An experimental study on smectites as nitrogen conveyors in subduction zones

We performed high pressure and high temperature (HPHT) experiments on NH ₄-doped montmorillonite (~2 wt% of NH ₄) under pressures of 2.5, 4.0, and 7.7 GPa and temperatures from 200 to 700 °C. Each experiment was analyzed with XRD, FTIR, CHN elemental analysis, and SEM in order to determine the NH ₄-Smectite phase changes and their morphology, and the presence of ammonium in the runs. Our results show that smectite can easily transport nitrogen, speciated as ammonium (NH ₄ ⁺), incorporated into the smectite interlayer in mildly reducing environments to deeper levels in the Earth through cold thermal regime subduction zones. NH ₄-Smectite transforms into NH ₄-enriched micaceous phase (tobelite) through a NH ₄ ⁺-enriched interlayered I/S phase in relatively low pressures and temperatures (around 2.5 GPa and 500 °C). Tobelite is stable until more extreme conditions (7.7 GPa and 700 °C), together with lesser amounts of buddingtonite (an ammonium-bearing feldspar) kyanite, and garnet. Our experiments also show the effect of nitrogen in the feldspar stability, as potassic and sodic feldspar are stable up to ~5 GPa, while buddingtonite, is observed to be stable up to 7.7 GPa. Nitrogen can return to the surface once the stability of these nitrogen-enriched minerals is reached due to pressure or temperature increasing.