Petrology Lecture 3- Major and Minor Elements PDF

Petrology Lecture 3- Major and minor elements  CHAPTER 8 Introduction Igneous Lecture Outline Metamorphic Petrology 1. What are Major and Minor elements? 2. Whole rock geochemistry 2.1 definition 2.2 analytical methods 2.3 anal...

Petrology Lecture 3- Major and minor elements  CHAPTER 8 Introduction Igneous Lecture Outline Metamorphic Petrology 1. What are Major and Minor elements? 2. Whole rock geochemistry 2.1 definition 2.2 analytical methods 2.3 analytical reports 3. Use of major and minor elements 3.1 CIPW Norm 3.2 Harker diagram 3.3 AFM diagram 3.4 Magma series 1. What are Major and Minor elements Element Wt % Oxide Atom % O 60.8 Si 59.3 21.2 Al 15.3 6.4 Abundance of the elements Fe 7.5 2.2 in the Earth’s crust Ca 6.9 2.6 Mg 4.5 2.4 Na 2.8 1.9 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  2. Whole rock geochemistry 2.1 definition Whole rock (bulk rock) geochemical analysis- is a chemical analysis representative of a rock obtained by -crushing, pulverizing and grinding a representative sample -running the pressed/fused powder or a dissolved portion (after vigorous acid attack) in a spectroscopic instrument. Whole rock analysis determines major , minor and trace elements. 2. Whole rock geochemistry 2.2 Analytical method Modern Spectroscopic Techniques XRF (X-ray Fluorescence) ICP-MS (Inductively Coupled Plasma Mass spectrometry) INAA (Instrumental Neutron Activation Analysys) Figure 8.1. The geometry of typical spectroscopic instruments. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.  2. Whole rock geochemistry 2.3 Analytical results Major & minor elements: those that predominate in any rock analysis (Si, AI, Fe, Mn, Mg, Ca, Na, K and P). - Concentrations expressed as a weight per cent (wt %) of the oxide - Usually are the main constituents of rock-forming minerals - Sum of the major element oxides ca 100 wt% - Total iron is determined and expressed as FeOTot or Fe2O3Tot. Only titration can determine FeO or Fe2O3 Fe2O3 converted to FeO by multiplying by 0.8998. FeO be converted to Fe2O3 by multiplying by 1.1113 Example: Calculate FeO* for ZV14 FeO*=2.62*0.8998+8.90=11.26 wt% Whole-rock geochemistry of komatiite flows from the Belingwe greenstone belt, Zimbabwe (Data from Nisbet et al., 1987) in Rollinson 1993- Using Geochemical data. Longman Publishing  2. Whole rock geochemistry 2.3 Analytical results Trace elements: Concentrations < 0.1 wt%. Concentrations expressed in parts per million (ppm=106) more rarely in parts per billion (ppb= 109) Usually are not the main constituents of rock forming minerals (sometimes accessory minerals). Present as replacement in the lattice of minerals, rims, matrix glass etc Provide the most important information in terms of fingerprinting petrogenetic processes (along with isotopic data) 1 wt%=10 000ppm Whole-rock geochemistry of komatiite flows from the Belingwe greenstone belt, Zimbabwe (Data from Nisbet et al., 1987) in Rollinson 1993- Using Geochemical data. Longman Publishing  2. Whole rock geochemistry 2.3 Analytical results - Volatiles: e.g. H2O, CO2 and S may be discriminated in the major element analysis. - H2O+ Water present in the mineral lattice (e.g. micas, amphiboles), is released by heating at T> 110 °C - H2O- Water present as dampness in the rock powder. Eliminated by heating at T< 110°C. Not part of the rock! - LOI (loss on ignition)- Total volatile content of the rock is determined by ignition at 1000 °C. Most common form of presentation of volatiles. Whole-rock geochemistry of komatiite flows from the Belingwe greenstone belt, Zimbabwe (Data from Nisbet et al., 1987) in Rollinson 1993- Using Geochemical data. Longman Publishing  2. Whole rock geochemistry 2.3 Analytical results Table 8-3. Chemical analyses of some representative igneous rocks 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 Peridotite: average for K2O 0.34 1.10 1.62 4.30 5.24 Lizard (Green, 1964); H2O+ 3.91 0.95 0.83 1.10 1.57 other averages from LeMaitre (1976a). Total 98.75 99.06 99.3 99.50 99.23  3. Use of Major and Minor Elements ❑ rock classification (CIPW NORM and TAS diagram) ❑ magma differentiation (evolution) and fractional crystallization (Harker diagram and AFM diagram) ❑ distinguish families of magma type 3. Use of Major and Minor Elements 3.1 CIPW Norm Cross, Iddings, Pirsson (petrologists), Washington (geochemist ) Mode is the volume % of minerals seen Norm is a calculated “idealized” anhydrous mineralogy (wt%) Why use it? Calculate idealized mineralogy in volcanic rocks Calculate an approximate mineralogy from published whole- rock analyses when the mineralogy is not reported. Used in various classification schemes.  