Trace Elements. Today s lecture

Transcription

1 Trace Elements 300 Ni 200 ppm Zr SiO 2 wt. % Updates: M&M due date: Tuesday Today s lecture Topics: Trace element compositions Trace element behavior Partitioning Spider( dia)grams Spidergrams of crust 1

2 Magma series -- melting Groupings: alkaline (hotspots, continent) and subalkaline subalkaline: ridges, hotspots (or Alkaline Subalkaline %SiO 2 Types of incompatible elements high field strength elements (HFSE): HFSE + REE large ion lithophile elements (LILE): Alkali + Alkaline Earths W. White 2

3 Element Distribution Element fits in a crystal if similar: Ionic size (xl lattice) Charge (neutral crystal) Goldschmidt s rules 1. Ions of similar size (<15%) can replace each other 2. Ions of similar size and a charge difference of 1 can replace as long as neutrality is preserved 3. The ion with the higher ionic potential forms a stronger bond with the anions surrounding the crystal site W. White Chemical Fractionation The uneven distribution of an ion between two competing phases (melt-xl) Exchange equilibrium of a component i between two phases (solid and liquid) X i solid i (liquid) <=> i (solid) K = (mol) => D = (concentration by weight) X i liquid C s C l K = equilibrium constant (mol) D 3

4 Compatibility Compatibility defined w.r.t. minerals: D = distribution or partition coefficient incompatible elements are concentrated in the melt (K D or) D «1 compatible elements are concentrated in the solid (K D or) D» 1 D = C s C l Compatibility and minerals Compatibility depends on minerals and melts involved. Why? 4

5 Bulk distribution For a rock, determine the bulk distribution coefficient D for an element by adding up the minerals D Er = ( ) + ( ) + ( ) + ( ) = % olivine, D = % orthopyroxene, D = % clinopyroxene, D = % garnet, D = 4.7 Enrichment/depletion Bulk D important for crystallization: Early only olivine: D Ni >> 1 Þ Ni decreases in melt (depletion) D Zr << 1 in Ol, Px, Plag, Þ Zr increases in melt because bulk D < 1 (enrichment) ppm 300 Ni Zr SiO 2 wt. % 5

6 Trace element behavior Examples: Incompatible examples: K/Rb often used for amphibole: least incompatible in amph => controls K/Rb with its D values Sr and Ba actually compatible in plagioclase and alkali feldspar, resp. => start of fsp crystallization significantly changes bulk D and ends enrichment Compatible example: Ni strongly fractionated olivine Cr and Sc pyroxenes => Ni/Cr or Ni/Sc can distinguish the effects of olivine and augite in a partial melt or a suite of rocks produced by fractional crystallization REE Diagrams Plots of concentration as the ordinate (y-axis) against increasing atomic number Degree of compatibility increases from left to right across the diagram Concentration La Ce Nd Sm Eu Tb Er Dy Yb Lu 6

7 Odd-Even in the Solar System What s interesting/strange about this pattern? Element generation in stars prefers even numbered nuclei: sawtooth pattern Avoid this pattern in REE diagram by normalizing over Chondritic concentrations Normalized diagrams Chondrite normalized by Chondrite =? sample/chondrite ? La 58 Ce L 60Nd 62Sm 64 Eu 66 Tb 68Er 70 Yb 72 Lu 7

8 Spider Diagrams Including other elements Organized by increasing incompatibility (to the left) Different minerals+melts different ordering Rock/Chondrites Rb Ba Th Nb K La Ce Sr Nd Sm Zr Ti Gd Y Fig 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 REE/Spider Diagrams II 1. Compare enrichment between samples 2. Identify deviating concentrations: crystallizing phases/source mineralogy sample/chondrite 8.00 Eu* La Ce Nd 60 Sm Eu Tb 66 Er 68 Yb Lu Element Figure 9-5. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 8

9 Examples Fractional crystallization: (xls removed after they form: no equilibrium) W/ plag: Eu = removed W/ garnet: HREE = removed sample/chondrite % Ol 17% Opx 17% Cpx La 58 Ce 60 Nd 62Sm Eu 64 Tb Er Yb 70 Lu sample/chondrite % Ol 15% Opx 15% Cpx 10%Plag sample/chondrite % Ol 14% Opx 14% Cpx 14% Grt La Ce Nd Sm Eu Tb Er Yb Lu La Ce 60 Nd 62Sm Eu 64 Tb 66 Er 68 Yb 70 Lu 72 Batch Melting C C L O = 1 What s D i? D i (1- F) + F 1000 C L, C O = liquid, solid concentration F = fraction melt produced = melt/(melt + rock) D = D = 1 = even split, D < 1 = incompatible in minerals => enriched in melt D > 1 = compatible in minerals => depleted in melt C L /C O 1 D = 0.1 D = 0.5 D = 1 D = 2 D = 4 D = F Figure 9-2. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 9

10 Fractional melting, and others Separation of each melt drop as it formed C L /C O = (1/D) * (1-F) (1/D -1) Crystallization like melting Wall-rock assimilation Zone refining Combinations of processes Cox, Bell, Pankhurst The Composition of Earth s crust Oceanic crust» basalt, Continent best guess = based on shales (sample a bit of everything) Compared to primitive mantle - Crust has up to 100x the blue box elements: don t fit in crystal structure of mantle minerals, so escape with melt (that makes crust) Primitive mantle - Mantle has more of the brown box elements: do fit in mantle minerals, so melts get <1x 10

11 Oceanic vs continental crust G average oceanic crust ~ opposite of average continent for Rb-Na (pink box) G Increasing Rb-Nd difference right à left best explained with melting (incompatibility!) G Complementary pattern suggests melting relationship: primitive mantle melted to make early crust (surviving within continent) and residual mantle (oceanic crust source) Melting model G Overall pattern is similar, but with a few differences that need additional processes. 11

12 Continents vs arcs Continental crust similar to island arc andesite. Note the distinctive depletions in Ta and Nb, and large enrichments in Pb (vs. neighbor elements) Supports arguments that the bulk of continental crust is produced in arcs. Figure Comparison of incompatible element concentrations in a silicous andesite from the Banda arc (red stars) with the range of estimated concentrations in the continental crust (gray field). Both share a relative depletion in Nb and Ta and a relative enrichment in Pb. modified from White, Geochemistry 12