Measuring the pulse of the Iceland plume

地幔柱是固态对流的地幔上升流，它提供了热量和物质从地球深处转移到地表的主要途径。它们造成了世界上大部分的板块内火山活动（如夏威夷），它们的形成伴随着大规模的岩浆喷发，其规模是今天所见不到的。这些大规模的火山事件产生了巨大的火成岩省，其中最大的两个省与二叠纪-三叠纪（西伯利亚）和白垩纪-古新世（德干）边界的全球大灭绝事件相吻合。了解地幔柱的性质和起源是地球科学的首要目标。冰岛可以说是地球上研究地幔柱性质的最佳地点，因为在海洋扩张中心并置一个巨大的地幔融化异常，使得异常地幔减压到浅层，因此融化到最大可能的程度。与较小的熔体组分相比，较大的熔体组分对地幔成分和温度的估计更不模糊。冰岛下面的地幔似乎比北大西洋的上地幔更热，而且在成分上也不同，但温度和成分在产生融化异常中的相对重要性尚不清楚。甚至有人认为，冰岛的融化异常完全是由于地幔源的额外肥沃，因此根本不需要地幔柱。无论融化异常的来源是什么，它的影响似乎是脉冲的。横跨冰岛南部雷克雅内斯海岭（RR）的基底地形的历时v型脊（VSRs）在重力场中表现得最为清楚，它代表了约500万年时间尺度上的地壳厚度变化。这里提出的主要问题是：(1)这种地壳厚度的变化是由温度、地幔肥力或两者的变化引起的；(2)羽流流出是径向的还是沿RR通道流出的。2007年8月，史蒂夫·琼斯博士（都柏林三一学院）向爱尔兰海洋研究所提交了一份申请，申请在RV凯尔特探险家号上使用船舶时间，从vrs中挖掘样本，以解决这些问题，并迅速得到资助和安排。疏浚巡航（CE0806）在那一年的4月和5月进行，我们现在从两个VSRs中获得了一套玄武岩样本。我们建议对这些样品进行分析，并将其化学和同位素组成与邻近轴向带的玄武岩进行比较，从而使我们能够说在VSR形成时，山脊轴下的地幔是否比现在更热和/或更肥沃。邻近的山脊轴线在一段时间前被疏浚，这些样本的数据可在已发表的文献中找到。我们还从南部RR的一个长且以前未采样的剖面中恢复了样本，该剖面可能与第二个突出的VSR的投影相交。如果可以清楚地确定交点，那么就有可能确定vsr是略微弯曲（如径向流预测的那样）还是直（通道流）。除了对我们了解地球深部过程的重要性外，这里提出的工作也将对古海洋学和油气勘探产生影响。地幔温度的波动应引起格陵兰-冰岛-法罗海岭的水深相关波动，这将影响北大西洋北部成分水向南的流动。这里提出的工作应该促进我们对地幔和地表过程之间联系的理解。短暂的热引起的隆升沉降事件也影响被动大陆边缘，有证据表明这种振荡影响了北海的沉积记录。了解这些波动显然对石油行业很有意义。

Mantle plumes are solid-state convective mantle upwellings that provide the principal means by which heat and material are transferred from the deep Earth to the surface. They are responsible for much of the world's intraplate volcanism (e.g. Hawaii) and their initiation is accompanied by massive outpourings of magma on a scale not seen today. These massive volcanic episodes produce large igneous provinces, two of the largest of which coincided with global mass-extinction events at the Permian-Triassic (Siberia) and Cretaceous-Palaeocene (Deccan) boundaries. Understanding the nature and origin of mantle plumes is a first-order goal of the Earth Sciences. Iceland is arguably the best place on Earth to investigate the nature of mantle plumes because the juxtaposition of a large mantle-melting anomaly on an oceanic spreading centre allows the anomalous mantle to decompress to shallow levels and therefore melt to the maximum possible extent. Large melt fractions allow much less ambiguous estimates of mantle composition and temperature than do smaller melt fractions. The mantle beneath Iceland appears to be hotter than the ambient North Atlantic upper mantle and is also compositionally different, but the relative importance of temperature and composition in generating the melting anomaly is unclear. It has even been argued that the Iceland melting anomaly is entirely due to extra fertility in the mantle source and therefore doesn't require a mantle plume at all. Whatever the source of the melting anomaly, its effects appear to be pulsing. Diachronous V-shaped ridges (VSRs) in basement topography straddling the Reykjanes Ridge (RR) south of Iceland, expressed most clearly in the gravity field, represent variations in crustal thickness produced on a time scale of ~5 million years. The principal questions posed here are: (1) whether this variation in crustal thickness is caused by variations in temperature, mantle fertility, or both; and (2) whether plume outflow is radial or channelled down the RR. An application for ship time on the RV Celtic Explorer to dredge samples from the VSRs in order to address these questions was submitted to the Irish Marine Institute by Dr Steve Jones (Trinity College, Dublin) in August 2007 and was funded and scheduled extremely quickly. The dredging cruise (CE0806) took place in April and May of that year and we now have a suite of basalt samples from two of the VSRs. We propose to analyse these samples and compare their chemical and isotopic composition with that of basalt from the adjacent axial zone, thus allowing us to say whether the mantle beneath the ridge axis at the time of VSR formation was hotter and/or more fertile than it is at present. The adjacent ridge axis was dredged some time ago and data from these samples is available in the published literature. We also recovered samples from a long and previously unsampled section of the southern RR where it is likely to intersect the projection of the second prominent VSR. If the point of intersection can be clearly identified then it will be possible to determine whether the VSRs are slightly curved (as predicted by radial flow) or straight (channelled flow). Beside its importance to our understanding of deep-Earth processes, the work proposed here will also have implications for palaeoceanography and hydrocarbon exploration. Fluctuation in mantle temperature should cause correlated fluctuation in water depth along the Greenland-Iceland-Faroes ridge, and this will affect the flow of northern component water southwards from the North Atlantic. The work proposed here should advance our understanding of the link between mantle and surface processes. Transient thermally induced uplift-subsidence events also affect passive continental margins, and there is evidence that such oscillations have influenced the sedimentary record in the North Sea. Understanding these oscillations will clearly be of interest to the oil industry.