Geology 302 Excercises in Geochemistry of the Earth s Interior I

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1 Geology 302 Excercises in Geochemistry of the Earth s Interior I These excercises are designed to get you thinking about chemical and physical processes that took place early on during the formation of the Earth and later as the mantle and crust evolved. Most of the plotting and calculations are best done in spreadsheet format. Please show your work to ensure partial credit. 1. Crystallization of Mid Ocean Ridge Basalt (MORB). The accompanying Excel spreadsheet onm the course webpage contains a worksheet with 47 major element analyses of MORB wholerock and glass samples recovered from the East Pacific Rise. All of these samples contain <10% phenocrysts of Forsterite and Anorthite in the mode. A. Could any of these MORBs be a primary magma? Recall that primary magmas in equilibrium with Fo 91 olivine have molar Mg#s, that is 100*Mg 2+ /(Mg 2+ + Fe 2+ ), of >68. To calculate the Mg# on a molar basis, you will need to divide the wt.% oxide values for MgO and FeO* by their gramformula-weights of 40.3 and 71.8, respectively, before calculating the ratio. If none are primary, which lava is the most primitive (Mg-rich) composition? Which one quenched from the least primitive, therefore most differentiated magma? B. Plot an X-Y variation diagram of MgO (X) vs. CaO (Y) of these compositions. Use a scale of 0-20 wt% on the Y axis and 0-50% on the X axis. C. In your diagram, plot the positions of Forsterite, Diopside, and Anorthite (from handout), and answer the following questions. 1. Can the fractional crystallization of any of these three minerals alone be responsible for the evolution from most primitive to least primitive magmas? If so which mineral? 2. It is more likely that the phenocryst phases in the modes of these rocks played a role in controlling differentiation. In what proportion must plagioclase and forsterite fractionate together from the most primitive lava to explain the variation in the less primitive lavas? (Hint-use the lever rule) 3. Imagine a 1 kg parcel of the most primitive MORB in your diagram. Which is more effective at generating the observed range in MORB compositions from high to low Mg# s: (1) fractionation of 100 grams of diopside alone, or fractionation of 100 grams of plagioclase and olivine in the proportions you determined above? How can you tell?

2 2. A second worksheet in the Excel workbook contains major element, trace element, and Sr, Nd, and Pb isotope analyses from 15 samples of lava flows erupted from three different volcanoes during the Quaternary (last 1.8 Ma). A. The geochemical behavior of K and U differ markedly when temperatures are hot enough to volatilize K (leaving U as a refractory solid). In contrast, K and U behave similarly at lower temperatures (<2000 o C) during normal melting and crystallization processes in the terrestrial planets. Make a plot of K/U vs ppm K for the 15 samples using logarithmic scales as in Figure 5.4 of Brown and Mussett (1993). 1. Overall how do the K/U and K of these volcanic samples compare to the carbonaceous chondrite CI, the most primitive consituent known in our solar system? What is the likely reason for the differences? 2. Given the data from our solar system, would you describe the variation in K/U among the samples as small or large? Is this consistent with when these magmas were produced? 3. How do these lavas (magmas) compare to MORB? i.e., are they more differentiated? less differentiated? or the same? On the basis of the K-U data, is it possible that the SNC meteorites, that are only 1300 Ma, and which are thought to come from Mars, originated by processes broadly similar to those which produced the volcanoes from which the 15 samples come? What name would you assign to the volcanic samples which most closely match the SNC meteorites? (Hint look at the major element compositions silica contents, etc.). B 1. Olivine is the predominant mineral in the silica-poor lavas, and with respect to K and U it has identical mineral-melt D values of Starting with the most primitive lava (highest Mg# = 100*Mg/(Mg+Fe 2 ), basically the highest MgO content) and using the Rayleigh equation for perfect fractional crystallization (below), calculate the percentage of this magma that must crystallize olivine to generate the most K and U rich lava compositions. C C L o L = F ( D 1) (Hint: see Wilson, 1989, p. 89, for guidance) 2. Make a Harker diagram of ppm K vs. ppm U. Plot the lava samples and your crystallization models for K and U (at say 5 wt.% increments of crystallization) on the diagram. Does the model accurately explain the origin of the silica-rich lavas? What may be the cause(s) for any discrepencies?

