Differentiation of Magmas By Fractional Crystallization
Karl R. Wirth
St. Paul, MN 55105
* 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
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 students indicate that the exercise is both enjoyable and informative. Student knowledge of fractional crystallization and magmatic differentiation, as indicated by exam results and course projects, has increased significantly since incorporating this exercise into the curriculum.
A Brief History of the Theory of Magmatic Differentiation
The origin and evolution of the theory of magmatic differentiation is rich with history. However, the history of this important concept is rarely examined in detail in most textbooks; many currently available petrology textbooks only trace the theory of differentiation back to N.L. Bowen. This contrasts with many biology textbooks and curricula in which there is much greater emphasis on the historical development of key concepts. It is important to reveal some history to students, not only to deepen their understanding of the important concepts, but also to help them understand the process of science itself. Several recent publications (Pearson, 1996; Young, 1998; Estes et al., 2000) describe key aspects of the history of the theory of differentiation and are the basis of the summary provided below. Prior to the development of the idea of magmatic differentiation in the late nineteenth century, magmas were generally regarded as originating from two distinct sources, one silica rich and the other silica-poor. Intermediate lavas were explained as being a mixture of these two sources. At the time, some geologists suggested these different magma sources originated from concentrically distributed zones within the earth and others argued for a secular change in erupted magma compositions. Early ideas about differentiation began to emerge in the mid to late-1800’s with the recognition of mineralogical and chemical similarities among suites of igneous rocks (Young, 1998). However, it was Charles Darwin who first integrated field observations with available experimental data into a coherent model of differentiation (e.g., Pearson,...
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