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Thin section #94 Slide 2 MC-200 Slide 3 Liquids and residuum of melted pyrolite Figure 10-9 After Green and Ringwood (1967). Earth Planet. Sci. Lett. 2, 151-160. Slide 4 Initial Conclusions: l Tholeiites favored by shallower melting 25% melting at < 30 km tholeiite 25% melting at 60 km olivine basalt l Tholeiites favored by greater % partial melting 20 % melting at 60 km alkaline basalt incompatibles (alkalis) initial melts 30 % melting at 60 km tholeiite Slide 5 Primary magmas l Formed at depth and not subsequently modified by FX or Assimilation l Criteria Highest Mg# (100Mg/(Mg+Fe)) really parental magma F Experimental results of lherzolite melts s Mg# = 66-75 s Cr > 1000 ppm s Ni > 400-500 ppm s Multiply saturated Slide 6 Summary l A chemically homogeneous mantle can yield a variety of basalt types l Alkaline basalts are favored over tholeiites by deeper melting and by low % PM l Fractionation at moderate to high depths can also create alkaline basalts from tholeiites Slide 7 Preference for mineral phase Preference for melt Plot of ionic radius vs. ionic charge for trace elements of geological interest. Ionic radii are quoted for eight-fold coordination to allow for comparison between elements. From Rollinson (1993). Ionic charge vs. radius Slide 8 l Incompatible elements commonly two subgroups based on the ratio of valence to ionic radius: F Smaller, highly charged high field strength (HFS) elements (REE, Th, U, Ce, Pb 4+, Zr, Hf, Ti, Nb, Ta) 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 Compatible elements (small, low valence) include: Compatible elements (small, low valence) include: Major elements (Fe, Mg) and trace elements Major elements (Fe, Mg) and trace elements (Ni, Cr, Cu, W, Ru, Rh, Pd, Os, Ir, Pt, and Au) (Ni, Cr, Cu, W, Ru, Rh, Pd, Os, Ir, Pt, and Au) Slide 9 Incompatible elements HREEs are less incompatible Slide 10 Relative ionic radii for common valences and coordination numbers Compatible elements Slide 11 Preference for mineral phase Preference for melt Plot of ionic radius vs. ionic charge for trace elements of geological interest. Ionic radii are quoted for eight-fold coordination to allow for comparison between elements. From Rollinson (1993). Ionic charge vs. radius Slide 12 Trace Elements Note magnitude of major element changes From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 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). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. wt % Slide 13 Figure 9-1. Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Note magnitude of trace element changes Trace Elements ppm ppm Slide 14 Table 9-6 A brief summary of some particularly useful trace elements in igneous petrology Element Use as a petrogenetic indicator Ni, Co, CrHighly compatible elements. Ni (and Co) are concentrated in olivine, and Cr in spinel and clinopyroxene. High concentrations indicate a mantle source. V, TiBoth show strong fractionation into Fe-Ti oxides (ilmenite or titanomagnetite). If they behave differently, Ti probably fractionates into an accessory phase, such as sphene or rutile. Zr, HfVery incompatible elements that do not substitute into major silicate phases (although they may replace Ti in sphene or rutile). Ba, RbIncompatible element that substitutes for K in K-feldspar, micas, or hornblende. Rb substitutes less readily in hornblende than K-spar and micas, such that the K/Ba ratio may distinguish these phases. SrSubstitutes for Ca in plagioclase (but not in pyroxene), and, to a lesser extent, for K in K- feldspar. Behaves as a compatible element at low pressure where plagioclase forms early, but as an incompatible at higher pressure where plagioclase is no longer stable. REEGarnet accommodates the HREE more than the LREE, and orthopyroxene and hornblende do so to a lesser degree. Sphene and plagioclase accommodates more LREE. Eu 2+ is strongly partitioned into plagioclase. YCommonly incompatible (like HREE). Strongly partitioned into garnet and amphibole. Sphene and apatite also concentrate Y, so the presence of these as accessories could have a significant effect. Table 9-6. After Green (1980). Tectonophys., 63, 367-385. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Slide 15 Trace elements as a tool to determine paleotectonic environment l Useful for rocks in mobile belts that are no longer recognizably in their original setting l Can trace elements be discriminators of igneous environment? l Approach is empirical on modern occurrences l Concentrate on elements that are immobile during low/medium grade metamorphism Slide 16 Table 18-4. A Classification of Granitoid Rocks Based on Tectonic Setting. After Pitcher (1983) in K. J. Hs (ed.), Mountain Building Processes, Academic Press, London; Pitcher (1993), The Nature and Origin of Granite, Blackie, London; and Barbarin (1990) Geol. Journal, 25, 227-238. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Slide 17 Figure 9-8. (a) after Pearce and Cann (1973), Earth Planet, Sci. Lett., 19, 290-300. (b) after Pearce (1982) in Thorpe (ed.), Andesites: Orogenic andesites and related rocks. Wiley. Chichester. pp. 525-548, Coish et al. (1986), Amer. J. Sci., 286, 1-28. (c) after Mullen (1983), Earth Planet. Sci. Lett., 62, 53-62. Slide 18 REE data for oceanic basalts Figure 10-13a. REE diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989). increasing incompatibility Slide 19 Spider diagram for oceanic basalts increasing incompatibility Figure 10-13b. Spider diagram for a typical alkaline ocean island basalt (OIB) and tholeiitic mid-ocean ridge basalt (MORB). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Data from Sun and McDonough (1989). Slide 20 REE data for UM xenoliths Figure 10-14 Chondrite-normalized REE diagrams for spinel (a) and garnet (b) lherzolites. After Basaltic Volcanism Study Project (1981). Lunar and Planetary Institute. LREE enriched LREE depleted or unfractionated LREE depleted or unfractionated LREE enriched