Trace Element Models: Melting and Crystallization

GEOL 567 Name______Global Geochemical Systems

Problem Set Two Trace Element Models: Melting and Crystallization 50 points Due 2 March 05

Part One: Mantle Melting

Using the bulk composition, eutectic composition, initial concentrations of Ni, Rb, Ba, and Sr, and partition coefficients in Tables 1 and 2, calculate the trace element compositions of partial melts of mantle lherzolite. Use a non-modal batch melting model for F values from 0 to 1 in increments of 0.1. You can do this with a calculator or a spreadsheet. Plot the results on the following diagrams: f vs. CL/Co for all four elements Ba and Sr vs. Rb Ba/Rb, Ba/Sr ratios vs. the differentiation index of your choice (F, Ni, Rb...) Write an abstract summarizing the results (incompatible element enrichments and compatible element depletions, relative enrichments as a function of partition coefficient, element ratios that are useful to describe partial melting relations, ...... )

Part Two: Fractional Crystallization of Basalt

Choose one partial melt (basalt) calculated in Part One. Calculate the concentrations of Ni, Ba, Rb, and Sr during fractional crystallization consistent with a 2:1 plagioclase/olivine phenocryst assemblage (67% plag and 33% ol), using the partition coefficients in Table 2.

Plot the results on the following diagrams: F vs. CL/Co for all four elements Ba and Sr vs. Rb Ba/Rb, Ba/Sr ratios vs. the differentiation index of your choice (F, Ni, Rb...) Write an abstract summarizing the results, similar to your abstract for Part One. Compare the behavior of Sr in the melting model of Part One to the crystallization model of Part Two. Why did the behavior of Sr change?

Part Three: Application: Fractional Crystallization or Magma Mixing??

REE data for Taos Plateau basalts, andesites, and a dacite exposed in the Red River Gorge near Questa, NM are reproduced in Table 3 (McMillan and Dungan, 1986, JGR, 91:6029- 6045). Plot these data on a chondrite-normalized REE diagram.

Calculate and compare models of fractional crystallization and magma mixing for the evolution of this suite of lavas, assuming that basalt RG-197 is the parent magma. Partition coefficients and phenocryst assemblages are included in Table 3. For mixing, assume that dacite RG-206 was the silicic component. Plot the results of both models on REE diagrams and write an abstract interpreting the results.

TABLE 1. Bulk (point X) and eutectic (point Y) modal compositions for lherzolite melting used in problem 1 (from Wilson, 1989).

Modal Aug Modal Opx Modal Ol

Eutectic (P values) 0.68 0.22 0.10 Bulk (X values) 0.25 0.20 0.55

TABLE 2. Initial concentrations (problem 1) and partition coefficients (problems 1-2).

Initial Element Concentration D ol D opx D aug D pl Ni 2000 10.0 4.0 2.0 0.01 Ba 100 0.001 0.001 0.001 0.2 Rb 5 0.001 0.001 0.001 0.07 Sr 100 0.001 0.010 0.070 2.2 TABLE 3. Data for problem 3. Analyses from McMillan and Dungan (1986).

RG-197 RG-202 RG-203 RG-204 RG-206

SiO2 50.2 51.5 54.5 56.1 60.5

La 12.5 14.5 24.8 29.9 40.9 Ce 28 32 53 64 82 Sm 3.6 3.8 5 5.5 6.5 Eu 1.18 1.18 1.46 1.54 1.63 Tb 0.67 0.64 0.64 0.68 0.69 Yb 2.30 2.33 2.03 1.92 1.59 Lu 0.381 0.350 0.304 0.294 0.254

Modal phenocryst content (in % of total rock): ol 5 3 7 7 0 aug 0 0 0 0 10 plag 0 2 3 3 0

Partition coefficients:

D ol D aug D pl La 0.005 0.02 0.10 Ce 0.005 0.04 0.10 Sm 0.01 0.15 0.10 Eu 0.01 0.30 0.30 Tb 0.01 0.30 0.10 Yb 0.01 0.60 0.10 Lu 0.01 0.60 0.10 Problem Set Two Trace Element Models: Melting Crystallization, and Mixing Evaluation Criteria

Part One: Partial melting of lherzolite Technical content of the abstract ____Trace element composition of the required partial melts are correctly calculated. (2) ____The required diagrams (total of 8) are presented. (2) ____Writer describes the behavior of trace elements during melting as a function of partition coefficient. (2) _____Writer discusses the variation in trace element ratios during partial melting, and why certain ratios are diagnostic for melting processes. (2) Writing ____Writer supports interpretations using data and diagrams. (2) ____Interpretations reflect clarity of thought and depth of analysis. (3) ____Standard edited English is used effectively. (1)

Part Two: Fractional crystallization of basaltic magma Technical content of the abstract ____Trace element composition of the required fractionated melts are correctly calculated (2) ____The required diagrams (total of 8) are presented. (2) ____Writer discusses the behavior of trace elements during fractional crystallization as a function of partition coefficient. (2) ____Writer discusses the variation in trace element ratios during fractional crystallization, and why certain ratios are diagnostic for the crystallization process. ____Writer compares the behavior of Sr in Part One and Part Two. (2) Writing ____Writer supports interpretations using data and diagrams. (2) ____Interpretations reflect clarity of thought and depth of analysis. (3) ____Standard edited English is used effectively. (1)

Part Three: Magma mixing and Rare Earth Elements Technical content of the abstract ____REE data of the Red River lavas are presented in a standard REE diagram. (2) ____REE concentrations in mixed magmas are calculated. (2) ____REE concentrations in mixed magmas are presented in a standard REE diagram. (2) ____REE concentrations in magmas produced by fractional crystallization of olivine from RG- 197 are presented. (2) ____Writer discusses the differences in REE distribution for mixing and fractional crystallization models. (4) Writing ____Writer supports interpretations using data and diagrams. (2) ____Interpretations reflect clarity of thought and depth of analysis. (3) ____Standard edited English is used effectively. (1)