An experimental study on smectites as nitrogen conveyors in subduction zones

We performed high pressure and high temperature (HPHT) experiments on NH4-doped montmorillonite (similar to 2 wt % of NH4) under pressures of 2.5, 4.0, and 7.7 GPa and temperatures from 200 to 700 degrees C. Each experiment was analyzed with XRD, FTIR, CHN elemental analysis, and SEM in order to determine the NH4-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 (NH4+), incorporated into the smectite interlayer in mildly reducing environments to deeper levels in the Earth through cold thermal regime subduction zones. NH4 Smectite transforms into NH4-enriched micaceous phase (tobelite) through a NH4+-enriched interlayered I/S phase in relatively low pressures and temperatures (around 2.5 GPa and 500 degrees C). Tobelite is stable until more extreme conditions (7.7 GPa and 700 degrees 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 stables until similar to 5 GPa, while buddingtonite, is stable at 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.