Ammonium loss and nitrogen isotopic fractionation in biotite as a function of metamorphic grade in metapelites from western Maine, USA

Ammonium fixed in micas of metamorphic rocks is a sensitive indicator both of organic-inorganic interactions during diagenesis as well as of the devolatilization history and fluid/rock interaction during metamorphism. In this study, a collection of geochemically well-characterized biotite separates from a series of graphite-bearing Paleozoic greenschist- to upper amphibolite-facies metapelites, western Maine, USA, were analyzed for ammonium nitrogen (NH4+-N) contents and isotopic composition (δ15NNH4) using the HF-digestion distillation technique followed by the EA-IRMS technique. Biotite separates, sampled from 9 individual metamorphic zones, contain 3000 to 100 ppm NH4+-N with a wide range in δ15N from +1.6‰ to +9.1‰. Average NH4+-N contents in biotite show a distinct decrease from about 2750 ppm for the lowest metamorphic grade (∼500 °C) down to 218 ppm for the highest metamorphic grade (∼685 °C). Decreasing abundances in NH4+ are inversely correlated in a linear fashion with increasing K+ in biotite as a function of metamorphic grade and are interpreted as a devolatilization effect. Despite expected increasing δ15NNH4 values in biotite with nitrogen loss, a significant decrease from the Garnet Zones to the Staurolite Zones was found, followed by an increase to the Sillimanite Zones. This pattern for δ15NNH4 values in biotite inversely correlates with Mg/(Mg + Fe) ratios in biotite and is discussed in the framework of isotopic fractionation due to different exchange processes between NH4+-NH3 or NH4+-N2, reflecting devolatilization history and redox conditions during metamorphism.