Geographical distribution of 3He/4He ratios and seismic tomography in Japan
Geographical distribution of 3He/4He ratios and seismic tomography in Japan
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DOI：
10.2343/geochemj.42.51
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发表时间：
2008-02
影响因子：
0.8
通讯作者：
Y. Sano;J. Nakajima
中科院分区：
文献类型：
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作者：
Y. Sano;J. Nakajima
编制了日本群岛天然气、火山流体和地下水的3He/4He比值(Ra)，并在俯冲带的构造格架中讨论了它们的地理分布，并精确确定了地震速度结构。在典型岛弧特征发育的日本东北部，垂直于海沟轴线的3He/4He比值、额弧的低Ra值和火山弧的高Ra值具有明显的反差。这可能反映了浅层地壳中是否存在高Ra的岩浆。作为原始氦的载体，源熔体可能在约150 km深的太平洋板块脱水形成于楔形地幔的低V区，并可沿板块向上、次平行流动，受S波速度扰动的很好约束。在日本西南部的中部地区和四国地区，除了距离火山前锋约100公里处发现了中Ra外，其余地区的Ra与日本东北部相似。在中部地区火山弧中观察到的高Ra可能与菲律宾海板块下方岩浆源区产生的地幔氦有关。四国地区的中Ra是由年轻板块的脱水作用解释的，具有中等的老化效应。这些特征与地震层析成像的结果再次一致。在日本西南部的近海地区，在额弧区观测到了异常高的Ra，该区域被称为“近海黑子”。由于与日本东北部相比，高Ra位于离火山前缘更宽的区域，菲律宾海板块下方产生的熔体可能会穿透到板块裂隙中，并可能通过上升流到达地壳浅层。
The 3He/4He ratios (Ra) of natural gas, volcanic fluid, and groundwater are compiled in the Japanese Islands and their geographical distributions are discussed in the tectonic frame work of subduction zone together with precisely-determined seismic velocity structures. In Northeastern (NE) Japan where typical island arc signatures are developed, there is a clear contrast of 3He/4He ratios perpendicular to the trench axis, low-Ra in the frontal arc and high-Ra in the volcanic arc. This may reflect the presence or absence of magma with high-Ra in the shallow crust. As a carrier of primordial helium, source melt may be generated in low-V zone of the wedge mantle by dehydration of Pacific slab at about 150 km deep and may flow upward sub-parallel to the slab, which is well constrained by S-wave velocity perturbation. In the Chugoku and Shikoku districts of Southwestern (SW) Japan, there is a geographical contrast of Ra similar to NE Japan except for the region at about 100 km from the volcanic front where medium-Ra was found. High-Ra observed in volcanic arc of the Chugoku district may be attributable to the mantle helium derived from the magma source generated below the Philippine Sea slab. Medium-Ra in the Shikoku district is explained by dehydration of the young slab with a moderate aging effect. These features are again consistent with the results of seismic tomography. In the Kinki district of SW Japan, anomalously high-Ra was observed in the frontal arc region that was called by “Kinki Spot”. Since the high-Ra is located at much wider region from the volcanic front when compared with NE Japan, the melt generated below the Philippine Sea slab may penetrate into the fissure of the slab tear and may arrive at the shallow crust by upwelling flow.