Chemical Petrology 1

Chemical Petrology 1: Major and Minor elements
(Chapter 8)

last update:09/25/06

Major element chemistry of rocks and minerals

What we basically deal with:

Abundance of the elements in the Earth’s crust

Major elements: usually greater than 1%

SiO₂ Al₂O₃ FeO* MgO CaO Na₂O K₂O H₂O

Minor elements: usually 0.1 - 1%

TiO₂ MnO P₂O₅ CO₂

Trace elements: usually < 0.1%

everything else

Ways to analyze rocks and minerals

1. Initial method - "wet"-chemical gravimetric/titration techniques

	possible to analyze all elements and oxidation states of transition elements, but tedious
	most commonly used pre-1975 - analyses for other modern techniques are commonly reported as weight % of the oxides due to the traditions of the wet chemical techniques
	preparation and technique: dissolve rock powders in acids and titrate
	used for: major and minor elements

2. Modern spectroscopic techniques are generally similar in approach

Atomic absorption spectrophotometry (AA)

• preparation and technique: dissolve rock powders in acids and and vaporize on gas arc

• used for: major, minor and some trace elements

• good for natural fluids

X-ray fluorescence spectroscopy

• preparation and technique: crush rock, expose to high energy X-ray source and determine the relative intensity of characteristic X-rays relative to standard

• used for: major, minor and selected trace elements

• easier and quicker than AA

Electron microprobe analysis (EMP)

• preparation and technique: polished thin sections or melt powder, polish and then bombard with electrons and analyze characteristic X-rays (down to µm range)

• used for: major, minor and trace elements

• easy to use, but homogeneity problems and expensive


Sample OL-4 - optical images	Sample IG-2 - optical images
Sample OL-4 - backscattered electron image	Sample IG-2 - backscattered electron image

Ion microprobe analysis

• preparation and technique: minerals in polished thin section or melt powder, polish and bombard with stream of oxygen ions making a crater of material blasted into a mass spectrometer (down to 10s of µm range)

• used for: major, minor and trace elements and isotopes - one variation will do geochronology

• not easy to use, but homogeneity problems and very expensive

Proton-induced X-ray emission (PIXE)

• preparation and technique: melt powder, polish, bombard with protons and analyze characteristic X-rays (down to 10s, of µm range)

• used for: major, minor and trace elements

• easy to use, but homogeneity problems and expensive equipment

Inductively coupled plasma spectroscopy (ICP)

• preparation and technique: powder rock, dissolve sample and vaporize into plasma relative to standards

• used for: major, minor and trace elements

• easy to use, quite accurate

Instrumental neutron activation analysis (INAA)

• preparation and technique: powder rock, expose to high-flux neutron source (e.g. synchrotron) and compare to standards

• used for: trace elements

• easy to use, quite accurate

Mass spectrometry, (MS)

• preparation and technique: powder rock, dissolve sample and introduce from heated filament

• used for: isotopes

• preparation difficult, but accurate

Other possibilities

synchrotron radiation source (e.g. CAMD) for microtomography, XANES, XAFS, microXRF, XRD...

Analytical results

Rocks reported in wt% oxides (except anions such as F, Cl), parts per million by weight (ppm) for trace elements. H₂O in rock analyses is reported as H₂O⁺ (structural water) and H₂O^-(adsorbed water).

There is always analytical uncertainty that should be reported. Typical uncertainties of EMP data are +/- 2% relative of the major elements (>10%) with greater relative uncertainties for the lower concentration elements (related to the counting statistics of the individual microprobe).

CIPW Norm

Norm is a calculated "idealized" mineralogy

CIPW norm - idealized or "synthetic" mineralogy based on a "recipe" keyed to the minerals that are first to crystallize (i. e. hypothetical crystallization sequence)

	especially useful for fine-grained or glassy rocks
	estimated using anhydrous mineral equivalents
	normative mineralogy is in molar (atomic) basis - thus in order to compare to the modes requires conversion using molecular volume or molecular weights
	useful for application of some mineralogical classification criteria

e.g. Silica-saturation mineralogically implies free quartz in the rock. For a fine-grained or glassy rock a CIPW norm calculation indicating a quartz-normative composition implies that silica-saturation.

Not all igneous minerals are among the normative minerals e.g. amphibole, biotite, muscovite as well as garnet, Al₂SiO₅ polymorphs and cordierite. Thus, it is necessary to use a little "petrologic sense" when interpreting norms

Mode is the volume % of minerals seen

Used for both classification and interpretation

Modes-- measure of the mineral content of a rock - very important for mineralogical classification of igneous rocks

• visually estimated modes (vol%) - only as good as the observer

point counts - identification of mineral at regular grid intervals - quite accurate (500-5000 points)

• weight modes = vol% x specific gravity

Variation Diagrams

How do we display chemical data in a meaningful way?

Major element variation diagrams for a suite of rocks (i.e. genetically-related or representative of a given area = petrogenetic province) are best. An additional dimension can be illustrated with contouring or projections techniques or combinations of elements.

