DIFFERENTIATION OF MAGMAS BY FRACTIONAL CRYSTALLIZATION THE M&M MAGMA CHAMBER

Transcription

1 Geol 2312 Igneous and Metamorphic Petrology Spring 2009 Name DIFFERENTIATION OF MAGMAS BY FRACTIONAL CRYSTALLIZATION THE M&M MAGMA CHAMBER Objective: This exercise is intended to improve understanding about the process of magmatic differentiation by fractional crystallization. The activity also provides an opportunity to utilize your knowledge of mineral stoichiometry, the IUGS classification system, and Excel spreadsheets. In lecture this week, we will be learning about the importance of crystal-melt fractionation driving magmatic differentiation, which accounts for much of the diversity we observed in igneous rock compositions. We ve also discussed magmatic differentiation, the process by which diverse rock types are generated from a single magma. This process of crystallization differentiation is most obviously manifest it the form of mafic layered intrusions, such as the Sonju Lake Intrusion. In such systems, growing crystals segregate themselves from the main magma reservoir by gravitationally accumulating on the floor (and to a lesser extent on the margins) of the magma chamber. The stratigraphic variations in mineralogy, texture and chemisty of these cumulate rocks record the progressive crystallization and differentiation of the magma. This exercise will, in fun and tasty way, attempt to model the crystallization differentiation process that occurs in mafic layered intrusions by taking into account the changing compositions of solid solution minerals and the residual magmas as they cool and crystallize. Warning!, no eating the minerals until they are all crystallized! Magma Chamber Teams: You will be doing this exercise in four teams of 4-5 members. Each team will have a large sheet of white construction paper with an ovoid area representing your magma chamber. On the upper left corner of the paper, give your intrusion a name. In the upper left corner, list the members of your team. Constructing the M&M magma chamber: In this exercise each major cation (e.g., Si, Ti, Al, ) will be represented by a different colored M&M. So as to compare everyone s magma chamber, we will agree upon a particular color for each cation before the exercise starts. Once we have established the color scheme, count out the appropriate number of M&M s for each cation that is indicated in the top panel of sheet A. Mix the M&M s and pile them in the magma chamber ovoid. Part 1: Fractional Crystallization and Differentiation of the Magma Chamber 1. First off, note the general proportions of the different cations (colors) in the magma chamber and their random mixture. This is the Parent Magma of the intrusion. We will now crystallize this random mixture in 10 steps, wherein each step will preferentially extract the higher temperature components of solid solution minerals (Fo in olivine, An in plagioclase) and will successively crystallize minerals in a manner that replicates the progressive cooling down through Bowen s reaction series. As we will see, this will drive the remaining magma to become enriched in lower temperature components and minerals. 2. Before starting the stepwise crystallization of the magma chamber, you will need to determine the chemical formulas and number of cations needed for each minerals that will be involved in the crystallization process (listed in Panel 1 of the data sheet). In Panel 2, write the mineral formulas of each mineral in the space beneath the mineral name and, in the column below, indicate the numbers of each cation in that mineral formula. 3. With each crystallization step, move the appropriate number of M&M s from the magma chamber to the floor of the magma chamber. (Note that the moles of the minerals crystallizing during each step are indicated in Panel 1.) Cluster the cations related for each mineral, but arrange the mineral assemblage as a cumulate layer across the floor of the intrusion. After

2 2 each crystallization step, record the number of cations that remains in the residual magma (3 rd panel, Sheet A), take a digital photograph of your magma chamber and draw a horizontal line across the top of each layer as you accumulate them. This line represents the cumulate floor of the magma chamber after each step of crystallization. 4. Before your fractionally crystallized magma chamber is dismantled (consumed!), describe below the general stratigraphic trends that you observe in your mafic layered intrusion. 5. Before class on Thursday, enter your data from the modeling above into the Excel spreadsheet that will be ed to you. This would be Panel 2 (mineral compositions) and Panel 3 (cations remaining in liquid). 6. In addition, for each crystallization step, complete the calculations in the same spreadsheet for: a. Modes of the cumulus minerals being removed to the chamber floor (Panel 4) b. Normalized modes of the minerals used for IUGS classification of mafic rocks (Ol-Pyx- Pl) and intermediate to felsic rocks (QAP). (Panel 5) c. Liquid composition (in cation percent) (Panel 6) d. fraction of liquid remaining (F) (Panel 6) e. Mg# = ([Mg]/[Mg+Fe]) (Panel 6) We will use this data to make plots in Excel in the computer lab on Thursday. Part 2: Analyzing and Plotting the Results (Meet in the Computer Lab on Thursday) 1. Generate x-y plots of the following: a. fraction of liquid remaining (F) vs. Si, and vs. Mg# (liquid line of descent) b. Si versus Ti, Al, Fe, Mg, Ca, Na, and K (Harker diagrams) c. indicate the appearance of minerals on the liquidus on each of the plots. 2. Next, use your data to determine the following (see diagrams in text): a. IUGS rock name for the cumulus layer formed during each crystallization event, and b. the rock name for the residual liquid remaining after each crystallization event (use the TAS classification, but for Si only; in our experiment alkali elements are unrealistically too high in abundance).

3 3 Reflecting on what you have learned, address the following on a separate sheet: 1. How are the terms compatible and incompatible defined (consult your text treatment of these terms)? From your results, which elements behaved compatibly during crystallization of this magma? Which behaved incompatibly? Do all of the elements exhibit the same behavior throughout the duration of crystallization? Explain. 2. Describe the changes in rock composition (proportions of cumulus minerals) that result from each increment of crystallization. Use IUGS rock terms and plots in your answer. 3. Describe the chemical changes that result in the residual magma after each increment of crystallization. 4. At what melt fraction (F) does the magma approximate andesite composition? Rhyolite composition? 5. Which aspects of this model magma chamber are realistic? Which are not? Discuss the ways that the model might be made more realistic. 6. Contrast the results of this model (fractional crystallization) with those of a model involving equilibrium crystallization.

4 4 M&M Magma Chamber Data Sheet Modes of Fractionated Minerals Cation Parent M&M Fractionating Minerals Melt Color Forsterite Fayalite Diopside Anorthite Albite K-feldspar Quartz Ilmenite Magnetite Si 184 Ti 5 Al 71 Fe 38 Mg 40 Ca 33 Na 23 K 6 Total 400 Minerals on Liquidus Minerals Fo Fa Di Anorthite Albite K-spar Quartz Ilmenite Magnetite Cumulus Mineral Modes Minerals Fo Fa Di Anorthite Albite K-spar Quartz Ilmenite Magnetite Normalized Mineral Modes for IUGS Classification Classification Mineral ol pyx plag alk qtz Cations Remaining in Liquid Elements Parent Melt Si 184 Ti 5 Al 71 Fe 38 Mg 40 Ca 33 Na 23 K 6 Total 400 % Liq 1.00 Liquid Composition (Cation %) Elements Parent Melt Si Ti 1.25 Al Fe 9.50 Mg Ca 8.25 Na 5.75 K 1.50 Total F (Liq. fract.) 1.00 Mg# 0.51 TAS Classif.

5 Materials needed: M&M s White paper roll Excel files Markers 5