Scales of preservation and root causes of mantle heterogeneities in the Iapetan Ocean convecting upper mantle

尽管地幔约占地球体积的80%，但其组成结构仍未被很好地了解，尽管这些知识对于限制地球在约45亿年的地质时间内的热和化学演化具有重要意义。我们缺乏对地幔的详细了解，部分原因是它很少暴露在我们的行星表面，这使得直接观察和研究变得困难。然而，最近的研究清楚地表明，不能假设地幔在成分上是均匀的或在地质时间上是静态的。来自大洋盆地的橄榄岩(深海橄榄岩)和蛇绿岩保存了化学和同位素在区域(100‘S公里)和小(厘米到米)尺度上不均匀的对流上地幔的证据。复杂的形成和蚀变上地幔历史涉及熔体枯竭、参照作用(最初的难熔残留物，如方辉橄榄岩通过熔体添加再次变成二辉橄榄岩)和熔体-岩石反应，但这些过程的原因、时间和分布没有得到很好的解决。蛇绿岩代表了部分或全部保存下来的被遮挡的大洋地幔的片状，是评估地幔不均匀的时间、原因和程度的特别有价值的资源，因为它们允许基于现场的观察与对否则无法获得的地幔物质的地球化学调查相结合。此外，蛇绿岩保存了一系列大洋地幔岩性(如方辉橄榄岩、二辉橄榄岩和纯橄榄岩)，这种变化允许详细评估作用于保存下来的地幔的事件的分布和相对时间。与深海橄榄岩不同的是，一些蛇绿岩的一个独特属性是存在豆荚状铬铁矿矿层，通常位于岩石莫霍面区域，通常与铂族元素矿化有关。蛇绿岩豆荚状铬铁矿的形成时间和成因一直存在争议，但有人认为它们代表了超俯冲带环境中的集中熔体通道带。设得兰(英国)和勒卡(挪威)超俯冲带蛇绿岩由大洋岩石圈组成，在大洋中脊两侧约620 Ma分离，随后在约130 Ma后在陆壳上方产生，各自位于伊阿普图斯洋北部的对岸。PI和项目合作伙伴已经对设得兰蛇绿岩进行的一项初步研究表明，它保存了在Iaptus地幔的整个生命周期中发生的熔体枯竭、渗流和参考事件的复杂序列的证据。从先导数据集进行的关键观察是，后期地幔事件只部分叠加了早期地幔过程形成的成分不均一性，与俯冲带相关的相对较高的部分熔融程度非常有效地产生了这种不均一性。这一重要观察结果将在拟议的研究中得到验证：1)将设得兰研究扩展到更详细的程度；2)纳入对保存完好的Leka蛇绿岩样本的比较研究；3)与形成于其他(例如大洋中脊)构造背景的蛇绿岩现有的地球化学和同位素数据集进行比较。为了实现这一目标，将利用Re-Os同位素系统和高度亲铁元素(Os、Ir、Ru、Rh、Pt、Pd、Re、Au)丰度测量的强大组合来区分产生地幔不均一性的过程，如熔体枯竭、参考作用和熔岩反应。因此，将深入了解一块大洋地幔的化学演化和其中成分不均一性的发展，从露头到大洋板块尺度，在土卫八大洋的大部分时间里。

Although the terrestrial mantle comprises ~80 vol.% of our planet, its compositional architecture is not well understood despite the importance such knowledge holds for constraining Earth's thermal and chemical evolution over ~4.5 billion years of geological time. Our lack of detailed insight into the mantle stems in part from the fact that it is rarely exposed at our planets surface, making direct observation and study difficult. It is clear from recent study, however, that the mantle cannot be assumed to be compositionally homogenous or static over geological time. Peridotites from the ocean basins (abyssal peridotites) and from ophiolites preserve evidence for a convecting upper mantle that is chemically and isotopically heterogeneous at regional (100's km) and small (cm-to-m) scales. Complex formation and alteration upper mantle histories involving processes of melt-depletion, refertilisation (whereby originally refractory residues such as harzburgites become lherzolites again via melt addition) and melt-rock reaction have been held responsible, but the causes, timing and distribution of such processes are poorly resolved.Ophiolites, which represent partially-to-wholly preserved slivers of obducted oceanic mantle, are particularly valuable resources for assessing the timing, causes and extent of mantle heterogeneity, as they allow field-based observation to be coupled with geochemical investigation on otherwise inaccessible mantle material. Furthermore, ophiolites preserve a range of oceanic mantle lithologies (e.g., harzburgites, lherzolite and dunite) and such variation allows detailed assessment of the distribution and relative timing of events acting upon the mantle that is preserved. A distinctive attribute of some ophiolites, which contrasts with abyssal peridotites, is the presence of podiform chromitite seams, typically in the region of the petrological Moho, which are often associated with Platinum-group element mineralization. The timing and genesis of ophiolite podiform chromitites is controversial, but it has been suggested that they represent zones of focused melt channeling in supra-subduction zone settings.The Shetland (UK) and Leka (Norway) supra-subduction zone ophiolites comprise oceanic lithosphere separated at ~620 Ma on either side of a mid-ocean ridge and subsequently obducted over continental crust ~130 Ma later, each on opposite sides of the northern Iapetus Ocean. A pilot study already carried out on the Shetland ophiolite by the PI and Project Partner reveals that it preserves evidence for a complex sequence of melt depletion, percolation and refertilisation events that occurred over the lifetime of the Iapetus mantle. The critical observation made from the pilot dataset is that later mantle events only partially overprint the compositional heterogeneities developed from earlier mantle processes and that the relatively high degrees of partial melting associated with the supra-subduction zone are very effective at generating such heterogeneity. This important observation will be tested in the proposed research by 1) extending the Shetland study to greater levels of detail; 2) inclusion of a comparative study of carefully selected samples from the well-preserved Leka ophiolite; 3) drawing comparisons with existing geochemical and isotopic datasets from ophiolites that formed in other (e.g., mid-ocean ridge) tectonic settings. In order to achieve this, the powerful combination of the Re-Os isotopic system and highly-siderophile element (Os, Ir, Ru, Rh, Pt, Pd, Re, Au) abundance measurements will be utilised to discriminate between the processes responsible for generating mantle heterogeneities such as melt depletion, refertilisation and melt-rock reaction. Thus, profound insight will be gained into the chemical evolution of a piece of oceanic mantle and the development of compositional heterogeneity therein, from outcrop to oceanic plate scales, over much of the lifetime of the Iapetus Ocean.