Quantifying the thermal structure and hydrothermal fluid fluxes in the lower oceanic crust

新的洋壳是由海底火山活动沿着大洋中脊形成的。海水向下迁移到这个新的可渗透的地壳中，并随着它的循环而逐渐加热，因为深层存在岩浆。海水衍生的流体在地壳中迁移时会发生化学修饰，因为它们会与所经过的岩石发生反应。当这些流体离开海底时，它们富含金属和硫化氢，导致形成经济上重要的火山托管的大量硫化物矿床，并为独特的生物生态系统提供营养。海洋和洋壳之间的化学交换改变了两者的组成。在更广泛的地球系统内，这种交换在调节海洋的组成和进入俯冲带的输入方面发挥着重要作用，这影响了弧火山的组成，并促进了地幔的不均一性。 在我们对洋壳如何形成和热液改变的理解上，一个关键的缺口与位于海底下2公里处的深成层序有关。在大洋中脊及其附近，岩浆侵入与热液冷却和蚀变之间的相互作用控制着深成地壳的热和化学演化。为了接近这些关键岩石，我们利用了“构造窗口”，在那里，地壳下部的暴露被构造过程揭开了屋顶，从而完全绕过了上地壳。在太平洋有两个这样的构造窗口，赫斯和皮托迪普斯，这是全球仅有的这些关键深成岩被绘制和采样的位置。在过去的20年里，我参加了四次潜水器/水下机器人采样巡航，并领导了两次国际海洋钻探探险，为我的拟议研究带来了全球独一无二的样本套件。 在下一个赠款周期，我的研究小组将使用这个独特的样本套件来测试相互竞争的假设，即地壳是如何通过岩浆作用在快速扩张的大洋中脊形成的，随后又是通过热液流动和反应冷却和改变的。我们还将建立岩浆和热液对下地壳S和亲磷元素收支的控制的定量模式，以扩大我们对下地壳对这些元素的全球旋回的贡献的理解。

New ocean crust is built along mid-ocean ridges by submarine volcanic activity. Seawater migrates down into this new permeable crust, and is progressively heated as it circulates due the presence of magma at depth. Seawater-derived fluids are chemically modified as they migrate through the crust because they react with the rocks through which they are passing. When these fluids exit the seafloor they are enriched in metals and H2S, leading to the formation of economically important volcanic hosted massive sulfide deposits and providing nutrients for unique biological ecosystems. The chemical exchange between the oceans and the ocean crust acts to modify the composition of both. Within the broader earth system this exchange plays an important role in regulating the composition of the ocean and the inputs into subduction zones, which influences the composition of arc volcanoes and contributes to heterogeneity in the mantle. A critical gap in our understanding of how the ocean crust is built and hydrothermally altered is related to the plutonic sequence that resides >2 km beneath the seafloor. At and near mid-ocean ridges, the interplay between magmatic intrusion and hydrothermal cooling and alteration controls the thermal and chemical evolution of the plutonic crust. To gain access to these critical rocks we take advantage of “tectonic windows,” where lower crustal exposures were unroofed by tectonic processes, thus bypassing the upper crust entirely. In the Pacific Ocean there are two such tectonic windows, Hess and Pito Deeps, and these are the only locations globally where these critical plutonic rocks have been mapped and sampled. Over the last 20+ years, I have participated in four submersible/ROV sampling cruises and led two international ocean drilling expeditions, leading to a globally unique sample suite for my proposed research. Over the next grant cycle, my research group will use this unique sample suite to test competing hypotheses for how crust is built at fast-spreading mid-ocean ridges by magmatic processes and subsequently cooled and altered by hydrothermal flow and reaction. We will also build a quantitative model for the magmatic and hydrothermal controls on the S and chalcophile element budget of the lower crust to expand our understanding of how the lower crust contributes to the global cycling of these elements.