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的层饼模型不能解释最近从较慢的扩展速率的大洋中脊(MOR)观测到的结果。我们现在认识到，拆离断层有助于板块扩张，火成岩地壳可能是不均匀的、薄的或甚至没有的。地轴正下方的地幔岩石圈可能对玄武岩熔体在流向海底的过程中的产生、运输、储存和变质机制产生强烈的影响。消除这些不同过程的相对重要性并评估它们对地球上最丰富的岩浆类型--MORB喷发的影响是一个首要问题，但由于难以进入现代海洋的整个地壳部分，这几乎是一个棘手的问题。蛇绿岩，如Smail(阿曼北部)，是通过在俯冲带上方扩散形成的，因此在地幔融化和熔体演化受到含水流体影响的环境中形成。阿曼东南海岸外的Masirah岛(图1)几乎是独一无二的，它是从真正的开阔大洋MOR中挖掘出的海洋岩石圈的碎片，因此提供了一个开创性的机会，为缓慢扩张的岩石圈的结构和从地幔到海底的MORB熔体产生和改造的综合过程提供了重要的新见解。Masirah与更知名的Smail蛇绿岩无关，它形成于侏罗纪冈瓦瓦兰裂谷期间原印度洋(缓慢扩张的)MOR，并在白垩纪-古新世侵位到阿拉伯东南边缘。20世纪70年代至90年代的背景调查表明，该岛由两个逆冲片组成，每个片包含一个完整的大洋地壳序列，其特征是非常薄(约0.5公里)的深成地壳和不均匀的正常到富集型MORB玄武岩成分(Peters&Mercoli，1998)。然而，尚未对地幔熔融和岩石圈吸积过程进行现代或深入的研究。在这个项目中，我们特别寻求了解：(A)马西拉的岩浆管道系统和由此产生的大洋地壳结构；(B)同位素多样的地幔熔体在通往海底的途中发生均一化的情况；(C)上升熔体与地幔岩石圈相互作用的程度；以及(D)辉长岩中的反应性多孔流动，而不是分离结晶作用对MORB成分的影响(李森伯格等人，2013年)--学生将通过广泛的野外工作，然后对来自马西拉地幔、深成岩段和岩浆岩/熔岩的矿物和块状岩石样品进行综合的主要、微量元素和同位素研究。

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.