10:45 〜 12:15
[BCG06-P07] In-situ analyses of inclusions containing organic compounds discovered from Tahitian harzburgite xenolith
キーワード：タヒチ島、マントル捕獲岩、有機物、多環芳香族、炭素X線吸収端微細構造（XANES）、メルト包有物
The speciation of C-O-H in the Earth’s mantle has been long debated. Observational studies reported inclusions containing organic compounds from minerals in kimberlite pipes [e.g.,1]. Sugisaki and Mimura [2] applied gas chromatography mass spectrometry (GC-MS) for a variety of igneous rocks and detected heavier hydrocarbons (n-alkane) only from mantle derived rocks. They proposed three possibilities for the formation of such hydrocarbons; 1) they were inorganically synthesized in the Earth’s mantle, 2) they were delivered by meteorites and comets to the early Earth and were preserved in the mantle, or 3) biogenic hydrocarbons were recycled via subduction cycle. However, it has been still unclear how such hydrocarbons are distributed in the peridotites. Here, we report organic compounds in aligned, hence secondary melt inclusions in a clinopyroxene grain of Tahitian harzburgite xenolith found through in-situ microscopic analyses for secondary trapped melt inclusions in a clinopyroxene
In the clinopyroxene grain, synchrotron radiation X-ray nano-tomography (SR-XnCT) imaged an inclusion array composed of 4 phases, i.e., platinum-group minerals (PGM), Fe-Ni-Cu sulfides (BMS), silicate glasses and light-element substances [3]. Raman spectra of the light-element substances has broad peaks at 1360±, 1600± and 2900± cm-1 and wide-field fluorescence imaging technique demonstrated that the light-element substances exhibit the fluorescence [4]. These results indicate that the light-element substances are composed of polycyclic aromatic hydrocarbons. For further Carbon (C) K-edge X-ray absorption near-edge structure (XANES) analyses by scanning transmission X-ray microscopy (STXM) at the beamline BL-19A of the synchrotron facility Photon Factory (Tsukuba, Japan), we made a thin (about 500 nm-thick) section of an inclusion array in the same clinopyroxene grain using focused ion beam-scanning electron microscope (FIB-SEM) system at Kyoto University.
C K-edge XANES spectra obtained from the light-element substances have a variety of peaks at 285.2 eV (aromatic carbon/C=C), 286.8 eV (C≡N/C=O), 287.5 eV (aliphatic carbon) and 290.9 eV (carbonate/CO2). At 291.5 eV, no significant peaks are seen; thus, aromatic component is not high-molecular weight hydrocarbons like graphite or graphene. Peaks at 287.5 and 290.9 eV were not detected in inclusions which cropped out during FIB-procedure, indicating that they are contained in liquid or gaseous phase. On the other hand, peaks at 285.2 and 286.8 eV were detected in all the spectra obtained from light-element substances. This implies that aromatic/C=C component and C≡N/C=O are in the solid part of the light-element substances.
Our new XANES spectra demonstrated that the light-element substances bear various functional groups including aromatic carbon/C=C, C≡N/C=O, aliphatic carbon and CO2/carbonate-bearing phase. Their peaks were obtained only from the light-element substances within the inclusions, and thus, these are not derived from contamination. The high pressure (> 2 GPa) in the Earth’s mantle may be one of the possible explanations for existence of such hydrocarbons [e.g., 5]. Sokol et al. [6] synthesized mixtures of light alkanes, unsaturated hydrocarbons and O-bearing species from C-O-H fluid under upper mantle condition (5.5-7.8 GPa and 1100-1400 ℃). Otherwise, the polymerization of low-weight hydrocarbons may be attributed to some catalytic effects by transition metals which are often used as catalysts in Fischer-Tropsch type (FTT) reaction [7], because transition metal sulfides often coexist with the hydrocarbons in the studied inclusions.
[1] Garanin et al. (2011) Moscow Univ. Geol. Bull. [2] Sugisaki and Mimura (1994) Geochim. Cosmochim. Acta. [3] Mitsukawa et al. (2022) JpGU. [4] Mitsukawa et al. (2022) JAMS. [5] Kutcherov and Krayushkin (2010) Rev. Geophys. [6] Sokol et al. (2019) Phys. Earth Planet. Inter. [7] Etiope and Lollar (2013) Rev. Geophys.
In the clinopyroxene grain, synchrotron radiation X-ray nano-tomography (SR-XnCT) imaged an inclusion array composed of 4 phases, i.e., platinum-group minerals (PGM), Fe-Ni-Cu sulfides (BMS), silicate glasses and light-element substances [3]. Raman spectra of the light-element substances has broad peaks at 1360±, 1600± and 2900± cm-1 and wide-field fluorescence imaging technique demonstrated that the light-element substances exhibit the fluorescence [4]. These results indicate that the light-element substances are composed of polycyclic aromatic hydrocarbons. For further Carbon (C) K-edge X-ray absorption near-edge structure (XANES) analyses by scanning transmission X-ray microscopy (STXM) at the beamline BL-19A of the synchrotron facility Photon Factory (Tsukuba, Japan), we made a thin (about 500 nm-thick) section of an inclusion array in the same clinopyroxene grain using focused ion beam-scanning electron microscope (FIB-SEM) system at Kyoto University.
C K-edge XANES spectra obtained from the light-element substances have a variety of peaks at 285.2 eV (aromatic carbon/C=C), 286.8 eV (C≡N/C=O), 287.5 eV (aliphatic carbon) and 290.9 eV (carbonate/CO2). At 291.5 eV, no significant peaks are seen; thus, aromatic component is not high-molecular weight hydrocarbons like graphite or graphene. Peaks at 287.5 and 290.9 eV were not detected in inclusions which cropped out during FIB-procedure, indicating that they are contained in liquid or gaseous phase. On the other hand, peaks at 285.2 and 286.8 eV were detected in all the spectra obtained from light-element substances. This implies that aromatic/C=C component and C≡N/C=O are in the solid part of the light-element substances.
Our new XANES spectra demonstrated that the light-element substances bear various functional groups including aromatic carbon/C=C, C≡N/C=O, aliphatic carbon and CO2/carbonate-bearing phase. Their peaks were obtained only from the light-element substances within the inclusions, and thus, these are not derived from contamination. The high pressure (> 2 GPa) in the Earth’s mantle may be one of the possible explanations for existence of such hydrocarbons [e.g., 5]. Sokol et al. [6] synthesized mixtures of light alkanes, unsaturated hydrocarbons and O-bearing species from C-O-H fluid under upper mantle condition (5.5-7.8 GPa and 1100-1400 ℃). Otherwise, the polymerization of low-weight hydrocarbons may be attributed to some catalytic effects by transition metals which are often used as catalysts in Fischer-Tropsch type (FTT) reaction [7], because transition metal sulfides often coexist with the hydrocarbons in the studied inclusions.
[1] Garanin et al. (2011) Moscow Univ. Geol. Bull. [2] Sugisaki and Mimura (1994) Geochim. Cosmochim. Acta. [3] Mitsukawa et al. (2022) JpGU. [4] Mitsukawa et al. (2022) JAMS. [5] Kutcherov and Krayushkin (2010) Rev. Geophys. [6] Sokol et al. (2019) Phys. Earth Planet. Inter. [7] Etiope and Lollar (2013) Rev. Geophys.