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 ³He/⁴He 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 ³He/⁴He 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.

中文翻译：

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南海槽熊野前臂盆地的地幔衍生流体

在IODP探险队338和348期间获得的12个钻井泥浆气体样品中的稀有气体丰度和同位素数据，为日本东南部Kumano前臂盆地下方内增生棱镜中的流体成因和流体运移提供了新见解。样品来自三个相邻的钻孔（C0002F，C0002N和C0002P），其深度在海床以下950至3050米（mbsf）之间，由空气以及地幔衍生和地壳流体的不同贡献组成。空气校正后的³ He / ⁴他的比例来自实验。338落在0.44±0.24 Ra和3.26±0.28 Ra之间。＃1400和＃1800样品（样品数量表示以mbsf为单位的深度）受放射氦的影响更大（1.33±0.34 Ra分别为0.44±0.24 Ra），而样品＃950的无空气氦同位素组成明显更高（3.26） ±0.28 Ra）。Exp浓度在950 mbsf，1100 mbsf，1400 mbsf和1800 mbsf附近增强，表明在这些深度有活跃的流体流动。实验期间收集到的另外两个样品（＃1700和＃1850）的氦同位素组成。在一年后的实验中获得了338份样品。348的变量较小（1.74-2.46 Ra，平均³ He / ⁴他的比率为〜2.4 Ra），并且在采样过程中未观察到ra异常。对于这些样品，我们假设通过含氦流体扩散流通过增生棱镜的沉积物而迁移。相比之下，来自Exp的950、1400和1800 mbsf的ra活动增加，氦同位素的组成变化更大。338可以更好地解释为通过暂时渗透的地层的通道化流体和主动流体流动，这可能是由埋藏的信任断层的重新激活引起的短期事件。＃950的氦同位素组成与目前俯冲菲律宾海板块（〜3.4 Ra）的估计氦同位素值非常吻合，而1400和1800 mbsf的流体表明来自地壳的放射性氦的输入，可能是地面来源。