9:38 AM - 9:53 AM
[SGL17-03] Helium isotope ratios of cuttings in the Nankai Trough
Keywords:Helium Isotopes, Nankai Trough, Cuttings
In the subduction zones of the circum Pacific regions, specific helium isotope ratio (3He/4He) distributions have been obtained on the axes that intersect perpendicular to the trench [1]. As a representative of the fore-arc region of island arcs, i.e. the trench side of volcanic front in northeastern Japan shows crustal values lower than 1Ra (Ra is the atmospheric isotopic ratio, 1.4 × 10-6). On the volcanic arc region, the ratios reach a maximum of 8 Ra with upper mantle signature. Such step-like variations are also observed in the Chugoku and Shikoku regions of southwest Japan, where isotopic ratios reach 3 Ra on the forearc region [2]. These studies were obtained by collecting and measuring hot springs, groundwater, and natural gas on land. In this study, we attempted to extend the distribution of helium isotope ratios to the seaward side of the forearc by analyzing rock samples drilled from accretionary complexes beneath the forearc basin of the Nankai Trough.
The sample used for analysis was a cuttings of sedimentary rock collected at Site C0002 of IODP, which was drilled on the southeastern margin of the Kumano Basin off the Kii Peninsula [3]. A sample of approximately 2 g was cut and ultrasonically cleaned with pure water, then dried and set in a vacuum ball mill. The sample was crushed under ultra-high vacuum and the gaseous components were extracted. Helium and neon were purified using an activated carbon trap at low temperature and a titanium getter at high temperature. After measuring the 4He/20Ne ratio in a quadrupole mass spectrometer, the neon was separated by a cryogenic trap, and the helium was introduced into a noble gas mass spectrometer to measure the 3He/4He ratio. The data obtained were normalized against atmospheric standard helium. For some of the powder samples after crushing, the gaseous components were extracted by a furnace and the 3He/4He ratio was measured.
Helium concentration, 3He/4He ratio, and 4He/20Ne ratio of a total of 21 samples of cuttings collected almost every 100 m from about 1000 m to 2900 m below the seafloor were measured by vacuum crushing. 3He/4He ratio ranged from 0.90 Ra to 3.36 Ra, while 4He/20Ne ratio varied from 3.8 to 66. These values were generally consistent with the noble gas compositions of the mud water used during riser drilling [4]. The 3He/4He ratio, corrected for atmospheric helium, increased from 1000 m to 1400 m and then tended to decrease with depth. The 3He/4He ratio obtained by heating the powder sample varied from 0.08Ra to 0.26Ra and was of apparent crustal origin. Therefore, the over 3Ra values observed by the crushing are not due to cosmic dust-derived helium in marine sediments, but indicate a contribution from mantle-derived helium.
Reference [1] Sano & Wakita, 1985, JGR 90, 8729-8741. [2] Dogan et al. 2006, Chem. Geol. 233, 235-248. [3] Fukuchi et al., 2017, G-Cubed 18, 3185-3196. [4] Wiersberg et al., 2018, PEPS 5, 79.
The sample used for analysis was a cuttings of sedimentary rock collected at Site C0002 of IODP, which was drilled on the southeastern margin of the Kumano Basin off the Kii Peninsula [3]. A sample of approximately 2 g was cut and ultrasonically cleaned with pure water, then dried and set in a vacuum ball mill. The sample was crushed under ultra-high vacuum and the gaseous components were extracted. Helium and neon were purified using an activated carbon trap at low temperature and a titanium getter at high temperature. After measuring the 4He/20Ne ratio in a quadrupole mass spectrometer, the neon was separated by a cryogenic trap, and the helium was introduced into a noble gas mass spectrometer to measure the 3He/4He ratio. The data obtained were normalized against atmospheric standard helium. For some of the powder samples after crushing, the gaseous components were extracted by a furnace and the 3He/4He ratio was measured.
Helium concentration, 3He/4He ratio, and 4He/20Ne ratio of a total of 21 samples of cuttings collected almost every 100 m from about 1000 m to 2900 m below the seafloor were measured by vacuum crushing. 3He/4He ratio ranged from 0.90 Ra to 3.36 Ra, while 4He/20Ne ratio varied from 3.8 to 66. These values were generally consistent with the noble gas compositions of the mud water used during riser drilling [4]. The 3He/4He ratio, corrected for atmospheric helium, increased from 1000 m to 1400 m and then tended to decrease with depth. The 3He/4He ratio obtained by heating the powder sample varied from 0.08Ra to 0.26Ra and was of apparent crustal origin. Therefore, the over 3Ra values observed by the crushing are not due to cosmic dust-derived helium in marine sediments, but indicate a contribution from mantle-derived helium.
Reference [1] Sano & Wakita, 1985, JGR 90, 8729-8741. [2] Dogan et al. 2006, Chem. Geol. 233, 235-248. [3] Fukuchi et al., 2017, G-Cubed 18, 3185-3196. [4] Wiersberg et al., 2018, PEPS 5, 79.