Chapter 9 Spring 2010 - Trace Elements: comparison with...

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Unformatted text preview: Trace Elements: comparison with major elements The magnitude of change of the major elements during magmatic processes is frequently small. Trace elements span a wider range of geochemical characteristics and so can change concentration more than most major elements. Diagrams for 310 volcanic rocks from Crater Lake, Oregon Cascades. Data compiled by Rick Conrey. Red lines show possible "trends" The concentration of a major element in a phase is commonly buffered by the system. Major elements commonly vary little in a phase as the system composition changes. At the given temperature here (~1580C) we could vary the amount of liquid in the system from 0 to 100% by varying the system composition from Fo34 to Fo69, with no change in the olivine composition. Trace elements are not buffered like this. Major element buffering Trace elements: usually not buffered and so can change a lot In this example set of lavas, notice how these trace elements change concentration over a wide range: 250 ~0 ppm for Ni ~60 ~250 ppm for Zr Also notice that the changes are in opposite directions. Harker Diagram for Crater Lake. From data compiled by Rick Conrey. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. m Trace elements: wide range of behavior m M m m M M M m M M M M M M, m = typical Major, minor elements Partitioning of trace elements between phases At equilibrium, a trace element in the different phases of a system typically has different concentrations. The partitioning of a trace element "i" between two phases, in this example a solid crystalline phase and magmatic liquid, is defined by the equation: Kdisolid/liquid = C Where: Kdi C isolid /Ciliquid is the partition coefficient for element i between the solid phase and the liquid phase. is the concentration of element i in the solid phase. isolid *the amount of a gas absorbed by a liquid is in proportion to the pressure of the gas above the liquid, Partitioning of trace elements and Henry's Law* 1. Trace element concentrations are low by definition, and so are usually in the Henry's Law region of concentration. 1. This means that the concentration (more precisely activity) of element i in the various phases varies directly (linearly) with concentration in the system. 1. Thus, if XNi in the system doubles, the XNi in all phases will double. 1. This does not mean that X in all phases is the same. Ni Rather the X in each phase will vary in proportion to the Kd > 1: Compatible elements are concentrated in the solid part of a system; that is, the elements are comparatively compatible with the crystalline structures of at least one of the solid phases present. Compatible and incompatible trace elements Kd < 1: Incompatible elements are concentrated in the liquid part of a system; that is, the elements are comparatively incompatible with all of the crystalline structures of solid phases present. Examples of typical incompatible elements: Kd < 1 High field strength (HFS) elements: Smaller, high charge elements typically including REE3+, Th4+, U4+, Zr4+, Hf4+, Ti4+, Nb5+, Ta5+. These elements are typically almost insoluble in aqueous fluids and so are relatively immobile during alteration. Large ion lithophile (LIL) elements: Larger, low charge elements typically including K+, Rb+, Cs+, Ba2+, Pb2+, Sr2+. These elements are typically quite soluble in fluid phases, especially aqueous fluids, and so are mobile during alteration. The term "high field strength" refers to the high surface charge gradient on these ions, calculated as the ratio: (formal charge/ionic radius2). The term "lithophile" refers to the tendency of these elements to form oxygen compounds and end up in high concentrations in crustal rocks. Incompatible element behavior The expected behavior of incompatible elements is to increase in concentration with crystallization. This statement assumes that the set of magmas in question is related largely by crystallization, which is not necessarily correct. Examples of typical compatible elements: Kd > 1 Medium-size first row transition metals (Cr3+, Co2+, Ni2+). Chalcophile elements: Elements that partition strongly into sulfide liquid or solid phases (platinum group elements, gold, silver, copper, many semimetals). The term "chalcophile" refers to the affinity these elements have for sulfur. The expected behavior of compatible elements is to decrease in concentration with crystallization. Compatible element behavior This statement also assumes that the suite of rocks in question is related to one another largely by crystallization, which is not necessarily the case. Actual partitioning behavior varies Crystal/liquid partition coefficients vary with: Mineral of interest. The partitioning behavior of elements is different for olivine, pyroxene, plagioclase, garnet, spinel, etc. Liquid composition of interest. Typically, many elements become more compatible with increasing silica content of the magma. Mineral composition. The partitioning behavior of elements changes with site size and shape which vary with mineral composition. Temperature. Pressure. Crystal/liquid partitioning in basalt, several elements Compatible elements Incompatible elements Crystal/liquid partitioning in basalt and dacite Compatible elements Incompatible elements The Rare Earth Elements (REE) Steady decrease in lanthanide ionic