4/5/2017 - Concord University · 4/5/2017 1 Chemical Petrology • Analysis of rocks - major,...

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Chemical Petrology• Analysis of rocks - major, minor, and trace elements

• Looking for meaningful patterns in a data set: major-element variation diagrams & magma differentiation (fractional crystallization, magma mixing)

• Igneous rock & magma series classification using chemistry

• Alkaline vs. subalkaline• Tholeiitic vs. calc-alkaline• Peralkaline, metaluminous, peraluminous

• Relative abundances of rock types

• Magnitude of major-element vs. trace-element changes during partial melting or fractional crystallization

• Substitution of trace elements for major elements in the major mineral phases & chemical fractionation / distribution coefficients

• Categorizing trace-elements based on their behavior in igneous processes

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Major elements: usually greater than 1%SiO2 Al2O3 FeO* MgO CaO Na2O K2O H2O

Minor elements: usually 0.1 - 1%TiO2 MnO P2O5 CO2

Trace elements: usually < 0.1%everything else

Element Wt % Oxide Atom %O 60.8Si 59.3 21.2Al 15.3 6.4Fe 7.5 2.2Ca 6.9 2.6Mg 4.5 2.4Na 2.8 1.9

Abundance of the elementsin the Earth’s crust

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A typical rock analysisWt. % Oxides to Atom % Conversion

Oxide Wt. % Mol Wt. Atom prop Atom %

SiO2 49.20 60.09 0.82 12.25

TiO2 1.84 95.90 0.02 0.29

Al2O3 15.74 101.96 0.31 4.62

Fe2O3 3.79 159.70 0.05 0.71

FeO 7.13 71.85 0.10 1.48MnO 0.20 70.94 0.00 0.04MgO 6.73 40.31 0.17 2.50CaO 9.47 56.08 0.17 2.53

Na2O 2.91 61.98 0.09 1.40

K2O 1.10 94.20 0.02 0.35

H2O+ 0.95 18.02 0.11 1.58

(O) 4.83 72.26Total 99.06 6.69 100.00

Must multiply by # of cations in oxide

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Table 8-3. Chemical analyses of somerepresentative igneous rocks

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 2.79FeO 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

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e.g. Bivariate (x-y) diagrams

Harker diagramfor

Crater Lake, OR

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).

5Variation DiagramsHow do we display

chemical data in a meaningful way?

Bowen’s reaction series

Fig 4.11 Understanding Earth

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This will be different at high pressure

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Rollinson's Using Geochem Data

Crystallization

Ternary Variation Diagrams Example: AFM diagram

(alkalis-FeO*-MgO)

Figure 8.2. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal communication).

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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.

Removal of mineral phases drives the remaining melt away from the composition of the individual mineral or weighted mean composition of the mineral assemblage

e.g. removal of S from P drives the remaining liquid toward D

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Models of Magmatic Evolution

crystallization

Harker diagram Smooth trends

Model with 3 assumptions:

1 Rocks are related by fractional crystallization

2 Trends = liquid line of descent

3 The basalt is the parent magma from which the others are derived

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.

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Kuehn-Negrini, 2010

Fractional Crystallization or something else?

Analysis of tephra glasses by electron microprobe

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Kuehn-Negrini, 2010

Analysis of tephra glasses by electron microprobe

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Kuehn-Negrini, 2010

Analysis of tephra glasses by electron microprobe Magma Series

Can chemistry be used to distinguish families of magma types?

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Early on it was recognized that some chemical parameters were very useful in regard to distinguishing magmatic groups

Total Alkalis (Na2O + K2O)

Silica (SiO2) and silica saturation

Alumina (Al2O3)

Alkali vs. Silica diagram for Hawaiian volcanics:Seems to be two distinct groupings: alkaline and subalkaline

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

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Fig

. 8.1

7. A

fter

Le

Mai

tre

(197

6) J

. Pet

rol.,

17,

589

-637

.

41,000 igneous rock analyses 16

The Basalt Tetrahedron and the Ne-Ol-Q base

Alkaline and subalkaline fields are again distinct

Figure 8.12. Left: the basalt tetrahedron (after Yoder and Tilley, 1962). J. Pet., 3, 342-532. Right: the base of the basalt tetrahedron using cation normative minerals, with the compositions of subalkaline rocks (black) and alkaline rocks (gray) from Figure 8-11, projected from Cpx. After Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548.

