Understanding the crust-mantle transition from fast-spreading mid-ocean ridges: experiments and analytical studies using ICDP OmanDP drill core samples

2017/18年冬季，ICDP钻探倡议“阿曼钻探项目”(OmanDP)成功地穿透了阿曼蛇绿岩的古地壳-地幔过渡，该蛇绿岩被认为是世界上最大、暴露最好、研究最多的海底岩石圈。岩心长400米(CM1)和300米(CM2)，回收成功率接近100%，这使它们成为岩石学、岩石学和地球化学研究的独特宝贵资源，并确保在快速扩张的山脊系统的最高地幔和最低地壳之间的神秘过渡过程中获得相干剖面。通过阿曼古地壳的壳-地幔转换而钻取的剖面，提供了连贯的最低层状辉长岩、块状辉长岩、斜长花岗岩和无缝隙的榴辉岩的长段，可被视为了解岩浆过程和地壳深部增生模式的罗塞塔石碑，这是现代海洋迄今无法到达的深度窗口。拟议的项目侧重于采用两种不同方法的几个科学目标：(1)在一项实验研究中，我们旨在模拟与俯冲带在“干”(如东太平洋隆起)和“湿”条件(伊豆-博宁-马里亚纳弧型地壳)之间过渡时启动俯冲带的背景有关的母体MORB型系统中的平衡和分离结晶。在一个子项目中，我们的目标是量化尖晶石/铬铁矿和MORB熔体之间的微量元素分布。(2)通过对CM1和CM2岩心岩石的岩石学和地球化学研究，阐明壳幔转换过程中块状纯橄榄岩的形成机制，建立地壳下部的分异/分馏模式。这两种互补方法的结果将为深入理解快速扩张地壳底部在初始俯冲环境下的岩浆作用提供基础，并将为快速扩张洋壳吸积的地球动力学提供基础知识。

In winter 2017/18, the ICDP drilling initiative "Oman Drilling Project" (OmanDP) successfully penetrated the paleo-crust-mantle transition of the Oman ophiolite, which is regarded as the world’s largest, best-exposed, and most-studied section of subaerially exposed oceanic lithosphere. The cores are 400 m (CM1) and 300 m (CM2) long and were recovered with a nearly 100 % success rate, making them a uniquely valuable resource for petrographical, petrological, and geochemical study and ensuring access to coherent sections throughout the enigmatic transition between the uppermost mantle and the lowermost crust of fast-spreading ridge systems. The drilled transects through the crust-mantle transition of the Oman paleo-crust, provide coherent long sections of the lowermost layered gabbros, massive dunites, troctolites, and wehrlites without any gaps, and can be regarded as a Rosetta Stone for understanding the magmatic processes and the mode of deep crustal accretion, in a depth window that was not accessible in recent oceans so far. The proposed project focuses on several scientific targets following two different methodological approaches: (1) In an experimental study we aim to simulate equilibrium and fractional crystallization in a parental MORB-type system relevant for a setting of initiation of a subduction zone at the transition between "dry" (like East Pacific Rise) and "wet" conditions (Izu-Bonin-Mariana arc-type crust). In a subproject we aim to quantify the trace element distribution between spinel/chromite and MORB melt. (2) The second approach focus on petrological and geochemical investigations on the rocks from the CM1 and CM2 drill cores in order to clarify the mechanism of massive dunite formation in the crust-mantle transition, and establish the exact mode of differentiation/fractionation within the lowermost crust. The results of both complementary approaches will provide a basis for a deep understanding of the magmatic processes at the base of fast-spreading crust in an environment of initial subduction, and will provide fundamental knowledge about the geodynamics of accretion of fast-spreading oceanic crust.