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triangular_plots_metamorphic_petrology - Triangular Plots...

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This document last updated on 15-Mar-2010 EENS 212 Petrology Prof. Stephen A. Nelson Tulane University Triangular Plots in Metamorphic Petrology Like igneous rocks, most metamorphic rocks are composed of 9 or more major elements. Thus, initially it would appear that we are dealing with a 9 or 10 component system. Recall, however, that the number of components in any given system is the minimum number required to define the composition of all phases in the system. Thus, since constituents like Na 2 O and K 2 O are not usually found as separate mineral phases, we can combine these with other constituents, like Al 2 O 3 and SiO 2 in the feldspars, and thus reduce the number of components required to define our system. This is especially important if we wish to graphically display chemical rock and mineral data in a way that is easily visualized. In fact, the best way to visualize such data is to attempt to reduce the number of components to 3, so that we can plot the compositions of rocks and minerals on a triangular composition diagram. Before discussing how this is done, we first review some of the principles of three component compositional diagrams. General Three Component Compositional Diagrams For the moment we will assume that we are in a true 3 component system and the minerals that we display on a triangular composition diagram is a set of mineral that are in equilibrium over a narrow range of temperature and pressure. We first look at how one of these diagrams might appear in the hypothetical system A, B, C. As shown on the diagram, at the pressure and temperature under consideration, there are seven possible minerals that can occur in this system, although we know from the phase rule that not all 7 can occur in the same rock. In general, the most common set of phases will be where the number of degrees of freedom, F, is 2, i.e. where pressure and temperature can vary by small amounts without changing the number of phases. For such a situation, the phase rule is: F= C + 2 - P and with F=2 & C=3, P = 3, which tells us that for this divariant assemblage, we will have three coexisting phases. Mineral phases that coexist with each other at this temperature and pressure are connected by lines, called tie lines . Triangular Plots in Metamorphic Petrology 3/15/2010 Page 1 of 14
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The tie lines divide the diagram into smaller compositional triangles, that tell us what minerals will coexist in a rock of any composition under the current temperature and pressure conditions. For example, for a rock with composition x , the divariant mineral assemblage is AB, A, A 2 C. If the rock composition is moved slightly so that it now has composition y, note that the mineral assemblage will change to AB, A 2 C, ABC. Within each compositional triangle, we can use the lever rule to estimate the proportions of the
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triangular_plots_metamorphic_petrology - Triangular Plots...

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