Metasomatism within the ocean crust
Metasomatism within the ocean crust
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DOI：
10.1007/978-3-642-28394-9_8
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发表时间：
2013
期刊：
影响因子：
--
通讯作者：
W. Bach;N. Jöns;F. Klein
中科院分区：
文献类型：
--
作者：
W. Bach;N. Jöns;F. Klein
从洋脊到海沟，洋壳与海水进行广泛的化学交换，这对确定海洋及其岩石基础的化学和同位素组成至关重要。虽然总的交换通量很大，但地壳岩石的一级交代变化一般较小（主要元素浓度的相对变化通常<10%）。剧烈的流体诱导交代质量转移仅限于流体通量非常高的区域，如热液上流区。在这些上升流带中，绿帘石化、绿泥石化和绢云母化是常见的，并且它们通常以硫化物矿化为特征，在热液喷口区下方形成显著的金属聚集。扩散交代作用在层状（辉长岩-粗玄岩-玄武质）地壳中处于次要地位，因为不同岩性之间的化学势差很小。然而，在非均质地壳（混合镁铁质-超镁铁质岩性）中，玄武岩岩性和橄榄岩之间的扩散质量转移非常常见。这些过程包括岩石圈地幔中辉长岩脉的rodingitization和接触辉长岩侵入体的蛇纹岩的滑石化。这些交代变化的驱动因素是粒间流体中主要溶质活动的强烈交叉接触差异。这些过程中的大多数发生在绿片岩相条件下，其中流体中二氧化硅和质子活性的差异最为明显。简单的地球化学反应路径模型为研究这些过程提供了强有力的工具。由于洋壳在其生命的大部分时间里都是水文活跃的，因此扩散交代带通常也受到流体流动的影响，因此流体引起的交代作用和岩性驱动的交代作用之间并不总是可能有明确的区别。不均匀的地壳通常沿着缓慢和超低速扩张的山脊，其中大部分的延伸是由断层（正断层和拆离断层）调节的。与均匀的镁铁质或超镁铁质岩石块体相比，镁铁质-超镁铁质接触面的水合程度更大，温度更高。因此，这些岩性接触在大岩性深度处变得机械薄弱，并且在大洋核杂岩的折返和隆起过程中易于捕获大部分应变。因此，交代作用在设置海洋岩石圈的流变学特性沿着缓慢的海洋扩张中心，其中-长度-包括全球洋中脊系统的一半。
From ridge to trench, the ocean crust undergoes extensive chemical exchange with seawater, which is critical in setting the chemical and isotopic composition of the oceans and their rocky foundation. Although the overall exchange fluxes are great, the first-order metasomatic changes of crustal rocks are generally minor (usually <10% relative change in major element concentrations). Drastic fluid-induced metasomatic mass transfers are limited to areas of very high fluid flux such as hydrothermal upflow zones. Epidotization, chloritization, and serizitization are common in these upflow zones, and they often feature replacive sulfide mineralization, forming significant metal accumulations below hydrothermal vent areas. Diffusional metasomatism is subordinate in layered (gabbroic-doleritic-basaltic) crust, because the chemical potential differences between the different lithologies are minor. In heterogeneous crust (mixed mafic-ultramafic lithologies), however, diffusional mass transfers between basaltic lithologies and peridotite are very common. These processes include rodingitization of gabbroic dikes in the lithospheric mantle and steatitization of serpentinites in contact to gabbroic intrusions. Drivers of these metasomatic changes are strong across-contact differences in the activities of major solutes in the intergranular fluids. Most of these processes take place under greenschist-facies conditions, where the differences in silica and proton activities in the fluids are most pronounced. Simple geochemical reaction path models provide a powerful tool for investigating these processes. Because the oceanic crust is hydrologically active throughout much of its lifetime, the diffusional metasomatic zones are commonly also affected by fluid flow, so that a clear distinction between fluid-induced and lithology-driven metasomatism is not always possible. Heterogeneous crust is common along slow and ultraslow spreading ridges, were much of the extension is accommodated by faulting (normal faults and detachment faults). Mafic-ultramafic contacts hydrate to greater extents and at higher temperatures than uniform mafic or ultramafic masses of rock. Hence, these lithologic contacts turn mechanically weak at great lithopheric depth and are prone to capture much of the strain during exhumation and uplift of oceanic core complexes. Metasomatism therefore plays a critical role in setting rheological properties of oceanic lithosphere along slow oceanic spreading centers, which – by length – comprise half of the global mid-ocean ridge system.