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

Abstract

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 ( NH 4 + -N ) contents and isotopic composition (δ 15N NH4) 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 NH 4 + -N with a wide range in δ 15N from +1.6‰ to +9.1‰. Average NH 4 + -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 NH 4 + 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 δ 15N NH4 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 δ 15N NH4 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 NH 4 + - NH 3 or NH 4 + - N 2 , reflecting devolatilization history and redox conditions during metamorphism.