Trends or correlations show up as patterns

Bivariate (x-y) diagrams

Harker diagram for Crater Lake

These data range from basalt to rhyolite in a smooth trend. This trend suggests the lavas are genetically related.

Parental magmas (those derived directly from partial melting of a source) are generally the most primitive (Si-poor and Mg-rich) and evolve along some a differentiation trend.

	Harker diagram Smooth trends Model with 3 assumptions: 1. Rocks are related by fractional crystallization 2. Trends = liquid line of descent 3. The basalt is the parent magma from which the others are derived
Results of modeling the B to BA sequence suggests that the bulk mineral extract (fractionating crystals) must have been a mix of olivine, diopside and plagioclase (An62).

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:

Seems to be two distinct groupings: alkaline and subalkaline

Plot of >41,000 igneous rock analyses on an alkali vs silica diagram. Note the general continuum in data.

AFM diagram: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series

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.

It is common to see similar rock types generated in both the tholeiitic and calc-alkaline series, but there are differences in mineral chemistry

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

Significant features:

(1) calc-alkaline magmas are restricted to subduction zone magmas

(2) tholeiitic magmas are the predominant type at mid-ocean ridges

Chemical Petrology 2: Trace elements and isotopes
(Chapter 9)

last update:09/25/06

	Note magnitude of major element changes Harker diagram for Crater Lake
	Compare to the range of trace elements Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Element Distribution

Goldschmidt’s rules (simplistic, but useful)

(1) 2 ions with the same valence and radius should exchange easily and enter a solid solution in amounts equal to their overall proportions

How does Rb behave? Ni?

The Shannon effective ionic radius (in angstroms) table gives relationship between coordination of the ion and ionic radius.

Chemical Fractionation

The uneven distribution of an ion between two competing (equilibrium) phases

Exchange equilibrium of a component i between two phases (solid and liquid)

i _(liquid) = i _(solid)

(eq. 9-2) K = aⁱ_solid/aⁱ_liquid= gXⁱ_solid/gXⁱ_liquid

K = equilibrium constant

	Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system
	Thus if X_Ni in the system doubles the X_Ni in all phases will double

This does not mean that X_Ni in all phases is the same, since trace elements do fractionate. Rather the X_Ni within each phase will vary in proportion to the system concentration

incompatible elements are concentrated in the melt

(K_D or D) « 1

compatible elements are concentrated in the solid

K_D or D » 1

For dilute solutions can substitute D for K_D:

D = C_S/C_L

Where C_S = the concentration of some element in the solid phase

Incompatible elements (goes into melt) commonly - two subgroups

Smaller, highly charged high field strength (HFS) elements (REE, Th, U, Ce, Pb⁴⁺, Zr, Hf, Ti, Nb, Ta)

Low field strength large ion lithophile (LIL) elements (K, Rb, Cs, Ba, Pb²⁺, Sr, Eu²⁺) are more mobile, particularly if a fluid phase is involved

Compatibility depends on minerals and melts involved.

Which are incompatible? Why?

For a rock, determine the bulk distribution coefficient D for an element by calculating the contribution for each mineral

(eq. 9-4) D_i = S W_A D_i

W_A = weight % of mineral A in the rock

D_i = partition coefficient of element i in mineral A

Example: hypothetical garnet lherzolite = 60% olivine, 25% orthopyroxene, 10% clinopyroxene, and 5% garnet (all by weight), using the data in Table 9-1, is:

D_Er = (0.6 · 0.026) + (0.25 · 0.23) + (0.10 · 0.583) + (0.05 · 4.7) = 0.366

Trace elements strongly partitioned into a single mineral

Ni partitioning into olivine (in Table 9-1) = 14

Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

	Incompatible trace elements concentrate ® liquid
	Reflect the proportion of liquid at a given state of crystallization or melting

Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Models of Magma Evolution

Batch Melting

The melt remains resident until at some point it is released and moves upward

Equilibrium melting process with variable % melting

(eq. 9-5) C_L/C_O = 1/(D(1-F)+F)

C_L = trace element concentration in the liquid

C_O = trace element concentration in the original rock before melting began

F = wt fraction of melt produced = melt/(melt + rock)

	A plot of C_L/C_O vs. F for various values of D_i using eq. 9-5 D_i = 1.0 D_i » 1.0 (compatible elements) Very low concentration in melt Especially for low % melting (low F) D_i << 1.0 (Highly incompatible elements) Greatly concentrated in the initial small fraction of melt produced by partial melting Subsequently diluted as F increases
As F ® 1 the concentration of every trace element in the liquid = the source rock (C_L/C_O ® 1) As F ® 1; C_L/C_O ® 1
As F ® 0 C_L/C_O ® 1/D_i If we know C_L of a magma derived by a small degree of batch melting, and we know D_i we can estimate the concentration of that element in the source region (C_O)
For very incompatible elements as D_i ® 0 equation 9-5 reduces to: (eq. 9-7) C_L/C_O = 1/F If we know the concentration of a very incompatible element in both a magma and the source rock, we can determine the fraction of partial melt produced

