Chemical and thermal constraints on focussed fluid flow in the lower oceanic crust
Chemical and thermal constraints on focussed fluid flow in the lower oceanic crust
复制标题
下洋壳集中流体流动的化学和热约束
DOI：
10.2475/06.2006.01
复制
发表时间：
2006
影响因子：
2.9
通讯作者：
R. N. Wilson
中科院分区：
文献类型：
--
作者：
L. A. Coogan;K. A. Howard;K. Gillis;M. Bickle;H. Chapman;A. Boyce;G. Jenkin;R. N. Wilson
从下洋壳的脊轴附近的热提取的机制是不受约束的，尽管它的重要性，了解深成复合体的吸积过程和与脊热液系统的质量通量。我们研究了阿曼蛇绿岩深成杂岩中的流体集中区在洋壳近轴冷却中的作用。在航空照片上发现的线性体，出现在101公里的间距，显示出广泛的热液流体流经100至50米宽的区域的证据。这些地区的流体流动开始于约800°C，并至少持续到较低的绿片岩相。锶同位素分析表明，通过这些区域的流体通量是足够的运输交代前从席状岩墙复杂的基础接近莫霍面。计算出的将交代前缘输送通过集中流体流动区的最小流体通量为101 x108 kgm−2。钙从橄榄石到单斜辉石的扩散交换模型表明，在集中流体流动区附近的冷却速率增强。从增强的冷却速率估计的热通量大致一致的流体通量从建模的Sr-同位素组成的样品从集中的流体流动区。流体和热通量的独立估计的组合，例如这些，可以提供比单独使用的任何一种方法更严格的约束的热历史。我们的研究结果表明，集中的流体流动可能在下大洋地壳的冷却中发挥了重要作用。下洋壳中显著的集中流体流动对预测大洋中脊热液环流相关的总质量通量具有重要意义。这是因为流过通道的流体在比流过岩体的流体更小的流体通量下变成化学岩石缓冲。因此，如果集中的流体流动是下洋壳热量损失的一个重要机制，那么从海脊热液系统流入海洋的化学通量可能比目前认为的要小。
The mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems. We have investigated the role of zones of focussed fluid flow in the plutonic complex of the Oman ophiolite in the near-axis cooling of the oceanic crust. Lineaments identified on aerial photographs, that occur at ∼1 km spacing, show evidence for extensive hydrothermal fluid flow through regions ∼10 to 50 m wide. Fluid flow is initiated in these regions at ∼800°C and continues at least into the lower greenschist facies. Strontium-isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the Moho. Computed minimum fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are ∼1x108 kgm−2. Modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones. Heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the Sr-isotopic composition of samples from the focussed fluid flow zones. The combination of independent estimates of the fluid and heat fluxes, such as these, can provide more rigorous constraints on the thermal history than either approach used in isolation. Our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust. Significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid-ocean ridges. This is because fluids flowing through channels become chemically rock-buffered at smaller fluid fluxes than those flowing pervasively through a rock mass. Thus, if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought.