Magma
Handout
Differentiation of Magmas By Fractional Crystallization
Karl R. Wirth
Geology Department
Macalester College
St. Paul, MN 55105 wirth@macalester.edu Purpose: * improve understanding of fractional crystallization and magmatic differentiation * utilize mineralogy concepts (e.g., stoichiometry) * utilizeclassification and chemical variation diagrams * provide practical experience designing and using spreadsheets * provide appreciation of the historical origins of an important concept in geology
Introduction
An understanding of the process of magmatic differentiation is essential to the study of the petrology of igneous rocks. Students at Macalester College typically first encounter this concept in the introductory geology course when they learn about Bowen’s reaction series and fractional crystallization. After a brief introduction to phase diagrams in the mineralogy course, students generally receive a thorough treatment of magmatic differentiation in the petrology course. Despite trying several different approaches to “teaching” the concepts of element incompatibility, fractional crystallization, and magmatic differentiation in the petrology course, it was clear that many students still did not have a firm understanding of the details of differentiation. In retrospect, this made sense; the students didn’t have any real-world experience with fractional crystallization, they had only heard or read about. Although it would be possible to have students conduct a phase diagram experiment to illustrate the differentiation process, this would likely require specialized equipment and would be time-intensive. The M&M® magma chamber exercise was developed to provide students first-hand experience with the process of magmatic differentiation by fractional crystallization using a simple experiment. This experiment is currently being used in both the petrology and introductory geology courses at Macalester College. Most
Differentiation of Magmas By Fractional Crystallization
Karl R. Wirth
Geology Department
Macalester College
St. Paul, MN 55105 wirth@macalester.edu Purpose: * improve understanding of fractional crystallization and magmatic differentiation * utilize mineralogy concepts (e.g., stoichiometry) * utilizeclassification and chemical variation diagrams * provide practical experience designing and using spreadsheets * provide appreciation of the historical origins of an important concept in geology
Introduction
An understanding of the process of magmatic differentiation is essential to the study of the petrology of igneous rocks. Students at Macalester College typically first encounter this concept in the introductory geology course when they learn about Bowen’s reaction series and fractional crystallization. After a brief introduction to phase diagrams in the mineralogy course, students generally receive a thorough treatment of magmatic differentiation in the petrology course. Despite trying several different approaches to “teaching” the concepts of element incompatibility, fractional crystallization, and magmatic differentiation in the petrology course, it was clear that many students still did not have a firm understanding of the details of differentiation. In retrospect, this made sense; the students didn’t have any real-world experience with fractional crystallization, they had only heard or read about. Although it would be possible to have students conduct a phase diagram experiment to illustrate the differentiation process, this would likely require specialized equipment and would be time-intensive. The M&M® magma chamber exercise was developed to provide students first-hand experience with the process of magmatic differentiation by fractional crystallization using a simple experiment. This experiment is currently being used in both the petrology and introductory geology courses at Macalester College. Most