Water in the sub-oceanic mantle: An infrared spectroscopic study of orthopyroxene from abyssal Atlantic and Pacific peridotite

水通常以微量的形式存在于名义上无水的地幔矿物中，对地球地幔的物理性质和动力学具有重大影响。正辉石是地幔水的重要容器。它可能含有几百ppm的水。正辉石比橄榄石含有更多的水，而且它比橄榄石更能在挖掘过程中保存地幔含水量，使其成为地幔水位的有用代表。以往的研究仅限于来自次大陆地幔的正辉石。海底地幔正辉石的第一批水数据是由DFG资助的自己的调查结果。最近发表的研究结果首次提供了有关海洋橄榄岩含水量的直接信息。我们的初步研究(i)证明了水存在于海底无水矿物中，（ii）获得了地幔中水含量及其变化的信息，（iii）表明减压导致的水损失没有发生，（iv）证实了部分蛇纹石化对正辉石中构造水没有影响。这些新数据令人惊讶，因为它们记录了亚大西洋地幔中不同的水含量，包括典型的水饱和度浓度水平（ODP-Leg 153）和低一个数量级的浓度水平（Leg 209）。来自东太平洋隆起的样本（来自正在进行的项目的未发表数据）显示出中间特征。自己先前的研究产生了各种意想不到的结果，这些结果对有关(i)上地幔中水的分布和再分布以及（ii）沿洋中脊挖掘地幔岩石所涉及的过程的模型产生了深远的影响。然而，新的发现很难解释，因为可用的数据集仍然非常有限。目前，挖掘速率，或者更确切地说，熔体耗尽后的地幔停留时间似乎是最重要的变量。因此，计划项目的主要重点是测试这一假设，并限制控制海底地幔含水量的参数。为此，将调查来自各ODP地点的海洋橄榄岩；样品的选择必须考虑到扩散速率、熔体损耗程度、矿物成分、平衡温度和压力的可变性。计划中的研究将提供关于地幔水的含量和分布的进一步信息，更重要的是，对扩张中心以下的动态过程提出限制。为了建立一个更一般的模型并检验初步的假设，绝对有必要对沿慢速和快速扩展脊的更多地点的橄榄岩进行研究。所建议的项目旨在弥补一个基本的知识差距，也将有助于更好地了解在扩张中心下面发生的动态过程，这些过程与挖掘地幔岩石有关。

Water, commonly present in trace amounts in nominally anhydrous mantle minerals, significantly influences the physical properties and dynamics of the Earth mantle. Orthopyroxene is a very important container for mantle water. It may incorporate several hundred ppm H2O. Orthopyroxene contains considerably more water than olivine, plus it is able to preserve mantle water contents during exhumation better than olivine rendering it a useful proxy for mantle water levels. Previous studies were restricted to orthopyroxene from the sub-continental mantle. The first water data for sub-oceanic mantle orthopyroxene are the result of own investigations, financed by DFG. The recently published results provide the first direct information on the water content in oceanic peridotite. Our initial study (i) proved that water is present in sub-oceanic anhydrous minerals, (ii) yielded information on the content of water and its variation in the mantle, (iii) showed that decompression-induced water loss did not occur, and (iv) confirmed that partial serpentinization has no affect on structural water in orthopyroxene. The novel data are surprising because they document contrasting water contents in the sub-Atlantic mantle and comprise concentration levels typical for water saturation (ODP-Leg 153) as well as those that are one order of magnitude lower (Leg 209). Samples from the East Pacific Rise (unpublished data from ongoing project) show intermediate characteristics. The own previous study yielded various unexpected results that have far-reaching consequences for models concerning (i) the distribution and re-distribution of water in the upper mantle and (ii) the processes that were involved in the exhumation of mantle rocks along mid-ocean ridges. However, the new findings are difficult to explain because the available data set is still very limited. At present, the exhumation rate or, better, mantle residence time after melt depletion seems to be the most important variable. Hence, the main focus of the planned project is to test this hypothesis and to constrain the parameters that govern the water contents in the sub-oceanic mantle. For this, oceanic peridotite from various ODP sites will be investigated; the samples have to be selected to account for variability in spreading rate, degree of melt depletion, mineral composition, equilibrium temperature and pressure. The planned study will provide further information on the content and distribution of mantle water and, more importantly, put constraints concerning dynamic processes below spreading centers. To develop a more general model and to test the preliminary hypothesis, it is absolutely necessary to study peridotite from more sites along slow- and fast-spreading ridges. The project suggested is aimed at closing a fundamental gap of knowledge, and will also help to better understand the dynamic processes that take place beneath spreading centers and that are involved in the exhumation of mantle rocks.