Subduction Initiation Investigated by Drilling of the Izu-Bonin-Mariana (IBM) Forearc

板块构造理论认为，板块在洋中脊形成，在俯冲带破坏，在俯冲带之上是被称为岛弧的火山链。俯冲带被比作一个工厂（“俯冲工厂”），物质（地幔、海洋地壳和沉积物）被送入工厂，在工厂内加工（通过脱水和熔化），并产生一系列产品（岩浆、矿床）。有许多“健康和安全”问题，包括地震、海啸和火山爆发。由于这个“俯冲工厂”大部分位于水下和地下，国际海洋发现计划（IODP）组织了三次海洋钻探探险（350-352），每次为期两个月，旨在更好地了解俯冲工厂是如何工作的。这些考察队都位于日本南部被称为IBM（伊豆-波宁-马里亚纳）地区的海洋，这是了解俯冲过程的关键自然实验室。每次探险都有一个特定的目标，探险352（本提案的重点）的职责是调查俯冲如何开始（即“工厂”如何打开）以及这个俯冲启动过程的产物。该提案的首席研究员朱利安·皮尔斯（Julian Pearce）是两位联合首席科学家之一。在日本南部的深海海沟的内墙中，形成于俯冲早期阶段的岩石暴露得特别好，而第352考察队计划在这里的博宁海沟进行钻探。在底部进行的疏浚工作已经证明，它是由一个类似海洋地壳的层序组成的，底部是地幔岩，上面是辉长岩，然后是岩脉和海底熔岩。这些岩石可以追溯到公元前52Ma，被认为代表了俯冲开始的第一阶段，当一个板块下沉，剥离并形成一个延伸带。上面是一组具有地球化学特征的火山岩，这些火山岩被认为代表了从扩张到弧火山作用的转变，最后是更常规的岛弧火山岩，标志着真正的俯冲开始，即俯冲板块既有水平成分，也有垂直成分。据估计，从俯冲开始到真正的俯冲开始大约需要8亿年的时间。然而，由于没有参考剖面，对俯冲起始的解释和假设测试受到了限制，因此我们的目标是通过钻探整个1-1.5公里厚的熔岩序列来获得参考剖面。许多研究将在钻井船上和实验室进行。作为联合首席科学家，PI对综合结果负有共同责任，在这种情况下，为地质学界提供一种俯冲起生火山作用的类型序列，可用于与现在在山带和其他地方的旧地质序列中暴露的火山岩中推断的俯冲起生序列进行比较。此外，他还负责分析三种同位素（Pb、Hf和Nd），作为俯冲起始过程的示踪剂，并对结果进行解释。为此，他的目标是与Rex Taylor （co-I）合作，Rex Taylor本人曾在IBM地区工作，同时也经营着一个同位素实验室，不仅分析Hf和Nd同位素，还专门从事Pb同位素比率的高精度分析。俯冲过程是复杂的，其元素有许多潜在的来源：俯冲带上方的地幔、俯冲地壳和各种类型的俯冲沉积物。Pb是俯冲沉积物的较好示踪剂，Hf-Nd是地幔物源和系统温度的较好示踪剂。通过分析从熔岩层序底部到顶部取样的这些同位素比值，我们旨在记录俯冲变量在从俯冲起始到真正俯冲的过渡过程中是如何变化的。

The theory of plate tectonics has plates created at mid-ocean ridges and destroyed at subduction zones, above which lie chains of volcanoes known as island arcs. Subduction zones have been likened to a factory (the 'Subduction Factory') whereby materials (mantle, oceanic crust and sediment) are fed into the factory, processed within the factory (by dehydration and melting), and create range of products (magmas, ore deposits). There are many 'health and safety' issues, including earthquakes, tsunamis and explosive volcanoes. Because much of this 'Subduction Factory' is located underwater and sub-surface, the International Ocean Discovery Program (IODP) has put together three marine drilling Expeditions (350-352), each two months long, aimed at a better understanding of how the subduction factory works. These Expeditions are all located in the ocean south of Japan known as the IBM (Izu-Bonin-Mariana) region, which is a key natural laboratory for understanding subduction processes. Each Expedition has a particular goal and Expedition 352 (the focus of this proposal) has the remit to investigate how subduction starts (i.e. how the 'Factory' opens) and the products of this subduction initiation process. Julian Pearce, the PI of this proposal, is one of its two co-chief scientists.The rocks formed at the earliest stages of subduction are particularly well exposed in the inner walls of the deep ocean trenches south of Japan and it is here, in the Bonin Trench, that Expedition 352 plans to drill. Dredging has already demonstrated at the base is made up of an oceanic crust-like sequence with mantle rocks at its base, overlain by gabbros, and then by dykes and submarine lavas. These date back to c. 52Ma and are believed to represent the first stage of subduction initiation, when one plate sinks, peels back and creates a zone of extension. This is overlain by a set of geochemically-distinctive volcanic rocks which are believed to represent the transition from spreading to arc volcanism, and finally by more conventional island arc volcanic rocks marking the point at which true subduction starts, i.e. when the subducting plate has a horizontal as well as vertical component. Estimates indicate that it took about 8 m.y. between subduction initiation and the start of true subduction. Interpretations, and testing of hypotheses for subduction initiation, are however, limited by the fact that there is no reference section, and so we aim to obtain that by drilling of the full 1-1.5 km thick lava sequence.Many studies will be carried out on the drilling ship and in the laboratory. As co-chief scientist, the PI has joint responsibility for synthesising the results, in this case to provide the geological community with a type sequence of subduction initiation volcanism which can be used for comparison with inferred subduction initiation sequences in volcanic rocks now exposed in old geological sequences in mountain belts and elsewhere. In addition, he has specific responsibility for analyzing three isotopes (of Pb, Hf and Nd) for use as tracers of the subduction initiation process, and for interpreting the results. For this he aims to work with Rex Taylor (co-I), who has worked in the IBM region himself and also runs an isotope laboratory that not only analyses Hf and Nd isotopes but also specializes in high precision analyses of Pb isotope ratios. The subduction process is complicated, with elements having a number of potential sources: mantle above the subduction zone, subducted crust and various types of subducted sediment. Pb is a particularly good tracer of subducted sediment, while Hf-Nd are good tracers of the provenance of the mantle and temperature of the system. By analyzing these isotope ratios in samples taken from bottom to top of the lava sequence, we aim to document the how subduction variables change during the transition from subduction initiation to true subduction.