Scientific Drilling into the San Andreas Fault: Fluid Transport Analysis - Rare Gas "Ages" of Matrix Porosity Fluids in Drill Core

科学钻探圣安德烈亚斯断层：流体输送分析——岩心基质孔隙流体的稀有气体“年龄”主要研究者martin Stute， Peter Schlosser， Gisela Winckler摘要目前对沿着圣安德烈亚斯断裂带（SAFZ）的低摩擦滑动的解释是高压下流体的存在。由于流体输运在时间和空间上都有很大的变化，因此在断裂带钻孔中对压力梯度和渗透率的原位测量将建立当前的条件，但几乎不能表明断层加压周期（几十年）的时间间隔内的压力/输运模式。哥伦比亚大学Lamont-Doherty地球观测站的研究人员进行的这项研究测试了一种方法，该方法利用从SAFZ深处获得的岩心中回收的孔隙流体中稀有气体浓度的分布来确定流体进出断裂带的流动。铀和钍是衰变系列元素，岩石中的钾产生4He和40Ar以及致核（21,22ne）和致裂（132,134,136Xe）稀有气体。由稀有气体浓度得出的流体“年龄”取决于生产速率和释放到孔隙流体中的速率。可以对孔隙流体中这些气体的垂直和水平剖面进行建模，以确定流体的流动速度和方向。用于这些分析的样品是通过对在EARTHSCOPE/SAFOD项目框架内从SAFZ的不同深度回收的新钻芯进行亚采样获得的。由于岩石中稀有气体的损失发生在数小时到数天的时间尺度上，因此需要对岩心进行二次采样以去除外层（已经丢失稀有气体），然后将内部部分放置在现场的高真空容器中。在数周至数月的时间内释放到顶空的稀有气体通过质谱法和总孔隙流体重量来确定。测量的一整套稀有气体同位素（3,4he, 20,21,22ne, 36,38,40ar, 84,86kr以及129,131,132,134,136Xe）用于将非大气（在岩石中产生并积聚在孔隙流体中）与大气稀有气体组分分离。该研究还研究了岩石中稀有气体的浓度和产生它们的元素的浓度，以确定基质孔隙流体中过量稀有气体丰度的源函数。上面描述的一些方法已经在浅层含水层研究和一些深层钻探中得到了大量应用，但它从未在深层科学钻探的背景下得到全面应用。这项研究将建立一种方法，为流体流动的来源、方向和大小提供关键信息，这些信息奠定了假设的基础，即超静流体压力是几十年时间尺度上沿SAFZ的低摩擦滑移的原因。这对了解SAFZ和地震预测具有广泛的意义，影响到加利福尼亚的数百万人。这项研究的更广泛影响包括培养一名研究生，并将这项研究的结果整合到哥伦比亚大学的地球科学课程中。这项工作也被纳入哥伦比亚大学危害与风险研究中心（CHRR）的研究和教育活动，该中心的使命是推进自然和环境危害的预测科学，并将科学与危害风险评估和管理相结合。

0454514StuteScientific Drilling into the San Andreas Fault: Fluid Transport Analysis - Rare Gas "Ages" of Matrix Porosity Fluids in Drill CoreMartin Stute, Peter Schlosser, Gisela Winckler Principal InvestigatorsAbstractThe currently favored explanation for the low friction slip along the San Andreas Fault Zone (SAFZ) is the presence of fluids under very high pressure. Since fluid transport is subject to wide variation in both time and space, the in situ measurement of pressure gradients and permeability in a fault zone drill hole will establish the current conditions but provide little indication of pressure/transport patterns over the time interval for pressurization cycles of the fault (decades). This study conducted by researchers from the Lamont-Doherty Earth Observatory of Columbia University tests a methodology to determine the flow of fluids into/from the fault zone using the distribution of rare gas concentrations in pore fluids recovered from cores obtained at depth in the SAFZ. Uranium and Thorium decay-series-elements, and Potassium in rocks produce 4He and 40Ar as well as nucleogenic (21,22Ne) and fissiogenic (132,134,136Xe) rare gases. Fluid 'ages' derived from rare gas concentrations depend on the production rates and the release rates into the pore fluids. Vertical and horizontal profiles of these gases in pore fluids can be modeled to determine fluid flow velocities and directions. Samples for these analyses are being obtained by sub-sampling fresh drill core recovered in the framework of the EARTHSCOPE/SAFOD project from various depths in the SAFZ. Because rare gas loss from the rock occurs over a timescales of hours to days, drill core is sub-sampled to remove the outer rind (which has already lost rare gases) and the internal sections are then placed in high vacuum containers in the field. The rare gases released to the headspace over a period of weeks to months are determined by mass spectrometry and the total pore fluid by weight. The entire suite of measured noble gas isotopes (3,4He, 20, 21, 22Ne, 36, 38, 40Ar, 84,86Kr, and 129,131, 132, 134, 136Xe) is used to separate the non-atmospheric (produced in the rocks and accumulated in pore fluids) from the atmospheric noble gas components. The study also investigates the concentrations of rare gases in the rocks and of the elements producing them to determine the source function for excess rare gas abundances in matrix pore fluids. Some of the approaches described above have seen numerous applications in studies of shallow aquifers and in a few cases of deep drilling, but it has never been used in its entirety in the context of deep scientific drilling. This study will establish the methodology providing key information for the source, direction and magnitude of fluid flow that underlie the hypothesis that superhydrostatic fluid pressures are responsible for low friction slip along the SAFZ on timescales of decades. This has wide ranging implications for the understanding the SAFZ and earth quake prediction, affecting millions of people in California. Broader impacts of the study include the training of a graduate student, and the integration of results of this study in the Earth Science curriculum at Columbia University. This work is also integrated in the research and educational activities of Columbia's Center for Hazards and Risk Research (CHRR) whose mission it is to advance the predictive science of natural and environmental hazards and the integration of science with hazard risk assessment and management.