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

水通常以微量存在于名义上无水的地幔矿物中，显著影响地幔的物理性质和动力学。斜方辉石是地幔水的重要容器。它可能含有几百ppm的H2O。斜方辉石比橄榄石含有更多的水，加上它能够在折返过程中保存地幔水含量比橄榄石更好，使其成为地幔水位的有用代理。以往的研究仅限于陆下地幔中的斜方辉石。大洋下地幔斜方辉石的第一个水数据是自己调查的结果，由DFG资助。最近发表的结果提供了大洋橄榄岩中水含量的第一个直接信息。我们的初步研究（i）证明了水存在于大洋下的无水矿物中，（ii）获得了地幔中水含量及其变化的信息，（iii）表明没有发生减压引起的水损失，（iv）证实了部分蛇纹岩化对斜方辉石中的结构水没有影响。这些新数据令人惊讶，因为它们记录了亚大西洋地幔中对比鲜明的水含量，包括水饱和度的典型浓度水平（ODP-153航段）以及低一个数量级的浓度水平（209航段）。东太平洋海隆的样本（未公布的数据来自正在进行的项目）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，对有关以下方面的模型产生了深远的影响：㈠上地幔中水的分布和再分布; ㈡沿沿着洋中脊的地幔岩石折返过程。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，折返速率，或者更好地说，熔体耗尽后的地幔停留时间似乎是最重要的变量。因此，计划中的项目的主要重点是检验这一假设，并限制决定海底地幔含水量的参数。为此，将对来自不同ODP地点的大洋橄榄岩进行调查;必须选择样品，以说明扩展速率、熔体消耗程度、矿物成分、平衡温度和压力的变化。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，将限制有关扩张中心以下的动力学过程。为了建立一个更普遍的模型并检验初步假设，绝对有必要对沿着慢速和快速扩张脊的更多地点的橄榄岩进行研究。建议的项目旨在填补知识的根本空白，也将有助于更好地了解扩张中心下方发生的动态过程，以及地幔岩石的折返。

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 Samples from the East Pacific Rise (unpublished data from ongoing project) show intermediate characteristics.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.