Wehrlites in the ICDP OmanDP drill cores: a key to understand the nature of the hydrous magmatism in subduction zone initiation settings

全球的俯冲过程已经被很好地理解了，但是对于俯冲是如何开始的却知之甚少。对俯冲带起始岩浆过程的认识主要是基于洋壳火山层序的岩石。在这里，众所周知，来自俯冲板块流体的水是驱动剂，有可能产生典型的含水岩浆系列（弧前玄武岩、安山岩、博宁岩）。相比之下，从相应的深成岩中几乎一无所知：从分异熔体中提取的分异晶体部分。它们通常形成层状堆积层序，表现在发育的俯冲带下的洋壳辉长岩部分，以蛇绿岩闻名。阿曼蛇绿岩是在陆地俯冲开始时形成的快速扩张海洋地壳的最佳模拟物，这种深层堆积序列暴露得很好，很容易接近，特别是在ICDP阿曼钻井项目（OmanDP）框架内回收的钻芯出现之后。在弧形成初期的岩石成因中，显示富水岩浆存在的关键岩石是由橄榄石和斜辉石组成的斜长石；斜长石的结晶，正如“正常”海洋地壳中辉长岩的典型共熔结晶一样，由于高水活动的存在而被延迟或完全抑制。这个为期一年的博士后项目重点研究在大多数OmanDP地壳岩心辉长岩序列中作为相干层出现的韦氏岩屑。我们计划开始一项综合的岩石层矿物相的主要和微量元素分析项目。所获得的分析数据将使我们能够建立一个详细的岩石学模型，研究在演化俯冲带之上扩张体系下洋壳增生过程中含水morb型体系的岩浆分异是如何进行的。结合岩石学建模和相关相图的应用，wehlitic相的主要元素组成将揭示不同水活度的结晶路径。为了估计存在间隙斜长石的微长石的结晶温度，我们将采用斜长石中ree -斜辉石测温法。斜辉石的微量元素分析将使我们能够通过应用分配系数来计算平衡熔体的微量元素含量，非常适合与火山序列的成分进行比较。我们的目的是分析岩屑中斜辉石的流体可动微量元素，以模拟在这种特殊的初始俯冲带起始环境下流体可动元素的富集。

The global process of subduction is well understood, but relatively little is known about how subduction starts. The progress in understanding the magmatic processes during subduction zone initiation is mostly based on rocks from the volcanic sequence of the oceanic crust. Here, it is well-known that water, derived from fluids of the subducting slab, is the driving agent with the potential to produce characteristic hydrous magmatic series (fore-arc basalts, andesites, boninites). In contrast, practically nothing is known from the corresponding plutonic rocks: the portion of fractionated crystals extracted from differentiating melts. These often form layered cumulate sequences, which are manifested in the gabbroic part of the oceanic crust under developing subduction zones, well-known from ophiolites. In the Oman ophiolite, the best-investigated analogue of fast-spreading oceanic crust generated during the initiation of subduction on land, such deep cumulate sequences are well exposed and easily accessible, especially since the advent of drill cores recovered in the frame of the ICDP Oman Drilling Project (OmanDP). Key rocks in the plutonic crust showing the presence of water-rich magmas during the petrogenesis of initial arc formation, are wehrlites, consisting of mainly olivine and clinopyroxene: the crystallization of plagioclase, as it is typical for the cotectic crystallization of gabbros in the "normal" oceanic crust, is delayed or completely suppressed due to the presence of high water activities. This one-year postdoc project focuses to investigate systematically wehrlites occurring as coherent layers within the gabbro sequences of most OmanDP crustal drill cores. We plan to start a comprehensive analysis program of mineral phases in the wehrlites for major and trace elements. The analytical data to be obtained will enable us to establish a detailed petrologic model, how the magmatic differentiation in hydrous MORB-type systems during the accretion of the lower ocean crust in spreading systems above evolving subduction zones proceeds. Major element compositions of the wehrlitic phases in combination with petrological modeling and application of relevant phase diagrams will reveal crystallization paths for variable water activities. For estimating the crystallization temperatures of those wehrlites where interstitial plagioclase is present, we will apply REE-in-plagioclase-clinopyroxene thermometry. Trace element analyses in clinopyroxene will enable us to calculate the trace element contents of equilibrium melts by applying partioning coefficients, well-suited for comparison with the compositions from the volcanic sequence. We aim to analyse clinopyroxene in wehrlites for fluid-mobile trace elements for modeling the fluid mobile element enrichment in such a special environment of initial subduction zone initiation.