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[SCG53-08] Abiotic Polycyclic Aromatic Hydrocarbons Preserved in Melt Inclusions from a Tahitian Mantle Xenolith
Keywords:Polycyclic Aromatic Hydrocarbons (PAHs), Peridotites, Organic Matter, Raman Spectroscopy, STXM/XANES, XnCT
Abiotic formation of organic matter (OM) in the Earth’s mantle has been proposed since the 1870s [1]. High-pressure and high-temperature experiments, along with theoretical studies, have demonstrated that OM can be synthesized from inorganic compounds, with simple hydrocarbon molecules such as methane tending to polymerize into more complex OM (e.g., [2]). In most such experiments, relatively simple saturated hydrocarbons are the primary products, while thermodynamic simulations have suggested that more complex OM, such as aromatic hydrocarbons, could be stable in the mantle [3]. Despite extensive research, the extent to which complex OM forms under mantle conditions remains poorly understood.
In this context, polycyclic aromatic hydrocarbons (PAHs) in xenocrysts from kimberlite pipes are of particular interest. PAHs have been detected in situ as well as through gas chromatography-mass spectrometry analyses of olivine, garnet, and diamond (e.g., [4]). Although these PAHs are interpreted to have an abiotic origin, their distribution in the mantle and formation processes remain poorly understood.
Here, we report PAHs in melt inclusions in a clinopyroxene (Cpx) grain from a spinel harzburgite xenolith collected on Tahiti, part of the Society hotspot track. To characterize the host Cpx grain, we conducted field-emission scanning electron microscopy with energy-dispersive spectroscopy (FE-SEM-EDS). We extracted three micro-samples containing melt inclusions using a focused ion beam (FIB) system and performed an integrated analytical approach for the micrometer-scale inclusions, including X-ray nano-computed tomography (XnCT), Raman microscopy, wide-field fluorescence microscopy, and X-ray absorption near-edge spectroscopy (XANES) with scanning transmission X-ray microscopy (STXM). FE-SEM-EDS analysis revealed that the host Cpx grain has lower Ti and Al contents and a higher Mg# than those of other Cpx grains in the same sample. The melt inclusions are of crack-healing origin, as indicated by XnCT analyses, which also revealed that they consist of silicate glasses, base-metal sulfides, platinum-group minerals, and C-O-H phases. Raman microscopy, wide-field fluorescence microscopy, and STXM/XANES analyses consistently confirmed the C-O-H phases are dominantly composed of PAHs. Furthermore, STXM/XANES analyses demonstrated that these PAHs contain few aliphatic chains or functional groups and coexist with CO and CO2 in the C-O-H phases.
The detected PAHs are unlikely to be contaminants, as their signals are exclusively observed within the inclusions. The lower Ti and Al contents and a higher Mg# of the host Cpx grain compared to other Cpx grains suggest a deeper origin [5]. Reduced species such as PAHs and CO are present in the C-O-H phases, and the redox state of the Earth's mantle is inferred to become more reduced with increasing depth [6]. Thus, the inclusions were likely trapped under reducing conditions at the base of the stability-field of spinel-bearing peridotites in the upper mantle. Based on experimental studies (e.g., [7]), we propose that the aromatization of simple hydrocarbon molecules occurred within the C-O-H phases of the inclusions, resulting in the abiotic formation of PAHs in the Earth's upper mantle. Whereas most high-pressure and high-temperature experiments have shown that methane predominantly polymerizes into simple saturated hydrocarbons, our findings suggest that PAHs can also be formed under certain conditions in the Earth's upper mantle.
[1] Wang et al. (2023), Acta Geologica Sinica - English Edition. [2] Kenney et al. (2002), Proceedings of the National Academy of Sciences. [3] Serovaiskii and Kutcherov (2020), Scientific Reports [4] Tomilenko et al. (2016), Doklady Earth Sciences. [5] Adam and Green (1994), Chemical Geology. [6] Frost and McCammon (2008), Annual Review of Earth and Planetary Sciences. [7] Wang et al. (2007), Science in China Series D: Earth Sciences.
In this context, polycyclic aromatic hydrocarbons (PAHs) in xenocrysts from kimberlite pipes are of particular interest. PAHs have been detected in situ as well as through gas chromatography-mass spectrometry analyses of olivine, garnet, and diamond (e.g., [4]). Although these PAHs are interpreted to have an abiotic origin, their distribution in the mantle and formation processes remain poorly understood.
Here, we report PAHs in melt inclusions in a clinopyroxene (Cpx) grain from a spinel harzburgite xenolith collected on Tahiti, part of the Society hotspot track. To characterize the host Cpx grain, we conducted field-emission scanning electron microscopy with energy-dispersive spectroscopy (FE-SEM-EDS). We extracted three micro-samples containing melt inclusions using a focused ion beam (FIB) system and performed an integrated analytical approach for the micrometer-scale inclusions, including X-ray nano-computed tomography (XnCT), Raman microscopy, wide-field fluorescence microscopy, and X-ray absorption near-edge spectroscopy (XANES) with scanning transmission X-ray microscopy (STXM). FE-SEM-EDS analysis revealed that the host Cpx grain has lower Ti and Al contents and a higher Mg# than those of other Cpx grains in the same sample. The melt inclusions are of crack-healing origin, as indicated by XnCT analyses, which also revealed that they consist of silicate glasses, base-metal sulfides, platinum-group minerals, and C-O-H phases. Raman microscopy, wide-field fluorescence microscopy, and STXM/XANES analyses consistently confirmed the C-O-H phases are dominantly composed of PAHs. Furthermore, STXM/XANES analyses demonstrated that these PAHs contain few aliphatic chains or functional groups and coexist with CO and CO2 in the C-O-H phases.
The detected PAHs are unlikely to be contaminants, as their signals are exclusively observed within the inclusions. The lower Ti and Al contents and a higher Mg# of the host Cpx grain compared to other Cpx grains suggest a deeper origin [5]. Reduced species such as PAHs and CO are present in the C-O-H phases, and the redox state of the Earth's mantle is inferred to become more reduced with increasing depth [6]. Thus, the inclusions were likely trapped under reducing conditions at the base of the stability-field of spinel-bearing peridotites in the upper mantle. Based on experimental studies (e.g., [7]), we propose that the aromatization of simple hydrocarbon molecules occurred within the C-O-H phases of the inclusions, resulting in the abiotic formation of PAHs in the Earth's upper mantle. Whereas most high-pressure and high-temperature experiments have shown that methane predominantly polymerizes into simple saturated hydrocarbons, our findings suggest that PAHs can also be formed under certain conditions in the Earth's upper mantle.
[1] Wang et al. (2023), Acta Geologica Sinica - English Edition. [2] Kenney et al. (2002), Proceedings of the National Academy of Sciences. [3] Serovaiskii and Kutcherov (2020), Scientific Reports [4] Tomilenko et al. (2016), Doklady Earth Sciences. [5] Adam and Green (1994), Chemical Geology. [6] Frost and McCammon (2008), Annual Review of Earth and Planetary Sciences. [7] Wang et al. (2007), Science in China Series D: Earth Sciences.