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Ne Ab Q

1070 1060

1713

Ab + Tr

Tr + L

Ab + LNe + L

Liquid

Ab + L

Ne + Ab

ThermalDivide

Thermal divide separates the silica-saturated (subalkaline) from the silica-undersaturated (alkaline) fields at low pressure

Cannot cross this divide by FX, so can’t derive one series from the other (at least via low-P FX)

Different phase assemblages form when these magmas crystallize.

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AFM diagram: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series

Figure 8.14. AFM diagram showing the distinction between selected tholeiitic rocks from Iceland, the Mid-Atlantic Ridge, the Columbia River Basalts, and Hawaii (solid circles) plus the calc-alkaline rocks of the Cascade volcanics (open circles). From Irving and Baragar (1971). After Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548.

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These two magma series follow different trajectories during (fractional) crystallization.

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

Plate Margin Within Plate

A world-wide survey suggests that there may be some important differences between the three series

After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer

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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.

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Trace Elements

Generally don’t form their own phases or govern the appearance of a phase (like K is required to make K-feldspar). Rather, they enter the various phases present by substituting for other elements.

Show a range of behaviors so can be sensitive indicators of igneous processes

e.g. amount of fractional crystallization + minerals involved% partial melting of source rockstype of source rock (minerals present)tectonic setting (e.g. subduction-related or not)

23Note the relative magnitudes of major and trace element changes

Winter Figure 8.2 Winter Figure 9.1.

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Classification of the trace-elements26


Rare-earth element abundances are often presented as normalized values

to simplify interpretation.

Chondrite-normalized REE diagramSolar system abundances

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Brownlow's Geochemistry

Rare-earth element abundances of mafic rocks by type 28


Mean trace element abundances by rock type(“spider” diagrams)

Classification of the trace-elements

Compatible – prefer the solid phases during melting/crystallization

Incompatible – prefer the liquid phase during melting/crystallization

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Element DistributionGoldschmidt’s rules (simplistic, but useful)

1. 2 ions with the same valence (charge) and radius should exchange easily and enter a solid solution in amounts equal to their overall proportions

How does Rb behave?

What about Ni?

What major elements do they mimic?

What phases do they enter?

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Strong tendency to lose outermost electrons

Strong tendency to gain electrons

Inert (noble) gases

Transition elements

Lanthanides

Actinides

Tend to share, gain, or lose electrons

See: http://www.webelements.com/webelements/index.html

Atomic number

Chemical symbol

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Goldschmidt’s rules2. If 2 ions have a similar radius and the same

valence: the smaller ion is preferentially incorporated into the solid over the liquid

Fig. 6.10. Isobaric T-X phase diagram at atmospheric pressure After Bowen and Shairer (1932), Amer. J. Sci. 5th Ser., 24, 177-213. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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3. If 2 ions have a similar radius, but different valence: the ion with the higher charge is preferentially incorporated into the solid over the liquid

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Incompatible elements commonly two subgroups

Smaller, highly charged high field strength (HFS) elements(REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta)

Low field strength large ion lithophile (LIL) elements(K, Rb, Cs, Ba, Pb2+, Sr, Eu2+) are more mobile, particularly if a fluid phase (H2O-rich) is involved

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Trace elements strongly partitioned into a single mineral (relatively compatible with solid phase)

Ni - olivine in Table 9-1 = 14

Figure 9.1a. Ni Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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Incompatible trace elements concentrate in liquid

Reflect the proportion of liquid at a given state of crystallization or melting

Figure 9.1b. Zr Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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Distribution coefficients(a.k.a. partition coefficients)

A type of equilibrium constant.

Used to describe the distribution of trace elements between two phases.

Most typically used for distribution between solid & liquid phases, but sometimes also used for distribution between two solids.

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liquid

solidD c

cK

Like other equilibrium constants, this relationship is affected by both pressure and temperature. It is also affected by the composition of the liquid and often by oxygen fugacity.

concentration of the trace element in a specific solid phase

concentration of the same trace element in the liquid phase

Distribution coefficients

If multiple solid phases are present, a bulk distribution coefficient may be calculated for each trace element of interest.

