The fluid history of the Mount Mica and Irish Pit pegmatite localities is examined with mineral oxygen and hydrogen isotope data from samples that document the crystallization history of these rocks. High δ ¹⁸O values (> +11‰) from both pegmatites suggest anatexis of a metasedimentary source. Quartz-tourmaline oxygen isotope geothermometry calculations indicate cooling of Mount Mica from 760°C to 363°C, but those from the Irish Pit were magmatic (> 650°C). Oxygen and hydrogen isotopes reveal different fluid histories for these pegmatites: (1) High-D/H fluids, potentially of external origin, dominated the Irish Pit and Mount Mica pocket zone, most likely following pegmatite emplacement. (2) An internal magmatic fluid that evolved from δD = –115‰ and δ ¹⁸O = +14.5‰ to δD = 70‰ and δ¹⁸ O = +11.5‰ as the wall, intermediate, and core zones of the Mount Mica pegmatite cooled over a 400°C temperature range under closed system conditions.