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The Unsolved Mysteries of Petrology

The story of a nameless rock.
by Sarah Appleton on 20 April 2011

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Transcript of The Unsolved Mysteries of Petrology

One day in Petrology... my professor handed me this rock. She assigned me the duty of identifying this rock. & I thought ACK!!! After the initial moment of panic, I took a closer look and thought, This will be easy. Its Granite. To prove it I did a modal analysis. It wasn't Granite. Guess what? Now I needed further proof to support my claim that it was Alkali-feldspar syenite. What better way than with a homemade thin section? The Game Plan Cut and prepare the billet Frost the slide Mount the billet Trim and grind thin section Polish the thin section What Really Happened & & So, after hours and hours and hours of grinding I had a thin section. Now that I had a thin section I could begin an in depth look at the mineralogy and textures. I created a petrographic report for my sample. I learned a lot about my rock including that it was aphanitic rather than phaneritic. This changed the name of my rock from Alkali-feldspar syenite to trachyte. My next step was to research my rock to answer a few questions. Mainly... (1) How did the rock form? So I began to comb throught the literature and there were thousands of sources. And I found 3 Sources Chronology, chemistry, and origin of trachytes from Hualalai Volcano, Hawaii by Brian L. Cousens, David A. Clauge, and Warren D. Sharp

Origin of the trachyte-quartz trachyte-peralkalic rhyolite suite of the Oligocene Paisano volcano, Trans-Pecos Texas by Don F. Parker

Growth rate of alkali feldspars in decompression-induced crystallization experiments in a trachtic melt of the Phlegraen fields (Napoli, Italy) by Marta Calzolaio, Fabio Arzilli, and Michael R. Carroll.
So I read them and took notes. I learned a lot about how trachytic magmas form. Trachytic magmas can come from three different sources. (1) hydrous basaltic magmas
(2) basaltic magma
(3) continental crust These magmas, however, cannot just magically become a trachytic magma. At least without Fractional Crystallization In both the study in Hawaii and the study in Texas the magma that erupted pooled and then underwent fractional crystalization and then became a tracytic magma. Magnetite inside a masked green grain Alkali phenocryst surrounded by the masked green mineral. Trachytic texture as it flows around the green mineral. Alkali feldspar phenocryst showing zoning. This is an example of how trachytic magmas formed in Hawaii. So I decided to make a poster of my project. Attempt one... Attempt two... Much better The End (Cousens et al., 2003) My Sample
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