CLASSROOM EARTH The Pegmatite Puzzle: Insights from Mineral Intergrowth Textures Emily Yoder, SA-9933 - Central Michigan University
My name is Emily Yoder and I am currently a junior at Central Michigan University, double majoring in Geology and Geography (Geographic Information Science concentration). I have been a student member of AIPG since Spring 2019 and am currently the CMU Student Chapter Secretary. During my sophomore year, I became involved in undergraduate research on mineral intergrowths in pegmatites with Dr. Sirbescu. My favorite parts of research have been learning new concepts and research methods, as well as growing as a writer. This past spring, I wrote a proposal for CMU’s Undergraduate Summer Scholars Program scholarship and grant, and was incredibly honored to receive this funding for my research for Summer 2020. My current goal is to attend graduate school to study volcanology, so I am thankful for this opportunity to learn important skills for my future. I highly recommend becoming involved in undergraduate research for any students who are considering it and am glad to be able to share my research here!
Pegmatites are intrusive igneous rocks of granitic compo- sition with unique textural features. Notably, they can have coarse crystals of 2.5 cm to over 10 m in length and special mineral intergrowth textures. The cooling rate of pegmatites is debated because extrapolating the slow cooling rates for common intrusive igneous rocks to the coarse crystals found
in pegmatites suggests an incredibly long duration of crystal- lization, perhaps as long as the age of the Earth. Some theo- ries suggest that pegmatites formed by a long, slow cooling process. However, other models of pegmatite crystallization use geologic and experimental evidence to show that cooling actually occurs far more rapidly and at unusually low (under- cooled) temperatures. As a familiar example, pure water can be placed in a freezer and undercooled to a temperature far below its freezing point and remain a liquid, then crystallize to unusual ice needles within seconds. Similarly, pegmatite- forming magma may cool below its typical crystallization tem- perature when it intrudes much colder host rocks (the freezer) and crystals with unusual textures may grow rapidly. While typical granites form around 700°C, studies have proposed that crystallization in pegmatites may occur as low as 400°C.
The key to solving this pegmatite puzzle may be found in a skeletal intergrowth between quartz and tourmaline, which has been the focus of the research I am currently working on with Dr. Sirbescu at CMU. We are studying samples from the granitic Emmons pegmatite in Oxford County, Maine. Emmons is about 260 Ma, rich in exotic lithium-cesium and tantalum minerals, and zoned, meaning mineralogy and texture varies distinctly from the outer edge of the pegma- tite to its core. Crystallization begins at the outermost zones in contact with the surrounding host rock. The quartz- tourmaline intergrowth is found in the hanging wall zone, where tourmaline up to 45 cm long has grown towards the core, perpendicular to the host rock. This oriented ‘comb texture’ is especially vis- ible in the field (Fig. 1). From these more ideal (euhedral) crystals, tourmaline continues to branch out into anhedral shapes, with each individual crystal like a tree and its branches. What is puzzling is that the tree “trunk” and branches are in optical continuity, so they are a single tourmaline crystal, best viewed in thin sections (Fig. 2). Quartz intergrows with the skeletal tourmaline branches. As they grow simultaneously from the magma, the two minerals seem to be competing for space.
Figure 1. Comb texture tourmaline (black) from the wall zone of the Emmons pegmatite; growth direction shown by yellow arrows (photo by Dr. Sirbescu, 2018).
www.aipg.org
Both tourmaline and quartz can also trap microscopic fluid and melt inclu-
Jan.Feb.Mar 2021 • TPG 49
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