Lecture 1: Introduction, partition coefficients and data presentation Modeling of trace element and radiogenic isotopic data in igneous petrology Lecture 1: Introduction, partition coefficients and data presentation

Aims Understanding what geochemical data can and cannot be used for SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total 59.65 0.75 16.14 6.87 0.12 4.06 6.52 3.25 1.84 0.14 0.77 100.11 Ba Rb Sr Pb Th U Zr Nb Y Sc V Cr 473 68 287 11 6 2 162 5 21 26 177 59 Ni Cu Zn Ga 17 30 67 16 24.7 193.2 69.4 24 23.7 67.4 85.4 324.8 Cs La Ce Pr Nd Sm Eu Gd 22.6 151.6 6.4 6.9 447.4 17.5 37.6 4.7 18.5 4 1.1 3.9 Tb Dy Ho Er Tm Yb Lu Hf Ta 0.6 3.6 0.8 2.1 0.3 2.3 0.4 4.5 0.9 9.7 Understanding what geochemical data can and cannot be used for Knowing how to manipulate and present geochemical data Use and application of mathematical models to geological processes The limitations and assumptions of these models Microsoft Excel (!)

Overview of a geochemical analysis SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total 59.65 0.75 16.14 6.87 0.12 4.06 6.52 3.25 1.84 0.14 0.77 100.11 Ba Rb Sr Pb Th U Zr Nb Y Sc V Cr 473 68 287 11 6 2 162 5 21 26 177 59 Ni Cu Zn Ga 17 30 67 16 24.7 193.2 69.4 24 23.7 67.4 85.4 324.8 Cs La Ce Pr Nd Sm Eu Gd 22.6 151.6 6.4 6.9 447.4 17.5 37.6 4.7 18.5 4 1.1 3.9 Tb Dy Ho Er Tm Yb Lu Hf Ta 0.6 3.6 0.8 2.1 0.3 2.3 0.4 4.5 0.9 9.7 Major elements: measured in weight percent (wt%), sum to 100% (+/-) Trace elements: measured in parts per million (ppm), 10-6 g per g of sample, µg per g. (micrograms). 1 wt% = 10,000 ppm. Do not form stoichiometric constituents in important mineral phases.

Classification on the basis of geochemical behaviour Large Ion Lithophile Elements (LILE) Cations with large radii and low electric charges (Rb, Cs, Ba, Pb, Tl, Sr, Eu), tend to occur in higher abundances in more evolved rocks (felsic). Tend to substitute for K and sometimes Ca. High field strength elements (HFSE) Cations with smaller radii and higher electric charges (U4+, Th4+, Be2+, Mo6+, W6+, Nb5+, Ta5+, Sn4+, Zr4+, REE3+) which don’t really substitute for any major ions in common silicate minerals, may tend to form minerals in their own right (uraninite, zircon). (Figure from Rollinson (1993))

Why are trace elements so useful? Trace elements in rocks/magmas do not affect the chemical or physical properties of the system as a whole to a significant extent Trace element: present in so low concentrations that they behave passively and do not influence geochemical processes Trace elements do not control the appearance or disappearance of major minerals Behaviour is controlled by element-matrix (element-mineral) reactions, not element-element reactions Different elements have different chemical properties (charge, size etc), therefore different elements behave in different ways during different geological processes

Where do trace elements live? In simple terms, trace elements replace major cations with similar size and charge in minerals. A A B C B For example in this hypothetical mineral, major element B could be replaced by trace element D, but not C or E. D B A A E B

LIL HFS Examples: Rb replacing K in biotite and alkali feldspar Sr and Eu replacing Ca in plagioclase Ni replacing Fe and Mg in olivine Cr and V replacing Fe in magnetite Hf replacing Zr in zircon

Incorporation into minerals Goldschmidt’s rules of substitution (1930’s). 1. Ions of one element can extensively replace those of another in ionic crystals if their radii differ by less than about 15%. 2. Ions whose charges differ by one unit substitute readily for one another provided electrical neutrality of the crystal is maintained. If the charges of the ions differ by more than one unit, then substitution is generally slight. 3. When two different ions can occupy a particular position in a crystal lattice, the ion with the higher ionic potential forms a stronger bond with the anions surrounding the site. 4. Substitution may be limited, even when the size and charge criteria are satisfied, when the competing ions have different electronegativities and form bonds of different ionic character. e.g. Na+ and Cu+ have the same charge and radius, so according to rules 1-3, Cu+ should replace Na+ in minerals such as albite (NaAlSi3O8) or halite (NaCl). This does not happen because Cu forms bonds that are more covalent than Na.

Incorporation into minerals “camouflage”: when trace element has same charge and similar ionic radius to major element. e.g. Zr4+ (0.80 Å) and Hf4+ (0.79 Å). Zircon crystals (ZrSiO4) accept both Zr & Hf (but Hf has lower abundance in nature). “capture”: when trace element enters a crystal preferentially because it has a higher ionic potential (charge/radius ratio) than ions of the major element. e.g. K-feldspar may incorporate Ba2+ (1.44 Å) or Sr2+ (1.21 Å) in place of K+ (1.46 Å). Needs a coupled substitution of Al3+ for Si4+ to preserves electrical neutrality of the crystal lattice. “admission”: entry of trace element with a lower ionic potential than the major ion because it has either a lower charge or a larger radius, or both. e.g. calcite replacement of Ca2+ (1.08 Å) by Sr2+ (1.21 Å). The extent to which ions are admitted to a particular lattice site decreases as the difference in radii increases. Ba2+ (1.44 Å) is less abundant in calcite than Sr2+ (1.21 Å).

