Heat flow determinations of ODP Hole 111-504B

Hole 504B in the eastern equatorial Pacific has been the focus of five scientific drilling expeditions since it was first drilled in 1979. During these five legs, a series of temperature logs has been obtained over a time span of almost 8 yr, documenting the geothermal and hydrologic state of the oceanic crust in this region. Immediately following reentry at the onset of ODP Leg 111 operations, a high-resolution temperature probe was lowered into the borehole and a precise record of temperature vs. depth in Hole 504B was recorded down to 1300 mbsf. As was observed during previous legs, the temperature gradient in the upper 400 m was reduced, indicating that downhole flow of cool ocean waters through the casing continued, though at a diminished rate. As subhydrostatic pressures in the upper basement have gradually diminished, the volume of flow has decayed from an estimated 6000-7000 L/hr in late 1979 to about 80 L/hr during Leg 111. At depths below 480 mbsf, a predominantly conductive heat transfer environment enabled the temperature gradient log to be analyzed with respect to lithology on both fine and broad scales. Anomalies in the gradient log in the cased section through the sedimentary column were found to correspond to biostratigraphic age markers and/or sharp changes in sediment composition and texture. Broad variations in temperature gradient within the basement correlated with large-scale porosity trends. Conductive heatflow estimates depict a systematic reduction with depth, ranging from approximately 196 mW/m**2 in the sediments to 120 ± 17 mW/m**2 at 1300 mbsf. Possible causes for this observation were examined from several perspectives, but none was suitably convincing. A fluid instability analysis indicated the likely existence of convection cells within the borehole and substantiated the hypothesis of mixing within the borehole postulated from isotopic and chemical studies of borehole waters. However, such mixing of borehole fluids does not provide an adequate explanation for the heatflow variations, and the disparity between surficial and deep values of heat flow remains unresolved.东赤道太平洋的504B孔自1979年首次钻探以来，已成为五次科学钻探航次的重点研究对象。在这五轮钻探期间，研究人员在近8年的时间跨度内获取了一系列温度测井（temperature logs），记录了该区域洋壳的地热与水文状态。在大洋钻探计划（Ocean Drilling Program, ODP）第111航次作业启动时重新下入钻孔后，研究人员随即下放高分辨率温度探头（high-resolution temperature probe）至井内，获取了504B孔温度随深度变化的精准记录，测量深度达1300 mbsf（meters below seafloor，海底以下米数）。 正如此前航次所观测到的，上部400米范围内的温度梯度（temperature gradient）有所降低，表明低温海水沿套管向下的渗流仍在持续，不过流速已有所减缓。随着上部基底的亚静水压力（subhydrostatic pressures）逐渐消散，渗流量已从1979年末估算的6000~7000升/小时衰减至大洋钻探计划第111航次期间的约80升/小时。 在480 mbsf以下的深度区间，热传递以传导为主，因此可针对岩性（lithology）的精细与宏观尺度分别分析温度梯度测井数据。研究发现，套管段沉积柱中的梯度测井异常与生物地层年龄标志层（biostratigraphic age markers）、以及沉积物组成与结构（sedimentary composition and texture）的突变均存在对应关系；基底内温度梯度的大范围变化则与大尺度孔隙度趋势（porosity trends）相关联。 传导热流（conductive heatflow）估算值随深度呈现系统性降低，从沉积物中的约196 mW/m²变化至1300 mbsf处的120 ±17 mW/m²。研究从多个视角探讨了这一现象的潜在成因，但尚未找到令人信服的合理解释。流体不稳定性分析（fluid instability analysis）表明，井内可能存在对流单元（convection cells），同时证实了基于井内水样同位素与化学研究（isotopic and chemical studies）提出的井内混合假说。然而，井内流体的此类混合并不能充分解释热流变化，地表与深部热流值之间的差异仍未得到解决。