3 C. Make a plot of the 143 Nd/ 144 Nd ratio vs the 87 Sr/ 86 Sr ratio for the lava samples. Comparing to Figure 2.7 in Wilson (1989), what group of basalts might these lavas most closely be associated with? Hint: the MORB-normalized spiderdiagram at right plot a silica-poor sample from each of the 3 volcanoes. What can you say about the tectonic setting and origin of the lavas? Lava sample/morb Shishaldin Kanaga Seguam 1 MORB 0.1 Sr K Rb Ba Th Ta Nb Ce P Zr Hf Sm Ti Y Yb Element

4 3. U-Pb geochronology. Using the equations: D = N(e 8t -1) 206 Pb = 238 U(e 8t -1); 8 = x10 10 yr Pb = 235 U(e 8t -1); 8 = x10 10 yr -1 Construct a U-Pb concordia diagram for 4560 Ma of Earth History. Begin at an intial time of 0 years and increment time in 200 million year steps. Add the following data points obtained by ion microprobe from zircon crystals in the Jack Hills conglomerate, Western Australia: 206 Pb/ 238 U 207 Pb/ 235 U Draw a best fit line through the array of data. What can you conclude about the age and history of these zircons?

5 4. Partial melting of the mantle A third spreadsheet in the workbook available on the course website is set up to calculate the chondrite-normalized REE patterns in liquids generated from the batch modal melting of peridotite (equation 1 from lecture 5). It does this by tabulating the bulk distribution coefficient, D, for each rare earth relement based on the modal percentage of the minerals comprising the source rock. The modal percentages can be adjusted in the spreadsheet (they should total 100%) to model the melting of different mineral assemblages. Once you have chosen the modal percentages, you can vary F, the fraction of melt produced, to generate an REE pattern (black values in diagram). The spreadsheet uses a batch melting equation (Wilson, p. 63, Eq. 1) Using the spreadsheet, answer the following questions: A. MORB that forms the Pacific Ocean crust typically has a light rare earth depleted pattern (NMORB, see Wilson, 1989, p. 142, Fig. 5.42). Given the REE concentrations of N-MORB in the spreadsheet (values and REE pattern in red), is it possible to melt peridotite with chondritic abundances of REE (values in green) to produce Pacific MORB? Use the following modal % of minerals in the peridotite: 50% Olivine, 10% Opx, 40% Cpx, and try varying the percent melt between 10 and 30% (F = 0.1 to 0.3). B. Try adding 10% modal garnet to the source and reducing the olivine to 40%. What is the effect of garnet on the REE patterns of the liquid produced? What controls the observed effect? Are these patterns more, or less, similar to N-MORB than the liquids you were able to produce in in part A? C. The column in blue color allows you to adust the concentration of the different REE in the source rock by factors either less than or greater than the condritic values. At a fixed value of F = 0.25 (as we will see later, most petrologists think about 20-30% melting of the mantle is necessary to generate MORB), determine the factors that you need to adjust the source concentrations by, in order to generate the MORB. D. Explain why the chondritic mantle fails to produce MORB (hint: in lecture 2 and lecture 4 we reviewed the connection between chondritic meteorites and the earth and radiogenic isotopic and trace element evididence concerning the long-term evolution of the earth s mantle.

6 5. Sm-Nd geochronology and isotopic evolution of Nd in Earth A. The Sm and Nd isotope ratios measured in amphibole, cpx, opx, and plag. In a basalt and a rhyolite are as follows: basalt 147 Sm/ 144 Nd 143 Nd/ 144 Nd amphibole cpx opx plag rhyolite plag cpx opx amphibole Calculate the age of each lava flow using a spreadsheet program and the isochron method. Note that the analytical uncertainties (errors) on the slope of each isochrons correspond to ± 1% of thecalculated ages. B. Could these lavas have erupted at the same time? C. Could these lavas have been generated by melting a common source material? Could the source material for either lava be depleted upper mantle? (Hint: we suspect that he depleted upper mantle began to evolve by about 2.8 Ga and had an initial 143 Nd/ 144 Nd ratio of and a 147 Sm/ 144 Nd ratio of about Using this information you can calculate the 143 Nd/ 144 Nd ratio in the depleted mantle at the time of eruption of the lavas you determined above).