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(综合海洋发现方案)框架内对下部洋壳进行钻探研究的目的是了解洋壳的形成过程。在东太平洋隆起的赤道太平洋赫斯深裂谷，IODP 345探险队在IODP历史上首次从快速扩张的海脊钻探了下部深成地壳的相干岩芯，有可能回答关于快速扩张的下部洋壳的演化和沉积的长期争论的问题。钻探的岩心显示了壮观的模式分层，首次验证了蛇绿岩模型用于解释EPR地壳的有效性。这些原始下部深成岩的块状岩石成分，结合以前浅层深成岩和玄武岩的地球化学数据，首次可靠地估计了在快速扩张的山脊上形成的地壳的平均成分，与上(玄武岩)和下(辉长岩)地壳区分的母质熔体相对应。利用探险345获得的这些新发现，并利用探险345发现的独特样品，该项目旨在了解下地壳从快速扩张的脊状岩浆演化的确切机制，重点是浅地幔和地壳中发生的熔体/岩石相互作用对原生晶体堆积骨架的形成及其变质作用。该项目采用多方法方法：专题(1)侧重于天然岩石的岩石学/地球化学，方法是利用显微分析工具对在Hess Deep钻探的辉长岩的矿物相进行分析，重点是斜方辉石的作用和熔体/岩石相互作用的作用。我们对原始辉长岩中的单斜辉石橄榄晶进行了初步的显微分析工作，在其中观察到了一些不能用分离结晶解释的微量元素的显着分带，表明了后者的存在。主题(2)是一个实验项目，其方法是模拟熔体/岩石相互作用以及Hess深部块状地壳平均体系中的平衡结晶和接近完美的分离结晶，该体系可被视为Hess深部地壳的母体熔体。实验数据与天然赫斯深辉长岩的对比，将使我们能够约束赫斯深部地壳的岩浆演化，确切的分异/分馏机制，以及熔体/岩石相互作用的作用。综合结果的影响有可能验证/拒绝关于快速扩张的大洋脊处地壳增长过程的长期假说，并将促进我们对熔体如何从地幔通过下地壳输送的理解。

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.