Volatile fluxes through the Big Bend section of the San Andreas Fault, California: Helium and carbon-dioxide systematics
Volatile fluxes through the Big Bend section of the San Andreas Fault, California: Helium and carbon-dioxide systematics
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DOI：
10.1016/j.chemgeo.2012.09.007
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发表时间：
2013-02
期刊：
影响因子：
3.9
通讯作者：
J. Kulongoski;D. Hilton;P. Barry;B. Esser;D. Hillegonds;K. Belitz
中科院分区：
文献类型：
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作者：
J. Kulongoski;D. Hilton;P. Barry;B. Esser;D. Hillegonds;K. Belitz
为了调查挥发物的来源及其与圣安德烈亚斯断层系统 (SAFS) 的关系，从南加州 SAFS 大弯段附近的井中采集了 18 个地下水样本，并分析了氦和碳的丰度和同位素。经过气泡夹带校正后，4He 浓度在 4.15 到 62.7 (×10−8) cm3STPg−1H2O 之间变化。3He/4He 比率在 0.09 到 3.52 RA 之间变化（其中 RA=air3He/4He），与样品中高达 44% 的地幔氦含量一致。对 10 个样品的子集进行了主要挥发相 (CO2) 的分析，即地幔-地壳系统中氦的假设载流相：CO2/3He 比率从 0.614 到 142 (×1011)，δ13C (CO2) 值从 -21.5 到 -11.9‰ 变化（相对于 PDB）。3He/4He 比率和 CO2 浓度最高位于 SAFS 附近的 Mil Potrero 和 Cuddy 山谷的油井中。升高的 3He/4He 比率被解释为地幔挥发通量通过 SAFS 被地壳中产生的放射性 He 稀释的结果。具有最高 3He/4He 比率的样品也具有最低的 CO2/3He 比率。氦同位素组合、He-CO2 元素关系和地下水挥发物的 δ13C (CO2) 值揭示了地幔和深层地壳（变质）流体起源的混合物。流体在高静水压力下流入地震带可能会引起断层破裂，并将挥发物转移到浅层地壳中。我们计算出沿 SAFS 的向上流体流速为 147mma−1，比之前的估计高出 37 倍（Kennedy 等，1997）。然而，利用新确定的 SAFS 特征，我们计算出沿 SAFS 的 3He 总通量为 7.4×103cm3STPa−1(0.33mol3Hea−1)，CO2 通量为 1.5×1013cm3STPa−1(6.6×108mola−1)，约为先前估计的 1%。沿着SAFS大弯曲部分的较低通量表明，仅地幔挥发物的通量不足以在地震带中引起超静水压力；然而，结果表明地壳（变质）流体是 CO2 挥发预算的主要组成部分，这可能代表了 SAFS 中断层弱化压力所需的额外通量。
To investigate the source of volatiles and their relationship to the San Andreas Fault System (SAFS), 18 groundwater samples were collected from wells near the Big Bend section of the SAFS in southern California and analyzed for helium and carbon abundance and isotopes. Concentrations of4He, corrected for air-bubble entrainment, vary from 4.15 to 62.7 (×10−8) cm3STPg−1H2O.3He/4He ratios vary from 0.09 to 3.52 RA(where RA=air3He/4He), consistent with up to 44% mantle helium in samples. A subset of 10 samples was analyzed for the major volatile phase (CO2) — the hypothesized carrier phase of the helium in the mantle–crust system: CO2/3He ratios vary from 0.614 to 142 (×1011), and δ13C (CO2) values vary from −21.5 to −11.9‰ (vs. PDB).3He/4He ratios and CO2concentrations are highest in the wells located in the Mil Potrero and Cuddy valleys adjacent to the SAFS. The elevated3He/4He ratios are interpreted to be a consequence of a mantle volatile flux though the SAFS diluted by radiogenic He produced in the crust. Samples with the highest3He/4He ratios also had the lowest CO2/3He ratios. The combined helium isotope, He–CO2elemental relationships, and δ13C (CO2) values of the groundwater volatiles reveal a mixture of mantle and deep crustal (metamorphic) fluid origins. The flux of fluids into the seismogenic zone at high hydrostatic pressure may cause fault rupture, and transfer volatiles into the shallow crust. We calculate an upward fluid flow rate of 147mma−1along the SAFS, up to 37 times higher than previous estimates (Kennedy et al., 1997). However, using newly identified characteristics of the SAFS, we calculate a total flux of3He along the SAFS of 7.4×103cm3STPa−1(0.33mol3Hea−1), and a CO2flux of 1.5×1013cm3STPa−1(6.6×108mola−1), ~1% of previous estimates. Lower fluxes along the Big Bend section of the SAFS suggest that the flux of mantle volatiles alone is insufficient to cause the super hydrostatic pressure in the seismogenic zone; however, results identify crustal (metamorphic) fluids as a major component of the CO2volatile budget, which may represent the additional flux necessary for fault weakening pressure in the SAFS.