GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY

Lecture 7Major, Minor and Trace Element Chemistry of Igneous Rocks

February 8, 2016

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

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 3.26

99.97 1.6174 100.00

WHOLE ROCK ANALYSIS OF A BASALT

Wt%Molecular

Wt.Wt%/Mol. Wt. Mole%

SiO2TiO2Al2O3Fe2O3FeOMnOMgOCaONa2OK2OP2O5H2O+

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

SPECTROSCOPIC 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 SERIESRELATED TO TECTONIC PROVINCESCharacteristicSeries 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

Basalt BasalticandesiteAndesite Dacite

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-AlkalineAFM 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

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

Fe+2 156 A Mg+2 145 A

TRACE ELEMENT COMPATIBILITYCompatibility – 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

leIn

com

patib

le

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

BEHAVIOR OF TRACE ELEMENTS DURING FRACTIONAL

CRYSTALLIZATIONRayleigh 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

leIn

com

patib

le

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

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.000.100

1.000

10.000

100.000

Tr(Yb)Tr(Ce)Ce/YbTr(Ni)

0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.000.100

1.000

10.000

100.000

OG(Yb)OG(Ce)Ce/YbOG(Ni)

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 REEFrom 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 DIAGRAMSSORTING EFFECTS OF PARTIAL MELTING VS. FRACTIONAL

CRYSTALLIZATION

From 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