of layering. magma differentiation, fractional crystallization, and the development of layering. The Skaergård intrusion formed when Tholeiitic magma was emplaced about 55 The Skaergård intrusion formed when Tholeiitic magma was emplaced about 55 million years ago (boundary Paleocene and Eocene, PETM). The body is essentially a 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 single pulse of magma, which crystallized from the bottom upward and the top downward. The intrusion is characterized by exceptionally well-developed cumulate downward. The intrusion is characterized by exceptionally well-developed cumulate crystal layers of Olivines, Pyroxenes, Plagioclases, and Magnetite.crystal layers of Olivines, Pyroxenes, Plagioclases, and Magnetite.

http://minerva.union.edu/hollochk/skaergaard/introduction.htm

SkaergårdSkaergårdModel 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/skaergaard/introduction.htm

Models of Magmatic Models of Magmatic EvolutionEvolution

hypothetical set of related volcanics.

Oxide B BA A D RD R

SiO 2 50.2 54.3 60.1 64.9 66.2 71.5

TiO 2 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

Harker diagrams Harker diagrams Oxide vs SiOOxide vs SiO22

– Smooth trendsSmooth trends– 3 assumptions:3 assumptions:

1 Rocks are related by 1 Rocks are related by FractionationFractionation

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

3 Basalt is the parent 3 Basalt is the parent magma from which the magma from which the others are derivedothers 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 Since the solid basalt probably had no K, 46.5% probably had no K, 46.5% SiOSiO22 is interpreted to be the is interpreted to be the

concentration in the bulk concentration in the bulk SiOSiO2 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 oxidesoxides

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.

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

An alternative norm calculation An alternative norm calculation based on molecular proportions and based on molecular proportions and cationscations

Uses the equivalent weights . In the Uses the equivalent weights . In the case of CaO, the Equivalent Weight is case of CaO, the Equivalent Weight is the Molecular weight. In the case of the Molecular weight. In the case of Al2O3 or Na2O the equivalent weight Al2O3 or Na2O the equivalent weight is half the molecuar weightis half the molecuar weight

Cation Norm ExampleCation Norm Example Wt% oxide values Wt% oxide values

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

Then CIPW rules Then CIPW rules except cations are except cations are allocated differently. allocated differently. In the case of CIPW In the case of CIPW norm the proportion norm the proportion of components of components allocated to Albite is allocated to Albite is Na/Al/Si = 1:1:6 on Na/Al/Si = 1:1:6 on the basis of combined the basis of combined oxygen, whereas in oxygen, whereas in Cation Norm the Cation Norm the Albite allocation is 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 basis, rather the result is recalculated result is recalculated as a molecular as a molecular percentage.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

Extrapolate the other curves Extrapolate the other curves back BA back BA B B blue line and blue line and read off X of oxideread off X of oxide

Oxide Wt% Cation Norm

SiO2 46.5 ab 18.3TiO2 1.4 an 30.1Al2O3 14.2 di 23.2Fe2O3* 11.5 hy 4.7MgO 10.8 ol 19.3CaO 11.5 mt 1.7Na2O 2.1 il 2.7K2O 0Total 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)

Magma SeriesMagma SeriesCan chemistry be used to distinguish Can chemistry be used to distinguish

familiesfamilies of magma types? of magma types?

Early on it was recognized that some Early on it was recognized that some chemical parameters were very useful in chemical parameters were very useful in regard to distinguishing magmatic groupsregard to distinguishing magmatic groups

– Total Alkalis (NaTotal Alkalis (Na22O + KO + K22O)O)

– Silica (SiOSilica (SiO22) and silica saturation) and silica saturation

– Alumina (AlAlumina (Al22OO33))

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: alkalinealkaline and and subalkalinesubalkaline

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

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

The Basalt Tetrahedron and the Ne-Ol-Q baseThe Basalt Tetrahedron and the Ne-Ol-Q baseAlkaline and Subalkaline fields are again distinctAlkaline 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

Ne Ab Q

1070 1060

1713

Ab + Tr

Tr + L

Ab + LNe + L

Liquid

Ab + L

Ne + Ab

ThermalDivide

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 pressureat 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 fractionationfractionation. 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 FieldAlkaline Field

Figure 8-13

F

A M

Calc-alkaline

T

ho 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-alkalinecalc-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

AFM diagram showing “typical” areas for various extents of evolution from primitive magma types. Tholeites go through a Ferro-Basalt stage before continuingtowards Rhyolite.

A world-wide survey suggests that there may be A world-wide survey suggests that there may be some important differences between the three seriessome 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

*