Download
1 / 17
240 likes | 592 Views
Geol 2312 Igneous and Metamorphic Petrology. Lecture 9 Major and Minor Element Chemistry of Igneous Rocks. February 11, 2009. Whole Rock Analysis of a Basalt. Molecular Wt. Wt%/ Mol. Wt. Wt%. Mole%. Major elements : usually > 1 wt.% control properties of magmas
E N D
Geol 2312 Igneous and Metamorphic Petrology Lecture 9 Major and Minor Element Chemistry of Igneous Rocks February 11, 2009
Whole Rock Analysis of a Basalt Molecular Wt. Wt%/ Mol. Wt. Wt% Mole% 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 structural water Trace Elements (ppm) adsorbed water 1 wt.% = 10,000 ppm 1 ppm = 0.0001 wt.%
Analytical Techniques • 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
Rock - Peridotite Basalt Andesite Rhyolite Phonolite SiO2 42.26 49.20 57.94 72.82 56.19 TiO2 0.63 1.84 0.87 0.28 0.62 Al2O3 4.23 15.74 17.02 13.27 19.04 Fe2O3 3.61 3.79 3.27 1.48 2.79 FeO 6.58 7.13 4.04 1.11 2.03 MnO 0.41 0.20 0.14 0.06 0.17 MgO 31.24 6.73 3.33 0.39 1.07 CaO 5.05 9.47 6.79 1.14 2.72 Na2O 0.49 2.91 3.48 3.55 7.79 K2O 0.34 1.10 1.62 4.30 5.24 H2O+ 3.91 0.95 0.83 1.10 1.57 Total 98.75 99.06 99.3 99.50 99.23 Chemical Analyses of Common Rock Types that Approximate Magma Compositions 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 Numbers show the order that mineral are figured. See Winter (2001) Appendix for instructions. 13) 14) 15) 11) 12) 7) 4) 8) 5) 9) 10) 1) 2) 6) 3)
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 Liquid Lines of Descent Variation of major and minor oxide abundances vs. SiO2 (thought to be and indication of the evolved character of a magmatic system) The “Daly” Gap Real or an artifact of the variation of SiO2 concentration with differentiation 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). Evolved Primitive
Differentiation Indexes from Winter (2001)
Magma SeriesRelated to Tectonic Provinces Sub- alkaline Na2O + K2O SiO2
Subalkaline Discrimination Diagrams Fe2O3 + FeO AFM Diagram Tholeiitic--Calc-Alkaline boundary after Irvine and Baragar (1971). Can. J. Earth Sci., 8, 523-548 MgO Na2O + K2O
Tectonic Province Discrimination Diagrams Rollinson (1993)
Incompatability of Trace ElementsPartition Coefficients (Cs/Cl)
Incompatability of Trace Elements Incompatible elements commonly two subgroups • Smaller, highly charged high field strength (HFS) elements (REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta); relatively immobile during metamorphism and alteration • Low field strength large ion lithophile (LIL) elements (K, Rb, Cs, Ba, Pb2+, Sr, Eu2+) are more mobile, particularly if a fluid phase is involved Best to plot concentratoin of trace elements relative to some standard composition
Trace Element Normalization Plots (Spider Diagrams) Positive Anomaly Enriched Rock/Standard Comp* Depleted Negative Anomaly Most Least Incompatible Elements (likes magma) Compatible Elements (likes minerals) • Common Standard Compositions for Normalizing • Chondritic meteorite • Avg. Mid-ocean Ridge Basalt (MORB) • Primitive Mantle • Primitive Ocean Island Basalt (OIB)
Rare Earth Element (REE) Spider Diagram Light REE Heavy REE Likes Pl Likes Garnet
Interpreting REE DiagramsPartial Melting of the Mantle Degree of Partial Melting (F) 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.
