magmadiff - Magmatic Differentiation Page 1 of 16 EENS 2120...

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This document last updated on 30-Jan-2012 EENS 2120 Petrology Prof. Stephen A. Nelson Tulane University Magmatic Differentiation Chemical Variation in Rock Suites Soon after geologists began doing chemical analyses of igneous rocks they realized that rocks emplaced in any given restricted area during a short amount of geologic time were likely related to the same magmatic event. Evidence for some kind of relationship between the rocks, and therefore between the magmas that cooled to form the rocks came from plotting variation diagrams. A variation diagram is a plot showing how each oxide component in a rock varies with some other oxide component. Because SiO 2 usually shows the most variation in any given suite of rocks, most variation diagrams plot the other oxides against SiO 2 as shown in the diagram here, although any other oxide could be chosen for plotting on the x-axis. Plots that show relatively smooth trends of variation of the components suggested that the rocks might be related to one another through some process. Of course, in order for the magmas to be related to one another, they must also have been intruded or erupted within a reasonable range of time. Plotting rocks of Precambrian age along with those of Tertiary age may show smooth variation, but it is unlikely that the magmas were related to one another. If magmas are related to each other by some processes, that process would have to be one that causes magma composition to change. Any process that causes magma composition to change is called magmatic differentiation. Over the years, various process have been suggested to explain the variation of magma compositions observed within small regions. Among the processes are: 1. Distinct melting events from distinct sources. 2. Various degrees of partial melting from the same source. 3. Crystal fractionation. 4. Mixing of 2 or more magmas. 5. Assimilation/contamination of magmas by crustal rocks. 6. Liquid Immiscibility. Initially, researchers attempted to show that one or the other of these process acted exclusively Page 1 of 16 Magmatic Differentiation 1/30/2012 http://www.tulane.edu/~sanelson/eens212/magmadiff.htm
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to cause magmatic differentiation. With historical perspective, we now realize that if any of them are possible, then any or all of these processes could act at the same time to produce chemical change, and thus combinations of these processes are possible. Still, we will look at each one in turn in the following discussion. Distinct Melting Events One possibility that always exists is that the magmas are not related except by some heating event that caused melting. In such a case each magma might represent melting of a different source rock at different times during the heating event. If this were the case, we might not expect the chemical analyses of the rocks produced to show smooth trends on variation diagrams. But, because variation diagrams are based on a closed set of numbers (chemical analyses add up to 100%), if the weight% of one component increases, then the weight percent
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This note was uploaded on 02/01/2012 for the course EENS 2120 taught by Professor Nelson during the Spring '11 term at Tulane.

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magmadiff - Magmatic Differentiation Page 1 of 16 EENS 2120...

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