Testing the mantle plume hypothesis through IODP drilling on the Louisville Seamounts

地幔热柱是固态对流地幔上涌，它提供了热量和物质从地球深处转移到地表的主要途径。它们是世界上大部分板内火山活动的罪魁祸首(例如夏威夷)，火山活动的启动伴随着大规模的岩浆喷发，规模之大为今天所未见。这些大规模的火山喷发产生了巨大的火成岩省(LIP)，其中两个最大的火成岩省与二叠纪-三叠纪(西伯利亚)和白垩纪-古新世(德干)界线的全球大灭绝事件重合。地球上最大的嘴唇是安通爪哇高原(OJP)，它完全在海平面以下喷发，因此对环境的影响有限。验证地幔热柱假说，以及如果确实存在地幔热柱，则了解其性质和来源是地球科学的首要目标，并已成为几次深海钻探探险的动机，其中包括作为本提议基础的海底钻探探险330。地幔热柱假说的早期预测之一是，热柱在地幔中保持固定达数千万年之久，其表面表现为热点轨迹(海洋岛屿和海山链)，可用来定义绝对的板块运动。这一预测在夏威夷-帝王海山链(HESC)的ODP197航段上得到了检验，并被证明是无效的。对岩心的古地磁研究表明，在81-47 Ma之间，该热点向南漂移了约15度，这意味着早期夏威夷热柱相对于地球自转轴的运动速度约为40 mm/年。尽管这一发现并不一定削弱羽流假说，但它确实表明羽流并不总是固定的。对这一运动的一种可能的解释是，它反映了这一时期普遍向南的地幔流动(地幔‘风’)。IODP Expeation 330旨在通过对同一时期形成的路易斯维尔海山链(LSC)部分进行类似的研究来检验地幔“风”假说。LSC位于南太平洋，与HESC大致平行，是唯一可以检验该假说的热点链。我在IODP探险330中的角色将是研究钻芯的岩石学和地球化学，我建议用三种方式测试地幔热柱假说。(1)我建议利用玄武岩的主要元素组成来估算海山形成时的地幔温度；(2)利用微量元素数据来评估地幔来源的成分。有人认为LSC是由地幔热柱产生的，其初始阶段是巨大的OJP LIP的原因。痕量元素数据，加上其他船上科学家正在进行的同位素研究，应该可以让我们检验这一假设。OJP是在ODP192航次期间进行钻探的，我在该航段担任联席首席科学家。最后(3)我建议使用氦同位素数据(与SUERC的芬利·斯图尔特博士合作获得)来寻找深地幔参与LSC形成的证据。在来自HESC的玄武岩中(以及其他可能与羽流有关的玄武岩中)发现了高3He/4He，并被认为暗示着原始3He的输入，无论是来自深部地幔还是可能来自地核。

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 (LIPs), two of the largest of which coincided with global mass-extinction events at the Permian-Triassic (Siberia) and Cretaceous-Palaeocene (Deccan) boundaries. The largest of all Earth's LIPs, the Ontong Java Plateau (OJP), was erupted entirely below sea level and consequently had only a limited environmental impact. Testing the mantle plume hypothesis and, if plumes do indeed exist, understanding their nature and origin, are first-order goals of the Earth Sciences, and have provided the motivation for several DSDP, ODP and IODP deep-sea drilling expeditions including IODP Expedition 330 which is the basis of the present proposal. One of the early predictions of the mantle plume hypothesis is that plumes remain fixed in the mantle for tens of millions of years and that their surface manifestation as hotspot trails (chains of ocean islands and seamounts) can be used to define absolute plate motions. This prediction was tested during ODP Leg 197 on the Hawaii-Emperor seamount chain (HESC) and shown to be invalid. Palaeomagnetic studies on the cores showed that the hotspot drifted about 15 degrees southwards between 81 and 47 Ma, implying motion of the early Hawaiian plume of about 40 mm/yr with respect to the Earth's rotation axis. Although this finding doesn't necessarily weaken the plume hypothesis, it does show that plumes aren't always fixed. One possible explanation for the motion is that it reflects a general southward mantle flow (mantle 'wind') over this period. IODP Expedition 330 is designed to test the mantle 'wind' hypothesis by carrying out a similar study on the part of the Louisville seamount chain (LSC) that formed over the same period. The LSC is located in the south Pacific Ocean, runs roughly parallel to the HESC, and is the only hotspot chain on which the hypothesis can be tested. My role in IODP Expedition 330 will be to study the petrology and geochemistry of the drill core, and I propose to test the mantle plume hypothesis in three ways. (1) I propose to use the major element composition of basalt to estimate the mantle temperature at the time of seamount formation and (2) trace element data to assess the composition of the mantle source. It has been proposed that the LSC were produced from a mantle plume whose initiation phase was responsible for the giant OJP LIP. Trace element data, coupled with isotope studies being carried out by other shipboard scientists, should be allow us to test this hypothesis. The OJP was drilled during ODP Leg 192, on which I served as Co-Chief Scientist. Finally (3) I propose to use helium-isotope data (obtained in collaboration with Dr Finlay Stuart at SUERC) to look for evidence of deep-mantle involvement in the formation of the LSC. High 3He/4He has been found in basalt from the HESC (and in other possibly plume-related basalts) and is thought to imply an input of primordial 3He, either from the deep mantle or possibly the core.