Petrology Lecture 4 - Trace Elements PDF

Petrology Lecture 4- Trace Elements  CHAPTER 9 Introduction Lecture Outline Igneous Metamorphic Petrology 1. What are trace elements? 2. Element distribution 3. Chemical fractionation 4. Model of magma evolution ...

Petrology Lecture 4- Trace Elements  CHAPTER 9 Introduction Lecture Outline Igneous Metamorphic Petrology 1. What are trace elements? 2. Element distribution 3. Chemical fractionation 4. Model of magma evolution  1. What are trace elements? Trace elements Decreasing Atomic Radius generally act as impurities within the mineral structure and distributed in consistent proportions in all phase Transition metals- Sc, Ti, V, Cr, Mn, Co, Ni, Cu, Zn Decreasing Atomic Radius Lanthanides - REE Others- Rb, Sr, Y, Zr, Nb, Cs, Ba, Hf, Ta, Pb, Th, U  1. What are trace elements? Note magnitude of major vs minor element changes in a suite of rocks Figure 8.2 (right) and 9.2. (left) 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. 2. Element Distribution Goldschmidt’s rules (simplistic, but useful)  A) 2 ions with the same valence and radius should exchange easily and enter a solid solution in amounts equal to their overall proportions. Trace elements will follow behavior of major elements (see table)  B) If 2 ions have a similar radius and the same valence: the smaller ion is preferentially incorporated into the solid over the liquid  C) If 2 ions have a similar radius, but different valence: the ion with the higher charge is preferentially incorporated into the solid over the liquid  Using Goldschmidt’s rule and the periodic table : 1) to what element will Rb behave similarly? Ni? 2) Which ion is smaller Fe or Mg? 3) Mg/Fe in Ol is > or 1) Rb behaves than in liquid phase (melt) 3. Chemical Fractionation Chemical fractionation: exchange equilibrium of a component i between two phases (solid and liquid) Distribution constant KD Xi= mol fraction of component i in the solid or liquid phase When components are very dilute (trace el) Distribution coefficient or partition coefficient CS= Concentration of trace element in solid phase (ppm or wt %) CL= Concentration of trace element in liquid phase (same 7  3. Chemical Fractionation  Incompatible elements are Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace concentrated in the melt KD or Elements in Basaltic and Andesitic Rocks D «1 Olivine Opx Cpx Garnet Plag Amph Magnetite  Compatible elements are Rb 0.01 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.04 0.06 0.012 1.83 0.46 concentrated in the solid K Use Table 9.1 to determine whether D or Ba 0.01 0.013 0.026 0.023 0.23 0.42 D is»an Rb 1 incompatible or a Ni 14.0 5.0 7.0 0.955 0.01 6.8 29. compatible element. What about Cr 0.7 10.0 34.0 1.345 0.01 2.0 7.4 La 0.007 0.03 0.056 0.001 0.148 0.544 2. Ba? Ni? Cr? Eu? Ce 0.006 0.02 0.092 0.007 0.082 0.843 2. Rare Earth Elements Nd 0.006 0.03 0.23 0.026 0.055 1.34 2. Rb is incompatible because D1 Er 0.026 0.23 0.583 6.56 0.02 1.74 1.5 Yb 0.049 0.34 0.542 11.5 0.023 1.642 1.4 Lu 0.045 0.42 0.506 11.9 0.019 1.563 Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated 8 3. Chemical Fractionation Incompatible elements commonly divided into two subgroups  Smaller, highly charged High Field Strength Elements HFSE: REE, Th, U, Ce, Pb4+, Zr, Hf, Ti, Nb, Ta  Low field strength Large Ion Lithophile Elements LILE: K, Rb, Cs, Ba, Pb2+, Sr, Eu2+ are more mobile, particularly if a fluid phase is involved 9  3. Chemical Fractionation  For a rock, determine the bulk distribution coefficient D for an element by calculating the contribution for each mineral eq. 9.4: Di =  WA Di A WA = weight % of mineral A in the rock Di = partition coefficient of element i in A mineral A  Table 9-1. Partition Coefficients (CS/CL) for Some Commonly Used Trace Elements in Basaltic and Andesitic Rocks Olivine Opx Cpx Garnet Plag Amph Magnetite Rb 0.010 0.022 0.031 0.042 0.071 0.29 Sr 0.014 0.040 0.060 0.012 1.830 0.46 Ba 0.010 0.013 0.026 0.023 0.23 0.42 Ni 14 5 7 0.955 0.01 6.8 29 Cr 0.70 10 34 1.345 0.01 2.00 7.4 La 0.007 0.03 0.056 0.001 0.148 0.544 2 Ce 0.006 0.02 0.092 0.007 0.082 0.843 2 Nd Rare Earth Elements 0.006 0.03 0.230 0.026 0.055 1.340 2 Sm 0.007 0.05 0.445 0.102 0.039 1.804 1 Eu 0.007 0.05 0.474 0.243 0.1/1.5* 1.557 1 Dy 0.013 0.15 0.582 1.940 0.023 2.024 1 Er 0.026 0.23 0.583 4.700 0.020 1.740 1.5 Yb 0.049 0.34 0.542 6.167 0.023 1.642 1.4 Lu 0.045 0.42 0.506 6.950 0.019 1.563 Data from Rollinson (1993). * Eu3+/Eu2+ Italics are estimated Example: hypothetical garnet lherzolite = 60% olivine, 25% orthopyroxene, 10% clinopyroxene, and 5% garnet (all by weight), using the data in Table 9.1, is: DEr = (0.6 x 0.026) + (0.25 x 0.23) + (0.10 x 0.583) + (0.05 x 4.7) = 0.366 4. Models of Magma Evolution - Batch melting Batch Melting is the simplest model for an equilibrium process involving a solid and a liquid.  The melt remains resident until at some point it is released and moves upward  Equilibrium melting process with variable % melting CL = trace element concentration in the liquid CO = trace element concentration in the original rock before melting F = wt fraction of melt produced = melt/(melt + rock) 12  4. Models of Magma Evolution - Batch melting A plot of CL/CO vs. F for various values of Di Di = 1.0, no fractionation Di = liquid/solid for any F 1.0 Values of F > 0.4 are unlikely for batch melting since melt should separate and rise before Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 13  4. Models of Magma Evolution - Batch melting Compatible elements Di >> - Will occur in very low concentration in melt 1.0 (comparing to their abundance in source!!!) - Especially for low % melting (low F) Figure 9.2. Variation in the relative concentration of a trace element in a liquid vs. source rock as a fiunction of D and the fraction melted, using equation (9.5) for equilibrium batch melting. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. 14  4. Models of Magma Evolution - Batch melting Highly incompatible elements Di isolated from reaction with the remaining liquid CL/CO = F (D -1) CL Concentration of some element in the residual magma Co Concentration of some element in original magma F Amount of melt remaining 21