3. Use of Major and Minor Elements 3.1 CIPW Norm Cross, Iddings, Pirsson (petrologists), Washington (geochemist ) Silica saturation & the norm: Silica-oversaturared- norm contains Qtz Silica-saturared- Maybe some Qtz, no “Si-deficient minerals” (Ol, Neph, Felptd) Silica-undersaturared- Si-deficient minerals present, no Qtz or Opx 3. Use of Major and Minor Elements 3.2 Harker diagrams: magma differentiation and fractional crystallization BASIC 1. Melts crystallize from a liquid to a solid over a range of temperature (and P). 2. Most mineral phases crystallize over a range T; the number of minerals tends to increase as temperature decreases. 3. Minerals usually crystallize sequentially, generally with considerable overlap. 4. Solid solution minerals change composition during cooling. 5. The melt composition also changes during crystallization. 6. The minerals that crystallize, as well as the sequence in which they form, depend on the T-P-X of the melt. 7. The nature and pressure of any volatile components (H2O or CO2) can also affect T crystallization and the mineral sequence. 13 3. Use of Major and Minor Elements Basalt-> Rhyolite 3.2 Harker diagrams: magma Cpx Pl Ol+Cpx differentiation and fractional crystallization Ol+Cpx Cpx+Pl Fe-Ti Oxds Residual www.keywordsking.com- enrichment Aptt During crystal fractionation (or fractional crystallization): Residual enrichment - solids are removed from melt - melt follows liquid line of descent Figure 8.2. Harker variation diagram for 310 analyzed volcanic rocks - Description before interpretation!!! 14 from Crater Lake (Mt. Mazama), Oregon Cascades. In Winter 2001 3. Use of Major and Minor Elements Basalt-> Rhyolite 3.2 Harker diagrams (binary variation diagram): magma differentiation and fractional crystallization Interpreting suites of rocks (genetically related or from same area) Differentiation index: SiO2 , Zr, #Mg=Mg/Mg+FeO*, MgO. Primary magmas: derived directly by partial melting of mantle, no differentiation Derivative (evolved) magmas experienced some form of differentiation. Parental magma: most primitive found in an area/suite. Others derived from it Figure 8.2. Harker variation diagram for 310 analyzed volcanic rocks from 15 Crater Lake (Mt. Mazama), Oregon Cascades. In Winter 2001 3. Use of Major and Minor Elements 3.3 AFM diagram (Ternary Variation Diagram): magma differentiation and fractional crystallization A= Alkalis (Na2O+K2O) F= FeO* M= MgO This chemical variation strongly implies a genetic relationship or evolutionary process… Figure 8.3. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal 16 communication). 3. Use of Major and Minor Elements 3.4 Magma series Early on it was recognized that some chemical parameters were very useful in regard to distinguishing magmatic types ❖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  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.  AFM diagram: can further subdivide the subalkaline magma series into a tholeiitic and a calc-alkaline series Red circles- - Tholeiitic series: Stronger tholeiitic rocks increase in FeO*/MgO at from Iceland, the early -> delayed oxide Mid-Atlantic formation -> reduced Ridge, the anhydrous magmas Columbia River - Calc-alkaline series: Basalts, and FeO*/MgO never increases Hawaii. > early oxide formation - > oxidized hydrous Purple circles- magmas calc-alkaline rocks of the Cascade volcanics. 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. Figure 8.15. Plot of wt.% Al2O3 vs. anorthite content of the normative plagioclase, showing the distinction between the tholeiitic and calc-alkaline series. From Irvine and Baragar (1971).  Alumina saturation indices Based on alkali-alumina ratio: (Na2O + K2O) / Al2O3 Useful for very felsic rocks Peralkaline [Al2O3 < (Na2O + K2O)] Peraluminous [Al2O3 > (CaO + Na2O + K2O)] Metaluminous [Al2O3 < (CaO + Na2O + K2O)] but Al2O3 > (Na2O + K2O)] Figure 8.10- (peraluminous granitic rocks from the Achala Batholith, Argentina) (Shand, 1927) with analyses of 22 (Lira and Kirschbaum, 1990). In S. M. Kay and C. W. Rapela (eds.), Plutonism from Antarctica to Alaska. 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. Figure 8.16. Wt.% K2O vs. Na2O diagram subdividing the alkaline magma series into High-K-, K-, and Na-sub- series. After Middlemost (1975). Copyright © with permission from Elsevier Science.  A world-wide survey suggests that there may be some important differences between the three series… Characteristic Plate Margin Within Plate Series Convergent Divergent Oceanic Continental Alkaline yes yes yes Tholeiitic yes yes yes yes Calc-alkaline yes 25 After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer