CLASSROOM EARTH
and can reveal the relative timing of fluid and melt inclusions. For example, trails of inclusions that cut across growth zones were formed later than inclusions that stop at the edge of a growth zone, similar to the cross-cutting principle used for relative dating.
Figure 2. Quartz (white) and tourmaline (pink/brown) intergrowth shown in thin section (200 m thick); note the larger central tourma- line and the smaller branches intergrown with quartz; tourmaline growth direction shown by yellow arrow and one boundary between growth zones shown by red dotted line (photo by Emily Yoder, 2020).
sions, which are an important source of data related to the pegmatite’s original environment of formation. As a mineral grows, melt and/or fluid can become trapped in small spaces caused by imperfections in the mineral. Upon cooling, the trapped melt crystallizes into microcrystals within the tiny cavity, while the fluid separates into an aqueous liquid and
CO2 gas bubble and may also precipitate daughter crystals (Fig. 3). Melt inclusions are significant because they indi- cate when melt was present during crystallization and may give clues about the melt’s composition. Numerous fluid and melt inclusions are well-preserved in Emmons tourmaline and quartz, which is a good evidence that the minerals were growing rapidly (at disequilibrium). My objective is to extract information from these inclusions about the composition, pres- sure, and temperature conditions during rapid crystallization of the pegmatite.
Tourmaline is especially important in my study because (1) its widening shape clearly indicates direction of growth from its point of attachment towards its branching intergrowth with quartz, and (2) single crystals have growth zones indicated by color variations under the microscope (Fig. 2). These color bands are an important indicator of the stages of crystallization
After initial qualitative observations, we started collect- ing microthermometric data to understand the temperature and pressure conditions during the pegmatite’s formation. Microthermometry consists of measuring temperatures at which phase changes take place in fluid inclusions during freezing-heating cycles on a microscope stage. For the Emmons samples, I carefully monitored the inclusions as I froze the samples down to about –100°C and heated them up to about 350°C. Essentially, the temperature data we obtained allows us to estimate the composition and density of the fluid trapped in the bubbles, as well as the temperature/pressure conditions during quartz-tourmaline crystallization. We can correlate this data with the locations of inclusions and propose a model for the pegmatite’s formation.
Currently, we are processing the raw fluid inclusion micro- thermometric data and have already found on average 415°C trapping temperatures for an estimated pressure of 300 MPa. That is very low, indicating that the pegmatite magma was highly undercooled in the hanging wall. Going forward, we are planning to collect qualitative and quantitative data from a continuous sequence of samples from the pegmatite border towards the core, in order to understand whether the processes and conditions were changing during crystallization. We hope to solve the puzzle of these unusual textures and present this research at a 2021 national or international conference.
References
Falster, A.U., et al. (2019). The Emmons Pegmatite, Greenwood, Oxford County, Maine. Rocks and Minerals, Vol 94, Issue 6, 498-519. DOI: 10.1080/00357529.2019.1641021.
Goldstein, R. & Reynolds, J. (1994). Systematics of Fluid Inclusions in Diagenetic Minerals. SEPM Society for Sedimentary Geology, Short Course Notes No. 31. https://
doi.org/10.2110/scn.94.31.
Hulsbosch, N., et al. (2019). Evaluation of the petrogenetic significance of melt inclusions in pegmatitic schorl-dravite from graphic tourmaline-quartz assemblages: Application of LA-ICP-QMS analyses and volume ratio calculations. Geochimica et Cosmochimica Acta, Vol. 244, 308-335.
https://doi.org/10.1016/j.gca.2018.10.023.
London, D. (2008). Pegmatites. R. F. Martin (Ed.). Mineralogical Association of Canada.
Nabelek, P.I., et al. (2010). The role of H2O in rapid emplace- ment and crystallization of granite pegmatites: resolving
the paradox of large crystals in highly undercooled melts. Contributions to Mineralogy and Petrology, Vol. 160, 313- 325.
https://doi.org/10.1007/s00410-009-0479-1.
Shepherd, T.J., et al. (1985). A practical guide to fluid inclu- sion studies.
Sirbescu, M.C. & Nabelek, P.I. (2003). Crustal melts below 400°C. Geology, Vol. 31, No. 8, 685-688. https://doi. org/10.1130/G19497.1.
Figure 3. In tourmaline, a fluid inclusion with three phases: liquid
water (blue arrow), liquid CO2 (green arrow), gaseous CO2 (red arrow); also note small crystal (melt) inclusion (orange arrow) (photo by Emily Yoder, 2020).
50 TPG •
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Sirbescu, M.C., et al. (2017). Experimental Crystallization of Undercooled Felsic Liquids: Generation of Pegmatitic Texture. Journal of Petrology, Vol. 58, Issue 3, 539-568.
https://doi.org/10.1093/petrology/egx027.
www.aipg.org
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