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扩展速度较慢的洋中脊（MOR）观测到的结果。我们现在认识到，剥离断层有助于板块扩张，火成岩地壳可能是不均匀的、薄的，甚至不存在。地轴正下方的地幔岩石圈对玄武岩熔体向海底的生成、输送、储存和变质机制具有潜在的强烈影响。分解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，是一个一级问题，但几乎是棘手的，因为很难获得现代海洋中完整的地壳剖面。蛇绿岩，如Semail（阿曼北部），其益处值得怀疑，因为蛇绿岩是在俯冲带之上扩张形成的，因此是在地幔熔融和熔融演化受含水流体影响的环境中形成的。阿曼东南海岸外的Masirah岛（图1）几乎是独一无二的，因为它是从一个真正的开放海洋MOR中挖掘出来的海洋岩石圈碎片，因此提供了一个开创性的机会，为缓慢扩张的岩石圈结构和从地幔到海底的MORB熔体产生和改造的综合过程带来重要的新见解。与更著名的Semail蛇绿岩无关，Masirah形成于侏罗纪冈瓦纳兰裂谷时期的原印度洋（缓慢扩张的）more，并于白垩纪-古新世被置于阿拉伯东南部边缘。20世纪70年代至90年代的背景调查表明，该岛由两个逆冲断层片组成，每个断层片都包含一个完整的海洋地壳序列，其特征是非常薄（~0.5km）的深部地壳和非均匀的正-富- morb玄武岩组成（Peters & Mercolli, 1998）。然而，没有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)岩浆管道系统和由此产生的马西拉海洋地壳结构；(b)同位素多样的地幔在通往海底的途中融化均匀；(c)上升熔体与地幔岩石圈相互作用的程度；(d)辉长岩中有多少反应性多孔流动，而不是部分结晶，改变了MORB组成（Lissenberg等人，2013）——学生将通过广泛的实地工作，然后对来自Masirah地幔、深部构造剖面和片状岩脉/熔岩的矿物和大块岩石样本进行综合的主要元素、微量元素和同位素研究来解决这个问题。

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.