GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 9

Major and Minor Element Chemistry

of Igneous Rocks

February 11, 2009

Major elementsMajor elements: : usually > 1 wt.%

control properties of magmas

major constituents of essential minerals

Minor elementsMinor elements: : usually 0.1 – 1 wt.%

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

Trace elementsTrace elements: : usually < 0.1 wt.%

substitutes for major and minor elements in essential and accessory minerals

49.2 60.09 0.8188 50.622.03 95.9 0.0212 1.3116.1 101.96 0.1579 9.762.72 159.7 0.0170 1.057.77 71.85 0.1081 6.690.18 70.94 0.0025 0.166.44 40.31 0.1598 9.8810.5 56.08 0.1872 11.583.01 61.98 0.0486 3.000.14 94.2 0.0015 0.090.23 70.98 0.0032 0.200.7 18.02 0.0388 2.400.95 18.02  0.0527 3.2699.97 1.6174 100.00

WHOLE ROCK ANALYSIS OF A BASALT

Wt%Molecular

Wt.Wt%/Mol. Wt. Mole%

SiO2

TiO2

Al2O3

Fe2O3

FeOMnOMgOCaONa2OK2OP2O5

H2O+

H2O-

Ba 5Co 32Cr 220Ni 87Pb 1.29Rb 1.14

Sr 190Th 0.15U 0.16V 280Zr 160La 5.1

Trace Elements (ppm)

structural water

1 wt.% = 10,000 ppm1 ppm = 0.0001 wt.%

adsorbed water

ANALYTICAL TECHNIQUES

Energy Source AbsorptionDetectorSample

EmissionDetector

Output withabsorption trough

Output withemission peak

Absorbedradiation

Emittedradiation

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 PhonoliteSiO2 42.26 49.20 57.94 72.82 56.19TiO2 0.63 1.84 0.87 0.28 0.62Al2O3 4.23 15.74 17.02 13.27 19.04Fe2O3 3.61 3.79 3.27 1.48 2.79

FeO 6.58 7.13 4.04 1.11 2.03MnO 0.41 0.20 0.14 0.06 0.17MgO 31.24 6.73 3.33 0.39 1.07CaO 5.05 9.47 6.79 1.14 2.72Na2O 0.49 2.91 3.48 3.55 7.79K2O 0.34 1.10 1.62 4.30 5.24H2O+ 3.91 0.95 0.83 1.10 1.57

Total 98.75 99.06 99.3 99.50 99.23

Magma - Ultramafic Mafic Intermed. Felsic Alkalic

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

1) 2)

3)

4) 5)

6)

7) 8) 9)

10)

11)

13)

12)

14) 15)

Numbers show the order that mineral are figured.See Winter (2001) Appendix for instructions.

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

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).

The “Daly” GapReal or an artifact of the variation of SiO2 concentration with differentiation

Variation of major and minor oxide abundances vs. SiO2 (thought to be and

indication of the evolved character of a magmatic system)

Primitive Evolved

LiquidLines of Descent

DIFFERENTIATION INDEXES

from Winter (2001)

MAGMA SERIESMAGMA SERIESRELATED TO TECTONIC PROVINCES

CharacteristicSeries Convergent Divergent Oceanic ContinentalAlkaline yes yes yesTholeiitic yes yes yes yesCalc-alkaline yes

Plate Margin Within Plate

35 40 45 50 55 60 65 70 750

2

4

6

8

10

12

14

16

Na2O+K2O

SiO2

Picro-basalt

BasaltBasalticandesite

AndesiteDacite

Rhyolite

Trachyte

TrachydaciteTrachy-andesite

Basaltictrachy-andesiteTrachy-

basalt

TephriteBasanite

Phono-Tephrite

Tephri-phonolite

Phonolite

Foidite

Na 2

O +

K2O

SiO2

Sub-alkaline

SUBALKALINE DISCRIMINATION DIAGRAMS

40506070809010010

15

20

Al2O3

AN

Tholeiitic

Calc-Alkaline

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

Na2O + K2O

Fe2O3 + FeO

MgO

TECTONIC PROVINCE DISCRIMINATION DIAGRAMS

Rollinson (1993)

Zr Y*3

Ti/100

C

D A

B

Island- arc A,B

Ocean-floor B

Calc-alkali B,C

Within-plate DD

TECTONIC PROVINCE DISCRIMINATION DIAGRAMS

MnO*10 P2O5*10

TiO2

CAB

IAT

MORB

OIT

OIA

MgO Al2O3

FeO*

OrogenicOcean Ridge

Ocean Island

Con.

S.C.I.

Zr Sr/2

Ti/100

B

C

A

Island-arc A

Calc-alkali B

Ocean-floor C

INCOMPATABILITY OF TRACE ELEMENTS

PARTITION COEFFICIENTS (CS/CL)

Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace

Elements in Basaltic and Andesitic Rocks

Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.010 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.040 0.060 0.012 1.830 0.46 Ba 0.010 0.013 0.026 0.023 0.23 0.42 Ni 14 5 7 0.955 0.01 6.8 29Cr 0.70 10 34 1.345 0.01 2.00 7.4La 0.007 0.03 0.056 0.001 0.148 0.544 2Ce 0.006 0.02 0.092 0.007 0.082 0.843 2Nd 0.006 0.03 0.230 0.026 0.055 1.340 2Sm 0.007 0.05 0.445 0.102 0.039 1.804 1Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1Dy 0.013 0.15 0.582 1.940 0.023 2.024 1Er 0.026 0.23 0.583 4.700 0.020 1.740 1.5Yb 0.049 0.34 0.542 6.167 0.023 1.642 1.4Lu 0.045 0.42 0.506 6.950 0.019 1.563Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated

Rar

e E

arth

Ele

men

ts

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)

Most LeastIncompatible Elements

(likes magma)Compatible

Elements(likes minerals)

Roc

k/S

tand

ard

Com

p*

Common Standard Compositions for Normalizing• Chondritic meteorite• Avg. Mid-ocean Ridge Basalt (MORB)• Primitive Mantle• Primitive Ocean Island Basalt (OIB)

Enriched

DepletedNegative Anomaly

Positive Anomaly

RARE EARTH ELEMENT (REE) SPIDER DIAGRAM

Light REE Heavy REELikes Pl Likes Garnet

INTERPRETING REE DIAGRAMSINTERPRETING REE DIAGRAMSPARTIAL MELTING OF THE MANTLE

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.

Degree of Partial Melting (F)