Chemistry of Igneous Rocks

Nov. 2, 2009

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

WHOLE ROCK ANALYSIS OF A BASALTWt%MolecularWt.Wt%/Mol. Wt.Mole%Trace Elements (ppm)structural water1 wt.% = 10,000 ppm1 ppm = 0.0001 wt.%adsorbed water

49.260.090.818850.622.0395.90.02121.3116.1101.960.15799.762.72159.70.01701.057.7771.850.10816.690.1870.940.00250.166.4440.310.15989.8810.556.080.187211.583.0161.980.04863.000.1494.20.00150.090.2370.980.00320.200.718.020.03882.400.9518.020.05273.2699.971.6174100.00

SiO2TiO2Al2O3Fe2O3FeOMnOMgOCaONa2OK2OP2O5H2O+H2O-

Ba5Co32Cr220Ni87Pb1.29Rb1.14

Sr190Th0.15U0.16V280Zr160La5.1

ANALYTICAL TECHNIQUESWhole 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 UMDs SEM - X-ray Diffractometry(XRD)Incident X-rays are diffracted by characteristic mineral structure

CHEMICAL ANALYSES OF COMMON ROCK TYPES THAT APPROXIMATE MAGMA COMPOSITIONSMagma - UltramaficMaficIntermed. Felsic Alkalic

CIPW NORMATIVE CALCULATIONSMode is the volume % of minerals observedNorm 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 PLOTSObjective: to show the co-variation of elemental components that may give insight to magmatic processes such as- partial meltingmagma mixingcountry rock assimilation/contaminationfractional crystallization (or crystallization differentiation)Types:bivariate (X-Y)triangularnormalization plots (spider diagrams)

MOST PLOTS ARE APPROPRIATE FOR LIQUID COMPOSITIONS ONLY!!!

HARKER VARIATION DIAGRAMSWinter (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 differentiationVariation of major and minor oxide abundances vs. SiO2 (thought to be and indication of the evolved character of a magmatic system)PrimitiveEvolvedLiquidLines of Descent

DIFFERENTIATION INDEXESfrom Winter (2001)

INTERPRETING TRENDS ON VARIATION DIAGRAMSRollinson (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 WinterExtraction CalculationsAddition-Subtraction Diagram

INTERPRETING TRENDS ON VARIATION DIAGRAMSScattered Trendsnot all liquidsnot comagmaticpolybaric fractionationsample heterogeneityvaried data sources

MAGMA SERIESRELATED TO TECTONIC PROVINCESNa2O + K2OSiO2Sub-alkalineWinter (2001) Figure 8.11. Total alkalis vs. silica diagram for the alkaline and sub-alkaline rocks of Hawaii. After MacDonald (1968). GSA Memoir 116

Sheet1

CharacteristicPlate MarginWithin Plate

SeriesConvergentDivergentOceanicContinental

Alkalineyesyesyes

Tholeiiticyesyesyesyes

Calc-alkalineyes

Sheet2

Sheet3

SUBALKALINE DISCRIMINATION DIAGRAMSAFM DiagramTholeiitic--Calc-Alkaline boundary after Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548Na2O + K2OFe2O3 + FeOMgO

ALUMINA/ALKALI DISCRIMINATION DIAGRAMSWinter (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 DIAGRAMSRollinson (1993)

Figure 9.8Examples 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 PROCESSESTransition MetalsRare Earth ElementsGoldschmidts (1937) Rules of Element AffinityTwo 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)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)AlkalisPreciousMetals

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 1Kd(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 ELEMENTSPARTITION COEFFICIENTS (CS/CL)Compatible

Sheet1

OlivineOpxCpxGarnetPlagAmphMagnetiteMineralModeDensityWt propWt%Bulk D

Rb0.0100.0220.0310.0420.0710.29ol153.6540.1776900296

Sr0.0140.0400.0600.0121.8300.46cpx333.4112.20.3692003949

Ba0.0100.0130.0260.0230.230.42plag512.7137.70.4531095755

Ni14570.9550.016.829garnet400

Cr0.7010341.3450.012.007.4Sum303.901.00

La0.0070.030.0560.0010.1480.5442Rb0.05

Ce0.0060.020.0920.0070.0820.8432Sr0.85

Nd0.0060.030.2300.0260.0551.3402

Sm0.0070.050.4450.1020.0391.8041

Eu0.0070.050.4740.2430.1/1.5*1.5571

Dy0.0130.150.5821.9400.0232.0241

Er0.0260.230.5834.7000.0201.7401.5

Yb0.0490.340.5426.1670.0231.6421.4

Lu0.0450.420.5066.9500.0191.563

Data from Rollinson (1993).

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Rare Earth Elements

BEHAVIOR OF TRACE ELEMENTS DURING PARTIAL (BATCH) MELTINGCL/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)CompatibleIncompatibleBEHAVIOR 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 CRYSTALLIZATIONFrom Rollinson (1993)CompatibleIncompatibleBulk 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 CRYSTALLIZATIONF (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/CoCL/CoTroctoliteOlivine Gabbro

TRACE ELEMENT BEHAVIOR DURING FRACTIONAL CRYSTALLIZATIONEXAMPLE FROM THE SONJU LAKE INTRUSIONE. Compatible Elements

RARE EARTH ELEMENT(REE)DIAGRAMSCOMPARES RATIOS AND NORMALIZES TO A STANDARD COMPOSITIONLight REEHeavy REEFrom Rollinson (1993)Fractional crystallization increases the REE abundance, but has a neglible effect on the REE patternREE 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 ratiosPartial Melting - significant change in REE ratios

TRACE ELEMENT NORMALIZATION PLOTS (SPIDER DIAGRAMS)Most LeastIncompatible Elements(likes magma)Compatible Elements(likes minerals)Rock/Standard Comp*Common Standard Compositions for Normalizing Chondritic meteorite Avg. Mid-ocean Ridge Basalt (MORB) Primitive Mantle Primitive Ocean Island Basalt (OIB)EnrichedDepletedNegative AnomalyPositive Anomaly

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