xiiia.rocas igneas quimica

7/28/2019 XIIIA.rocas Igneas Quimica

1/17

Major and Minor Element Chemistry

of Igneous Rocks


2/17

Whole Rock Analysis of a Basalt

Major elements :usually > 1 wt.%control properties of magmasmajor constituents of essentialminerals

Minor elements :usually 0.1 1 wt.%

substitutes for major elements inessential minerals or may form smallamounts of accessory mins.

Trace elements :usually < 0.1 wt.%substitutes for major and minor elements in

essential and accessory minerals

49.2 60.09 0.8188 50.62

2.03 95.9 0.0212 1.31

16.1 101.96 0.1579 9.76

2.72 159.7 0.0170 1.05

7.77 71.85 0.1081 6.69

0.18 70.94 0.0025 0.16

6.44 40.31 0.1598 9.8810.5 56.08 0.1872 11.58

3.01 61.98 0.0486 3.00

0.14 94.2 0.0015 0.09

0.23 70.98 0.0032 0.20

0.7 18.02 0.0388 2.40

0.95 18.02 0.0527 3.26

99.97 1.6174 100.00

Wt%Molecular

Wt.Wt%/Mol. Wt. Mole%

SiO 2 TiO2

Al2O 3 Fe 2O 3 FeOMnO

MgOCaONa 2OK2OP 2O 5 H2O+ H2O -

Ba 5

Co 32

Cr 220

Ni 87

Pb 1.29

Rb 1.14

Sr 190

Th 0.15

U 0.16

V 280

Zr 160

La 5.1

Trace Elements (ppm)

structural water

1 wt.% = 10,000 ppm

1 ppm = 0.0001 wt.%

adsorbed water


3/17

Analytical Techniques

Energy Source AbsorptionDetector Sample

EmissionDetector

Output withabsorption trough

Output withemission peak

Absorbedradiation

Emittedradiation

Whole Rock Analyses- X-ray Fluorescence (XRF)

X-rays excite inner shell electrons producingsecondary X-rays

- Inductively Coupled Plasma (ICP)dissolved rock mixed with Ar gas is turned intoplasma which excites atoms; generates X-rays

- Instrumental Neutron Activation (INAA)nuclei bombarded with neutrons turning atomsradioactive; measure emitted X-rays

- Mass Spectrometry(MS)atoms ionized and propelled through a curvedelectromagnet which seperates the ions byweight (good for isotope analysis)

Mineral Chemical Analyses- Electron Microprobe (EM)

incident electron beam generates X-rayswhich whose characteristic wavelengths

are measured (WDS)- Energy Dispersive Spectrometry (EDS)incident electron beam generates X-rayswhich whose characteristic energies aremeasured; attached to UMDs SEM

- X-ray Diffractometry(XRD)Incident X-rays are diffracted bycharacteristic mineral structure


4/17

Chemical Analyses of Common RockTypes that Approximate Magma

CompositionsRock - 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

Magma - Ultramafic Mafic Intermed. Felsic Alkalic


5/17

CIPW Normative CalculationsMode is the volume % of minerals observed

Norm is the weight % of minerals calculated fromwhole rock geochemical analyses by distributingmajor elements among rock-forming minerals

1)2)

3)

4)5)

6)

7)8)9)

10)

11)

13)

12)

14)15)

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


6/17

Geochemical Plots

Objective: to show the co-variation of elemental components thatmay 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)


7/17

Harker VariationDiagrams

Winter (2001) Figure 8-2. Harker variation diagram for 310analyzed volcanic rocks fromCrater Lake (Mt. Mazama),

Oregon Cascades. Data compiledby Rick Conrey (personalcommunication).

The Daly Gap Real or an artifact of the variation of SiO 2

concentration withdifferentiation

Variation of major andminor oxide abundancesvs. SiO 2 (thought to be andindication of the evolved character of a magmatic system)

Primitive Evolved

LiquidLines of Descent


8/17

Differentiation Indexes

from Winter (2001)


9/17

Magma SeriesRelated to Tectonic ProvincesCharacteristic

Series Convergent Divergent Oceanic Continenta l Alkaline 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

Picro-basalt

BasaltBasalticandesite

AndesiteDacite

Rhyolite

Trachyte

TrachydaciteTrachy-andesite

Basaltictrachy-andesiteTrachy-

basalt

TephriteBasanite

Phono-Tephrite

Tephri-phonolite

Phonolite

Foidite

N a

2 O

+

K 2

O

SiO 2

Sub-alkaline


10/17

Subalkaline Discrimination Diagrams

40506070809010010

15

20

Al 2O 3

AN

Tholeiitic

Calc-Alkaline

AFM DiagramTholeiitic--Calc-Alkalineboundary after Irvine andBaragar (1971). Can. J. EarthSci., 8, 523-548

Na 2O + K 2O

Fe 2O3 + FeO

MgO


11/17

TectonicProvinceDiscrimination Diagrams

Rollinson (1993)


12/17

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 P 2O 5*10

TiO 2

CAB

IAT

MORB

OIT

OIA

MgO Al 2O 3

FeO*

Orogenic

Ocean Ridge

Ocean Island

Con.

S.C.I.

Zr Sr/2

Ti/100

B

C

A

Island-arc A

Calc-alkali B

Ocean-floor C


13/17

Incompatability of Trace ElementsPartition Coefficients (C s/C l)

Table 9-1 . Partition Coefficients (C S /C L) for Some Commonly Used TraceElements in Basaltic and Andesitic Rocks

Olivine Opx Cpx Garnet Plag Amph MagnetiteRb 0.010 0.022 0.031 0.042 0.071 0.29Sr 0.014 0.040 0.060 0.012 1.830 0.46

Ba 0.010 0.013 0.026 0.023 0.23 0.42Ni 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). * Eu 3+ /Eu 2+ Italics are estimated

R a r e

E a r t

h E l e m e n

t s


14/17

Incompatability of Trace Elements

Incompatible elements commonly two subgroups

Smaller, highly charged high field strength (HFS)elements (REE, Th, U, Ce, Pb 4+ , Zr, Hf, Ti, Nb, Ta);relatively immobile during metamorphism andalteration

Low field strength large ion lithophile (LIL) elements(K, Rb, Cs, Ba, Pb 2+ , Sr, Eu 2+) are more mobile,particularly if a fluid phase is involved

Best to plot concentratoin of trace elementsrelative to some standard composition


15/17

Trace Element Normalization Plots(Spider Diagrams)

Most LeastIncompatible Elements

(likes magma)Compatible

Elements(likes minerals)

R o c

k / S t a n

d a r d

C o m p

*

Common Standard Compositions for Normalizing Chondritic meteorite Avg. Mid-ocean Ridge Basalt (MORB)

Primitive Mantle Primitive Ocean Island Basalt (OIB)

Enriched

Depleted Negative

Anomaly

Positive Anomaly


16/17

Rare Earth Element (REE)Spider Diagram

Light REE Heavy REELikes Pl Likes Garnet


17/17

Interpreting REE DiagramsPartial Melting of the Mantle

Winter (2001) Figure 9-4. Rare Earth concentrations(normalized to chondrite) for meltsproduced at various values of F viamelting of a hypothetical garnetlherzolite using the batch melting

model (equation 9-5). From Winter (2001) An Introduction to Igneousand Metamorphic Petrology.Prentice Hall.

Degree of PartialMelting (F)

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