An experimental study on smectites as nitrogen conveyors in subduction zones

Abstract

Abstract We performed high pressure and high temperature (HPHT) experiments on NH4-doped montmorillonite (~2\u202fwt% of NH4) under pressures of 2.5, 4.0, and 7.7\u202fGPa and temperatures from 200 to 700\u202f°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\u202fGPa and 500\u202f°C). Tobelite is stable until more extreme conditions (7.7\u202fGPa and 700\u202f°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\u202fGPa, while buddingtonite, is observed to be stable up to 7.7\u202fGPa. Nitrogen can return to the surface once the stability of these nitrogen-enriched minerals is reached due to pressure or temperature increasing.