Re-fertilizing the Earth´s mantle: crust-mantle interaction in (ultra)high pressure settings

了解碰撞环境下岩石圈的化学演化意味着了解和量化地壳和岩石圈地幔之间的物质交换。这种交换是一个复杂的双向过程：岩石圈地幔和俯冲岩石释放的熔体和流体渗入地壳，引起熔合、元素重新分配，从而导致地壳分化，同时，俯冲的地壳物质重新滋养枯竭的地幔，更新其产生玄武岩岩浆的能力。地壳-地幔相互作用过程可以直接研究造山橄榄岩，即嵌入碰撞造山带的岩石圈地幔部分。造山带橄榄岩明显显示出地幔非均质性，反映在厘米到米制的辉石岩和榴辉岩体的存在上。实验研究表明，地幔非均质性是玄武岩（MORB和OIB）组成变化的重要原因。因此，了解涉及辉石岩和榴辉岩的熔融相关过程是揭示壳幔相互作用、碰撞环境和地幔相关岩浆活动之间联系的最直接方法。近年来，在波西米亚地块造山带橄榄岩中的辉石岩和含金刚石变质沉积岩中的榴辉岩中首次发现了保存完好的玻璃状和结晶熔融包裹体。辉石岩中的熔体具有花岗质成分，具有明显的俯冲相关微量元素特征，而榴辉岩的初步数据显示，在深度100 ~ 100 km处产生了花岗质熔体。而地幔（橄榄岩）熔融主要生成玄武岩，榴辉岩熔融主要生成特隆赫岩-闪长岩-花岗闪长岩（TTG）系列熔体。因此，在这些岩石类型的地幔深度的变质高峰期间，花岗岩类融化的存在是完全出乎意料的，迄今为止也无法解释。我们的项目旨在通过多学科方法解决这个谜题，包括辉石岩和榴辉岩的MI研究、实验岩石学、相平衡建模和同位素研究。我们将确定熔体生成反应和熔体来源，并描述源区域的物理化学参数如何影响熔体化学。同位素研究将为确定这些与熔体有关的过程发生的时间框架以及熔体的化学亲和力提供坚实的约束。我们将定义哪些化学示踪剂可用于识别和量化碰撞环境中地幔和地壳对岩浆活动的不同贡献。总之，我们雄心勃勃的项目将把辉石岩和榴辉岩中天然MI的丰富新地球化学数据与实验和热力学计算的结果结合起来，以阐明碰撞环境下岩石圈的演化，为地壳-地幔相互作用过程如何塑造大陆地壳的造山根提供一幅完整的画像。

Understanding the chemical evolution of the lithosphere in collisional settings means to understand and quantify the exchange of material between crust and lithospheric mantle. Such exchange is a complex, two-way process: melts and fluids released from both lithospheric mantle and subducted rocks infiltrate the crust, causing anatexis, element redistribution, and thus crustal differentiation, while at the same time, subducted crustal material re-fertilizes the depleted mantle, renewing its capability for production of basaltic magmas. Crust-mantle interaction processes can be directly investigated in orogenic peridotites, portions of lithospheric mantle embedded in collisional orogens. Orogenic peridotites clearly show evidence of mantle heterogeneity, reflected by the presence of centimetric to metric bodies of pyroxenite and eclogite. Experimental studies suggest that mantle heterogeneity contributes substantially to the variability in composition of basalts, both MORB and OIB. Hence, understanding melt-related processes involving pyroxenite and eclogite is the most direct way to unravel the link between crust-mantle interaction, collisional settings, and mantle-related magmatism.Preserved glassy and crystallized melt inclusions (MI) were recently discovered for the first time in pyroxenites in orogenic peridotites and in eclogites in diamond-bearing metasediments of the Bohemian Massif. The melt in the pyroxenites has granitoid composition, with a clear subduction-related trace element signature, whereas preliminary data on the eclogites show a granitic melt generated at depth >100 km. However, mantle (peridotite) melting is expected to generate basalts, whereas eclogite melting mainly produces melts in the Trondhejmite-Tonalite-Granodiorite (TTG) series. Thus the presence of a granitoid melt during peak metamorphism at mantle depth in these rock types is completely unexpected and, to date, unexplained. Our project aims to solve this enigma with a multidisciplinary approach integrating MI studies in pyroxenites and eclogites, experimental petrology, phase equilibria modeling, and isotopic studies. We will identify melt-producing reactions and source of melts, and describe how the physicochemical parameters at the source region influence melt chemistry. Isotope studies will provide solid constraints to define the timeframe in which these melt-related processes occurred, as well as on the chemical affinity of the melt. We will define which chemical tracers can be used to identify and quantify the different contributions from mantle and crust to magmatism in collisional settings. In conclusion, our ambitious project will merge a wealth of novel geochemical data from natural MI in pyroxenites and eclogites with results from experiments and thermodynamic calculations to clarify lithosphere evolution in collisional settings, providing a complete portrait of how crust-mantle interaction processes shape the orogenic roots of the continental crust.