Using basaltic crystal mushes to understand the mechanisms of crust formation and the deep structure of basaltic volcanoes

大洋和大陆S地壳分别由镁铁质或镁铁长英质结晶岩组成。深部镁铁质地壳单元的形成机制和结构尚不清楚。特别是，我们还不知道原生地幔在下地壳分异过程中的化学变化以及镁铁质岩浆结晶的时间尺度。最终在地球表面喷发的熔融的比例也没有得到很好的限制。要破译S地壳的大尺度岩性变化、岩浆分异机制和估计地壳内的质量转移，一个非常有用的方法是使用喷发时经常带到地表的晶体碎屑。它们代表在地壳中形成的堆积岩，因此是确定冻结的、高度结晶的部分地壳岩浆室的矿物学的独特机会，否则无法进入。该项目的主要目标是加深我们对大洋和大陆玄武岩火山之下深部岩浆作用的理解，并破译地壳形成的主要机制。我们将进行高温实验，以限制晶体蘑菇在喷发前的形成和解聚，并确定它们在地壳中的热物理状态。我们还将利用实验数据提出新的、准确的矿物熔体平衡公式，适用于地壳压力(&lt；7kbar)，可用于估计地壳中的岩浆储存条件。这些结果将与对来自4个构造背景的榴辉玄武岩进行统计、结构和地球化学分析相结合：大洋中脊(加拉帕戈斯扩张中心)、大洋高原(沙茨基隆起)、大陆泛滥玄武岩(蛇河平原)和大陆弧(智利南部火山带)。我们将使用玄武岩样品的高质量成像，结合地球化学图和晶体尺寸分布来区分与熔体平衡的斑晶和深结晶泥中形成的晶体。然后，将利用晶体的成分(主要和微量元素)和结构及其熔融包裹体的成分来研究玄武岩火山的深部管道系统和岩浆储存条件。特别是，我们将确定(非均质)地幔熔融的作用以及壳内分异对地壳的形成、生长和组成以及对喷发岩浆的地球化学变异性的影响的过程和时间。总而言之，这个项目将提高我们对不同构造背景下地壳的岩性和地球化学变异性的理解，并将增加对晶体泥浆流变学的理解，晶体泥浆流变学是岩浆分异的关键参数。我们的结果也将对其他玄武岩省有用，这些省的地壳过程对喷发岩浆的多样性起着重要作用。

Most of the oceanic and continental Earth s crust is respectively made up of mafic or mafic-felsic crystalline rocks. The mechanisms of formation and the structure of the deep, mafic, crustal units are not well understood. In particular, we do not yet know how primary mantle-derived chemically change during differentiation in the lower crust and the timescales of mafic magma crystallization. The proportion of melt finally erupting at the surface of the Earth is also poorly constrained. An extremely useful way to decipher the large-scale lithological variability of the Earth s crust, the mechanisms of magma differentiation and to estimate mass transfer in the crust is to use crystal mush fragments commonly brought to the surface during eruptions. They represent cumulate rocks formed in the crust and therefore are a unique opportunity to determine the mineralogy of frozen, highly crystalline, parts of crustal magma chambers, inaccessible otherwise.The main objective of this project is to refine our understanding of deep magmatic processes beneath oceanic and continental basaltic volcanoes and to decipher the dominant mechanisms of crust formation. We will perform high-temperature experiments to constrain the formation and disaggregation of crystal mushes prior to eruption and determine their thermo-physical state in the crust. We will also use experimental data to propose new and accurate formulations of mineral-melt equilibria, applicable at crustal pressure (<7 kbar) that can be used to estimate magma storage conditions in the crust. These results will then be combined with a statistically-based, textural and geochemical analysis of phyric basalts from 4 tectonic settings: mid-ocean ridge (Galapagos Spreading Centre), oceanic plateau (Shatsky Rise), continental flood basalt (Snake River Plain) and continental arc (South Volcanic Zone of Chile). We will use high-quality imaging of basaltic samples together with geochemical mapping and crystal size distribution to discriminate between phenocrysts in equilibrium with the melt and crystals formed in deep crystal mushes. The composition (major and trace elements) and texture of crystals together with the compositions of their melt inclusions will then be used to investigate the deep plumbing systems and magma storage conditions of the basaltic volcanoes. In particular, we will identify the role of (heterogeneous) mantle melting and the processes and timescales of intra-crustal differentiation on the formation, growth and composition of the crust and on the geochemical variability of erupted magmas. All together, this project will improve our understanding of the lithological and geochemical variability of the crust in various tectonic settings and will add to the current understanding of crystal mush rheology, a critical parameter for magma differentiation. Our results will also be useful for other basaltic provinces where crustal processes play a major role on the diversity of erupted magmas.