G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 7 Major, Minor and Trace Element Chemistry...
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Transcript of G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 7 Major, Minor and Trace Element Chemistry...
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GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGY
Lecture 7Major, Minor and Trace Element Chemistry of Igneous Rocks
February 8, 2016
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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
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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
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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
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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.
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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)
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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
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DIFFERENTIATION INDEXES
from Winter (2001)
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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
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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
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TECTONIC PROVINCE DISCRIMINATION DIAGRAMS
Rollinson (1993)
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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
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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
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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)
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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
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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
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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
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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)
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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
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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
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TRACE ELEMENT BEHAVIOR DURING FRACTIONAL CRYSTALLIZATION
EXAMPLE FROM THE SONJU LAKE INTRUSION
E. Compatible Elements
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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
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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
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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