Trace Elements Ni Zr ppm wt. % SiO 40 50 60 70 80 100 200 300 100 200 300 Ni SiO 2 Zr ppm wt. % TE often vary by > 103 very useful since so sensitive to distr. & fractionation

Today’s lecture Updates: Topics: Finish major elements Trace element compositions Trace element behavior Partitioning Spider diagrams TE often vary by > 103 very useful since so sensitive to distr. & fractionation

Magma Evolution Harker diagram Smooth trends Model with 3 assumptions: 55 65 75 45 2 3 4 8 6 10 14 16 Al2O3 MgO CaO Fe2O3 Na2O K2O Wt. % SiO2 B BA A D RD R Harker diagram Smooth trends Model with 3 assumptions: 1 Rocks are related 2 Trends = liquid line of descent (mineral control) 3 The basalt is the parent magma from which the others are derived B=basalt, BA=basaltic-andesite, A=andesite, D=dacite, RD=rhyo-dacite, R=rhyolite

Magma Series

Alkali vs. Silica -- Hawaiian volcanics: 12 10 8 6 4 2 35 40 45 50 55 60 65 %SiO2 %Na2O + K2O Alkaline Subalkaline Figure 8-11. Total alkalis vs. silica diagram for the alkaline (open circles) and sub-alkaline rocks of Hawaii. After MacDonald (1968). GSA Memoir 116

Evolving rocktypes, subalkaline subdivision (F) Rhyolite Dacite Andesite Basaltic Andesite Ferro-Basalt Basalt Calc-alkaline Tholeiitic B-A A D R FeO + Fe2O3 K2O + Na2O (A) MgO (M)

Occurrence of different series Calc-alkaline only in subduction zones Tholeiitic series anywhere Alkaline not at Mid Ocean Ridges After Wilson (1989). Igneous Petrogenesis. Unwin Hyman - Kluwer

The Basalt Tetrahedron Di Q En Critical plane of silica undersaturation Ne Ab Plane of silica saturation Fo Ol Ne Ab Opx Q Alkaline field Subalkaline field Dividing line 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.

Types of incompatible elements W. White

Element Distribution Element fits in a crystal if similar: Ionic size (xl lattice) Charge (neutral crystal) Goldschmidt’s rules Ions of similar size (<15%) can replace each other Ions of similar size and a charge difference of 1 can replace as long as neutrality is preserved The ion with the higher ionic potential forms a stronger bond with the anions surrounding the crystal site W. White therefore some TE's will follow similar major E Periodic Table is next slide

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) i (liquid) <=> i (solid) K = (mol) => D = (concentration by weight) K = equilibrium constant (mol) ≈ D Cs Cl Xi solid Xi liquid

Compatibility Cs Cl D =

Compatibility and minerals Not exact, since D varies with the composition of mins & melt

Bulk distribution For a rock, determine the bulk distribution coefficient D for an element by adding up the minerals DEr = (0.6 · 0.026) + (0.25 · 0.23) + (0.10 · 0.583) + (0.05 · 4.7) = 0.366 60% olivine, D = 0.026 25% orthopyroxene, D = 0.23 10% clinopyroxene, D = 0.583 5% garnet, D = 4.7

Enrichment/depletion 40 50 60 70 80 100 200 300 Ni SiO 2 Zr ppm wt. % TE often vary by > 103 very useful since so sensitive to distr. & fractionation

Trace element behavior Examples of using trace element ratios to evaluate crystallizing minerals: 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 K/Rb for amphibole: Usually behave similarly, so ~ constant ratio Unless amphibole which has a D of about 1.0 for K and 0.3 for Rb almost all K and Rb reside in it Melt of amphibole-bearing rock will -> decrease K/Rb in the partial melt Other factors being equal, a magma produced by partial melting of an amphibole-bearing source rock would have a lower K/Rb than one derived from amphibole-free source High absolute K or Rb could also  an amphibole-bearing source, but may result from other causes (high phlogopite, or an alkali-enriched fluid) The ratio is more indicative of amphibole due to the different D values Fractional crystallization of amphibole would also -> low K/Rb ratio in the evolved liquid

