Publication Date:
2023-04-19
Description:
The current PhD thesis focuses on phengite and majorite as hosts for nitrogen in the deep Earth. NH4-phengite end member was synthesized in piston-cylinder experiments at 700 °C and 4.0 GPa, and characterized by scanning electron microscopy (SEM), X-ray powder diffraction (XRPD) with Rietveld refinement, and Fourier transform infrared (FTIR) spectroscopy. Its devolatilization behavior (loss of OH and NH4+) was studied by means of in situ micro-FTIR spectroscopy under low and high temperatures (T) using a Linkam cooling-heating stage, and under high pressure (P) conditions in diamond anvil cell (DAC) experiments. To define the extent of NH4+ substitution for K+ in phengite at the same P-T conditions, NH4+/K+ exchange coefficients between aqueous fluids and phengite were experimentally determined in piston-cylinder experiments using the "synthesis exchange technique". Results show that at these P-T conditions, NH4+ partitions equally between phengite and coexisting fluid. Majorite was synthesized in multi-anvil experiments at P-T conditions of mantle transition zone and uppermost part of the lower mantle (18-23 GPa & 1450-1500 °C). Majorite as well as other run products were characterized by electron probe microanalysis (EPMA), XRPD with Rietveld refinement, vibrational spectroscopy (FTIR and Raman spectroscopy) and transmission electron microscopy (TEM). The effect of oxygen fugacity on controlling nitrogen speciation in the host mineral is elaborated here. It is shown that majoritic garnet can host N in different forms (N3- and NH4+) under different redox conditions by different mechanisms. The current PhD thesis therefore supports the argumentation that nitrogen can be introduced to the mantle through subduction, or in other words, that nitrogen in the mantle can be of recycled origin.
Language:
English
Type:
info:eu-repo/semantics/doctoralThesis
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