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The purpose of this exercise is to link volcanic rock petrology (hand samples and thin sections) with volcanic rock analytical data to hypothesize on crystal fractionation/accumulation control on lava composition. This lab exercise starts with thin section and hand sample sets of Iceland lavas. To the extent possible, each set has a range of lava compositions including picrites, multiply saturated basalts, and silicic lavas. Also provided is a spreadsheet table of Iceland lava chemical compositions and phenocryst compositions. During the lab, students first identify phenocryst and other phases in their sample set, and compile a table of lava mineralogy. The students then take the spreadsheet of chemical analyses and plot them according to location: Rekjaness, Askja, and Hekla, and they also plot the phenocryst compositions (phenocryst composition changes with magma evolution are ignored. Students then use the rock-derived phenocryst information and phase diagrams (looked at elsewhere in class) to decide which lavas were probably precipitating which phenocryst phases. They then draw crystal fractionation/accumulation control lines on their graphs to illustrate hypothetical controls on magma evolution. They also look at the continuity of data to decide if silicic rocks are related to the basalts by a continuum process (like crystal fractionation, e.g., Hekla) or by a non-continuum process (like partial melting of a different source rock, e.g., Askja). This exercise has proven useful in my course to strengthen the link between rock petrology and rock suite chemical variation. Without something of this sort, I have found that most students have a hard time understanding the meaning of chemical variation diagrams of an igneous suite. This lab follows a phase diagram study of igneous rocks, particulary Fo-An-Di which is most pertinent to the Iceland lavas. It also directly follows an introduction to control lines that is based on a series of Kilauea lavas (illustrated in another submission to this archive). Note that crystal fractionation and accumulation are certainly not the only processes that control these Iceland magma compositions. This exercise is illustrative only of these processes, however.
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