Chapter 8: Major Elements - Whitman College

Chapter 8: Major Elements - Whitman College

Chapter 8: Major Elements Wet-chems: gravimetric/volumetric Chapter 8: Major Elements Modern Spectroscopic Techniques Figure 8.1. The geometry of typical spectroscopic instruments. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Element O Si Al Fe Ca Mg Na

Wt % Oxide Atom % 60.8 59.3 21.2 15.3 6.4 7.5 2.2 6.9 2.6 4.5 2.4 2.8 1.9 Abundance of the elements in the Earths crust

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 A typical rock analysis Must multiply by # of cations in oxide 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 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 CIPW Norm Mode is the volume % of minerals seen Norm is a calculated idealized mineralogy Variation Diagrams How do we display chemical data in a meaningful way? Bivariate (x-y)

diagrams Harker diagram for Crater Lake 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). Bivariate (x-y) diagrams Harker diagram

for Crater Lake 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). Ternary Variation Diagrams Example: AFM diagram (alkalis-FeO*-MgO) Figure 8.3. AFM diagram for Crater Lake volcanics, Oregon Cascades. Data compiled by Rick Conrey (personal

communication). Models of Magmatic Evolution Table 8-5. Chemical analyses (wt. %) of a hypothetical set of related volcanics. Oxide SiO 2 TiO 2 Al2O3 Fe2O3* MgO CaO Na 2O K2O LOI Total B

50.2 1.1 14.9 10.4 7.4 10.0 2.6 1.0 1.9 99.5 BA A D 54.3 60.1 64.9 0.8

0.7 0.6 15.7 16.1 16.4 9.2 6.9 5.1 3.7 2.8 1.7 8.2 5.9 3.6 3.2 3.8 3.6 2.1

2.5 2.5 2.0 1.8 1.6 99.2 100.6 100.0 RD 66.2 0.5 15.3 5.1 0.9 3.5 3.9 3.1 1.2 99.7

R 71.5 0.3 14.1 2.8 0.5 1.1 3.4 4.1 1.4 99.2 B = basalt, BA = basaltic andesite, A = andesite, D = dacite, RD = rhyo-dacite, R = rhyolite. Data from Ragland (1989) Figure 8.6. 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. Harker diagram Smooth trends Model with 3 assumptions: 1 Rocks are related by FX 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. Extrapolate BA B and further to low SiO2 K2O is first element to 0 (at SiO2 = 46.5) 46.5% SiO2 is interpreted to be the concentration in the bulk solid extract and

the blue line the concentration of all other oxides 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. Extrapolate the other curves back BA B blue line and read off X of mineral extract Results: Remove plagioclase, olivine, pyroxene and Fe-Ti oxide

Oxide Wt% Cation Norm SiO2 TiO2 Al2O3 Fe2O3* MgO CaO Na2O K2O Total 46.5 1.4

14.2 11.5 10.8 11.5 2.1 0 98.1 ab an di hy ol mt il 18.3 30.1

23.2 4.7 19.3 1.7 2.7 100 Then repeat for each increment BA A etc. Figure 8.8. Variation diagram on a cation basis for the fractional crystallization of olivine, augite, and plagioclase to

form BA from B (Table 8-6). From Ragland (1989). Basic Analytical Petrology, Oxford Univ. Press. Figure 8.9. Equilateral triangle showing the solution to the bulk mineral extract (shaded area) best fitting the criteria for the variation diagrams in Figure 8-8. From Ragland (1989). Basic Analytical Petrology, Oxford Univ. Press. Magma Series Can chemistry be used to distinguish families of magma types? 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 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, 1070 Thermal divide separates the silica-saturated

(subalkaline) from the silica-undersaturated (alkaline) fields at low pressure Cannot cross this divide by FX, so cant derive one series from the other (at least via low-P FX) 1713 Liquid Thermal Divide Ne + L Ab + LAb + L Ne + Ab Ne

Tr + L 1060 Ab + Tr Ab Q 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 MidAtlantic 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.

F ic o l ei h it T Calc-alkaline A M 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 8.10 a. Plot of CaO (green) and (Na2O + K2O) (red) vs. SiO2 for the Crater Lake data. Peacock (1931) used the value of SiO2 at which the two curves crossed as his alkali-lime index (dashed line). 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.

Fig. 8.17. After Le Maitre (1976) J. Petrol., 17, 589-637. A world-wide survey suggests that there may be some important differences between the three series Characteristic Plate Margin Series Convergent Divergent Alkaline yes Tholeiitic yes yes Calc-alkaline yes

Within Plate Oceanic Continental yes yes yes yes After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer Figures I dont use Figure 8.4. Pearce element diagram of 0.5(Fe + Mg)/K vs. Si/K for two Hawaiian picritic magma suites. From Nicholls and Russell (1990). Figure 8.5. Pearce element diagrams for basalts (dark circles) and picrites

(light circles) erupted from Kilauea, Hawaii, between November, 1967 and August, 1968. After Nicholls (1990). 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). Figure 8.16. Wt.% K2O vs. Na2O diagram subdividing the alkaline magma series into High-K-, K-, and Na-subseries. After Middlemost (1975). Copyright with permission from Elsevier Science.

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