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

东太平洋海隆赫斯深裂谷下地壳熔体/岩石相互作用和分异结晶：分析与实验相结合的研究

• 批准号: 279047586
• 负责人: Professor Dr. Jürgen Koepke
• 金额: --
• 依托单位: Institut für Mineralogie
• 依托单位国家: 德国
• 项目类别: Infrastructure Priority Programmes
• 财政年份: 2015
• 资助国家: 德国
• 起止时间: 2014-12-31 至 2019-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/279047586?language=en
• 关键词: Melt; rock; interaction; fractional; crystallization

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.

综合海洋发现计划（IODP）框架下海洋地壳钻探研究的目的是了解海洋地壳的构造过程。在赤道太平洋东太平洋隆起的赫斯深裂谷（Hess Deep Rift）， IODP 345考考队在IODP历史上首次钻取了来自快速扩张脊的深部深部深部地壳的一致岩心，这有可能回答关于快速扩张的下大洋地壳演化和增生的长期争论的问题。钻取的岩心显示出壮观的模态分层，首次验证了蛇绿岩模型在解释EPR地壳中的应用。这些原始下深部岩体的大块岩石组成，结合以往浅层岩体和玄武岩的地球化学数据，提供了在快速扩张脊形成的地壳的平均组成的第一个可靠的估计，对应于上（玄武岩）和下（辉长岩）地壳区分的母熔融体。根据345考察队获得的这些新发现，以及345考察队获得的独特样品的可用性，本项目旨在了解快速扩张脊下地壳岩浆演化的确切机制，重点研究初级晶体堆积框架的形成及其在浅地幔和地壳中发生的熔融/岩石相互作用的改变。本项目采用多方法方法：主题(1)侧重于天然岩石的岩石学/地球化学，通过将微分析工具应用于Hess Deep钻探的辉长岩的矿物相，重点关注正辉石的作用和熔体/岩石相互作用的作用。我们对发生在原始辉长岩中的斜辉石暗晶的初步微量分析工作表明，我们观察到一些微量元素有明显的分带，这是不能用分馏结晶来解释的。主题(2)是一个实验项目，模拟了在Hess Deep大块地壳平均对应的系统中熔体/岩石相互作用以及平衡结晶和接近完美的分数结晶，该系统可视为Hess Deep地壳的母熔体。将实验数据与赫斯深辉长岩进行对比，将有助于约束赫斯深地壳的岩浆演化、分异/分馏的确切机制以及熔体/岩石相互作用的作用。综合结果的含义有可能验证/拒绝关于快速扩展的洋脊的地壳增生过程的长期假设，并将推进我们对熔体如何从地幔通过下地壳运输的理解。