Water in mantle eclogite: Origin, distribution and physical consequences

通过地球物理手段越来越详细地研究了大部分难以接近的深层岩石圈的组成、结构和热状态，并通过越来越复杂的数值模拟揭示了其动力学。深层岩浆夹带的捕虏体的输入提高了这两种方法的准确性。然而，榴辉岩作为以橄榄岩为主的克拉通岩石圈的一个次要但不可忽视的组成部分，其物理性质尚未得到系统的研究。像它们的橄榄岩对应物一样，这些岩石中的一些已经交代，并且可能是隐蔽的水合作用。地幔榴辉岩是由~50%斜辉石组成的岩石，预计对岩石圈水平衡有重要贡献，但对其水含量、富集性质和富集时间的研究很少，对地球物理观测的影响几乎是未知的。本研究的主要目的是：(1)利用傅里叶变换红外光谱测定不同克拉通和构造背景下8个特征特别明显的地幔榴辉岩套中斜辉石、石榴石±金红石的水含量及其分布。结合已发表的橄榄岩数据，这将使含榴辉岩的区域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限制；(2)通过对绢云母进行Ar-Ar测年，确定以含绢云母榴辉岩为代表的主要水化事件的时间，可能与水进入名义上无水矿物有关；(3)评价克拉通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.