Accretion of the lower oceanic crust: Reconciling evidence of hydrothermal fluid fluxes with mineral cooling rates from ODP Hole 1256D, IODP Exp335

海洋地壳覆盖了地球表面的三分之二，并在板块构造循环中不断循环。新的洋壳是沿着沿着大洋中脊形成的，这是一条海底火山链，存在于两个构造板块之间的边界上，最终将在俯冲带返回地幔。今天产生的大部分海洋地壳都是在快速扩张的海脊上形成的，在那里，两个构造板块以每年80毫米以上的速度相互远离。在这些快速扩张的海脊上形成的洋壳具有相对简单的地层。上地壳，即洋壳顶部1-2公里（总厚度约6- 7公里），由喷发的熔岩流组成，覆盖在侵入性的补给岩脉上。下地壳（约5公里厚）由称为辉长岩的深成岩石组成，辉长岩代表结晶岩浆房。产生下地壳（约5公里厚）的岩浆过程还没有得到很好的理解，主要是由于下地壳的采样稀疏。通过对蛇绿岩（现在侵位在大陆上的洋壳碎片）的研究，提出了两种下地壳增生的端元模型，即“辉长岩冰川”和“席状岩床”模型。它们的主要区别在于熔体侵入和结晶的位置。熔体内的热量的去除必须在脊轴的几公里内有效地实现，并对吸积模型的可行性提出了严格的热约束。来自下地壳的热量可以通过传导到周围的岩石中以及通过加热渗入地壳并将热量对流到海底的海水热液来提取。热液流体通过流体-岩石化学反应记录在火成岩中，产生新的次生矿物。这些矿物的存在既取代了原生火成岩矿物，也填充了裂缝，形成热液脉。这两种吸积模型的热可行性与洋壳中热液流体的规模和分布密切相关，多岩床模型要求下地壳中有广泛的热液冷却。从下地壳的完整部分回收的样品将为测试这些模型提供机会。上、下地壳之间的界面是下地壳岩浆热传递到上地壳对流热液的主要边界，称为传导边界层。在现代大洋地壳中，大洋钻探计划和综合大洋钻探计划仅在ODP/IODP 1256 D孔中对上大洋地壳和上/下地壳过渡区的完整剖面进行过一次采样，需要持续钻探约6个月才能到达该边界。在这个钻孔中，岩浆活动和热液过程的复杂相互作用记录在回收的火成岩中。在这项研究中，热液流体通量的大小在导电边界层将使用流体岩石反应的地球化学示踪剂计算。锶同位素是这一任务的理想选择，并已在几项研究中得到广泛和成功的应用。然后将这些结果组合成热模型，该模型将使用1256 D孔的岩浆观测作为边界条件。该模型将包括岩浆侵入传导边界层，并计算穿过边界的热通量，最终将用于测试吸积模型。

Ocean crust covers ~ two thirds of the Earth's surface and is constantly recycled through the plate tectonic cycle. New ocean crust is created along mid ocean ridges, a submarine chain of volcanoes that exist at the boundaries between two tectonic plates, and will eventually be returned to the mantle at subduction zones. Much of the ocean crust produced today is forming at fast spreading ridges, where the two tectonic plates are moving away from each other at rates >80 mm/yr. Ocean crust formed at these fast spreading ridges has a relatively simple stratigraphy. The upper crust, the top 1-2 km of ocean crust (total thickness ~ 6-7km), is composed of erupted lava flows that overlie intrusive feeder dikes. The lower crust (~5 km thick) is made up of plutonic rocks called gabbros that represent crystallised magma chambers. The magmatic processes that generate the lower crust (~5 km thick) are not well understood primarily due to the sparse sampling of the lower crust. From studies of ophiolites, pieces of the ocean crust that are now emplaced on the continents, two end member models for the accretion of the lower crust have been proposed, the "gabbro glacier" and "sheeted sills" models. They primarily differ in the location of melt intrusion and crystallisation. The removal of the heat within the melt has to be effectively achieved within a few kilometres of the ridge axis, and places strict thermal constraints on the feasibility of the accretion models. Heat from the lower crust can be extracted by conduction into the surrounding rock and by heating seawater-derived hydrothermal fluids that percolate into the crust and convect heat away to the seafloor. The hydrothermal fluids are recorded in the igneous rocks by fluid-rock chemical reactions that create new secondary minerals. These minerals are present both replacing primary igneous minerals and filling fractures to form hydrothermal veins. The thermal feasibility of the two accretion models is intimately linked to the magnitude and distribution of hydrothermal fluids in the ocean crust, with the multiple sills model requiring extensive hydrothermal cooling in the lower crust. Samples recovered from an intact section of the lower crust will provide opportunity to test these models. The interface between the upper and lower crust is the principal boundary over which magmatic heat from lower crust is transferred to the convecting hydrothermal fluids in the upper crust, and is called the conductive boundary layer. A complete section of upper ocean crust and the upper/lower crust transition has only been sampled once in modern ocean crust, in ODP/IODP Hole 1256D by the Ocean Drilling Program and Integrated Ocean Drilling Program and required ~6 months of continuous drilling to reach this boundary. In this borehole, the complex interplay of magmatism and hydrothermal processes are recorded in the igneous rocks recovered. In this study, the magnitude of hydrothermal fluid fluxes in the conductive boundary layer will be calculated using geochemical tracers of fluid rock reaction. Sr isotope are ideal for this task and have been used extensively and successfully in several studies. These results will then be combined into a thermal model that will use the magmatic observation from Hole 1256D as boundary conditions. The model will include magmatic intrusions into the conductive boundary layer and calculate the heat flux across the boundary and ultimately will be used to test accretion models.