GEOL 5310 ADVANCED IGNEOUS AND METAMORPHIC PETROLOGY

Chemistry of Igneous Rocks

Nov. 2, 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)

MOST PLOTS ARE APPROPRIATE FOR MOST PLOTS ARE APPROPRIATE FOR

LIQUID COMPOSITIONS ONLY!!!LIQUID COMPOSITIONS ONLY!!!

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)

INTERPRETING TRENDS ON VARIATION DIAGRAMS

Rollinson (1993)

Figure 8.7. Stacked variation diagrams of hypothetical components X and Y (either weight or mol %). P = parent, D = daughter, S = solid extract, A, B, C = possible extracted solid phases. For explanation, see text. From Ragland (1989). Basic Analytical Petrology, Oxford Univ. Press. (From Winter

Extraction CalculationsAddition-Subtraction Diagram

INTERPRETING TRENDS ON VARIATION DIAGRAMS

Scattered Trends

-not all liquids

-not comagmatic

-polybaric fractionation

-sample heterogeneity

-varied data sources

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

Winter (2001) Figure 8.11. Total alkalis vs. silica diagram for the alkaline and sub-alkaline rocks of Hawaii. After MacDonald (1968). GSA Memoir 116

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

ALUMINA/ALKALI DISCRIMINATION DIAGRAMS

Winter (2001) Figure 18.2. Alumina saturation classes based on the molar proportions of Al2O3/(CaO+Na2O+K2O) (“A/CNK”) after Shand (1927).

Common non-quartzo-feldspathic minerals for each type are included. After Clarke (1992). Granitoid Rocks. Chapman Hall.

Winter (2001) Figure 8-10 b. Alumina saturation indices (Shand, 1927) with analyses of the peraluminous granitic rocks from the Achala Batholith, Argentina (Lira and Kirschbaum, 1990). In S. M. Kay and C. W. Rapela (eds.), Plutonism from Antarctica to Alaska. Geol. Soc. Amer. Special Paper, 241. pp. 67-76.

TECTONIC PROVINCE DISCRIMINATION DIAGRAMS

Rollinson (1993)

Figure 9.8  Examples of discrimination diagrams used to infer tectonic setting of ancient (meta)volcanics. (a) after Pearce and Cann (1973), (b) after Pearce (1982), Coish et al. (1986). Reprinted by permission of the American Journal of Science, (c) after Mullen (1983) Copyright © with permission from Elsevier Science, (d) and (e) after Vermeesch (2005) © AGU with permission.

TECTONIC PROVINCE DISCRIMINATION DIAGRAMS

TRACE ELEMENTS IN IGNEOUS PROCESSES

Transition Metals

Rare Earth Elements

Goldschmidt’s (1937) Rules of Element Affinity1.Two ions with the same valence and radius should exchange easily and enter a solid solution in amounts equal to their overall proportions (e.g. Rb~K, Ni~Mg, Mn~Fe)

2.If two ions have a similar radius and the same valence: the smaller ion is preferentially incorporated into the solid over the liquid (e.g., Mg > Fe in Olivine)

Ionic Field Strength (Charge/Radius)Alkalis

PreciousMetals

TRACE ELEMENT COMPATIBILITY

Compatibility – degree to which an element prefers to partition into the solid over the liquid phase .

Kd(i)1 – Mineral-Liquid Partition Coefficient for element i in mineral 1

Kd(i)1 = C(i)

mineral 1/ C(i)liquid (C(i) - concentration of element i in wt. %)

Kd(i)1

> 1 – Compatible, Kd(i)1

< 1 – Incompatible

D(i) – Bulk Rock Partition Coefficient for element i

D(i) = x1 Kd(i)1 + x2 Kd(i)

2 + x3 Kd(i)

3 + .... (x1 – proportion of mineral 1)

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

Compatible

BEHAVIOR OF TRACE ELEMENTS DURING PARTIAL (BATCH) MELTING

CL/Co = 1/[D(i)(1-F) + F]

F - Fraction of LiquidD(i)- Bulk Distribution Coefficient for Element i

As D(i) 0 (strongly IE)

CL/Co ≈ 1/F

Normal Range of Partial Melting in 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).

Degree of Partial Melting (F)

From Rollinson (1993)

Com

patib

leC

ompa

tible

Inco

mpa

tible

Inco

mpa

tible

BEHAVIOR OF RARE EARTH ELEMENTS DURING PARTIAL (BATCH) MELTING OF THE

MANTLE

BEHAVIOR OF TRACE ELEMENTS DURING FRACTIONAL

CRYSTALLIZATION

Rayleigh Distillation: CL/Co = F(D

(i)-1)

F - Fraction of Liquid RemainingD(i)- Bulk Distribution Coefficient for Element i

From Rollinson (1993)

BEHAVIOR OF TRACE ELEMENTS DURING FRACTIONAL CRYSTALLIZATION

From Rollinson (1993)

Com

patib

leC

ompa

tible

Inco

mpa

tible

Inco

mpa

tible

Bulk Rock Partition Coefficient of Ce,Yb, and Nifor Crystallization of:

1) Troctolite (70% Pl, 30% Ol)

D(Ce) = xPl Kd(Ce)Pl

+ xOl Kd(Ce)Ol

= .7*.103 + .3*.007 = 0.092

D(Yb) = xPl Kd(Yb)Pl

+ xOl Kd(Yb)Ol

= .7*.07 + .3*.065 = 0.069

D(Ni) = xPl Kd(Ni)Pl

+ xOl Kd(Ni)Ol

= .7*.01 + .3*25= 7.5

2) Olivine Gabbro (63% Pl, 12% Ol, 25% Cpx)

D(Ce) = xPl Kd(Ce)Pl

+ xOl Kd(Ce)Ol + xCpx Kd(Ce)

Cpx

= .63*.103 + .12*.007 + .25*.09 = 0.088

D(Yb) = xPl Kd(Yb)Pl

+ xOl Kd(Yb)Ol + xCpx Kd(Yb)

Cpx

= .63*.07 + .12*.065 + .25*.09 = 0.074

D(Ni) = xPl Kd(Ni)Pl

+ xOl Kd(Ni)Ol + xCpx Kd(Ni)

Cpx

= .63*.01 + .12*25 + .25*8 = 5

TRACE ELEMENT BEHAVIOR DURING FRACTIONAL CRYSTALLIZATION

F (fraction of liquid remaining)

Rayleigh Distillation: CL/Co = F(D-1)

Conclusions: Fractional crystallization of mafic magmas gradually increases the concentrations of similarly incompatible elements, but has a minimal effect on their ratios; and strongly decreases the concentrations of compatible elements

F (fraction of liquid remaining)

CL/Co CL/CoTroctolite Olivine Gabbro

TRACE ELEMENT BEHAVIOR DURING FRACTIONAL CRYSTALLIZATION

EXAMPLE FROM THE SONJU LAKE INTRUSION

E. Compatible Elements

RARE EARTH ELEMENT(REE)DIAGRAMSCOMPARES RATIOS AND NORMALIZES TO A STANDARD

COMPOSITION

Light REE Heavy REE

From Rollinson (1993)

Fractional crystallization increases the REE abundance, but has a neglible effect on the REE pattern

REE commonly normalized to chondrite composition – thought to approximate the unfractionated composition of the earth.

Fractional crystallization of olivine from a komatiitic melt

REE RATIO DIAGRAMSFrom Rollinson (1993)

Fractional Crystallization - minimal change in

REE ratios

Partial Melting - significant

change in REE ratios

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