Explain how the percentage of silica in magma increases during crystallization despite the fact that silicate minerals are being removed throughout the crystallization process.

Fractional Crystallization of Magma

Objective

In this exercise, you will gain an understanding of magmatic differentiation by fractional crystallization using a model magma chamber. The activity was adapted from materials developed by Dr. Karl Wirth at Macalester College (http://www.macalester.edu/geology/wirth/CourseMaterials.html)

Introduction

In lecture you learned about the importance of fractionation in generating the diversity of observed igneous rock compositions (e.g., basalt, andesite, dacite, rhyolite). Magmatic differentiation is the process by which diverse rock types are generated from a single magma. There are many ways to accomplish differentiation, but one of the most common is by fractional crystallization, a two-stage process that involves: (1) the formation of crystals from a melt, and (2) mechanical separation of crystals and melt.

 

In 1844, Charles Darwin described lava flows from the Galápagos Islands in which the lowest flows contained greater proportions of feldspar crystals. These observations led Darwin to propose that density differences between crystals and melt would result in mechanical separation of these two phases and the formation of different magma types. This process, known today as gravity settling, was the focus of detailed experimental studies during by 1950s by N.L. Bowen (as in Bowen’s Reaction Series). Today, several additional mechanisms of crystal-melt fractionation are also recognized, including flow segregation and filter pressing.

 

Instructions

Materials & Supplies

1 bag of “magma”

White board

Markers

 

Directions

1. Construct the magma chamber.

 

1. In this exercise, each major cation in the magma (e.g., Si, Mg, Al) is represented by a different colored wooden bead. To simplify the activity, we assume that the magma chamber contains enough oxygen anions to form all of the minerals that crystallize; so, we will make all calculations in cation atomic percent (rather than oxide weight percent, which is commonly reported for rocks). We also assume that there is no solid solution in minerals crystallizing from the magma, and that there are no volatiles in the magma.

 

1. Before you begin, complete Table 1 (Mineral Compositions) by filling in each mineral formula and determining the proportions of cations in each mineral that will crystallize from the magma.

 

1. Check that your bag of “magma” has the correct number of cations (beads) for each color (for example, 184 green beads for Si, 71 yellow beads for Al, etc.). NOTE that each bag of beads has a different color scheme – be sure to check the note inside of the bag to identify your color scheme. The starting number of beads for each cation is listed in Table 2 ( Cations Remaining in Liquid).

 

1. Mix the beads together and place them in a magma chamber that you draw on one half of the white board. This represents the “liquid” end of your magma chamber.

 

1. Note the general proportions of the different colors in the magma chamber.

 

1. Begin fractional crystallization.

 

1. On the other half of the white board, draw 10 horizontal lines and label these with line “1” at the bottom and line “10” at the top. Each line represents one crystallization step in the table below.

 

1. Starting with Step 1, remove the appropriate number of beads from the “liquid” magma during each crystallization step in the table, below. As minerals crystallize, move them to the labeled layers on the “solid” region of the magma chamber.

 

Mineral	Crystallization Step
	1	2	3	4	5	6	7	8	9	10
Forsterite	2	3	4	3	2
Fayalite			1	2	2	1
Diopside				1	1	4	2	2	1	1
Anorthite		1	2	3	3	4	3	3	1	1
Albite						1	3	6	5	7
Orthoclase									2	2
Quartz									1	4

 

1. After each crystallization step, record the number of cations remaining in the liquid for each element (the number of each color of bead left in the “liquid”) in Table 2.

 

1. For each crystallization step, calculate the relative proportions of each element remaining in the magma as a percentage of the total number of elements remaining after that crystallization step [for example, if you remove 10 atoms of Mg and 5 atoms of Si in Step 1, %Mg = (30)/(355)]. Record this information in Table 3 (Magma Composition). You will need to perform these calculations in a spreadsheet and submit it with this homework.

 

1. Also calculate the proportion of magma remaining (f) in each crystallization step by dividing the number of cations (beads) remaining in the liquid by the original total number of cations in the liquid [for example, if you remove 10 atoms of Mg and 5 atoms of Si in Step 1, f = (355/370)]. Record this information in Table 3 (or include this in a spreadsheet).

