Exposed ocean crust on Masirah Island, SE Oman: crustal accretion and melt evolution at a slow-spreading (Jurassic) mid-ocean ridge

传统的海洋地壳结构层饼模型不能解释最近从较慢的扩张速率的大洋中脊（莫尔）的观测。我们现在认识到，拆离断层有助于板块扩张，而火成地壳可能是不均匀的、薄的，甚至不存在。轴下地幔岩石圈对玄武岩熔体的生成、输运、储存和改性机制具有重要影响。对这些不同过程的相对重要性进行反卷积，并评估它们对地球上最丰富的岩浆类型MORB喷发的影响，是一个一级问题，但由于难以获得现代海洋中的完整地壳部分，因此几乎是棘手的。蛇绿岩，如塞梅尔（阿曼北部），是有疑问的好处，已形成的俯冲带以上的蔓延，因此在环境中，地幔熔融和熔体演化的影响，由含水流体。（图1），是一个从真正的开放海洋莫尔中挖出的海洋岩石圈碎片，因此，提供了一个突破性的机会，带来了重要的新的见解慢扩张岩石圈的结构和综合过程的MORB熔体生产和修改，从地幔到海底。Masirah与更著名的Semail蛇绿岩无关，它形成于侏罗纪冈瓦纳大陆裂谷期间原印度洋的（缓慢扩展的）莫尔，并于侏罗纪-古新世侵位于阿拉伯东南缘。20世纪70年代至90年代的背景调查显示，该岛由两个逆冲断层片构成，每个逆冲断层片都包含一个完整的洋壳序列，其特征是非常薄（约0.5公里）的深成地壳和不均匀的正常-富集-MORB玄武岩成分（Peters和Mercolli，1998年）。然而，没有现代或深入的研究，重点是地幔熔融和岩石圈吸积过程。在这个项目中，我们特别试图了解：（a）Masirah的岩浆管道系统和由此产生的海洋地壳结构;（B）同位素多样性地幔熔体在通往海底的途中均匀化;（c）上升熔体与地幔岩石圈相互作用的程度;以及（d）辉长岩中有多少反应性多孔流动，而不是分离结晶，改变了MOR B成分（Lissenberg等人，2013年）-学生将通过广泛的实地工作，然后通过综合的主要，微量元素和矿物和散装岩石样品的同位素研究从Masirah地幔，深成岩段和席状岩脉/熔岩地址。

Conventional layer-cake models for ocean crustal structure cannot explain recent observations from slower spreading-rate mid-ocean ridges (MOR). We now recognise that detachment faults assist plate spreading and the igneous crust may be heterogeneous, thin or even absent. Mantle lithosphere directly beneath the axis potentially exerts a strong influence on the mechanisms of generation, transport, storage and modification of basaltic melt en-route to the seafloor. Deconvolving the relative importance of these different processes and assessing their effects on erupted MORB, the most abundant magma type on Earth, is a first-order problem, yet is almost intractable because of the difficulty of accessing full crustal sections in the modern oceans. Ophiolites, such as Semail (N Oman), are of questionable benefit, having formed by spreading above subduction zones and therefore in an environment where mantle melting and melt evolution is affected by hydrous fluids.Masirah Island, off the SE coast of Oman (Fig. 1), is near-unique in being an exhumed fragment of oceanic lithosphere from a genuine open-ocean MOR, hence offering a ground-breaking opportunity to bring significant new insights into the structure of slow-spreading lithosphere and the integrated processes of MORB melt production and modification, from mantle to seafloor. Unrelated to the much better-known Semail ophiolite, Masirah formed at a (slow-spreading) MOR in the proto-Indian Ocean during rifting of Gondwanaland in the Jurassic and was emplaced onto the SE Arabian margin in the Cretaceous-Palaeocene. Background investigations in the 1970s-90s showed the island to be formed of two thrust slices, each containing a complete oceanic crustal sequence characterised by a remarkably thin (~0.5km) plutonic crust and heterogeneous normal- to enriched-MORB basalt compositions (Peters & Mercolli, 1998). No modern or in-depth study focusing on mantle melting and lithospheric accretion processes has however been made. In this project we seek specifically to understand: (a) the magma plumbing system and resulting oceanic crustal structure of Masirah; (b) where isotopically-diverse mantle melts homogenise en-route to the seafloor; (c) the extent to which ascending melts interact with mantle lithosphere; and (d) how much reactive porous flow, rather than fractional crystallisation, in gabbro modifies MORB compositions (Lissenberg et al., 2013) - which the student will address via extensive field work followed by an integrated major-, trace-element and isotope study of minerals and bulk-rock samples from the Masirah mantle, plutonic section and sheeted dykes/lavas.