Water in mantle eclogite: Origin, distribution and physical consequences

地球物理手段越来越详细地调查了基本上无法进入的深层岩石圈的组成、结构和热状态，而日益复杂的数值模拟揭示了其动力学。这两种方法的准确性提高了输入夹带在深部岩浆捕虏体。然而，榴辉岩作为以橄榄岩为主的岩石圈中不可忽视的一部分，其物理性质尚未得到系统的研究。像橄榄岩一样，这些岩石中的一些已经被交代，并可能神秘地水合。地幔榴辉岩作为由~50%的单斜辉石组成的岩石，被认为对岩石圈水收支有重要贡献，但对其H2O含量的研究很少，其富集的性质和时间以及对地球物理观测的影响几乎是未知的。（1）利用傅立叶变换红外光谱法测定了不同构造背景下8个特征异常明显的地幔榴辉岩套中单斜辉石、石榴石±金红石中H_2O的含量及其分布。结合已发表的橄榄岩成分，这将使区域性含榴辉岩大陆岩石圈的体积密度、地震速度和电导率得到前所未有的详细和精确的约束：（2）通过金云母Ar-Ar定年，确定以含金云母榴辉岩为代表的主要水化事件的时间，可能与H2O引入名义上的无水矿物有关;（3）探讨了岩石圈流变学和稳定性的意义。

The composition, structure and thermal state of the largely inaccessible deep lithosphere are inter-rogated by geophysical means with increasing detail, and its dynamics revealed by increasingly sophisticated numerical modelling. The accuracy of both approaches is enhanced by input from xenoliths entrained in deep-seated magmas. However, the physical properties of eclogites, as a subordinate but non-negligible part of dominantly peridotitic cratonic lithosphere, have not been systematically investigated. Like their peridotitic counterparts, some of these rocks have been metasomatised and possibly cryptically hydrated. As rocks consisting of ~50% clinopyroxene, mantle eclogites are expected to significantly contribute to the lithospheric water budget, but there are few studies on their H2O contents, and the nature and timing of their enrichment, and effects on geophysical observables are virtually unknown. The main aims of this study are thus: (1) To determine the H2O content in - and distribution between - clinopyroxene, garnet ± rutile in eight exceptionally well-characterised mantle eclogite suites from various cratons and tectonic settings by Fourier-Transformed Infrared Spectroscopy. Combined with published com¬po¬sitions for peridotite, this will allow the bulk densities, seismic velocities and electrical conductivities of regional eclogite-bearing continental lithospheres to be constrained with unprecedented detail and accuracy; (2) To determine the timing of the major hydration event represented by phlogopite-bearing eclogite, possibly linked to H2O introduction into nominally anhydrous minerals, by Ar-Ar dating of phlogopite; (3) evaluate implications for the rheology and stability of cratonic lithosphere.