15:30 〜 15:45
[SGC35-07] Hydrocarbon seep dynamics revealed by noble gas and stable isotope analysis of gas inclusions of authigenic carbonates: A comparison of samples from Tatar Strait, Russia; Joetsu Basin, Japan; and Brejo do Espinho, Brazil.
★Invited Papers
キーワード：Carbonate, Noble Gas, Helium isotopes, Methane Derived Authigenic Carbonates, Methane Seep
Gas inclusions within methane-derived authigenic carbonates (MDACs) can potentially provide information regarding the history of both hydrocarbon accumulation and microbial mineralization. We present noble gas and stable isotopic results from such gas inclusions in Joetsu Basin, eastern margin of Japan Sea, as well as Tatar Strait, northern Japan Sea.
Long sections of massive gas hydrate have been recovered from Joetsu Basin during several cruises during the past decade; however similar recovery of gas hydrate in Tatar Strait for geochemical and isotopic studies has been challenging. This is because the shallow hydrates in Tatar Strait tend to form thin laminations which melt when by the time the cores are brought to the surface or shortly afterwards. We present a technique for determining the noble gas and stable carbon isotopic composition of MDAC voids, as a workaround where gas hydrate sampling is not possible. Such MDACs are frequently encountered in gas hydrate chimneys and seeps, such as those found in Joetsu Basin and Tatar Strait permitting a direct comparison of gas between the two localities. The noble gas isotopic composition was determined for gas inclusions in small pieces (<2g) of MDAC, at the AORI Noble Gas Facility which were surface-cleaned, placed under a vacuum and crushed. For comparison, a microbial dololomite from the Brejo do Espinho lagoon in Brazil was also analyzed.
Helium isotope ratios for Joetsu Basin MDACs are similar to those determined previously for hydrates from the same gas chimneys, which indicate the involvement of mantle gases in shallow hydrate accumulation and MDAC precipitation. The deeply buried Joetsu Basin MDACs show accumulation of radiogenic 4He in the inclusions during long-term burial. In Tatar Strait, the shallow authigenic carbonates associated with laminar hydrate have radiogenic 3He/4He ratios and 40Ar/36Ar ratios, indicative of the migration of deep fluids in contact with radiogenic source in gas chimneys there. The lagoon dolomite from Brazil shows an atmospheric 40Ar/36Ar ratio, combined with a 3He/4He ratio is possibly influenced by decay of tritium in shallow groundwater at the time of precipitation.
In addition, the stable carbon isotopic composition of the MDAC inclusion CO2 was determined for samples from the three sites. We compare with the stable isotopic composition of the inclusions with that of the carbonates themselves as well as with the Joetsu Basin methane, and discuss differences in the origin as they are related to the gas inclusions. The Tatar Strait samples in this study were collected as part of a bilateral research agreement between the RAS Pacific Oceanological Institute and Meiji University’s Gas Hydrate Research Laboratory. The Joetsu Basin samples used in this investigation were collected by Meiji University’s Gas Hydrate Research Laboratory under the commission of AIST from 2013-2015 as part of the methane hydrate research project funded by METI (Ministry of Economy, Trade and Industry), Japan.
Long sections of massive gas hydrate have been recovered from Joetsu Basin during several cruises during the past decade; however similar recovery of gas hydrate in Tatar Strait for geochemical and isotopic studies has been challenging. This is because the shallow hydrates in Tatar Strait tend to form thin laminations which melt when by the time the cores are brought to the surface or shortly afterwards. We present a technique for determining the noble gas and stable carbon isotopic composition of MDAC voids, as a workaround where gas hydrate sampling is not possible. Such MDACs are frequently encountered in gas hydrate chimneys and seeps, such as those found in Joetsu Basin and Tatar Strait permitting a direct comparison of gas between the two localities. The noble gas isotopic composition was determined for gas inclusions in small pieces (<2g) of MDAC, at the AORI Noble Gas Facility which were surface-cleaned, placed under a vacuum and crushed. For comparison, a microbial dololomite from the Brejo do Espinho lagoon in Brazil was also analyzed.
Helium isotope ratios for Joetsu Basin MDACs are similar to those determined previously for hydrates from the same gas chimneys, which indicate the involvement of mantle gases in shallow hydrate accumulation and MDAC precipitation. The deeply buried Joetsu Basin MDACs show accumulation of radiogenic 4He in the inclusions during long-term burial. In Tatar Strait, the shallow authigenic carbonates associated with laminar hydrate have radiogenic 3He/4He ratios and 40Ar/36Ar ratios, indicative of the migration of deep fluids in contact with radiogenic source in gas chimneys there. The lagoon dolomite from Brazil shows an atmospheric 40Ar/36Ar ratio, combined with a 3He/4He ratio is possibly influenced by decay of tritium in shallow groundwater at the time of precipitation.
In addition, the stable carbon isotopic composition of the MDAC inclusion CO2 was determined for samples from the three sites. We compare with the stable isotopic composition of the inclusions with that of the carbonates themselves as well as with the Joetsu Basin methane, and discuss differences in the origin as they are related to the gas inclusions. The Tatar Strait samples in this study were collected as part of a bilateral research agreement between the RAS Pacific Oceanological Institute and Meiji University’s Gas Hydrate Research Laboratory. The Joetsu Basin samples used in this investigation were collected by Meiji University’s Gas Hydrate Research Laboratory under the commission of AIST from 2013-2015 as part of the methane hydrate research project funded by METI (Ministry of Economy, Trade and Industry), Japan.