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Geol 2312 Igneous and Metamorphic Petrology. Lecture 9 Major and Minor Element Chemistry of Igneous Rocks. February 11, 2009. Whole Rock Analysis of a Basalt. Molecular Wt. Wt%/ Mol. Wt. Wt%. Mole%. Major elements : usually > 1 wt.% control properties of magmas

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Geol 2312 Igneous and Metamorphic Petrology

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Geol 2312 igneous and metamorphic petrology

Geol 2312 Igneous and Metamorphic Petrology

Lecture 9

Major and Minor Element Chemistry

of Igneous Rocks

February 11, 2009


Whole rock analysis of a basalt

Whole Rock Analysis of a Basalt

Molecular

Wt.

Wt%/

Mol. Wt.

Wt%

Mole%

Major elements:

usually > 1 wt.%

control properties of magmas

major constituents of essential minerals

Minor elements:

usually 0.1 – 1 wt.%

substitutes for major elements in essential minerals or may form small amounts of accessory mins.

Trace elements:

usually < 0.1 wt.%

substitutes for major and minor elements in essential and accessory minerals

structural water

Trace Elements (ppm)

adsorbed water

1 wt.% = 10,000 ppm

1 ppm = 0.0001 wt.%


Analytical techniques

Analytical Techniques

  • Whole Rock Analyses

  • - X-ray Fluorescence (XRF)

    • X-rays excite inner shell electrons producing secondary X-rays

  • - Inductively Coupled Plasma (ICP)

    • dissolved rock mixed with Ar gas is turned into plasma which excites atoms; generates X-rays

  • - Instrumental Neutron Activation (INAA)

    • nuclei bombarded with neutrons turning atoms radioactive; measure emitted X-rays

  • - Mass Spectrometry(MS)

    • atoms ionized and propelled through a curved electromagnet which seperates the ions by weight (good for isotope analysis)

  • Mineral Chemical Analyses

  • - Electron Microprobe (EM)

    • incident electron beam generates X-rays which whose characteristic wavelengths are measured (WDS)

  • - Energy Dispersive Spectrometry (EDS)

    • incident electron beam generates X-rays which whose characteristic energies are measured; attached to UMD’s SEM

  • - X-ray Diffractometry(XRD)

    • Incident X-rays are diffracted by characteristic mineral structure


Chemical analyses of common rock types that approximate magma compositions

Rock -

Peridotite

Basalt

Andesite

Rhyolite

Phonolite

SiO2

42.26

49.20

57.94

72.82

56.19

TiO2

0.63

1.84

0.87

0.28

0.62

Al2O3

4.23

15.74

17.02

13.27

19.04

Fe2O3

3.61

3.79

3.27

1.48

2.79

FeO

6.58

7.13

4.04

1.11

2.03

MnO

0.41

0.20

0.14

0.06

0.17

MgO

31.24

6.73

3.33

0.39

1.07

CaO

5.05

9.47

6.79

1.14

2.72

Na2O

0.49

2.91

3.48

3.55

7.79

K2O

0.34

1.10

1.62

4.30

5.24

H2O+

3.91

0.95

0.83

1.10

1.57

Total

98.75

99.06

99.3

99.50

99.23

Chemical Analyses of Common Rock Types that Approximate Magma Compositions

Magma - UltramaficMaficIntermed. Felsic Alkalic


Cipw normative calculations

CIPW Normative Calculations

  • Mode is the volume % of minerals observed

  • Norm is the weight % of minerals calculated from whole rock geochemical analyses by distributing major elements among rock-forming minerals

Numbers show the order that mineral are figured.

See Winter (2001) Appendix for instructions.

13)

14)

15)

11)

12)

7)

4)

8)

5)

9)

10)

1)

2)

6)

3)


Geochemical plots

Geochemical Plots

Objective: to show the co-variation of elemental components that may give insight to magmatic processes such as-

  • partial melting

  • magma mixing

  • country rock assimilation/contamination

  • fractional crystallization

    (or crystallization differentiation)

    Types:

  • bivariate (X-Y)

  • triangular

  • normalization plots (spider diagrams)


Harker variation diagrams

Harker Variation Diagrams

Liquid

Lines of

Descent

Variation of major and minor oxide abundances vs. SiO2 (thought to be and indication of the evolved character of a magmatic system)

The “Daly” Gap

Real or an artifact of the variation of SiO2 concentration with differentiation

Winter (2001) 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).

Evolved

Primitive


Differentiation indexes

Differentiation Indexes

from Winter (2001)


Magma series related to tectonic provinces

Magma SeriesRelated to Tectonic Provinces

Sub-

alkaline

Na2O + K2O

SiO2


Subalkaline discrimination diagrams

Subalkaline Discrimination Diagrams

Fe2O3 + FeO

AFM Diagram

Tholeiitic--Calc-Alkaline boundary after Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548

MgO

Na2O + K2O


Tectonic province discrimination diagrams

Tectonic Province Discrimination Diagrams

Rollinson (1993)


Tectonic province discrimination diagrams1

Tectonic Province Discrimination Diagrams


Incompatability of trace elements partition coefficients c s c l

Incompatability of Trace ElementsPartition Coefficients (Cs/Cl)


Incompatability of trace elements

Incompatability of Trace Elements

Incompatible elements commonly  two subgroups

  • Smaller, highly charged high field strength (HFS) elements (REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta); relatively immobile during metamorphism and alteration

  • 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

    Best to plot concentratoin of trace elements relative to some standard composition


Trace element normalization plots spider diagrams

Trace Element Normalization Plots (Spider Diagrams)

Positive

Anomaly

Enriched

Rock/Standard Comp*

Depleted

Negative

Anomaly

Most Least

Incompatible Elements

(likes magma)

Compatible

Elements

(likes minerals)

  • Common Standard Compositions for Normalizing

  • Chondritic meteorite

  • Avg. Mid-ocean Ridge Basalt (MORB)

  • Primitive Mantle

  • Primitive Ocean Island Basalt (OIB)


Rare earth element ree spider diagram

Rare Earth Element (REE) Spider Diagram

Light REE

Heavy REE

Likes Pl

Likes Garnet


Interpreting ree diagrams partial melting of the mantle

Interpreting REE DiagramsPartial Melting of the Mantle

Degree of Partial Melting (F)

Winter (2001) Figure 9-4.

Rare Earth concentrations (normalized to chondrite) for melts produced at various values of F via melting of a hypothetical garnet lherzolite using the batch melting model (equation 9-5). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


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