A helium isotope cross‐section study through the San Andreas Fault at seismogenic depths
A helium isotope cross‐section study through the San Andreas Fault at seismogenic depths
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DOI：
10.1029/2006gc001388
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发表时间：
2007-01
期刊：
影响因子：
3.7
通讯作者：
T. Wiersberg;J. Erzinger
中科院分区：
文献类型：
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作者：
T. Wiersberg;J. Erzinger
对圣安德烈亚斯深部断层观测站(SAFOD)主孔116~3943m深度的19个泥石气样品进行了惰性气体同位素分析，分析了孕震深度流体的成因和空间变异性。测得的3He/4He比值在0.40~1.02Ra之间(Ra为大气3He/4He比值1.39×10−6)，4He/20Ne值在0.33~4.92之间，揭示了总氦的三个组分的混合物：(1)大气氦，(2)具有地壳特征的氦，(3)来自地幔的氦。空气校正后的~3He/~4He比值在0.2Ra~0.9Ra之间。2117-3196m深度的样品具有相对恒定的氦同位素组成(0.35-0.46Ra)，表明该剖面太平洋板块中∼有5%的氦来自地幔。在太平洋板块向北美板块过渡的过程中，地幔氦的贡献略有增加，在北美板块(低于∼3500m井深)达到最大值∼12%。根据我们的观察，我们认为圣安德烈亚斯断层对更深层次的流体流动起到了作用，但更多来自地幔的流体通过位于圣安德烈亚斯断裂带(SAFZ)北美板块上的其他更具渗透性的断层上升。浅层流体在北美板块可渗透围岩中的横向扩散可以解释3He/4He比值随距离SAF的增加而增加的现象。
We have analyzed noble gas isotopes in 19 mud gas samples from 116–3943 m borehole depth of the San Andreas Fault Observatory at Depth (SAFOD) Main Hole in the context of origin and spatial variability of fluids occurring at seismogenic depths. The measured 3He/4He ratios range between 0.40 and 1.02 Ra (Ra is the atmospheric 3He/4He ratio of 1.39 × 10−6), with 4He/20Ne values between 0.33 and 4.92, revealing a mixture of three components to the total helium: (1) atmospheric helium, (2) helium with a crustal signature, and (3) mantle‐derived helium. The air‐corrected 3He/4He ratios fall between 0.2 Ra and 0.9 Ra. Samples from the 2117–3196 m depth show a relatively constant helium isotope composition (0.35–0.46 Ra), indicating that ∼5% of the helium in this section the Pacific Plate is derived from the mantle. The contribution of mantle‐derived helium increases slightly in the transition from the Pacific Plate to the North American Plate and reaches maximal values of ∼12% on the North American Plate (below ∼3500 m borehole depth). On the basis of our observations, we suggest that the San Andreas Fault plays a role for fluid flux from greater depths, but higher amounts of mantle‐derived fluids rise up through other, more permeable faults, situated on the North American Plate of the San Andreas Fault Zone (SAFZ). Lateral fluid dispersion at shallow depths through permeable country rock of the North American Plate may explain the observed increase in 3He/4He ratios with increasing distance to the SAF.