Assessment of Helium Isotopes near the Japan Trench 5 Years after the 2011 Tohoku-Oki Earthquake
Assessment of Helium Isotopes near the Japan Trench 5 Years after the 2011 Tohoku-Oki Earthquake
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DOI：
10.1021/acsearthspacechem.8b00190
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发表时间：
2019-04-01
影响因子：
3.4
通讯作者：
Sano, Yuji
中科院分区：
文献类型：
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作者：
Escobar, Ma Teresa;Takahata, Naoto;Sano, Yuji
氦同位素比值是幔源流体的重要示踪剂。自然界中不同的储层具有不同的氦同位素特征，这使得有可能识别各种来源及其混合。在2011年东北冲地震发生5年后，我们继续通过测量氦同位素来研究震中附近可能的流体喷发。我们于2016年在R/V Shinsei Maru上的日本海沟向陆坡采集了海水和沉积物样本。使用多芯取样器收集沉积物。使用CTD玫瑰花系统从不同深度获得海水样品。通过离心从沉积物中提取孔隙水。然后将来自每个样品的气体从溶液中析出，并引入配备有活性炭捕集器和Ti吸气剂的内部纯化系统中。氦同位素是用惰性气体质谱仪测量的。我们的数据显示，海底海水样品中的过量He-3比地震后测量的氦同位素比值所指示的要少，这表明没有高压流体导致先前观察到的过量He-3。此外，孔隙沃茨中He-3/He-4和Ne-20/He-4的比值表明，可能存在两种来源的混合：太平洋深层海水和深层流体。孔隙水氦同位素分布遵循非稳态扩散模型。这种观察的一个可能原因是上腔的再活动。30厘米的沉积物在地震期间，这可能改变了在浅沉积物孔隙沃茨的氦同位素剖面。
The helium isotope ratio is an important tracer of mantle-derived fluids. Different reservoirs in nature have distinct helium isotope signatures that make it possible to identify various sources and their mixing. At 5 years after the 2011 Tohoku-Oki earthquake, we continue to investigate possible fluid venting near the epicenter by measuring helium isotopes. We collected seawater and sediment samples in 2016 at the landward slope of the Japan Trench onboard R/V Shinsei Maru. Sediments were collected using a multiple corer. Seawater samples were obtained from different depths using a CTD rosette system. Pore water was extracted from sediments via centrifugation. Gas from each sample was then exsolved from solution and introduced into the in-house purification system equipped with charcoal traps and Ti getter. Helium isotopes were measured using a noble gas mass spectrometer. Our data show less excess He-3 in bottom seawater samples than indicated by helium isotope ratios measured after the earthquake, which suggests the absence of high-pressure fluids that contributed to excess He-3 observed previously. Furthermore, He-3/He-4 and Ne-20/He-4 in the pore waters show possible mixing between two sources: Pacific deep seawater and deep fluids. The pore water helium isotope profile follows a non-steady-state diffusion model. One possible reason for this observation is remobilization of the upper ca. 30 cm of sediments during the earthquake, which might have altered the helium isotope profile in the shallow sediment pore waters.