Helium-3 in different carriers from pelagic clays of DSDP Hole 91-596B and Core LR-44-GPC-3

To better understand the composition, characteristics of helium diffusion, and size distribution of interplanetary dust particles (IDPs) responsible for the long-term retention of extraterrestrial 3He, we carried out leaching, stepped heating, and sieving experiments on pelagic clays that varied in age from 0.5 Ma to ~90 Myr. The leaching experiments suggest that the host phase(s) of 3He in geologically old sediments are neither organic matter nor refractory phases, such as diamond, graphite, Al2O3, and SiC, but are consistent with extraterrestrial silicates, Fe-Ni sulfides, and possibly magnetite. Stepped heating experiments demonstrate that the 3He release profiles from the magnetic and non-magnetic components of the pelagic clays are remarkably similar. Because helium diffusion is likely to be controlled by mineral chemistry and structure, the stepped heating results suggest a single carrier that may be magnetite, or more probably a phase associated with magnetite. Furthermore, the stepped outgassing experiments indicate that about 20% of the 3He will be lost through diffusion at seafloor temperatures after 50 Myrs, while sedimentary rocks exposed on the Earth's surface for the same amount of time would lose up to 60%. The absolute magnitude of the 3He loss is, however, likely to depend upon the 3He concentration profile within the IDPs, which is not well known. Contrary to previous suggestions that micrometeorites in the size range of 50-100 µm in diameter are responsible for the extraterrestrial 3He in geologically old sediments [Stuart, F.M., Harrop, P.J., Knott, S., Turner, G., 1999. Laser extraction of helium isotopes from Antarctic micrometeorites: source of He and implications for the flux of extraterrestrial 3He flux to earth. Geochimica et Cosmochimica Acta, 63, 2653-2665, doi:10.1016/S0016-7037(99)00161-1], our sieving experiment demonstrates that at most 20% of the 3He is carried by particles greater than 50 µm in diameter. The size-distribution of the 3He-bearing particles implies that extraterrestrial 3He in sediments record the IDP flux rather than the micrometeorite flux.

为深入探究导致地外³He长期留存的行星际尘埃粒子（interplanetary dust particles，IDPs）的组成、氦扩散特征及粒径分布，我们对年龄介于0.5 Ma至约90 Myr的远洋黏土开展了浸出、阶梯加热与筛分实验。浸出实验结果表明，地质古老沉积物中的³He宿主相既非有机质，亦非金刚石、石墨、Al₂O₃、SiC等难熔物相，而是与地外硅酸盐、Fe-Ni硫化物及可能的磁铁矿相吻合。阶梯加热实验显示，远洋黏土的磁性与非磁性组分的³He释放谱极为相似。鉴于氦扩散大概率受矿物化学组成与结构调控，该实验结果提示存在单一载体：该载体可能为磁铁矿，或更可能为与磁铁矿伴生的物相。此外，阶梯脱气实验表明，在海底温度条件下，约20%的³He会在50 Myr后通过扩散发生流失；而暴露于地球表面的沉积岩在相同时长内的³He流失量可达60%。不过，³He的绝对流失量大概率取决于行星际尘埃粒子内部的³He浓度分布，而该分布目前尚不明确。与此前提出的“直径50~100 µm的微流星体是地质古老沉积物中地外³He来源”的观点[Stuart F.M.、Harrop P.J.、Knott S.、Turner G., 1999. 南极微流星体中氦同位素的激光提取：He的来源及地外³He通量抵达地球的意义. 地球化学与宇宙化学学报, 63, 2653-2665, doi:10.1016/S0016-7037(99)00161-1]相悖，我们的筛分实验证实，直径大于50 µm的粒子所携带的³He至多仅占总量的20%。含³He粒子的粒径分布特征表明，沉积物中的地外³He记录的是行星际尘埃粒子通量，而非微流星体通量。