[MIS26-P02] 白馬八方の蛇紋岩温泉に由来する炭化水素の分子および分子内炭素安定同位体解析
キーワード：蛇紋岩、炭化水素、分子内同位体分析、安定炭素同位体
Olivine is one of the major constituent minerals of various meteorites (Rubin, 1997). During serpentinization process of ultramafic rocks, water in contact with olivine is reduced to form molecular hydrogen (H2) (e.g., McCollom and Bach, 2009). The highly reduced (H2-rich) condition created by serpentinization is thermodynamically favorable for abiotic organic synthesis. Elevated concentrations of CH4 and higher hydrocarbon gases have been observed in serpentinite-hosted systems, regardless of continental or seafloor setting (e.g., Charlou et al., 2002; Proskurowski et al., 2008; Etiope et al., 2011; Szponar et al., 2013). Abiotic synthesis has been invoked to account for the carbon isotopic distribution among low-molecular weight hydrocarbons (e.g., Proskurowski et al., 2008). However, uncertainties still remain regarding specific abiotic production mechanisms for serpentinite-hosted systems. In this study, we report a new geochemical study of hydrocarbon gases (methane, ethane, propane, butane, pentane) from the borehole well at the on-land hot spring associated with serpentinization in Hakuba Happo, Japan. We have conducted position-specific as well as compound-specific stable carbon isotopic analyses of hydrocarbons.
The gas samples were collected from Hakuba Happo hot spring that lies on an serpentinized ultramafic rock body. Highly-alkaline hot spring water with temperature of around 50°C mainly contains N2, H2 and CH4 gases (Homma and Tsukahara, 2008; Suda et al., 2014). The concentrations of C1 to C5 hydrocarbons were determined by Gas Chromatography. A compound-specific carbon isotope measurement for C1-C5 hydrocarbons was performed by GC-C-IRMS coupled with an on-line pre-concentration system. A position-specific 13C composition in propane molecule (C3H8) was measured using the GC-Py-GC-C-IRMS (Gilbert et al., 2016) coupled with an on-line pre-concentration system.
The straight chain alkanes (n-alkanes) for the Happo sample show an isotopic depletion in 13C with increasing carbon number (δ13C1 > δ13C2 > δ13C3 > ...). This 13C depletion trend is very similar to those of some seafloor serpentinite-hosted hydrothermal systems (Proskurowski et al., 2008; Charlou et al., 2010), and undisputed abiogenic origin for the Murchison meteorite (Yuen et al., 1984). The observed isotopic trend can be explained by a simple polymerization model developed in this study. Our model assumes that, for any particular alkane, (i) all of the subsequently added carbon atoms that are bonded to the growing carbon chain have the same isotopic composition, and (ii) those are depleted in 13C with respect to the first carbon atom that initiates the carbon chain. The fit of this model suggests that n-alkanes for the Happo sample can be formed via polymerization from single-carbon compound (potentially methane) with a constant kinetic isotopic fractionation of -8.9 ± 1.0‰. To understand the type of polymerization mechanism, we next focus on the position-specific carbon isotopic compositions of hydrocarbons. For the first time, we applied a new method, namely position-specific 13C analysis of propane, to a natural sample derived from a serpentinite-hosted system. The difference of δ13C values between terminal and central carbon atom positions of propane molecule for the Happo sample was -1.2 ± 0.9‰. We show the important potential of the position-specific 13C analysis to identify different polymerization mechanisms that can not be discriminated by compound-specific isotopic analysis.
The gas samples were collected from Hakuba Happo hot spring that lies on an serpentinized ultramafic rock body. Highly-alkaline hot spring water with temperature of around 50°C mainly contains N2, H2 and CH4 gases (Homma and Tsukahara, 2008; Suda et al., 2014). The concentrations of C1 to C5 hydrocarbons were determined by Gas Chromatography. A compound-specific carbon isotope measurement for C1-C5 hydrocarbons was performed by GC-C-IRMS coupled with an on-line pre-concentration system. A position-specific 13C composition in propane molecule (C3H8) was measured using the GC-Py-GC-C-IRMS (Gilbert et al., 2016) coupled with an on-line pre-concentration system.
The straight chain alkanes (n-alkanes) for the Happo sample show an isotopic depletion in 13C with increasing carbon number (δ13C1 > δ13C2 > δ13C3 > ...). This 13C depletion trend is very similar to those of some seafloor serpentinite-hosted hydrothermal systems (Proskurowski et al., 2008; Charlou et al., 2010), and undisputed abiogenic origin for the Murchison meteorite (Yuen et al., 1984). The observed isotopic trend can be explained by a simple polymerization model developed in this study. Our model assumes that, for any particular alkane, (i) all of the subsequently added carbon atoms that are bonded to the growing carbon chain have the same isotopic composition, and (ii) those are depleted in 13C with respect to the first carbon atom that initiates the carbon chain. The fit of this model suggests that n-alkanes for the Happo sample can be formed via polymerization from single-carbon compound (potentially methane) with a constant kinetic isotopic fractionation of -8.9 ± 1.0‰. To understand the type of polymerization mechanism, we next focus on the position-specific carbon isotopic compositions of hydrocarbons. For the first time, we applied a new method, namely position-specific 13C analysis of propane, to a natural sample derived from a serpentinite-hosted system. The difference of δ13C values between terminal and central carbon atom positions of propane molecule for the Happo sample was -1.2 ± 0.9‰. We show the important potential of the position-specific 13C analysis to identify different polymerization mechanisms that can not be discriminated by compound-specific isotopic analysis.