radius with atomic number. Why the lanthanides are geochemically coherent Without normalizing, the result is a confusing Oddo-Harkins zig-zag pattern. Plotting REE concentrations without normalization 11 Log (Abundance in CI Chondritic Meteorite) 10 9 8 7 6 5 4 3 2 1 0 -1 -2 -3 0 H He C Ne MgSi Fe N S Ar Ca Ni Na Ti AlP K F Cl V B Sc O Li Sn Ba Pt Pb Th U 100 Be 10 20 30 40 50 60 70 80 90 Atomic Number (Z) 3 Eliminate Oddo-Harkins effect and make y-scale more functional by normalizing to a standard v estimates of primordial mantle REE v chondrite meteorite concentrations With chondrite normalization, the result is relatively smooth REE patterns. Plotting REE diagrams with chondrite normalization q Europium anomaly when plagioclase is a fractionating phenocryst or 3 a residual solid in source Result of Eu2+ substituting for Ca2+ 3 3 Anomaly is negitve (melt is depleted) in this example because the plag had not yet melted and retained the Eu Figure 9-5. REE diagram for 10% batch melting of a hypothetical lherzolite with 20% plagioclase, resulting in a pronounced negative Europium anomaly. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Spider Diagrams An extension of the normalized REE technique to a broader spectrum of elements Chondrite-normalized spider diagrams are commonly organized by (the author's estimate) of increasing incompatibility L R Different estimates different ordering (poor standardization) Fig. 9-6. Spider diagram for an alkaline basalt from Gough Island, southern Atlantic. After Sun and MacDonough (1989). In A. D. Saunders and M. J. Norry (eds.), Magmatism in the Ocean Basins. Geol. Soc. London Spec. Publ., 42. pp. MORB-normalized Spider Separates LIL and HFS Figure 9-7. Ocean island basalt plotted on a mid-ocean ridge basalt (MORB) normalized spider diagram of the type used by Pearce (1983). Data from Sun and McDonough (1989). From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Trace element ratios can identify equilibrium phases The ratios of trace elements are often superior to the absolute element concentration for identifying the role of a specific mineral. Here are two examples: 1. Ba/Sr as controlled by feldspars. 2. Ni/Cr as controlled by olivine and pyroxene. Summary of some useful trace elements Common use as a petrogenetic indicator Highly compatible elements. Ni and Co are concentrated in olivine, Cr in spinel Ni, Co, Cr, Sc and pyroxene, Sc in pyroxene. High concentrations indicate a mantle source and little fractional crystallization V and Ti are strongly partitioned into the Fe-Ti oxides ilmenite, magnetite, and titanomagnetite. If they behave differently, Ti is probably partitioning into an V, Ti accessory phase such as titanite or rutile. Very incompatible elements that do not substitute into major silicate phases. Zr, Hf, Nb, Ta They may replace Ti in titanite or rutile. Zr and Hf can be buffered by zircon crystallization. Incompatible elements that substitute for K in K-feldspar, micas, and amphibole. Rb substitutes less readily in amphibole than in K-feldspars or Ba, Rb micas, so K/Ba ratios may indicate the involvement of amphibole. Substitutes for Ca in plagioclase and to a lesser extent for K in K-feldspar, but not for Ca in pyroxenes and amphiboles or K in micas. Sr bahaves as a Sr compatible element where plagioclase is involved (e.g., at low pressure), and as an incompatible element under other circumstances. Generally incompatible except for Eu2+ in plagioclase-bearing systems, and HREE in garnet-bearing systems. Garnet, olivine, and OPX concentrate HREE, REE, Y CPX and amphibole concentrate MREE, and plagioclase and titanite concentrate LREE. After Green (1980). Tectonophysics, v. 63, p. 367-385; and Winter (2001) An Introduction to Igneous and Metamorphic Petrology, Prentice Hall. Element Trace elements as a tool to determine paleotectonics Trace elements in magmas are controlled by the source composition, melt generation processes, and by post-melting processes. Mantle compositions vary from one tectonic environment to another, because of the different regions from which the mantle rock comes and because of processes that modify the mantle composition. Similarly, post-melt generation magmatic processes differ to some extent from one tectonic environment to another. Trace elements are useful for indicating the tectonic environment of igneous rocks that are no longer recognizably in their original setting (e.g., old rocks). This approach is entirely empirical, based on modern occurrences. This approach generally concentrates on elements that are relatively immobile during hydrothermal alteration and low to medium grade metamorphism (e.g., high field strength elements and lanthanides). Example tectonic discriminant diagrams (basalts) Pearce, 1982, in Thorpe (ed.), Andesites: Orogenic andesites and related rocks, Wiley, Chichester, p. 525-548. WPB Within-plate basalts IAT Island arc tholeiites CAB Calc-alkaline basalts (arcs) MORB Mid-ocean ridge basalts OIT Ocean island tholeiite OIA Ocean island alkaline basalt Pearce and Cann, 1973, Earth and Planetary Science Letters, v. 19, p. 290-300. 290-300. Mullen, 1983, Earth and Planetary Science Letters, v. 62, p. 53-62. ...
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This note was uploaded on 05/02/2010 for the course ESCI 322 taught by Professor Evans during the Spring '10 term at Central Connecticut State University.

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