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

尽管地幔占地球总体积的约80%，但人们对其组成结构的了解并不充分，尽管这些知识对限制地球在约45亿年地质时间里的热化学演化具有重要意义。我们对地幔缺乏详细的了解，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，从最近的研究中可以清楚地看出，不能假定地幔的成分在地质时期是均匀的或静止的。来自海洋盆地（深海橄榄岩）和蛇绿岩的橄榄岩保存了对流上地幔在区域（100公里）和小尺度（厘米到米）上化学和同位素不均匀的证据。复杂的上地幔形成和蚀变历史涉及熔体耗损、再化作用（最初难熔的残余，如哈尔茨伯基岩，通过熔体添加再次变成橄榄岩）和熔体-岩石反应过程，但这些过程的原因、时间和分布尚未得到很好的解决。蛇绿岩代表了部分或全部保存的海洋俯冲地幔碎片，是评估地幔非均质性的时间、原因和程度的特别宝贵的资源，因为它们可以将实地观察与地球化学调查结合起来，对其他难以接近的地幔物质进行调查。此外，蛇绿岩保存了一系列的海洋地幔岩性（例如，哈尔茨伯基岩、伊黑橄榄岩和邓恩岩），这种变化可以详细评估作用于被保存的地幔的事件的分布和相对时间。与深海橄榄岩相比，蛇绿岩的一个独特特征是存在足状铬铁矿，通常在岩石学莫霍区，这通常与铂族元素矿化有关。蛇绿岩脚状铬铁矿的形成时间和成因存在争议，但有人认为它们代表了超俯冲带背景下的集中熔融通道带。设得兰（英国）和雷卡（挪威）的超俯冲带蛇绿岩由海洋岩石圈组成，它们分别在约620 Ma的洋中脊两侧分离，随后在约130 Ma的洋中脊两侧逆冲到大陆地壳上，分别位于Iapetus洋北部的两侧。PI和项目合作伙伴已经在设得兰蛇绿岩上进行了一项试点研究，表明它保留了在土卫八地幔的生命周期中发生的一系列复杂的融化耗尽、渗透和再化事件的证据。从试验数据中得出的关键观察结果是，后期的地幔事件只是部分叠加了早期地幔过程形成的非均质性，而与超俯冲带相关的相对高度的部分熔融非常有效地产生了这种非均质性。这一重要观察结果将在拟议的研究中得到检验：1)将设得兰群岛的研究扩展到更详细的层面；2)对从保存完好的Leka蛇绿岩中精心挑选的样品进行比较研究；3)与其他构造环境（如洋中脊）中形成的蛇绿岩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非均质性的过程，如熔体耗损，再肥化和熔融岩石反应。因此，在Iapetus海洋生命的大部分时间里，从露头到海洋板块尺度，我们将对一块海洋地幔的化学演化及其成分不均匀性的发展有深刻的了解。

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.