Partial Melting and Magma Composition

The crystallization of basaltic magma occurs over a temperature range of at least 200°C. Melting, the reverse process spans a similar temperature range. As rock begins to melt, those minerals with the lowest melting temperatures are the first to melt. Should melting continue, minerals with higher melting points begin to melt and the composition of the magma steadily approaches the overall composition of the rock from which it was derived. Most often, however, melting is not complete. The incomplete melting of rocks is known as partial melting, a process that produces most magma.

According to Bowen’s reaction series, rocks with a granitic composition are composed of minerals with the lowest melting (crystallization) temperatures—namely, quartz and potassium feldspar. Also note that as we move up Bowen’s reaction series, the minerals have progressively higher melting temperatures and that olivine, which is found at the top, has the highest melting point. When a rock undergoes partial melting it will form a melt that is enriched in ions from minerals with the lowest melting temperatures. The unmelted crystals are those of minerals with higher melting temperatures. Separation of these two fractions would yield a melt with a chemical composition that is richer in silica and nearer to the granitic (felsic) end of the spectrum than the rock from which it was derived.

In summary, Bowen’s reaction series is a simplified guide to understanding how partial melting produces rocks of various compositions. As rock begins to melt, those minerals with the lowest melting temperatures are the first to melt. As a result, partial melting produces magmas enriched in low-melting temperature components and richer in silica than the parent rock.

Task 2. Answer the following questions, using the vocabulary from Task 1.

1. What may happen if magma migrates through the crust?

2. What process is called assimilation?

3. Is the force of the injected magma often sufficient to dislodge blocks of “foreign” rock?

4. When does the process of magma mixing occur?

5. What did Bowen successfully demonstrate?

6. What process is called partial melting?

7. What happens to the minerals as we move up Bowen’s reaction series?

8. What does Bowen’s reaction series help to understand?

Task 3. Using the information from the texts above complete the following sentences:

1. A process when magma migrates through the crust and incorporates some of the surrounding host rock, is called … .

2. The incomplete melting of rocks is called … .

3. A process when one magma body intrudes another that has a different composition is called … .

Task 4. Choose the best answer:

1. As rock begins to melt, those minerals with the highest/the lowest melting temperatures are the first to melt.

2. When a rock undergoes partial melting it will form a melt that is enriched in ions from minerals with the highest/ lowest melting temperatures.

3. The unmelted crystals are those of minerals with higher/lower melting temperatures.

4. Partial melting produces magmas enriched in low-melting/high-melting temperature components.

5. Magma mixing may occur during the ascent of two/three chemically distinct magma bodies as the more buoyant mass overtakes the more slowly moving mass.

Task 5. Do the following: relate the classification of igneous rocks to Bowen’s reaction series, provide the examples.

Task 6. Define the following terms:

• magma assimilation;

• magma mixing;

• partial melting;

• magma composition.

Task 7. Read the short note. Did you know about it?

Quartz watches actually contain quartz crystals that allow them to keep time. Before quartz watches, timepieces used some sort of oscillating mass or tuning fork. Cogs and wheels converted this mechanical movement to the movement of the hands. It turns out that if voltage is applied to a quartz crystal, it will oscillate with a consistency that is hundreds of times better for timing than a tuning fork. Because of this property, and modern integrated-circuit technology, quartz watches are now built so cheaply that they are sometimes given away in cereal boxes. Modern watches that employ mechanical movements are very expensive indeed.