G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 9 Major and Minor Element Chemistry of Igneous Rocks February 11, 2009

Major elements : usually > 1 wt.% control properties of magmas major 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 W HOLE R OCK A NALYSIS OF A B ASALT Wt% Molecular Wt. Wt%/ Mol. Wt. Mole% SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K2OK2O P2O5P2O5 H2O+H2O+ H2O-H2O- Ba5 Co32 Cr220 Ni87 Pb1.29 Rb1.14 Sr190 Th0.15 U0.16 V280 Zr160 La5.1 Trace Elements (ppm) structural water 1 wt.% = 10,000 ppm 1 ppm = wt.% adsorbed water

A NALYTICAL T ECHNIQUES 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

C HEMICAL A NALYSES OF C OMMON R OCK T YPES THAT A PPROXIMATE M AGMA C OMPOSITIONS Rock - PeridotiteBasaltAndesiteRhyolitePhonolite SiO TiO Al2O Fe2O FeO MnO MgO CaO Na2O K2O H2O Total Magma - UltramaficMaficIntermed. Felsic Alkalic

CIPW N ORMATIVE C ALCULATIONS 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.

G EOCHEMICAL P LOTS 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)

H ARKER V ARIATION D IAGRAMS 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” Gap Real or an artifact of the variation of SiO 2 concentration with differentiation Variation of major and minor oxide abundances vs. SiO 2 (thought to be and indication of the evolved character of a magmatic system) Primitive Evolved Liquid Lines of Descent

D IFFERENTIATION I NDEXES from Winter (2001)

M AGMA S ERIES M AGMA S ERIES R ELATED TO T ECTONIC P ROVINCES Na 2 O + K 2 O SiO 2 Sub- alkaline

S UBALKALINE D ISCRIMINATION D IAGRAMS AFM Diagram Tholeiitic--Calc-Alkaline boundary after Irvine and Baragar (1971). Can. J. Earth Sci., 8, Na 2 O + K 2 O Fe 2 O 3 + FeO MgO

T ECTONIC P ROVINCE D ISCRIMINATION D IAGRAMS Rollinson (1993)

T ECTONIC P ROVINCE D ISCRIMINATION D IAGRAMS

I NCOMPATABILITY OF T RACE E LEMENTS P ARTITION C OEFFICIENTS (C S /C L )

I NCOMPATABILITY OF T RACE E LEMENTS Incompatible elements commonly  two subgroups F Smaller, highly charged high field strength (HFS) elements (REE, Th, U, Ce, Pb 4+, Zr, Hf, Ti, Nb, Ta); relatively immobile during metamorphism and alteration F Low field strength large ion lithophile (LIL) elements (K, Rb, Cs, Ba, Pb 2+, Sr, Eu 2+ ) are more mobile, particularly if a fluid phase is involved Best to plot concentratoin of trace elements relative to some standard composition

T RACE E LEMENT N ORMALIZATION P LOTS (S PIDER D IAGRAMS ) Most Least Incompatible 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) Enriched Depleted Negative Anomaly Positive Anomaly

R ARE E ARTH E LEMENT (REE) S PIDER D IAGRAM Light REEHeavy REE Likes Pl Likes Garnet

I NTERPRETING REE D IAGRAMS I NTERPRETING REE D IAGRAMS P ARTIAL M ELTING OF THE M ANTLE 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). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Degree of Partial Melting (F)