Japan Geoscience Union Meeting 2016

Presentation information

International Session (Oral)

Symbol S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Techtonophysics

[S-IT06] Interaction and Coevolution of the Core and Mantle

Mon. May 23, 2016 9:00 AM - 10:30 AM 304 (3F)

Convener:*Satoru Tanaka(Department of Deep Earth Structure and Dynamics Research Japan Agency for Marine-Earth Science and Technology), Taku Tsuchiya(Geodynamics Research Center, Ehime University), Chair:Tetsuo Irifune(Geodynamics Research Center, Ehime University), Hiroyuki Kagi(Geochemical Research Center, Graduate School of Science, University of Tokyo)

9:45 AM - 10:00 AM

[SIT06-04] Reaction between olivine and nitrogen at high pressure and high temperature

*Hiroyuki Kagi1, Toshinori Kubo1, Yukiko Hoshino1, Takehiko Yagi1, Ayako Shinozaki2, Hiroaki Ohfuji4, Satoshi Nakano5, Aiko Nakao6, Taku Okada3 (1.Geochemical Research Center, Graduate School of Science, The University of Tokyo, 2.Graduate School of Environmental Studies, Nagoya Univerisity, 3.ISSP, The University of Tokyo, 4.GRC, Ehime University, 5.NIMS, 6.RIKEN)

Keywords:nitrogen, mantle, olivine

Volatiles represented by hydrogen greatly affect deep-earth dynamics. Behavior of volatiles will contribute to understandings of budget of light elements, evolution of the earth, geodynamics and so on. Among volatile components, behavior of nitrogen in the deep earth is still unclear. Nitrogen is a main constituent of atmosphere and also exists in the crust by substituting potassium ions in silicate minerals. Chondrite-normalized nitrogen concentration of the bulk earth is one order of magnitude lower than those other volatiles such as He, Ne, Ar, H2O and so on. There could be a hidden nitrogen reservoir in the deep earth. We are going to test a possibility of a hidden nitrogen reservoir from high pressure and high temperature experiments.
San Carlos olivine or synthetic forsterite were loaded as a starting material in a diamond anvil cell with nitrogen in liquid state or compressed gas (180 MPa). After applying pressure at 5 GPa, a sample was heated using CO2 laser or fiber laser. X-ray diffraction patterns, SEM-EDS images, XPS spectra were obtained on the recovered samples.
XRD measurements on the recovered samples revealed the formation of enstatite (MgSiO3) suggesting the decomposition of Mg2SiO4 into MgSiO3 and MgO. This reaction is contrastive to the reaction occurring in H2 fluid (Shinozaki et al., 2013), Mg2SiO4 decomposed into SiO2 and MgO. Moreover, EDS mapping observations revealed that there are some Mg-rich (Si-depleted) spots.
XPS spectra shown in Fig. 1 revealed that nitrogen was detected from an olivine sample recovered from 5 GPa and 1700 K. Before Ar-sputtering, species assignable to NH4+ is dominant. Presumably, the sample surface is covered with adsorbed molecules. After Ar-sputtering, a broad band attributable to intrinsic nitrogen reacted with the mineral was observed. The present results suggest the formation of nitride species (N3-) in a mantle-derived silicate mineral. This study proposes nitride as a hidden nitrogen reservoir in the deep mantle.