Geochemical constraints on formation fluid ages, hydrothermal heat flux, and crustal mass transport mechanisms at Cajon Pass
Geochemical constraints on formation fluid ages, hydrothermal heat flux, and crustal mass transport mechanisms at Cajon Pass
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Cajon Pass 地层流体年龄、热液热通量和地壳质量传输机制的地球化学约束
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发表时间：
1992
期刊：
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通讯作者：
W. B. Clarke
中科院分区：
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作者：
T. Torgersen;W. B. Clarke
在卡洪山口科学钻孔的钻杆测试期间，对收集的钻孔流体进行了He、Ne和氦同位素测量。这些流体代表了由荧光素示踪剂测量确定的地层流体的主要成分（大于90%）。结果表明，地壳衍生的放射性氦源与过量的He浓度高达100倍的空气饱和水的条件。钻孔沃茨的气体含量和化学成分反映了从几个离散的、已识别的和相互连接不良的裂缝系统进入钻孔的地层沃茨的变化。这些条件表明过量4 He的局部来源，使得4 He模型年龄为33，000至5×106年，可用于该深度的地层流体。这一限制表明，通过水热过程可以去除总热流的<7%的上限。这些数据为低应力断裂提供了地球化学支持。这些模型年龄进一步表明流体输送速度的有效垂直分量为0.04-6 cm yr-1。压头和现场渗透率测量表明流速为10 - 5 cm/年。为了满足这两个现存的条件，流体交换必须发生（至少一次）在过去的33,000至5×106年的流动条件下比目前可操作的快得多。因此，该区域的流体流动是不连续的，长时间的缓慢流动被非常快速的输送周期所打断。这样的结论是一致的，与当地和一般的观察变质地质，并建议深部地壳流体输送机制，是机械和/或构造控制。
He, Ne, and helium isotopic measurements have been made on borehole fluids collected during drill stem tests at the Cajon Pass scientific drill hole. These fluids represent a dominant component (greater than 90%) of the formation fluids as determined from fluoroscein tracer measurements. The results indicate a crustally derived radiogenic helium source with excess He concentrations up to 100x air-saturated water conditions. The gas content and the chemical composition of the borehole waters reflect a variation in the formation waters entering the hole from several discrete, identified, and poorly interconnected fracture systems. These conditions indicate a local source for the excess 4He which allows 4He model ages of 33,000 to 5×106 years to be placed on the formation fluids at this depth. This constraint suggests an upper limit of <7% of the total heat flow can be removed by hydrothermal processes. These data thus provide geochemical support for a low stress fault. These model ages further indicate an effective vertical component of the fluid transport velocity of the order of 0.04–6 cm yr−1. Pressure head and in situ permeability measurements indicate flow velocities of 10−5 cm yr−1. To satisfy both of these extant conditions, fluid exchange must have occurred (as least once) in the last ∼33,000 to 5×106 years under flow conditions considerably faster than presently operable. Fluid flow in this region is therefore discontinuous with long periods of slow flow interrupted by periods of very rapid transport. Such a conclusion is consistent with both local and general observations of metamorphic geology and suggests a deep crustal fluid transport mechanism that is mechanically and/or tectonically controlled.