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.

为深入理解行星际尘埃粒子（interplanetary dust particles, IDPs）的组成、氦扩散特征以及对地外氦-3（extraterrestrial 3He）长期留存起到关键作用的粒径分布特征，我们对年龄跨度为0.5 Ma至约90 Myr的远洋黏土开展了浸出实验（leaching experiments）、阶梯加热实验（stepped heating experiments）与筛分实验（sieving experiments）。浸出实验结果表明，地质古老沉积物中3He的宿主相既非有机质，也非金刚石、石墨、氧化铝（Al₂O₃）、碳化硅（SiC）这类难熔相，而是与地外硅酸盐、铁镍硫化物（Fe-Ni sulfides），或许还有磁铁矿（magnetite）相吻合。阶梯加热实验显示，远洋黏土的磁组分与非磁组分的3He释放谱线极为相似。由于氦扩散大概率受矿物化学组成与结构调控，该阶梯加热结果指向单一载体相：该相可能为磁铁矿，或更大概率是与磁铁矿伴生的物相。进一步的分步脱气实验表明，在海底温度条件下，经过50 Myr后约20%的3He会通过扩散流失；而在地表暴露相同时长的沉积岩，其3He流失量可达60%。不过，3He流失的绝对量级大概率取决于IDPs内部的3He浓度分布，而目前该分布尚未被充分认知。与此前提出的「直径50~100 µm的微流星体（micrometeorites）是地质古老沉积物中地外3He来源」这一观点相反[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萃取：氦的来源及地外3He通量抵达地球的启示. 地球化学与宇宙化学学报（Geochimica et Cosmochimica Acta）, 63, 2653-2665, doi:10.1016/S0016-7037(99)00161-1]，我们的筛分实验证实，直径大于50 µm的颗粒所携带的3He至多仅占总量的20%。含3He颗粒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。