Partition coefficients Modeling of trace elements relies primarily on the PARTITION COEFFICIENT(KD) concentration of element in mineral concentration of element in melt KD greater than1 = compatible KD less than1 = incompatible In most systems we have more than one mineral so we define the bulk partition coefficient D D = xa.KD a/l + xb.KD b/l + xc.KD c/l …..

Role of charge and radius An illustration of the importance of ionic radii and charge on partitioning and the relative values of partition coefficients. e.g. clinopyroxene Ca(Mg,Fe)Si2O6. Trace elements whose charge and ionic radius most closely match that of the major elements (Ca, Mg, Fe) have the highest partition coefficients (KD’s) and are therefore most compatible.

Where do KD’s come from? How do we get info on KD’s? Natural samples: co-existing mineral and melt, such as crystals in a MORB glass. Synthetic experiments: investigate mineral at P & T of interest. in-situ analyses are best: ion-probe (SIMS) or laser ablation ICP-MS.

Predicting Partition Coefficients Earth and Planetary Science Letters, 210, 383-397 (2003) review article by Blundy & Wood on elastic strain modelling. Lattice strain and electrostatic models of how the disruption of crystal lattice around a trace element is minimised by relaxing neighboring ions and distributing surplus elastic and electrostatic energy through the lattice. r0(M) = radius of crystal lattice site, EM = elastic response, D0(M) = optimum KD = 1

Partition coefficients are not constants! Partition coefficients have been shown experimentally to vary with: Pressure Temperature Oxygen activity Crystal chemistry Water content of melt Composition of melt Therefore it is important to choose appropriate partition coefficients for the process you are trying to model, i.e. not low pressure, low temperature KD for rhyolites to model mantle melting! (Figure from Rollinson 1993)

Partition coefficients are not constants! Composition of the melt (e.g. silica content) is one of the most important factors (Figure from Rollinson 1993)

The Rare Earth Elements (REE, Lanthanides) Atomic number Name Symbol Valency Ionic radius (Å) 57 Lanthanum La 3+ 1.160 58 Cerium Ce 1.143 4+ 0.970 59 Praesodymium Pr 1.126 60 Neodymium Nd 1.109 61 Promethium Pm - 62 Samarium Sm 1.079 63 Europium Eu 1.066 2+ 1.250 64 Gadolinium Gd 1.053 65 Terbium Tb 1.040 66 Dysprosium Dy 1.027 67 Holmium Ho 1.015 68 Erbium Er 1.004 69 Thulium Tm 0.994 70 Ytterbium Yb 0.985 71 Lutetium Lu 0.977 The Rare Earth Elements (REE, Lanthanides)

REE All are generally 3+ and have similar ionic radii, but it decreases with increasing atomic number Eu3+ behaves as a typical REE at atmospheric conditions At reducing conditions (low oxygen activity) Eu3+ becomes Eu2+, ionic radius = 1.25, and has properties similar to Ca. Ce changes from Ce3+ to Ce4+ under very oxidizing conditions (e.g. weathering) REE behave as a coherent group, but differently in different minerals, and slightly different from each other.

Different minerals have slightly different partition coefficients for the REE so their removal during fractional crystallisation (or retention during partial melting) will have different and distinct effects on melt composition.

Oddo-Harkins effect Element R97/9 La 17.5 Ce 37.6 Pr 4.7 Nd 18.5 Sm 4.0 Eu 1.1 Gd 3.9 Tb 0.6 Dy 3.6 Ho 0.8 Er 2.1 Tm 0.3 Yb 1.9 Lu

Chondrite normalisation Element R97/9 Chondrite R97/9 norm La 17.5 0.235 56.45 Ce 37.6 0.603 46.53 Pr 4.7 0.089 38.52 Nd 18.5 0.452 30.83 Sm 4.0 0.147 20.51 Eu 1.1 0.056 14.97 Gd 3.9 0.197 15.06 Tb 0.6 0.0363 12.66 Dy 3.6 0.243 11.18 Ho 0.8 0.0556 11.14 Er 2.1 0.159 10.00 Tm 0.3 0.0242 9.26 Yb 1.9 0.163 11.66 Lu 0.0243 10.29 Chondrite values from Anders and Grevesse (1989)

Multi-element diagrams (a.k.a. Spiderdiagrams) Normalised to Primitive Earth (Primordial mantle or Chondrite) or MORB. Elements ordered in approx. increasing compatibility in a mantle melt Elements nearly all incompatible during melting and crystallization

Example: Andesites from Ruapehu, NZ Variable Ta LREE-enriched Variable Sr Negative Nb-Ta anomaly Enriched in incompatible elements Outliers Negative Ti Normalised to Chondrite values of Thompson (1982)

Example: Granitoids from NZ negative Ba and Sr negative P and Ti Normalised to MORB values of Pearce (1983)

Exercises Plotting KD for various minerals Plotting and describing REE diagrams Plotting spiderdiagrams Wrestling with Excel!