STABLE ISOTOPE EVIDENCE FOR CLOSED SYSTEM CRYSTALLIZATION OF ANATECTIC MELT: THE MOUNT MICA PEGMATITE, MAINE

Stable isotope analyses of quartz, schorl tourmaline, and muscovite from the Carboniferous Mount Mica Pegmatite document closed-system crystallization of an anatectic melt. High δ¹⁸O values of quartz (+15.5 ± 0.2‰, n = 8), tourmaline (+13.5 ± 0.3‰, n = 15), and muscovite (+12.5 ± 0.3‰, n = 11) similar to those from the host rock suggest anatexis of a local, metasedimentary source for pegmatite magma instead of a cogenetic origin with the Sebago Batholith (δ¹⁸O ~ +9‰, Dorais and Paige, 2000). Quartz-tourmaline ¹⁸O/¹⁶O geothermometry indicates cooling from 760°C to 363°C. Material-balance ¹⁸O/¹⁶O exchange modeling (Criss et al., 1986) that takes into account modal mineralogy demonstrates closed-system conditions during crystallization. A large range of δD values from tourmaline (–128 to –72‰, n = 17) and muscovite (–132 to –65‰; n = 14) initially suggest open-system infiltration of meteoric-hydrothermal fluids (Dyar et al., 1999), but Rayleigh modeling indicates that these minerals could have just as likely crystallized in the presence of late-stage water liberated from the nearby Sebago Batholith (also a source for boron). Tourmaline and muscovite H₂O contents are within stoichoimetric values, indicating primary δD values. These data reveal a fluid history involving an internal magmatic fluid that evolved from δD = –115‰ and δ¹⁸O = +14.5‰ to δD = –70‰ and δ¹⁸O = +11.5‰ as the wall, intermediate, and core zones of the pegmatite cooled over a 400°C temperature range under closed system conditions. In contrast, variable δ¹⁸O (+10.0 to +13.7‰, n = 5) and δD (–135 to –58‰, n = 7) values from pocket zone gem-quality elbaite tourmaline indicate a complex fluid history involving both low- and high-D/H fluids, potentially of external origin that infiltrated these zones of high permeability after pegmatite crystallization. The high-D/H fluids (δD ~ –20‰) were likely meteoric as paleomagnetic data (Torsvick et al., 2012) indicate New England was at low latitude during the Carboniferous.