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This is a laboratory-style investigation wherein students examine the petrography and major-element geochemistry of 6 samples of mid-ocean ridge basalt and related differentiated lavas recovered from the Cleft segment of southern Juan de Fuca Ridge, a medium spreading-rate MOR in the northeast Pacific Ocean. Lava types range from basalt to dacite. After some initial background information on basalts, the MOR environment, and the study area students investigate four thin sections, beginning with typical basalts and ending with a dacite. They are led through a series of directed questions that help them gain familiarity with commonly occurring minerals and textures in mid-ocean ridge lavas. Questions direct students toward the interpretation of quench-related textural features and crystallization sequence, as well as a few other textural observations and petrographic techniques. After proceeding through the initial four thin sections and associated questions student are then asked to undertake "full thin-section descriptions" of the remaining two samples. After investigating the thin-sections and determining a possible crystallization sequence from the petrographic data gathered (plagioclase followed by olivine followed by augitic clinopyroxene followed by pigeonite), students examine a P-T phase diagram to constrain possible pressures of formation. Discovering that crystallization pressures were low (less than ~ 0.75 GPa) students then examine a phase diagram of the olivine-plagioclase-augite-quartz system (olivine-quartz-augite ternary, projected from the plane of plagioclase saturation) [Walker, 1979]. Students draw 2 possible liquid lines of descent (LLD) onto the diagram, and then use their petrographic observations to qualitatively plot the samples along that LLD, determining a relative sequence of chemical evolution for the suite of samples. Lastly, given those determinations, student graph the major element data for the lava suite to infer paths of chemical evolution and the effects of fractional crystallization (possibly coupled with magma mixing).
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