Noble Gas, Halogen and Water Recycling into the Terrestrial Mantle

不相容的挥发性元素，如惰性气体和卤素，通过大洋中脊和其他火山活动区域，不断地从地幔带到地球表面。人们一直认为，这对这些物种来说是一个单向的过程。这一假设是在使用这些示踪剂来研究行星挥发性起源、惰性气体的增生过程、地幔动力学和大气演化，或跟踪海洋的卤素收支并评估海洋盐度在海洋生命演化和维持中的作用时做出的。曼彻斯特最近的研究表明，地表惰性气体正被俯冲到地幔深处，这一来源在地幔重惰性气体收支中占主导地位（Ballentine等人，Nature 2005； Holland和Ballentine， Nature 2006）。令人惊讶的是，地幔显示出与海水相同的Ar/Kr/Xe特征。这是太阳系中独一无二的元素/同位素组成，我们可以排除它是原始增生地幔成分的可能性。由于稀有气体的易挥发性，我们可以预期，在俯冲过程中，这一海水比例会受到干扰。那么保存海水特征的机制是什么呢？这些气体在海洋地壳的什么地方？它们是如何以及为什么被保存在俯冲板块中的？我们在提案中提出了新的试点数据，表明我们可以在脱水板块释放的流体中识别出相同的海水特征，这些流体已经被带到地幔中至少100公里的深度。通过曼彻斯特首创的一项技术，我们还表明，这种流体中的卤素，同样容易被分馏，与海洋孔隙流体中的值相同。看来稀有气体和卤素俯冲是有联系的。我们建议对海洋地壳和伴生沉积物进行第一次完整和系统的分析调查，以确定主导俯冲物质稀有气体和卤素预算的相或宿主的位置和元素特征。只要稍加努力，我们就能得到伴生水的丰度和氢同位素组成。我们还建议将试点研究结果扩展到另外两个具有对比热制度的地形，我们希望能够在那里取样并确定深层（~100公里）俯冲流体的惰性气体和卤素成分。这一组合数据集将首次以系统的方式将稀有气体、卤素和相关水从俯冲到不同热环境下的俯冲流体成分联系起来。这将使我们能够确定进入地幔的惰性气体的主要载体/载体，并利用我们对地幔中惰性气体浓度和对流行为的理解，开始建模并确定相关的俯冲卤素和水对各自的总地幔预算的影响，以及这些示踪剂在地幔系统中的演化——这些系统支撑着我们对地球的理解。

Incompatible and volatile elements like noble gases and the halogens are continually brought from the mantle to the Earth's surface at mid ocean ridges and other regions of volcanic activity. It has always been assumed that this is a one-way process for these species. This assumption is made when using these tracers to investigate planetary volatile origin, accretionary processes, mantle dynamics and atmosphere evolution in the case of the noble gases, or tracking the halogen budget of the oceans and assessing the role of ocean salinity in the evolution and sustenance of life in the oceans. Recent Manchester work suggests that surface noble gases are being subducted into the deep mantle and that this source dominates the mantle heavy noble gas budget (Ballentine et al., Nature 2005; Holland and Ballentine, Nature 2006). What is surprising is that the mantle shows an Ar/Kr/Xe signature identical to that of seawater. This is an elemental/isotopic composition unique in the solar system and we can rule this out as an original accretionary mantle component. Because of the volatile nature of noble gases we might expect this seawater ratio to be perturbed during the process of subduction. What then is the mechanism that preserves the seawater signature? Where in the oceanic crust are these gases found? How and why are they preserved in the subducting slab? We present new pilot data in our proposal showing that we can identify the same seawater signature in fluids released from dewatering slabs that have been taken to at least 100km depth in the mantle. With a technique pioneered at Manchester, we also show that the halogens in this fluid, which are equally susceptible to fractionation, are identical to values found in marine pore fluids. It would appear that noble gas and halogen subduction are linked. We propose the first complete and systematic analytical survey of the oceanic crust and associated sediment to identify the location and elemental character of the phases or hosts that dominate the noble gas and halogen budget of subducting material. For little extra effort we will also obtain the abundance and hydrogen isotopic composition of the associated water. We also propose to extend the pilot study results to two more terrains, with contrasting thermal regimes, where we expect to be able to sample and identify the noble gas and halogen composition of deep (~100km) subducted fluids. This combined data set will be the first to link noble gases, halogens and related water in a systematic way from subducting to subducted fluid composition in differernt thermal settings. This will enable us to identify the major carrier/ers of noble gas into the mantle and use our understanding of noble gas concentrations and convection behaviour in the mantle to start to model and identify the associated subducted halogen and water impact on the respective total mantle budget and the evolution of these tracers in the mantle system - systems that underpin our understanding of the Earth.