 

1. Use the results of the fractional crystallization exercise to answer the questions, below.

 

 

Problems in Fractional Crystallization

1. Use your lab materials to look up the formula for each mineral that will crystallize from the magma. Record formulas in the first row of the table, and identify each mineral group in the second row. Remember, no solid solution – so write pure endmember formulas. Second, record the number of cations per formula. Last, record the bead color for each cation.

 

Table 1. Mineral Compositions

	Bead Color	Forsterite	Fayalite	Diopside	Anorthite	Albite	Orthoclase	Quartz
Formula	—–
Mineral Group	—–
Si
Al
Fe
Mg
Ca
Na
K

 

1. Use the table on page 2 to determine what minerals are removed in each crystallization step. Record the cations remaining in the magma for each step in this table.

 

Table 2. No. Cations Remaining in Liquid

Cation	Start No.	Crystallization Step
		1	2	3	4	5	6	7	8	9	10
Si	184
Al	71
Fe	12
Mg	40
Ca	33
Na	23
K	7
Total	370

 

1. Convert the number of cations in each step to proportions by dividing each cation by the total number of cations in that step (see pg. 2 for an example). DO THIS IN A SPREADSHEET and submit the spreadsheet along with your homework. Set up your spreadsheet like the table below (DO NOT write answers in this table).

 

Table 3. Magma Composition

Cation	Start %	Crystallization Step
		1	2	3	4	5	6	7	8	9	10
Si	49.73
Al	19.19
Fe	3.24
Mg	10.81
Ca	8.92
Na	6.22
K	1.89
Total	100.00
Liquid Frac (f)	1.00

 

1. Compare the types of minerals removed at the beginning of crystallization with those removed in the middle and end of crystallization (i.e., mafic versus felsic minerals). Does this exercise largely follow Bowen’s reaction series? Why or why not?

 

1. Look back at the table under Procedure 2a- this table shows what minerals are removed from the magma during each crystallization step (1 to 10). Determine an appropriate simple intrusive igneous rock name (using Lab #5) for the assemblage of crystallized minerals at the 2nd, 7th, and 10th crystallization steps. The 4th step has been done as an example.

 

olivine (forsterite & fayalite), clinopyroxene (diopside), plagioclase feldspar (anorthite)

Step                 Minerals Removed                                    Rock Name

4 (example)                                                                                Olivine gabbro                       

2          _____________________________

7              _____________________________

10            _____________________________

 

1. Use the data from Table 3 to graph the following (using a spreadsheet program like Google Sheets or Excel). As a general rule, create graphs that occupy at least half of a sheet of paper. Make sure your axes are labeled and put a title on each graph. Turn in your graphs and answer the next questions based on these graphs.

 

1. On one graph, plot the liquid fraction remaining (f) on the y-axis, versus crystallization step (1 through 10) on the x-axis.

 

1. On a new graph, plot the % of each element below on the y-axis, versus crystallization step (1 through 10) on the x-axis. Use a different color for each element:

%Si

%Mg

%Al

%K

 

1. Describe the general trends you observe in each graph during crystallization (for example, increasing rapidly, decreasing slowly, remaining steady then increasing rapidly, etc.)

 

Liquid fraction remaining:

 

%Si:

 

%Mg:

 

%Al:

 

%K:

1. Which of the following three elements: Mg, Al, or K is most compatible (easily incorporated into a crystalline structure)? Which of these three elements is most incompatible (not easily incorporated)? How do you know?

 

1. Explain how the percentage of silica in magma increases during crystallization despite the fact that silicate minerals are being removed throughout the crystallization process.

 

1. Magmas are classified by their silica content according to the following:

Wt% SiO2	Magma name
45-52	Basalt
52-57	Basaltic andesite
57-63	Andesite
63-70	Dacite
>70	Rhyolite

 

Using the above classification scheme, classify the magma by %Si at the following steps. The magma composition in Step 4 has been done as an example.

 

Step     %Si (Table 3)          Magma Name

4 (example)     53.3             Basaltic andesite

Start     __________

2       __________

7       __________

10     __________

 

1. Which aspects of this model magma chamber are realistic? Which are not?  (Hint: think about the simplifications and assumptions in the model).  Suggest at least three ways to make the model more realistic.