Mantle-derived fluids in the Nankai Trough Kumano forearc basin

Abstract

Noble gas abundance and isotope data from 12 drilling mud gas samples obtained during IODP Expeditions 338 and 348 provide new insights on fluid origin and fluid migration in the inner accretionary prism below the Kumano forearc basin offshore SE Japan. The samples originate from three adjoining boreholes (C0002F, C0002N and C0002P) spanning depths between 950 and 3050 meters below sea floor (mbsf) and are composed of air and variable contributions of mantle-derived and crustal fluids. Air-corrected 3 He/ 4 He ratios of samples from Exp. 338 fall between 0.44 ± 0.24 Ra and 3.26 ± 0.28 Ra. Samples #1400 and #1800 (sample number denotes depth in mbsf) are more influenced by radiogenic helium (1.33 ± 0.34 Ra resp. 0.44 ± 0.24 Ra), whereas the air-free helium isotopic composition of sample #950 is clearly higher (3.26 ± 0.28 Ra). Enhanced radon activity around 950 mbsf, 1100 mbsf, 1400 mbsf, and 1800 mbsf suggests active flow of fluids at these depths. The helium isotopic composition of two other samples (#1700 and #1850) collected during Exp. 338 and of all samples obtained during the 1-year-later Exp. 348 are less variable (1.74-2.46 Ra with a mean 3 He/ 4 He ratio of~2.4 Ra) and no radon anomalies were observed during sampling. For these samples, we assume migration by diffusive flow of helium-bearing fluids through sediments of the accretionary prism. In contrast, elevated radon activities and the more variable helium isotopic composition of fluids from 950, 1400, and 1800 mbsf from Exp. 338 are better explained by channelized and active fluid flow through temporarily permeable strata, maybe as a short-term episodic flow event caused by reactivation of buried trust faults. The helium isotopic composition of #950 is in good agreement with estimated helium isotope values of the subducting Philippine Sea Plate (PSP) at present (~3.4 Ra), whereas fluids from 1400 and 1800 mbsf demonstrate the input of radiogenic helium from a crustal, probably terrestrial source.

Chemical and Noble Gas Isotope Compositions of Formation Gases from a 3 km Deep Scientific Borehole in the Koyna Seismogenic Zone, Western India

“…The continuous fluid analysis during drilling of the KTB (German Continental Deep Drilling Program) boreholes led to determining the composition of gases dissolved in drilling fluid and identifying several fluid flow zones with enhanced gas concentrations above the atmospheric abundances [7,19]. Subsequently, online monitoring of fluids and gases from circulating drilling mud has been successfully conducted in several scientific drilling projects in crystalline and sedimentary strata with a focus on fault zones, e.g., the San Andreas Fault Observatory at Depth (SAFOD) [7,[20][21][22], the Wenchuan Fault Zone drilling project [23], and the Nankai Trough Seismogenic Zone Experiment (NanTro-SEIZE) [24][25][26]. Online gas monitoring during drilling through fault zones provides information on the origin and spatial distribution of formation gases at seismogenic depths, gas migration, and the fault zone permeability architecture.…”

Thermogenic Petroleum Potential of the Nankai Subduction Zone, Offshore Sw Japan

“…Nishio et al (2015) reported evidence from mud volcanoes of fluids derived from depths of more than 20 km. Wiersberg et al (2019) also observed mantle-derived fluids in IODP borehole C0002, and inferred the existence of both diffusive flow and channelized flow within the accretionary prism complex. However, fluid migration paths from the potential source of thermogenic hydrocarbons in this region are poorly understood because deep-seated structures have not been clearly imaged on conventional seismic profiles.…”

“…The geochemical analysis of gas samples from mud volcanoes and the IODP C0002 borehole has demonstrated the existence of fluid migration paths in the accretionary prism complex from depth to shallow levels and to the seafloor (e.g. Toki et al, 2013;Nishio et al, 2015;Wiersberg et al, 2019). In addition, biogenic gases may be generated within shallow sediments in the Kumano Basin as a result of microbial activity, even in relatively low temperature environments (Ijiri et al, 2018).…”

Fluid Dynamics along the Nankai Trough: He Isotopes Reveal Direct Seafloor Mantle-Fluid Emission in the Kumano Basin (Southwest Japan)

“…Toki et al reported high crustal-derived He shares (resulting in 3 He/ 4 He ratios of approximately 0.1 Ra) measured in pore water samples collected at cold seeps at Oomine Ridge in the Nankai Trough . More recently, Wiersberg et al reported mantle-derived helium in the deep sediment column of the Kumano forearc basin . The 3 He/ 4 He ratios, measured in deep-drilling mud gas samples, were up to 3.3 Ra.…”

“…In this work, the three regions where sediment samples were collected are labeled as Seep, Landslide, and Colony (see Figure ). The Seep site corresponds to the area characterized by the cold seepage mentioned in Toki et al , The Landslide site is located in the proximity of IODP Site C0002 investigated by Wiersberg et al The Colony site, located in the outer ridge off Tokai, is characterized by the presence of a large-scale Calyptogena colony along the Daini Tenryu knoll being comprised mainly of dead shells with very few “patches” of living bivalves remaining. , The observation of such a bivalve colony is indicative of an active seepage providing nutrient-rich fluids (e.g., methane and various sulfides) on which these chemosynthetic organisms can feed.…”