Rare Earth Elements (REEs)

REE have contrasts and similarities in the D values:

All are incompatible

Also Note:

HREE are less incompatible; Especially in garnet

Eu can ® 2+ which conc. in plagioclase

REE Diagrams

Plots of concentration as the ordinate (y-axis) against increasing atomic number

Degree of compatibility increases from left to right across the diagram

Eliminate Oddo-Harkins effect and make y-scale more functional by normalizing to a standard

estimates of primordial mantle REE

chondrite meteorite concentrations

REE diagrams using batch melting model of a garnet lherzolite for various values of F:

Europium anomaly when plagioclase is a

fractionating phenocryst

or

a residual solid in source

Application of Trace Elements to Igneous Systems

1. Use like major elements on variation diagrams to document fractional crystallization, assimilation, etc. in a suite of rocks

More sensitive ® larger variations as process continues

Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

2. Identification of the source rock or a particular mineral involved in either partial melting or fractional crystallization processes

Garnet concentrates the HREE and fractionates among them

Thus if garnet is in equilibrium with the partial melt (a residual phase in the source left behind) expect a steep (-) slope in REE and HREE

Shallow (< 40 km) partial melting of the mantle will have plagioclase in the residuum and a Eu anomaly will result

Garnet and Plagioclase effect on HREE

Trace elements as a tool to determine paleotectonic environment

Useful for rocks in mobile belts that are no longer recognizably in their original setting

Can trace elements be discriminators of igneous environment?

Approach is empirical on modern occurrences

Concentrate on elements that are immobile during low/medium grade metamorphism

(a) after Pearce and Cann (1973), Earth Planet, Sci. Lett., 19, 290-300. (b) after Pearce (1982) in Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548, Coish et al. (1986), Amer. J. Sci., 286, 1-28. (c) after Mullen (1983), Earth Planet. Sci. Lett., 62, 53-62.

Isotopes

Same Z, different at. wt. (variable # of neutrons)

General notation for a nuclide:

As n varies ® different isotopes of an element

¹²C ¹³C ¹⁴C

Stable isotope: last ~ forever

Chemical fractionation is impossible - same electron configuration

Mass fractionation is the only type possible

Example: Oxygen Isotopes

^{16O
99.756% of natural oxygen
^{17O
0.039% "
^{18O
0.205% "}}}

Concentrations expressed by reference to a standard

International standard for O isotopes = standard mean ocean water (SMOW)

^{18O
and ¹⁶O are the commonly used isotopes and their ratio is expressed
as d:
d(¹⁸O/¹⁶O)
= 1000 x ((¹⁸O/¹⁶O)_sample
- (¹⁸O/¹⁶O)_SMOW)/(¹⁸O/¹⁶O)_SMOW

result expressed in per mille (‰)

Stable isotopes useful in assessing relative
contribution of various reservoirs, each with a distinctive isotopic signature

O and H isotopes - juvenile vs. meteoric
vs. brine water
d¹⁸O
for mantle rocks vs. surface-reworked sediments: evaluate contamination of
mantle-derived magmas by crustal sediments

Radioactive Isotopes
Unstable isotopes decay to other nuclides
The rate of decay is
constant, and not affected by P, T, X…
Parent nuclide =
radioactive nuclide that decays
Daughter nuclide(s) are
the radiogenic atomic products

Radioactive Decay

The Law of Radioactive Decay
(eq. 9-11) -(dN/dt)
is proportional to N (where N is
the # of parent atoms)
or -(dN/dt) = lN
(where l
is the decay constant)

(eq. 9-15) D = Ne^lt
- N= N(e^lt
-1)
® age
of a sample (t) if we know:

D the amount of the daughter
nuclide produced
N the amount of the original parent nuclide
remaining
l the
decay constant for the system in question

The U-Pb-Th System

Very complex system.

3 radioactive
isotopes of U: ²³⁴U, ²³⁵U, ²³⁸U
3 radiogenic
isotopes of Pb: ²⁰⁶Pb, ²⁰⁷Pb, and ²⁰⁸Pb

Only ²⁰⁴Pb is strictly
non-radiogenic

U, Th, and Pb are incompatible elements,
& concentrate in early melts
Isotopic composition of Pb in rocks =
function of

^{238U
® ²³⁴U
® ²⁰⁶Pb
(l =
1.5512 x 10^-10 a^-1)
^{235U
®
²⁰⁷Pb (l
= 9.8485 x 10^-10 a^-1)
²³²Th ®
²⁰⁸Pb (l
= 4.9475 x 10^-11 a^-1)}}}

	Concordia = Simultaneous co-evolution of ²⁰⁶Pb and ²⁰⁷Pb via:^{238U ® ²³⁴U ® ²⁰⁶Pb ^{235U ® ²⁰⁷Pb}}
	Discordia = loss of both ²⁰⁶Pb and ²⁰⁷Pb
Concordia diagram after 3.5 Ga total evolution