Mantle-derived fluids in the Nankai Trough Kumano forearc basin

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.