La Ce Nd Sm Eu Tb Er Dy Yb Lu REE Diagrams Concentration La Ce Nd Sm Eu Tb Er Dy Yb Lu

Odd-Even in the Solar System What’s interesting/strange about this pattern? Chondrite normalization: Normalize to chondrite meteorites to: 1) avoid Oddo-Harkins (even Z more common) 2) think chondrite = primitive earth, so can compare to initial distribution

Normalized diagrams ? sample/chondrite L 0.00 2.00 4.00 6.00 8.00 10.00 56 58 60 62 64 66 68 70 72 sample/chondrite L La Ce Nd Sm Eu Tb Er Yb Lu ?

Spider Diagrams 1 10 100 1000 Rb Ba Th Nb K La Ce Sr Nd Sm Zr Ti Gd Y Rock/Chondrites Order of elements based on estimates of increasing incompatibility from right to left in a "typical" mantle undergoing partial melting Elements are all incompatible (D<1) during most partial melting and fractional crystallization processes. The main exceptions are Sr, which may be compatible if plagioclase is involved, Y and Yb with garnet Ti with magnetite Troughs at these elements would indicate respective mineral involvement Oceanic basalts = large degrees of PM, their spider diagrams should reflect the trace element patterns of their source Less incompatible elements on the right-hand side should be less enriched during PM (particularly for small degrees of it), tilting the curve up on the left -> (-) slope Additionally, FX subsequent to magma segregation from the source should tip the pattern even further Fig. 9-5. 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. 313-345.

REE/Spider Diagrams II 0.00 2.00 4.00 6.00 8.00 56 58 60 62 64 66 68 70 72 Element sample/chondrite Eu* La Ce Nd Sm Eu Tb Er Yb Lu 2 1 Eu* is the value Eu “should” have if Eu+2 did not -> plagioclase Another example of how RATIOS can help Eu alone is inconclusive (low REE of low Eu) Sm/Eu is slope or Eu anomaly trough (Use Eu*/Eu anyway) Figure 9-5. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Examples 67% Ol 17% Opx 17% Cpx La Ce Nd Sm Eu Tb Er Yb Lu 0.00 2.00 4.00 6.00 8.00 10.00 56 58 60 62 64 66 68 70 72 sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu 67% Ol 17% Opx 17% Cpx 0.00 2.00 4.00 6.00 8.00 10.00 sample/chondrite 60% Ol 15% Opx 15% Cpx 10%Plag La Ce Nd Sm Eu Tb Er Yb Lu 0.00 2.00 4.00 6.00 8.00 10.00 56 58 60 62 64 66 68 70 72 sample/chondrite La Ce Nd Sm Eu Tb Er Yb Lu 57% Ol 14% Opx 14% Cpx 14% Grt

C 1 What’s Di? Di (1 F) F = - + Batch Melting D = 1 = even split, O = - + 0.1 1 10 100 1000 0.2 0.4 0.6 0.8 F D = 0.001 D = 0.1 D = 0.5 D = 1 D = 2 D = 4 D = 10 CL/CO CL, CO = liquid, solid concentration F = fraction melt produced = melt/(melt + rock) D = 1 = even split, D < 1 = incompatible in minerals => enriched in melt D > 1 = compatible in minerals => depleted in melt D = 1.0 No fractionation so CL/CO = 1 for all values of F Figure 9-2. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Fractional melting, and others Separation of each melt drop as it formed CL/CO = (1/D) * (1-F) (1/D -1) Crystallization like melting Wall-rock assimilation Zone refining Combinations of processes Can also apply the Rayleigh equation to Rayleigh fractional melting Cox, Bell, Pankhurst