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

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大型火成岩省（lip），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。地球上最大的爪哇高原，Ontong爪哇高原（OJP），完全在海平面以下喷发，因此对环境的影响有限。验证地幔柱假说，如果地幔柱确实存在，了解它们的性质和起源，是地球科学的一级目标，并为几个DSDP， ODP和IODP深海钻探探险提供了动力，包括IODP远征330，这是本提案的基础。地幔柱假说的早期预测之一是，地幔柱在地幔中固定存在数千万年，它们的表面表现为热点轨迹（海洋岛屿链和海底山），可以用来定义板块的绝对运动。这一预测在夏威夷-皇帝海山链（HESC）的ODP Leg 197中进行了测试，结果证明是无效的。对岩心的古地磁研究表明，热点在81 ~ 47 Ma之间向南漂移约15度，这意味着早期夏威夷羽流相对于地球自转轴的运动约为40 mm/年。虽然这一发现并不一定会削弱羽流假说，但它确实表明羽流并不总是固定的。对这种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（地幔“风”）。IODP远征330旨在通过对同一时期形成的路易斯维尔海山链（LSC）的一部分进行类似的研究来测试地幔“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设的热点链。我在IODP远征330中的任务是研究岩心的岩石学和地球化学，我建议用三种方法来检验地幔柱假说。(1)建议利用玄武岩主元素组成估算海山形成时的地幔温度；(2)利用微量元素数据评估地幔源成分。有人提出，LSC是由地幔柱产生的，其起始阶段是造成OJP巨大LIP的原因。微量元素数据，加上船上其他科学家进行的同位素研究，应该可以让我们检验这个假设。OJP是在ODP第192阶段钻探的，我担任联合首席科学家。最后，我建议使用氦同位素数据（与SUERC的Finlay Stuart博士合作获得）来寻找地幔深部参与LSC形成的证据。在HESC玄武岩（以及其他可能与羽状岩相关的玄武岩）中发现了高3He/4He，并被认为暗示了原始3He的输入，可能来自深部地幔，也可能来自地核。

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.