D = x1Kd1 + x2Kd2 + x3Kd3 etc.

x1, x2, and x3 represent the relative proportions of the individual phases(e.g. 0.2, 0.2, and 0.6)

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Trace element concentrations are in the Henry’s Law region of concentration, so their activity varies in direct relation to their concentration in the system

Thus if XNi in the system doubles the XNi in all phases will double (X=concentration)

Because of this, the ratios of trace elements are often superior to the concentration of a single element in identifying the role of a specific mineral

39 Sr and Ba - compatible or incompatible?

Sr is excluded from most common minerals except plagioclase

Ba similarly excluded except in alkali feldspar

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

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Compatible example: Ni strongly fractionated olivine > pyroxene

Cr and Sc pyroxenes » olivine

Ni/Cr or Ni/Sc can distinguish the effects of olivine and augite in a partial melt or a suite of rocks produced by fractional crystallization

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 7.4L 0 007 0 03 0 056 0 001 0 148 0 544 2

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Pearce (2014) Towards a protocol

for LA-ICP-MS analysis of rhyolitic

glass shards

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Variation in distribution

coefficients with melt composition

All of these but Eu have a 3+ charge, so the observed pattern largely reflects ionic

size.

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Variation in KD with melt composition

Basalts Rhyolites

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Variation in distribution

coefficients ionic radius and charge

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Effect of pressure on KD

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Effect of oxygen fugacity on KD

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Bro

wn

low

's G

eoch

emis

try

Enrichment/ depletion with partial melting

As F 0 CL/CO 1/Di

If we know CL of a magma derived by a small degree of batch melting, and we know Di

we can estimate the concentration of that element in the source region (CO)

CC

1Di(1 F) F

L

O

Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9-5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

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51


Crystallization

90% liquid

5% liquid

Enrichment with fractional

crystallization

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Trace elements during fractional

crystallization

Plagioclase fractional crystallization vs.

accumulation

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Trace elements during fractional

crystallization

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Element Use as a Petrogenetic Indicator

Ni, Co, CrHighly compatible elements. Ni and Co are concentrated in olivine, and Cr in spinel and

clinopyroxene. High concentrations indicate a mantle source, limited fractionation, or crystal accumulation.

Zr, HfVery incompatible elements that do not substitute into major silicate phases (although they

may replace Ti in titanite or rutile). High concentrations imply an enriched source or extensive liquid evolution.

Nb, TaHigh field-strength elements that partition into Ti-rich phases (titanite, Ti-amphibole, Fe-Ti

oxides. Typically low concentrations in subduction-related melts.

Ru, Rh, Pd, Re, Os, Ir, Pd

Platinum group elements (PGEs) are siderophile and used mostly to study melting and crystallization in mafic-ultramafic systems in which PGEs are typically hosted by sulfides. The Re/Os isotopic system is controlled by initial PGE differentiation and is applied to mantle evolution and mafic melt processes.

Sc Concentrates in pyroxenes and may be used as an indicator of pyroxene fractionation.

SrSubstitutes for Ca in plagioclase (but not in pyroxene), and, to a lesser extent, for K in K-

feldspar. Behaves as a compatible element at low pressure where plagioclase forms early, but as an incompatible element at higher pressure where plagioclase is no longer stable.

REE

Myriad uses in modeling source characteristics and liquid evolution. Garnet accommodates the HREE more than the LREE, and orthopyroxene and hornblende do so to a lesser degree. Titanite and plagioclase accommodates more LREE. Eu2+ is strongly partitioned into plagioclase.

YCommonly incompatible. Strongly partitioned into garnet and amphibole. Titanite and

apatite also concentrate Y, so the presence of these as accessories could have a significant effect.

Table 9.6 A Brief Summary of Some Particularly Useful Trace Elements in Igneous Petrology55

Figure 11.3 Position of the H2O-saturated ternary eutectic in the albite-orthoclase-silica system at various pressures. The shaded portion represents the composition of most granites. Included are the compositions of the Tuolumne Intrusive Series (Figure 4-32), with the arrow showing the direction of the trend from early to late magma batches. Experimental data from Wyllie et al. (1976). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

Surface, 33 km, and 100 km depths

Many silicic magmas approach the ternary eutecticEither fractional crystallization does take place or they are

minimum (eutectic) melts

Figure 12.15b. Cross-bedding in cumulate layers. Skaergård Intrusion, E. Greenland. Layering caused by different proportions of mafics and plagioclase. From McBirney and Noyes (1979) J. Petrol., 20, 487-554.

Figure 12.15a. Cross-bedding in cumulate layers. Duke Island, Alaska. Note also the layering caused by different size and proportion of olivine and pyroxene. From McBirney (1993) Igneous Petrology. Jones and Bartlett

Basalt pillows accumulating at the

bottom of a in granitic magma chamber,

Vinalhaven Island, Maine

Comingled basalt-Rhyolite Mt.

McLoughlin, OregonFigure 11.8 From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall

4/5/2017 - Concord University · 4/5/2017 1 Chemical Petrology • Analysis of rocks - major,...

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