Melt/rock interaction and fractional crystallization in the lower crust at Hess Deep Rift, East Pacific Rise: A combined analytical and experimental study

在IODP（综合大洋发现计划）框架下的下大洋地壳钻探研究的目的是了解大洋地壳的建造过程。在东太平洋隆起的赤道太平洋赫斯深裂谷，海洋大洋钻探项目第345次考察队在海洋大洋钻探项目历史上第一次从一个快速扩张的海脊钻取了深成下地壳较深部分的连贯岩心，有可能回答长期争论的关于快速扩张的下洋壳的演变和增生的问题。钻取的岩心显示出壮观的模态分层，首次验证了使用蛇绿岩模型解释EPR地壳。这些原始的下深圳湾岩体的岩石成分，结合以前的浅层次的深圳湾岩体和玄武岩的地球化学数据，提供了第一个可靠的估计地壳的平均成分形成在一个快速扩展的山脊，对应于父母的熔体从上（玄武岩）和下（辉长岩）地壳分化。利用345号考察队获得的这些新发现，以及345号考察队回收的独特样品，该项目旨在了解下地壳从快速扩张的海脊岩浆演化的确切机制，重点是原生晶体堆积框架的形成及其在浅地幔和地壳中发生的熔体/岩石相互作用的修改。该项目遵循多方法的方法：主题（1）侧重于天然岩石的岩石学/地球化学，通过使用微观分析工具应用于在赫斯深部钻探的辉长岩的矿物相，重点是斜方辉石的作用和熔体/岩石相互作用的作用。后者是由我们的初步微分析工作单斜辉石oikocrysts发生在原始辉长岩中，我们观察到一个显着的分区中的一些微量元素，不能解释的分离结晶。主题（2）是一个实验项目，模拟熔体/岩石相互作用以及平衡结晶和接近完美的分离结晶在一个系统中对应于赫斯深部地壳平均，这可以被视为母熔体赫斯深部地壳。与天然赫斯深辉长岩的实验数据的比较，将使我们能够约束赫斯深地壳的岩浆演化，分异/分馏的确切机制，熔体/岩石相互作用的作用。综合结果的影响有可能验证/拒绝长期的假设地壳增生过程中快速扩展的洋脊，并将推进我们的理解如何从地幔通过下地壳熔体运输。

Drilling studies of the lower oceanic crust in the framework of IODP (Integrated Ocean Discovery Program) have the objective of understanding the construction processes of the ocean crust. At the Hess Deep Rift in the equatorial Pacific at the East Pacific Rise (EPR), IODP Expedition 345 drilled, for the first time in the history of IODP, coherent cores of the deeper part of the lower, plutonic crust from a fast-spreading ridge, with the potential to answer long-debated questions on the evolution and accretion of fast-spread lower oceanic crust. The drilled cores show spectacular modal layering, validating for the first time the use of the ophiolite model for interpreting EPR crust. The bulk rock compositions of these primitive lower plutonics, in combination with previous geochemical data for shallow-level plutonics and basalts, provides the first reliable estimate of the average composition of crust formed at a fast-spreading ridge, corresponding to that parental melt from which the upper (basaltic) and lower (gabbroic) crust differentiate. With these novel findings obtained by Expedition 345, and with the availability of the unique samples recovered by Expedition 345, this project is aimed to understand the exact mechanism of the magmatic evolution of the lower crust from fast-spreading ridges, focusing both on the formation of the primary crystal cumulate framework and its modification by melt/rock interactions occurring in the shallow mantle and in the crust. This projects follows a multimethodical approach: Theme (1) focusses on the petrology/geochemistry of the natural rocks by using microanalytical tools to be applied to the mineral phases of the gabbroic rocks drilled at Hess Deep, with focus on the role of orthopyroxene and on the role of melt/rock interaction. The latter is indicated by our preliminary microanalytical work on clinopyroxene oikocrysts occurring in primitive gabbros in which we observed a significant zoning in some trace elements which cannot be explained by fractional crystallization. Theme (2) is an experimental project with the approach to simulate both melt/rock interaction as well as equilibrium crystallization and near-perfect fractional crystallization in a system corresponding to Hess Deep bulk crust average, which can be regarded as the parental melt for the crust at Hess Deep. The comparison of the experimental data with the natural Hess Deep gabbros will enable us to constrain the magmatic evolution of the Hess Deep crust, the exact mechanism of differentiation/fractionation, and the role of melt/rock interaction. The implications of the combined results have the potential to verify/reject long-lasting hypotheses on crustal accretionary processes at fast-spreading oceanic ridges and will advance our understanding how melts are transported from the mantle through the lower crust.