10:00 AM - 10:15 AM
[PPS07-05] Spatial distribution of Mg-containing organic compounds in carbonaceous chondrites.
Keywords:Carbonaceous chondrites, Magnesium, Metal-containing organic compounds, Molecular imaging
[Introduction]
Soluble organic compounds (SOM) from carbonaceous chondrites are important for understanding the origin of life and the chemical evolution of extraterrestrial organic matter in the early solar system. Recently, various CHO, CHNO, CHOS, CHNOS, and even CHN compounds have been reported in SOM through high-resolution mass spectrometry [Schmitt-Kopplin et al. (2010), Naraoka et al. (2017)]. On the other hand, Mg metal containing organic compounds (hereafter Mg-MOC) have also been identified in various meteorites, including carbonaceous chondrites [Ruf et al. (2017), Hashiguchi and Naraoka (2019) LPSC], suggesting that Mg-MOC may have formed through aqueous alteration [Ruf et al. (2017), Hashiguchi and Naraoka (2019) LPSC]. These compounds are considered important for understanding water-mineral-organic interactions on microplanetary surfaces. In this study, we investigated the spatial distribution of Mg-MOCs and their relationship with minerals using molecular imaging to clarify the formation process of Mg-MOCs in carbonaceous meteorites.
[Methods]
The samples were fragments from four carbonaceous chondrites: Tagish Lake (C2 ung), Nogoya (CM2), Murchison (CM2), and Allende (CV3). Fragments (~1 mm) with flat surface were selected by chipping and fixed to indium or low-melting-point alloys. These samples were subjected to desorption electrospray ionization (DESI) (Omni Spray 2D, Prosolia) and imaging mass spectrometry using an Orbitrap mass spectrometer (Q Exactive Plus, Thermo Scientific) at Kyushu University. Positive ions (m/z 50-500) were analyzed by methanol 100% spray (2-3 μL/min) with spray voltage 3 kV, and mass resolution of 140,000 (@ m/z 200). After DESI imaging, elemental mapping using FE-SEM/EDS was performed and compared with the spatial distribution of organic compounds.
[Results and Discussion]
Alkyl homologues of Mg-MOC (a total of 21 series: 13 series of CHOMg composition and 9 series of CHNOMg composition) were identified from all meteorite samples by DESI imaging within 2 ppm of mass precision. These were confirmed to be Mg-containing compounds based on the presence and abundance ratios of the Mg isotopologues. The sum of the identified Mg-MOC molecular species and their relative ion intensities followed the trend: Tagish Lake > Nogoya > Murchison ≈ Allende, with Tagish Lake exhibiting the highest number of molecular species (64) and the highest relative ion intensity.
Mg-MOC were found to be distributed in the matrix regions of all meteorites. Notably, in Tagish Lake and Nogoya, Mg-MOC were concentrated in Mg-containing phyllosilicate and carbonate-rich regions. In the Murchison meteorite, the relative ion intensity of Mg-MOC was higher in the carbonate-rich fragments, although no clear correlation with specific minerals was observed. In Tagish Lake, Nogoya, and Murchison meteorites, the number and relative ion intensity of the identified molecular species were consistent with the magnitude of aqueous alteration in the meteorites [e.g., Velbel et al. (2015)] and correlated with the spatial distribution of phyllosilicate and carbonate. This suggests that Mg-MOC were formed during aqueous alteration on the meteorite parent body and have been preserved on the surface and between the layers of altered minerals. In contrast, the Mg-MOC in the Allende meteorite may have formed during hydrothermal alteration on the parent body [e.g., Krot et al. (2000)] or during thermal metamorphism. Either way, it is suggested that these are thermally tolerant molecules that survived thermal metamorphism. These findings suggest that SOM in the carbonaceous chondrites interacted with Mg to form Mg-MOC, which remained stable and were preserved during both aqueous alteration and thermal metamorphism of the parent body.
Soluble organic compounds (SOM) from carbonaceous chondrites are important for understanding the origin of life and the chemical evolution of extraterrestrial organic matter in the early solar system. Recently, various CHO, CHNO, CHOS, CHNOS, and even CHN compounds have been reported in SOM through high-resolution mass spectrometry [Schmitt-Kopplin et al. (2010), Naraoka et al. (2017)]. On the other hand, Mg metal containing organic compounds (hereafter Mg-MOC) have also been identified in various meteorites, including carbonaceous chondrites [Ruf et al. (2017), Hashiguchi and Naraoka (2019) LPSC], suggesting that Mg-MOC may have formed through aqueous alteration [Ruf et al. (2017), Hashiguchi and Naraoka (2019) LPSC]. These compounds are considered important for understanding water-mineral-organic interactions on microplanetary surfaces. In this study, we investigated the spatial distribution of Mg-MOCs and their relationship with minerals using molecular imaging to clarify the formation process of Mg-MOCs in carbonaceous meteorites.
[Methods]
The samples were fragments from four carbonaceous chondrites: Tagish Lake (C2 ung), Nogoya (CM2), Murchison (CM2), and Allende (CV3). Fragments (~1 mm) with flat surface were selected by chipping and fixed to indium or low-melting-point alloys. These samples were subjected to desorption electrospray ionization (DESI) (Omni Spray 2D, Prosolia) and imaging mass spectrometry using an Orbitrap mass spectrometer (Q Exactive Plus, Thermo Scientific) at Kyushu University. Positive ions (m/z 50-500) were analyzed by methanol 100% spray (2-3 μL/min) with spray voltage 3 kV, and mass resolution of 140,000 (@ m/z 200). After DESI imaging, elemental mapping using FE-SEM/EDS was performed and compared with the spatial distribution of organic compounds.
[Results and Discussion]
Alkyl homologues of Mg-MOC (a total of 21 series: 13 series of CHOMg composition and 9 series of CHNOMg composition) were identified from all meteorite samples by DESI imaging within 2 ppm of mass precision. These were confirmed to be Mg-containing compounds based on the presence and abundance ratios of the Mg isotopologues. The sum of the identified Mg-MOC molecular species and their relative ion intensities followed the trend: Tagish Lake > Nogoya > Murchison ≈ Allende, with Tagish Lake exhibiting the highest number of molecular species (64) and the highest relative ion intensity.
Mg-MOC were found to be distributed in the matrix regions of all meteorites. Notably, in Tagish Lake and Nogoya, Mg-MOC were concentrated in Mg-containing phyllosilicate and carbonate-rich regions. In the Murchison meteorite, the relative ion intensity of Mg-MOC was higher in the carbonate-rich fragments, although no clear correlation with specific minerals was observed. In Tagish Lake, Nogoya, and Murchison meteorites, the number and relative ion intensity of the identified molecular species were consistent with the magnitude of aqueous alteration in the meteorites [e.g., Velbel et al. (2015)] and correlated with the spatial distribution of phyllosilicate and carbonate. This suggests that Mg-MOC were formed during aqueous alteration on the meteorite parent body and have been preserved on the surface and between the layers of altered minerals. In contrast, the Mg-MOC in the Allende meteorite may have formed during hydrothermal alteration on the parent body [e.g., Krot et al. (2000)] or during thermal metamorphism. Either way, it is suggested that these are thermally tolerant molecules that survived thermal metamorphism. These findings suggest that SOM in the carbonaceous chondrites interacted with Mg to form Mg-MOC, which remained stable and were preserved during both aqueous alteration and thermal metamorphism of the parent body.